As petroleum, production continues to decline in many parts of the globe, more operators are seeing well abandonment as a reality. Drilled wells are plugged and abandoned for different reasons of which the typical and operational reason is that the well has reached its economic limit or when drilled it was found to be a dry hole. (refer). According to Ide, T., et al 2006, well bore is taken to be high fluid transmittal pathway. Even with the current procedure of sealing and abandonment, individual wells have the tendency to loose their integrity due to various factors, which include but not limited to poor cementation, poor or ineffective plugging, and increase in formation pressure after abandonment, corrosion of casing (refer).
Safe and economical well abandonment are important to the industry from environmental and financial standpoints. Improper abandonment can require re-abandonment procedures to mitigate environmental contamination or to comply with updated regulations, causing an increased financial burden on the operator.
All wells drilled have a distinct life cycle with respect to its cost, duration, recovery, and value. Although these characteristics and attributes are specific to an individual drilled well, all producing wells pass through the same initial and final state, beginning with completion and ending with abandonment. After the drilling stage of a well and the target depth is reached, a decision to complete the well is made based on the reservoir attributes: is the well dry or is the hydrocarbon in place of economic value. Ultimately, every well becomes dormant because of reduced economic returns or technical problem. When a well stops producing, it either may be shut-in (SI), temporary abandoned (TA) or permanently abandoned (pa).
With ageing fields fast approaching their economic limit, abandonment is becoming increasingly frequent and many operators have to modify their abandonment procedure to fit the Wellbore condition and make certain that abandoned wells remain permanently sealed and prevent commingling while balancing the environmental objectives of abandonment and cost of actual abandonment. Wells, which are not abandoned appropriately, can become a major hazard to the underground source of drinking water and possibly the aquatic environment 
Shut in status (SI)
When a well is shut-in it is still flowing but its Christmas tree, SV, wing valves are all closed. Usually a well is shut-in if there is a technical or operational problem, which is believed to be temporary. There is no maximum time for a well to remain in shut-in status as long as it is regularly maintained according to regulatory requirement and procedures.
Temporary abandonment status (TA)
A well is said to be temporarily abandoned when the wellhead is removed and the producing interval is isolated with a plug and the casing is plugged below the mud line.
REASONS FOR ABANDONMENT
There are various reasons why a well is abandoned, these are:
END OF FIELD LIFE ABANDONMENT
Drilled wells must at one point in time be abandoned. Before a well reaches the point at which it has to be abandoned it passes through various stages in its life cycle; it begins with the survey and exploitation of an area for signs on hydrocarbon . This leads to a rewarding and exciting discovery of an accumulation of hydrocarbon deposit. This is followed by the acquired Data Processing stage and finally the drilling process. During drilling, the well is created by the use of drill bit and cased off at specific as drilling progresses.
Another fulfilling target is reached when the first hydrocarbon is produced a process which unfortunately eventually proceeds the declining period where the rate of hydrocarbon production decreases. However, successful enhanced oil recovery techniques often than not make this stage rewarding financially as it extends the life of the well .
When all enhanced oil recovery technique has been employed, and the cost of producing the well is no longer economically viable, the next process is abandonment; a stage not so welcomed by most operators as it means the cessation of production. Dry hole Abandonment
A drilled well is also abandoned when after drilling, the hole was found to be a dry hole.
Though abandonment is meant to be a permanent termination, the effect is felt for many years more than that of the short producing life of the well.
The main goal of any plugging and abandonment is to provide a permanent and effective isolation of fluids all along the subsurface formation in the different layers where they were enclosed prior to plugging, thereby preventing fluid migration and reduce environmental risks of contamination and prevent costly remedial jobs . To achieve this several significant intervals of the well must be filled and tightly closed with a sealant material from bottom hole to the surface with special attention paid to the production interval  and zones of high differential pressure and temperature. The material used for plugging differ depending on what type of well is being abandoned, for oil and gas well the material used is normally cement based materials, for water well, cement based as well as bentonite can be used to isolate the different intervals[ PUT UNDER CEMENT PLUG CHARACTERISTICS UNDERE INTEGRITY OF ABANDONED WELLS. The integrity of the abandoned well can fail for very many reasons such as plug failure, poor slurry design etc. A cement plug can fail to set at the desired location as cement slurry often has the tendency to fall through the lighter drilling fluid below it . Failure can also be as a result of downhole changes which may occur after the well has been abandoned .
Over the years, techniques for drilling and completion of hydrocarbon wells have continued to evolve. This drive for new technology for hydrocarbon recovery is due to the need to maximize hydrocarbon recovery while protecting the environment . The evolution of well abandonment techniques has been much slower than that of drilling and completion. This is because abandonment is considered a sunk cost .
The objectives of this thesis is to review the factors which contribute to the overall integrity of abandoned wells. These include, well parameters, cement placement techniques, casing integrity. These play an important role in the design, construction and actual execution of the abandonment project. In the abandonment of wells, the factors that contribute to the integrity differ depending on the wells. This is because each well is a unique entity and hence has to have independent well abandonment design.
