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Drilling fluid or mud is a fluid that is circulated through the drillstring and up the annulus back to surface under normal drilling operation. It is very important in the drilling process after the introduction of rotary drilling. The earlier uses were for well completions, but broadened with time to include coring, stuck pipe release/prevention, annulus packs, wellbore stabilization, HPHT applications, corrosive environments and directional drilling.
2.1.1 Functions of Drilling Fluid
Variety of additives was added into drilling fluid to carry out the following functions:
Counteracting formation pressure. The column of drilling mud in the drill pipe and annulus provides a hydraulic head that counteracts the pressure in the formation preventing blowout.
Protecting permeable zones from damages. Special drilling mud additives build a filter cake on the walls of the well, preventing drilling mud loss into permeable formation.
Removing cuttings from borehole. Jets of drilling mud exiting from drill bit carry drill cuttings away from the bit.
Cooling and lubricating the drill string and bit. Friction of rotating drill sting heats the drill pipe and the bit. Circulating of drilling mud cools the drill string and lubricates it where contact is made with the formation.
Suspending solids. The drilling mud has sufficient viscosity to suspend cuttings and weighting agents when drilling stops, as during addition of drill pipe.
Supporting parts of the weight of drill string. The steel rill pipe is supported in the hole in proportion to the volume and density of drilling mud displaced by the pipe in annulus.
Protecting, supporting, and stabilizing borehole wall.
Help in evaluation and interpretation of well logs.
2.1.2 Types of Drilling Fluids
There are three main types of drilling fluid which are (1) Water based mud; (2) Oil based mud and (3) Gases fluid
18.104.22.168 Water Base Mud
The term of water based mud is referring to any drilling fluid that the continuous phase is water. Several of additives can be also present in this mud. Water based mud contain of water phase, inert solids, a reactive solids phase and chemical additive. All of these parts affect the overall mud properties.
There are several types of water-based mud systems consist of:
Clean water and native muds, used to drill compact formations which are normally pressure (formation pressure equals hydrostatic pressure caused by formation fluids).
Dispersed muds - Lignosulphonate muds, used when the mud has to have following such characteristics such as relative high mud weight, used at moderately high formation temperature, low filtration loss required and high tolerance for contamination by drilling solids.
Nondispersed muds - KCl/ Polymer muds, used to drill water sensitive and sloughing shales as productive sand which are prone to formation damage, fresh water nondispersed muds are applied.
Flocculation muds, used to increase dynamically in filtration, viscosity and gel strength.
Salt-saturated muds, used to drill through salt domes and salt sections.
22.214.171.124 Oil Based Mud
Opposite to water based mud, oil based mud is a drilling fluid which contains crude or diesel oil as its continuous phase with only a small amount of water dispersed as droplets. This mud can have as little as 3% to 5% or as much as 20% to 40% (invert emulsions) water content. Oil based mud systems are applied when:
Drilling sensitive production zones or problem shales.
Drilling salt sections and formations that contain hydrogen sulfide.
Danger of stuck pipe problems
Drilling at bottom hole temperature that are permissible by water-based mud.
126.96.36.199 Gases Fluid
Gases drilling fluid consist of air, natural gases, mist, aerated mud and foams. This type of drilling fluid is used because it has lowest density that can prevent from formation damage and lost circulation. The use of gases fluid as circulating medium can be categorized into four drilling operation:
Air or dust drilling, which the circulating medium is air only, maintains the lowest possible downhole pressure.
Mist drilling, which water, containing a foaming agent (soap), is injected into the air stream at the surface and discharged as a wet mist.
Foam drilling, is often subdivided into stable- and stiff-foam drilling, used to improve hole-cleaning properties and drilling large-diameter holes.
Aerated mud, which both air and mud flow simultaneously in the annulus as air bubbles dispersed in a liquid to reduce ECD below that of water.
2.1.3 Mud Properties
Mud properties are very important to be evaluating to ensure the drilling fluid meets necessary criteria needed. These include mud weight, plastic viscosity, yield point, gel strength, filtrate and mud cake. All these rheological properties can be controlled by adding suitable additive.
188.8.131.52 Plastic Viscosity, Apparent Viscosity and Yield Point
Viscosity is a property of fluids and slurries that indicate their resistance to flow, defined as the ratio of shear stress to shear rate. The viscosity of fluids varies with pressure and temperature. The viscosity of most fluids is rather sensitive to changes in temperature, but relatively to pressure variations until they are exposed to high pressures. The effect of molecular weight on the viscosity of liquid is the liquid viscosity increase with increasing molecular weight.