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For instance, in a well where a fish is lost in hole the abandonment design has to taken into consideration remedial action or ways to set the cement plug as there may be no access to the bottom hole to set a bottom plug in the sump.
Abandoned wells can be a cause of concern due to their potential to act as path for flow between formations, which under normal circumstance are isolated including underground sources of drinking water, of great concern are those abandonment's with faulty plugs, compromised casing and those having cracks in the cement .
This work is aimed at highlighting the different factors which contribute to the integrity of an abandoned well. ( reorder and rewrite)
Well abandonment has come a long way since the first discovery of oil and gas, with the increasing awareness of the importance of environmental protection, the need to improve the processes of abandonment has now become a major concern for many operators, as abandoned wells are considered a possible conduit for fluid flow between different formations. According to C. H. Kelm et al, the objective of abandonment of a well must taken into consideration the need to do so in a best practices manner by examining the following fundamental aim of any abandonment process;
- The need to protect any hydrocarbon left in the pay zone of the formation drilled.
- The need to preserve and prevent contamination of freshwater zones (for onshore rigs) penetrated during the course of drilling the well.
- Avert of any contamination of the surface environment. For instance, in the case of vegetation, air pollution and marine environment.
- The need to abide by all regulatory requirements stated in during the abandonment.
In the past years many papers has been published on areas ranging from alternative plugging technique, self healing and expandable cement, improved cement slurry design, placement technique with the aim to reduce the cost of abandonment and improve the abandonment. Abandoned well in an oil field are sealed using a plugging material according to regulatory requirements. A perfect example of a plugging material in the ideal sense according to D.G Calvert et al 1994 is one, which can be pumped down the drilled hole, has the ability to harden in a reasonable time, and bond with the walls of the drilled formation and casing in order to prevent fluid flow from one formation to another. While regulations vary from place to place, the general practice involves plugging the Wellbore with a Portland cement material specifically designed for the isolation purpose. In his review of plugging and abandonment techniques, D. G. Calvert et al, stated that the cement mixture used in oil and gas vary depending on the type of hole is to be isolated.
Very few papers has been published that focuses on the integrity of the actual well after abandonment. Liversidge, D. et al. in his work on permanent plug and abandonment solution for the Northsea he presented case histories of the Brent South field abandonment project done using both class G cement with an expandable agent system and flexible cement according to the current stringent regulation.
Cement integrity preservation during well completion, production phases as well as during abandonment is of critical importance for long-term protection. In the past years numerous papers and texts in the area of cement sheath failure, improved flexible and expanding cement and related topics have been published, indicating the increasing need to improve well abandonment and reduce cost. Examples of works published include but not limited to (Bosma et al 2000), (Ravi et al. 2002), (Glessner et al., 2005), (Mainguy et al., 2007), (D. G calvert et al., 1994), (Locolier et al., 2006),( Liversidge et al., 2006). Although many papers have been written, very little work has been done to investigate the cement plug integrity after abandonment. The ascribed cause to this may be that permanent abandonment is considered a non-profit venture.
Mainguy M. et al., 2007 carried out an analysis of the probability of failure of cement plugs when subjected to varying compressive and tensile load using an ideal reservoir model designed to suit changes in the downhole condition. In his study he identified that there is a greater tendency for the material used to seal zones for abandonment to fail in wells situated where there is instability in the pressure, temperature and stress state due to changes that occur downhole. He concluded that when the plug is subjected to maximum tensile stress it failed due to the low tensile strength of the conventional class G cement. Though he suggested the use of pre-stressed cement as they adapt more to changes downhole, his work did not cover the problem of rock-cement de-bonding which is a problem that greatly reduces the sealing capacity of cement. In the study done by R.C. Smith et al., 1984, on the successful method of setting cement plug, he investigated the ongoing failures of cement plugs due to the instability caused by the difference between the density of the cement and the drilling mud. In his work, he suggested the use of mud thickened with bentonite before spotting the cement so as to allow a greater density difference. With respect to the problem of controlling the direction of flow of the cement slurry a diverter placed at the end of the tubing to help divert the direction of flow and improve stability. Drilling fluid can also be used as a plugging material by adding a cementitous additive. The additive can either be 'fly ash' of blast furnace slag which have the characteristics of a cement as they harden when the mixed with water.
Cement is not naturally occurring but manmade and like any other manmade material, it is expected to age, wear-off, and, degrade overtime under different subsurface condition, which may differ from the time it was initially set [W. Zhou et. al 2005].
Plugging oil well is a common operation, which is increasing as mature field reach the end of their producing life. In general, plugging and abandonment of a well involves filling a certain length of casing or open hole with a volume of cement mixture designed for it in order to provide adequate sealing against upward migration of formation fluid. After the cement plug is place in the desired location it is left to harden over time. The placement of the cement plug is a major part of abandonment, as failure of this will cause commingling of fluids from different formation. The setting and spotting of cement plugs can be done in various ways depending on the wellbore condition and regulatory requirement.
A review of the worldwide acceptable plugging procedure shows the a minimum of three cement plugs is required of which two are, the first plug is put in place by squeezing the cement plug through the perforation into the former producing zone in order to seal off any further influx of reservoir fluid into the Wellbore. The second plug is usually set towards the middle of the Wellbore or near a protective casing shoe. Finally the third plug is set about200- 300ft below the mud line. In general, the length of a plug ranges from 100to 200ft depending on the regulatory requirements. Any additional plug set is dependent on the well bore condition.
Although observations and studies show that cement plugs have the ability to perform as expected for up to several decades, uncertainty exists that the material can maintain its isolation integrity for several thousands of years. Recent study shows that abandoned wells in which CO2 was used in the enhanced oil recovery technique prior to abandonment have the potential to leak and allow CO2 migration notwithstanding the fact that the well has been properly abandoned [Scherer, G.W et al, 2005]. This is mainly due to corrosion and degradation of the casing and cement. This degradation and corrosion occurs when carbonic acid formed from the dissolution of CO2 in brine attacks the cement and casing [Scherer, G.W et al, 2005] a process, which is dependent on the temperature of the formation, cement composition, brine and the rock mechanics and composition. Potential leakage of reservoir fluids through degraded cement plugs is hence of primary concern.
Various work on inter-formational flow shows that there is still the possibility of flow between formations even with a successful plugging of different interval. This case can arise when the abandoned well is near an active well. Javandel et al developed the first analytical model; their model showed the possible of flow to an upper formation in response to a lower injection pressure build up in a lower formation. Striz and wiggings carried out further work by developing a coupled model to predict flow using a steady state approach to create a transient flow. This model can be used to developed abandoned fluid flow using available field data.
In recent studies, statistics show that in the US one in every three well drilled for hydrocarbon is dry and have to be plugged and abandoned[D.G Calvert, et al 1994]. Wells are drilled for various reasons ranging from industrial, oil and gas, to municipal uses, but in the end these well have to be abandoned [D.G Calvert, et al 1994]. Some wells were abandoned before any regulation and guidelines were defined, these wells may have either been plugged improperly or not plugged at all and these now poses a threat to the quality of the groundwater. For the aim of regulating bodies to be achieved i.e. achieving underground water protect and hence environmental protect, the operating companies must understand that following the different regulatory requirement alone is not sufficient to guarantee a lasting protection of the environment .
It is sometimes difficult for operators to abide by the regulatory requirements as well as developing a plan which would both serve to seal off the reservoir and provide long-term protection of the environment while justifying the overall cost in general .
Currently there is a high rise in abandonment of ageing and mature field which either have reached their economic limit or are no longer producing (refer).
Methods of ABANDONMENT
The initial stage of a decommissioning process is the plugging and abandonment of the wells, during this stage, the tubing, casing strings, and, conductors are cut below the mud line and removed, zones are sealed with cement plug to isolate the flow path between the reservoir fluids and other zones as well as the surface. Zones not sealed with cement plug are filled with mud with fluid having the proper weight and consistency to prevent movement of other fluids into the wellbore.
Most abandonment's follow a general methodology that is adjusted to meet individual well requirements. As procedures can and do change significantly for each well, cement plug design should frequently be attuned to reach minimum wait-on-cement (WOC) times with varying downhole conditions. Near-wellbore geology should be assessed, and the wellbore and annuli properly cleaned to avoid microannuli and poor cement bonding. Traditional techniques include cement squeezes, gel squeezes, and mechanical plugs such as bridge plugs and packers. Cement and gel technologies are mainly used for behind casing repair, and mechanical options are usually confined to plugging the casing.
In the general process of abandonment there as basic steps which are followed to ensure successful plug and abandonment program. This includes the planning process, wellbore equipment testing, designing, well geometry assessment.
The most essential decision after when to abandon a well  is how. Preparation is a key ingredient in plug and abandonment of a well. In order to abandon a well successfully careful planning and effective plugging and abandonment procedure is crucial to prevent gas or fluids from moving to the surface or to other subsurface formations. In addition to the environmental risks that come with poor seals, corrective plugging may be necessary, increasing the cost and difficulty of abandoning a well. However, operators and service companies have several options for obtaining complete, permanent abandonment.
For every well, there is a variation as each well P&A is unique and different. The techniques used to achieve this process are generally based on industry practice, research, and conformance with the relevant regulatory compliance requirement. The synthesis of practical knowledge, current technology and regulatory requirements results in the most effective wellbore plugging and abandonment possible.
Wellbore equipment testing.
A preliminary inspection and survey of the wellhead and wellbore condition is carried out to determine if the valves on the wellhead are in operable condition, if it is found not to be in operable condition they are hot-tapped. The wellbore is surveyed using a slickline unit to check for any obstructions in the well, to confirm measured depth and also to gauge the internal diameter of the tubing. After the survey and removal of the slickline, the annuli and tubing is filled with fluid using a well pump is installed at the wellhead to ascertain an injection rate into the perforations. The tubing and casing are also pressured up to check for integrity. Casing annuli are also pressure tested to check for communication problems between casing strings and to record the test pressure over a period of time. The integrity and reliability of the primary cement is assessed in order to ensure that the cement sheath is still providing isolation across the reservoir and the cap rock.