Plastic Viscosity (PV) in centipoise (cP) or milliPascal seconds (mPa•s) is calculated from mud viscometer data as:
PV (cP) = Θ600 - Θ300
Plastic viscosity is usually described as that part of resistance to flow caused by mechanical friction. Primarily, it is affected by:
Size and shape of solids
Viscosity of the fluid phase
The presence of some long chain polymer
The Oil-to-Water (O/W) or Synthetic-to-Water (S/W) ratio in invert-emulsion fluids
Type of emulsifiers in invert emulsion fluids.
An increase in plastic viscosity can mean an increase in the percent by volume of solids, a reduction in the size of the solid particles, a change in the shape of the particles or a combination of these. Any increase in the total surface area of solids exposed will be reflected in an increased plastic viscosity.
The effective viscosity is sometimes referred to as the Apparent Viscosity (AV). The apparent viscosity is reported as either the mud viscometer reading at 300 RPM (Θ300) or one-half of the meter reading at 600 RPM (Θ600).
Yield Point (YP) in pounds per 100 square feet (lb/100 ft2) is calculated from Fann VG meter data as:
YP (lb/100 ft2) = 2 x Θ300 - Θ600 or YP (lb/100 ft2) = Θ300 - PV
or in Pascals:
YP (Pa) = 0.4788 x (2 x Θ300 - Θ600)
YP (Pa) = 0.4788 x (Θ300 - PV)
Yield point, the second component of resistance to flow in a drilling fluid, is a measurement of the electrochemical or attractive forces in a fluid. These forces are a result of negative and positive charges located on or near the particle surfaces. Yield point is a measure of these forces under flow conditions and is dependent upon:
Surface properties of the fluid solids;
Volume concentration of the solids; and
Electrical environment of these solids (concentration and types of ions in the fluid phase of the fluid).
Yield point is that part of resistance to flow that may be controlled by proper chemical treatment. The yield point will decrease as the attractive forces are reduced by chemical treatment. Reduction of yield point will also decrease the apparent viscosity.
184.108.40.206 Gel Strength
Gel strength is the property exhibited by some fluids which form a gel structure while static and then become fluid again when shear is applied. Most water-base drilling fluids exhibit this property due to the presence of electrically charged particles or special polymers that link together to form a rigid matrix. Gel strength readings taken at 10-sec and 10-min intervals, and in critical situations at 30-min intervals, on the Fann VG meter provide a measure of the degree of gel strength present in the fluid. The strength of the gel formed is a function of the amount and type of solids in suspension, time, temperature, and chemical treatment. In other words, anything promoting or preventing the linking of particles will increase or decrease the gelation tendency of a fluid.
The magnitude of gelation, as well as the type of gel strength, is important in the suspension of cuttings and weight material. Gelation should not be allowed to become any higher than necessary to perform these functions. Excessive gel strengths can cause complications, such as the following:
Entrapment of air or gas in the fluid;
Excessive pressures when breaking circulation after a trip;
Reduction in the efficiency of solids-removal equipment;
Excessive swabbing while tripping out of the hole;
Excessive pressure surges while tripping in the hole;
Inability to get logging tools to the bottom.
220.127.116.11 Fluid Loss
A basic drilling fluid function is to seal permeable formations and control filtration (fluid loss). Potential problems related to thick filter cakes and excessive filtration include tight hole, increased torque and drag, stuck pipe, lost circulation, poor log quality, and formation damage. Adequate filtration control and the deposition of a thin, low-permeability filter cake are often necessary to prevent drilling and production problems. Potential problems from excessive filter cake thickness:
Tight spots in the hole that cause excessive drag;
Increased surges and swabbing due to reduced annular clearance;
Differential sticking of the drillstring due to increased contact area and rapid development of sticking forces caused by higher filtration rate;
Primary cementing difficulties due to inadequate displacement of filter cake;
Increased difficulty running casing.
Potential problems from excessive filtrate invasion:
Formation damage due to filtrate and solids invasion. Damaged zone too deep to be remedied by perforation or acidization. Damage may be precipitation of insoluble compounds, changes in wettability, and changes in relative permeability to oil or gas, formation plugging with fines or solids, and swelling of in-situ clays.
Invalid formation-fluid sampling test. Formation-fluid flow tests may give results for the filtrate rather than for the reservoir fluids.
Formation-evaluation difficulties caused by excessive filtrate invasion, poor transmission of electrical properties through thick cakes, and potential mechanical problems running and retrieving logging tools.
Logging tools (measuring filtrate altered properties rather than reservoir fluid properties).
Oil and gas zones may be overlooked because the filtrate is flushing hydrocarbons away from the wellbore, making detection more difficult.