A well control plan is designed to establish reservoir condition and subsequently the contingency responses to any event which may occur during the abandonment process.
DESIGN OF A WELL ABANDONMENT PROGRAM
Prior to plugging and abandoning a well, a review of the existing well design, record of past work, previous well performance and geologic and reservoir condition is carried out by the operator. The investigation of everything that may relate to the health and safety issues as well as regulatory requirements is also performed, after which the design of the abandonment program begins. The design is done based the existing wellbore and reservoir conditions depending on the findings from the review and investigation. This allows the operator to plan an abandonment program that will satisfy the goal of making the well safe from future resources. P&A design needs to be integrated in the planning of the well, and should be considered in a single budget. There are many factors which must be put into consideration in order to design an effective abandonment program , such as, the reservoir status, the integrity of the primary cement, hole cleaning and cement placement technique, the temperature and pressure of the well, the type of fluid in the well, the age of the well, the status of the cap rock.
- Fluid Type
Drilled wells produce fluids in liquid and gaseous form, wells which contain sour fluids i.e. sulphur rich would be expected to have accelerated corrosion rates and stress cracking depending on the age and wellbore construction, may impair the capacity to perform plug and abandonment, to mitigate this components which are corrosion resistant can be used.
- Reservoir status
In the design of P&A, it is necessary to consider the reservoir status concerning its stability, the current pressure and temperature, the pressure at the initial stages of well development and the permeability of the reservoir both horizontal and vertical. With the information, plug and abandonment is then designed to withstand the pressure of the well after finally reach equilibrium.
- Cap rock Status
It is also necessary to take into consideration the cap rock status i.e. is it still impermeable, has production activities induced fracture or has weathering taken effect.
Placing the Plugs
After the design and planning of the abandonment program, calculations must be made to determine the amount of cement required and the number of wiper plugs needed to separate the cement plugs from the rest of the fluids. The use of wiper plugs enables the formation of a stable platform on which the cement can be set. A wiper plug is placed in the wellbore, and then a predetermined quantity of cement slurry is pumped on top of it. Because of its weight, the slurry becomes a driving force. The slurry falls to the bottom of the hole, pushing the wiper plug ahead of it and forcing existing air and produced fluids back into the formation. Another plug and perhaps a bit more cement finish the job. In most wells, where there is one permaeble zone, one plug and one volume of cement and the surface plug are all that is needed. In other wells, additional wiper plugs, additional cement slurry, and probably spacers of water or drilling fluid are used consecutively until all of the air and fluid is forced out into the formation, there is zilch pressure on the pipe, and it is apparent from the returns that the whole wellbore is appropriately sealed. The quantity and kind of spacer fluid that can be used is dependent on individual state regulations. The remaining casing at the top of the well is cut off 3 ft below ground level.
Along with this general methodology, each region stipulates its own abandonment methods based on field conditions and local regulations as can be seen in the following examples.
P&A steps in Los Angeles Basin in as follows :
The abandonment program is prepared with the support of a qualified engineer.
A schematic showing the current mechanical condition of the well is prepared.
The geologic condition of the well, including the structure, faulting, and producing zones is assessed.
The depth and position of cement plug that will cover the producing zones and any potable water zones if applicable is measured and verified.
Choice of whether to use perforating or cavity shots is made.
The casing is pressure tested after setting cement retainers.
The different equipment required for the job is determined and assembled.
Estimate of abandonment/re-abandonment costs is made.
In contrast, the steps followed for the Hutton tension-leg platform (TLP) in the East
Shetland Basin of the North Sea involved three phases :
Perform standalone wireline intervention.
Perform drilling unit intervention to set the cement plugs after the first wireline plug has been set.
Cut casing 10 ft below the seabed and recover casing stumps.
Another abandonment performed in the North Sea followed a different procedure :
A permanent cement primary barrier placed immediately above the reservoir.
A secondary barrier placed as a back-up to the first barrier.
A third barrier then placed near the surface to isolate shallow water-bearing sands.
Severed completion tubing and recover wellhead.
In Western Canada, the traditional abandonment procedure of wells with casing vent flows included the following:
The source of the casing vent flow is estimated or determined.
If the source zone is shallower than the producing zone, the producing zone is abandoned.
The source zone is perforated. Depending on the feed rate obtained at the estimated source depth, either a bradenhead or a retainer squeeze is performed.
Retrievable tools are used as required.
Typically, Class G cement with Calcium Chloride and some fluid-loss control is utilized.
The slurry is placed and a static squeeze pressure of 7 MPa is attempted.
As needed, cement is drilled out and perforations are tested for seal.
Often, several attempts are made in order to obtain a static squeeze pressure of 7 Mpa on surface or mitigate the casing vent flow.
Techniques for Abandonment
The techniques used for plugging and abandonment of drilled well worldwide are generally based on industry practise. These techniques include;
- Coil tubing unit
- Rigless abandonment
COIL TUBING UNIT
The flexibility of coiled tubing has recently been tailored to develop rigless abandonment [15,16]. This method, focuses on harmonizing all well services to accomplish utmost efficiency. Coiled-tubing unit [fig....] abandonment, like any other method, is more effective when appropriate cementing procedure is used from the kick-off of the well, from original zonal isolation with the primary cement sheath to plugging and abandonment. Early prevention of microannulus can help operators obtain a complete final seal.
Five main criteria are recommended for optimal abandonment performance with coiled tubing:
- Mobility; All equipment should be mounted on wheels for increased mobility.
- Self-sufficiency; the service company provides nearly all activities.
- Dry location; Fluids are not drained on or near the wellsite.
- Single operation; the job is completed in one visit to the wellsite.
- Low mileage; Move time is reduced and transport optimized for maximum efficiency in unit and camp moves.
In this abandonment technique geological consideration like the type and condition of the reservoir and caprock formations are take into account. Permanent seals typically must be made between producing zones and at impermeable caprock formations. The condition and configuration of cement, perforations, tubulars, and downhole equipment are also considered.
In addition to providing complete, permanent seals, the use of coiled-tubing can help increase abandonment efficiency. This method can provide the following advantages:
- Increased tripping speeds
- Increased rig-move efficiency
- Precise placement of cement plugs; exact spotting of plugs at the interval of interest even in deep well as coil tubing can be reciprocated while pumping.
- Suitable for use on live wells; it is possible to run CTU for remedial cement squeeze in live well as the wellbore can be controlled using the BOP and stripper assembly.
- No need to pull production tubing; existing tubing and wellheads do not have to be removed to access the producing interval.
Success using the coil tubing method has been recorded in Oman.
REGULATORY REQUIREMENT FOR ABANDONMENT IN THE NORTHSEA AND USA
In the early years on the oil and gas industry, many wells were drilled and some were found to be dry and subsequently were abandoned without much consideration given to the manner in which the wells were abandoned. Sometimes tree stumps were thrown in the well as a means to plug it , during this era the preservation of the groundwater, in general, the environment was not a major issue, and there was no defined regulation by the oil states or agencies. During the tail end of the 1930's different states and agencies in the US started establishing regulations, this defined requirement to ensure better well abandonment [D.G Calvert, et al 1994].
The number of regulation guiding well abandonment has risen along with the rising need to protect the environment in countries around the world. Today most countries have some form of regulation that addresses well abandonment requirement; though these regulations are not uniform and differ from country to country and body to body, they provide a minimum standard for operating companies. For instance for the state of California in the United States of America, the different governing bodies have their own regulations which are as follows;
- Minerals Management Services (MMS): The basic plugging requirements are found in 30 CFR 250.110 Subpart G.
- Department of Conservation, Division of Oil: Gas, and Geothermal Resources (DOC)
- The California Code of Regulations Title 14 Division 2, Chapter 4 beginning with Section 1745 focuses on the fundamental plugging requirements.
- California State Lands Commission (CSLC): The fundamental plugging requirements in the California Code of Regulations Title 2 Section 2128(q).
Abandonment in the North Sea
In the North Sea as in US, the regulations differ. The different countries that make up the North Sea have their different governing bodies and subsequently different regulation. The law in the UK, Norway, Denmark and Holland holds the last operator of a well accountable and responsible to pay for all the cost incurred in permanently abandoning the well. It also holds them accountable for any leakage and any clean up that may be required in the event of a leak.
Abandonment programs in the Northsea are designed to meet the guidelines for abandonment issued by the operation association or government. For the UK sector of the north sea, abandonment guidelines is issued by the UKOOA, similarly for the Norwegian sector the guidelines are contained in the NORSOK/PTIL D-010 standard and for the Netherlands it is contained in the Dutch Mining Authority guidelines.
The different guidelines obtainable from the various sectors of the North sea and the world as a whole all veer towards the same goals and aims which are;
- Prevention of any hydrocarbon migration to the surface
- Prevention of any migration or commingling of wellbore fluids from one formation to another
- Preservation of the aquifer and fresh water zones by preventing any contamination by hydrocarbon fluids.
The different guideline specifies the regulatory requirements for achieving the various goals.
For the UK, the UKOOA guideline describes a permanent barrier to be one whose length is adequate enough to extend laterally across the entire wellbore.
It also stipulates barrier requirement for permeable zones and surfaces as
One barrier required where the zones is water bearing and normally pressured
Two barriers where the zone is water bearing but abnormally pressured.
Two barriers where the zone is hydrocarbon bearing
Second barrier of one zone may be first barrier of another
It gives no particular requirement for cement squeezing the perforation.
Barriers are also required where zones are in different pressure regimes.
For Norway the NORSOK standard principle can be summarized as follows;
Annular plugs are NOT acceptable for permanent abandonment rather it is a requirement to cut and pull the casing string after which a T-plug is set.
Explosive well head severance NOT permitted rather it is a requirement for mechanical systems such as abrasive jet to be used.