Figure 2.1 Filter cake in a formation
Drilling fluids are slurries composed of a liquid phase and solid particles. Filtration refers to the liquid phase of the drilling mud being forced into a permeable formation by differential pressure. During this process, the solid particles are filtered out, forming a filter cake (see Figure 2.1). If the liquid phase also contains an immiscible liquid such as brine in an oil-base mud then the immiscible liquid droplets will also be deposited in the filter cake and will assist in filtration control. Permeability refers to the ability of fluid to flow through porous formations. Mud systems should be designed to seal permeable zones as quickly as possible with thin, slick filter cakes. In highly permeable formations with large pore throats, whole mud may invade the formation (depending on the size of the mud solids). In such situations, bridging agents must be used to block the openings so the mud solids can form a seal. Bridging agents should be at least one-half the size of the largest openings. Such bridging agents include calcium carbonate, ground cellulose and a wide variety of other lost-circulation materials.
Nanotechnology is a generic term for application that works with matter that is so small that it exist in the atomic and molecular realm. This term refer to application of using nano-scale materials in engineering or ability to engineer material in nanometer scale. Thus, nanotechnology can be defined as the design and fabrication of materials, devices and systems with control at nanometer dimension. At the core of any process involving nanotechnology is nanometer (nm), which is one billionth of a meter and 10,000 times smaller than anything that the human eye can see. At this size, the substance's physical, chemical and biological properties are different from what they were at micrometer and larger scales. Nanomaterials can be nanoscale in one dimension (eg. surface films), two dimensions (eg. strands or fibres), or three dimensions (eg. particles). They can exist in single, fused, aggregated or agglomerated forms with spherical, tubular, and irregular shapes. Common types of nanomaterials include nanotubes, dendrimers, quantum dots and fullerenes.
Nanotechnology offers the transformation of oil and gas exploration and production. It could provide revolutionary solutions to problems related to upstream and downstream operation. For example, nanotechnology can lead to better understanding and control of rock/ fluid interactions and their effects on wellbore stability, fluid loss and formation damage, thereby leading to improved drilling efficiency. In oil and gas industry application, nanotechnology could be used to increase oppurtunities to develop geothermal resources by enhancing thermal conductivity, improving downhole separation, and aiding in the development of corrosive materials that could be used for geothermal-energy production. Nanotechnology also could help improve oil and gas production by making it easier to separate oil and gas in the reservoir, enhance the possibilities of developing unconventional and stranded gas resources. The oil and gas industry relies on the strength and stability of its material in all of its process. By building up such substaces on a nanoscale, it could produce equipment that is stronger, lighter and more resistant. Nano material can be made as light and elastic as silk yet as strong as steel.
In drilling industry, nanomaterial have great potential for a broad range in the drilling industry. Nanotechnology is not new, but its application in the oil industry is certainly in its infancy, including drilling application. Adding nano material into drilling mud can modify its properties and their suspension can provide many advantages. They can impart sedimentary, thermal, optical, mechanical, electrical, rheological and magnetic properties to a base fluid that can improve its performance. For record, oil exploration has used nanotechnology in drilling muds for the past 50 years. The nanoparticles in drilling muds are made of clays and are naturally occurring 1-nm-thick discs of aluminosilicates. These nanoparticles exhibit extraordinary rheological properties in water and oil. Recent studies indicates that successful applications of nanotechnology in drilling are likely to occur with synthetic nanoparticles, where size, shape and chemical interactions are carefully controlled to achieve the desired fluid properties and drilling performance.
Nanomaterial have unique properties due to their small size and high surface area per unit volume. As a result, they are found useful in many applications including oil and gas exploration and production. Nanomaterial appear to be stronger and more reactive than non-nanomaterials. The transition from micro- to nanoparticles leads to changes in physical as well as chemical properties of a material. Two of the major factors are the increase in the ratio of the surface area to volume, and the size of the particle. The increase in surface area-to-volume ratio, which increase as the particles get smaller, leads to an increasing dominance of the behavior of atoms on the surface area of particles over those in the interior of the particle: this affects the properties of the particles when they interact with other particles. Because of the higher surface area of the nanoparticles, the interaction with other particles within the mixture is greater, potentially leading to increased strength of the material, heat resistance and other properties of the mixture. Properties of nanomaterials depend highly on the shape, orientation and structure of individual nanoparticles.
Recent research which related to application of nanomaterial in drilling fluid is using nanoparticles to decrease differential pipe sticking and its feasibility in Iranian oil fields. These nanoparticles are carbon black particles in which added to drilling mud to perform some fuctions. This advantage of carbon black which has nanometer size particles causes to make a mud cake which is more continues and integrated ( it means that mud cake has low permeability). So, by having integrated and low permeability mud cake, it has less volume of filtrate and therefore mud cake thickness is less than usual cases. Besides that, there are scientists at China's Shandong University that are researching ways in which nanotechnology can be used to improved drilling process. In their specialized petroleum laboratory, they have developed an advanced fluid mixed with nanosized particles and superfine powder that significantly improve drilling speed.