Two barriers are required of which one must be cement; Primary barrier acceptable as one of the permanent barrier
All annular spaces are to be secured and sealed
The surface plug is required to be longer 200m in length and less than 50m below the seabed.
It is a requirement to isolate the perforations by cement squeezing and the use of mechanical barrier.
For the Netherlands, the Dutch mining Authority stipulates that;
The plugging of the wellbore should be properly done and durable.
For open-hole completions
It is required that a cement plug of minimum of 100m in length be used
A mechanical plug with cement on top
Reservoirs is plugged with cement; two reservoirs have to be sealed form each other with a cement plug of 100m in length or one equal to the length of the open hole.
For any perforations, DMA stipulates that a cement plug of a minimum of 100m in length placed from the top of perforation upwards or a mechanical plug with a 50m cement plug on top, or a mechanical plug with cement and perforation squeezed off.
Requirements for the reservoir is the same for open hole
Depending on what is involved in the execution of the abandonment various regulation govern the whole abandonment process, for instance in UK alone we have;
Offshore installation and well (design and construction) regulation 1996; these set of regulation addresses key issues relating to design and construction of offshore installation focusing on aspects of technical integrity and reliability of installations and wells. Section 13 of part IV focuses on well fluid containment throughout the wells life cycle. Section15 concentrates on well fluid containment after abandonment while, section 16 focuses on the materials to be used in well operation.
It stipulates the general duty for an operator to abandon a well in a manner so far as reasonably practicable that there can be no unplanned escape of fluid from the well and any risks to the health and safety of persons from it or anything in it or in strata are as low as reasonably practicable.
Offshore petroleum activities regulation 2005; these set of regulation govern the discharge of hydrocarbon in the UK waters. Any discharge into the UK waters are prohibited except with permit granted under these regulations and the permit stipulates the quantity of discharge.
Offshore chemical regulation 2000: These set of regulations govern the use and discharge of chemicals in the UK waters.
Offshore petroleum activities 2001: these set of regulation focuses on the protection and conservation of the fauna and flora and the marine environment in general.
Unavailability of Records
The availability of records is very important to planning and execution. If records are not available or the one available does not match the well configuration, then the planning success may be impacted. For instance in old wells , records may not exists and even if they do they may be incomplete and may provide inaccurate data on downhole conditions. Ideally operators should integrate abandonment plan in the initial plan of drilling of the well this allows changes made during drilling and production activities to be reflected in the abandonment plan if it is updated sequel to the changes.
Age, Production and Completion Operation of the Well
With time equipments and components of a well deteriorates and degrades from corrosion, wear, and normal operating conditions. This may affect the ability to perform the plug and abandonment operation. In addition, the construction practices used during the completion and production operation of the well may affect the P&A operations. For instance, the hydraulic fracturing of reservoir can induce cracks in the cement and remedial actions have to be taken before P&A can be done. On the other hand if the annuli is isolated with uncontaminated cement and the well is kept in good mechanical condition with regular workover operations then the P&A should be straight forward.
During the critical factors in P&A include;
- Wellbore accessibility
- Position of the plug
- Cutting and removal of tubular
- Placement of plugs
- Suitability of the sealing material
Documentation of wellbore accessibility i.e. if there is any tool lost in hole , location of packers. These can mitigate or complicate the design and construction of a well abandonment and hence should be consideration in the design. Rearrange
3.0 INTERGRITY OF ABANDONED WELLS
This chapter discusses the factors that affect abandonment. These critical factors include, cement plugs, primary cement sheath, well attributes,
3.1 CEMENT PLUG
In P&A process, based on the well condition and regulatory requirement, the plugs placed at the different zones should be designed to have specific properties to withstand any changes that may occur. These plugs are placed to at least the required minimum length about the zone. Despite the simplicity of plug-cementing operation, historically cement plugging has had a significant high rate of failure. Mud contamination is recognised as the most common reason for cement plug failure. This contamination drastically affects the cements compressive strength. However, even if the cement slurry was placed properly and initially fulfils its intended function, changes in downhole condition can destroy the integrity, which can lead to gas/fluid migration.
The loss of cement integrity and subsequently zonal isolation can be due to mechanical failure, de-bonding of cement from the casing or the formation, poor hole cleaning.
3.1.1 TYPES OF CEMENT USED FOR PLUGGING
18.104.22.168 Flexible Cement
Cement has long been recognized as the best material for sealing abandoned wells . However; recently developed sealant systems can provide better long-term wellbore integrity than Portland cement by increasing resistance to stress cracking and the formation of microannuli . These systems use an optimized distribution of flexible particles, which allows the set sealant to adapt to temperature and pressure changes. The ability to adapt facilitates the improvement of long-term zonal isolation for the well . Flexible sealant systems also have inherently low permeability, which helps them resist corrosive fluids. Their flexibility and linear expansion abilities are suitable for abandonment procedures.
3.1.2 CEMENT PLACEMENT TECHNIQUES
Cement plug placement is an important aspect in the whole abandonment process. Though the plugging material is important, the technique used to place the plugs is regarded as the most important aspect [D.G Calvert et al 1994], as there is a high tendency for the well fluids to contaminate and dilute the material used for the sealing process hence utmost care is put into consideration. Setting cement plugs is a well-known problem in the oil industry many failures have been recorded, the causes of these failures have been identified and addressed, techniques to address the improvement of these practices have been developed (e.g.???).
Depending on the wellbore condition and regulatory requirements, the typical cement placement methods include:
- The balance plug method
- The cement squeeze method
- The dump bailer method
22.214.171.124 THE BALANCED PLUG METHOD
In this method, the cement is pumped through tubing, work-string, coil tubing or down pipe until the level of the cement in the annulus is slightly below the top of the cement inside the tubing, then the tubing is slowly pulled up form the slurry. The cement then falls out of the pipe, filling the void space left as the pipe is removed.
Proper placement of cement using balanced plug method has remained a serious concern in the industry for many years. Many studies have been done to investigate plug cementing. Smith et al studied plug cementing in detail and made excellent recommendations for setting successful cement plugs. The recommendations included the placement of gelled fluid underneath the cement slurry and the use of a diverter tool to control the direction of flow of the cement. Bour et al recommended using reactive fluid system that develops a rapid gel is placed before pumping the cement in order to keep the cement slurry in place. Harestad et al have further improved these practises with the improvement of the balanced plug placement practices by the use of a mechanical cementing tool to prevent downward movement of slurry. In general, the use of fluid spacer ahead and after the cement helps in the proper placement of the cement.
126.96.36.199 CEMENT SQUEEZING
Squeezing is a process of using pressure to force the cement slurry to specified points, it is most commonly used to seal off reservoirs. This method of spotting cement is most favourable used to plug of perforations, it is also used to place cement plugs below any junk that my have been left in the well bore and to repair any poor cementing behind the casing. The various method of cement squeezing includes the bradenhead method and the bullhead method.
188.8.131.52.1 Bullhead Method:
This squeezing method involves pumping cement from the surface and forcing it down the borehole by the pump pressure from the surface.
184.108.40.206.2 Bradenhead Method: this method can be used as an alternative to bullheading, in this method, the cement is placed in the same manner as the balanced plug method but then the well is shut-in and extra pressure is applied at the surface from the casing valve to push the cement further down the wellbore.
220.127.116.11 DUMP BAILER METHOD
The dump bailer is a tool that contains a precise amount of cement. In this method, the exact volume of cement is lowered into the wellbore using a wireline, when the interval is reached the dumb bailer is either opened by electronic activation or on impact with a bridge plug. This method is used to place small quantities of cement, though it allows for precise placement of cement, it is not convenient for permanent plug and abandonment as it means many trips down the bore hole.
3.1.3 VERIFICATION AND TAGGING OF CEMENT PLUGS
To ensure proper plug placement it is required that the plugs be tagged. The two techniques mainly used to ensure appropriate placement are:
Use of open-ended pipe of pre-determined length to tag the plug
The use of wireline tools to tag the plug and verify the top of cement (TOC) by reading the counter on the wireline
18.104.22.168 Pressure Testing
As a requirement for abandonment, the cement plugs must be tested to confirm that the pressure isolation has been achieved.
3.2 PRIMARY CEMENT INTEGRITY
One of the main functions of cement is to prevent commingling of fluid between formations or to the surface throughout the life of the well, including after abandonment. Changes in the downhole condition such as change in temperature, pressure or tectonic effects can induce stresses that can alter the integrity of the cement sheath resulting in a loss of zonal isolation even with cementing operations providing initially provided a good hydraulic seal. When well abandonment process is taken into account in the early stages of the life of the well design, it makes provision for improved quality of primary cement between the casing and formation, this is an important factor that determines the success of well abandonment performed later. The abandonment plan must take into account the types of fluid contained in the reservoirs, the wells attribute, and the mechanical condition at the time of abandonment. Great advances have been made in cementing practices over the years. These advances include improvement in fluid displacement modelling as well as the development of slurries with improved chemical and rheological properties. These advances have gone a long way towards improving hydraulic isolation, but they do not address damage to the cement sheath that may occur days, months or years after the cement has set. The potential for loss of hydraulic isolation during or after a wells productive life represents a weak link in hydraulic isolation. A damaged cement sheath can allow migration of hydrocarbon and formation fluids. The use of novel material for well cementing which has the ability to repair in the event of any damage can offer significant protection against the loss of hydraulic isolation and prevent the migration of fluid.
Other CATEGORIES OF ABANDONED WELL
FACTORS THAT AFFECT CEMENT INTEGRITY
Mechanical failure occurs when either there is a large increase in wellbore pressure, wellbore temperature or the formation loading (creep), which leads to formation cracks. Mechanical failure can also be as a result of excessive shrinkage of cement slurry used. The characteristics of cement used may not be flexible when it sets, e.g., cement slurry design with high Young's modulus ratio and a passion ratio lower than that of the rock can lead to cement failure.
Formation of Microannulus
The formation of microannulus is as a result of debonding of either the casing from the cement of the cement from the formation. The cause of de-bonding can be due to radial displacement of the casing following wide changes in the downhole condition (temperature and pressure). This also negatively affects cement integrity. However, successful removal of the drilling mud and the filter cake during the initial stages of drilling decreases the risk of microannulus forming thereby increasing the bonding between the rock and cement for open hole and casing and cement for cased hole. In addition to this, the use of low shrinking cement as well as mathematical modelling to determine the maximum size of the microannulus in the event of changes in temperature and pressure.
Bulk shrinkage of cement slurry
The bulk shrinkage characteristics of ordinary Portland cement slurry as it sets can create small cracks and gaps which the creates a pathway for fluid to flow. Bulk shrinkage of conventional cement causes internal stress and if this stress is greater than the tensile strength then the cement cracks. On the other hand, if the stress is greater than the bond strength this results in cement de-bonding from the rock or casing. However, Bosma et al  recommends that to ensure a tight seal a cement slurry design with low Young's modulus and Poisson's ration close to 0.5 placed so that they would become thermally stressed.
Cement quality can also be compromised by well completion and workover operations such as hydraulic fracturing treatment, which can introduce cracks in the primary cement and excessive fluid losses during the primary cementing operation can cause flow paths.
Other causes of cement include:
- Tectonic forces can fracture set cement thereby creating flow paths
- Poor interfacial bonding and the use of highly permeable slurry systems
- The cement may not be resistant to corrosive fluids such as H2S
Regardless of the cause, loss of cement integrity is the failure of the main goals of well abandonment as this can result in fluid migration and impairment of zonal isolation.
CHALLENGES FACED IN PERMANENT WELL ABANDONMENT
In well abandonment project, some challenges may be encountered which may affect the overall success of the project. Some of these factors include:
- Improper understanding of the wellbore geometry, geology, accessibility, downhole equipment and is condition.
- Improper understanding of the reservoir pressure and fluid migration paths
- Inadequate data available on the history of the well such as well logs, diagrams because of time lapse between construction and abandonment.
Wellbore geometry is an important factor in well abandonment as every well has varying characteristics. In some wells overpressured formations are encountered during drilling, these abnormalities must be put into consideration in the design of any abandonment program, as they can be a cause of concern. The cement slurry recipe must be designed to withstand the pressure or temperature abnormalities in the various intervals.
Gas well with CVF issue
As earlier mentioned every well is different and has individual attributes. Therefore, plans for completion and abandonment are done according to the nature of the wellbore. For this case scenario, during drilling operations many gas bearing zones of different pressures were encountered prior to reaching the target depth. On completion pressures were observed in the annuli in different zones. Remedial actions were taken to repair the leak but this did not solve the problem, and eventually abandon the well.
The abandonment methodology used in this case is as follows;
- Conventional Portland cement was placed inside the 7`` casing and the 9 5/8''
- The 9 5/8'' casing was then milled out over a pre determined depth and underreamed to increase the diameter of the hole. Class G cement grade was then placed between this section.
- Microcement system was placed on top of the cement subsequently a 60liter was squeezed.
Though the initial repairs appeared successful, but after continued monitoring it was discovered that a zone was not fully isolated and the well had to be re-entered. Subsequently to further ensure long-term isolation both inside and outside the casing, a new abandonment plan was reviewed. It was concluded that the section to be isolated would be completely milled out. A cement plug designed to withstand the differential gas pressure placed form rock-face to rock-face.
The revised abandonment methodology included:
- Milling a 25m window in the 9 5/8'' casing just below the 13 3/8`` casing shoe. The diameter of the hole at the p 5/8`` casing was further increased to 22`` to reach the formation. The hole was then cleaned by circulating in drilling fluid.
- Flexible and expanding cement plug is then placed in the milled section and squeezed
- The bridge plug is set and pressure tested.
- The 9 5/7`` casing is cut and annular content are circulated and removed.
- High viscosity pill is spotted and subsequently a conventional cement slurry is placed in the 13 3/8`` casing.
- The 13 3/8`` casing is milled out just below the 20`` casing shoe and under-reamed to a diameter of 22`` to reach the formation and the cuttings is circulated out and removed.
- Flexible and expanding plug is placed in the underreamed section and squeezed.
- The bridge plug is set and pressure tested
- The high viscosity pill is spotted followed by the placement of conventional cement slurry in the 13 3/8 to the surface.
This abandonment methodology worked and provided complete isolation of the different pressured zones for this well with high differential gas pressure. For a normal well with straightforward wellbore and reservoir attributes, the initial abandonment program would have provided the intended long-term isolation of the different zones.
Risk analysis is the possibility that events or conditions in the future may not occur as expected. When throwing a dice, you may want a six but there is 25% chance that it will be a two. In abandoning, a well there is a high possibility that the permanent barriers placed across required interval would fail.
There is substantial risk in projecting the effectiveness of the permanent barriers and how long it would take nature to restore the pressure balance that existed before the well was drilled. This is because nature moves at its own pace, which is measured in geologic time not in years.