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Cuttings removal: During drilling there are small rock fragments that break from the wall of the borehole into the well. These fragments can cause problems like stuck pipe which can even lead to abandon the project. So the drilling fluid helps to circulate these small cutting to the top where they are removed from the fluid.
Pressure control: Inside the borehole, there are high pressures of fluids beneath the surface which push upwards and can cause blowouts. Drilling fluids controls this pressure by applying downward force which is greater than the upward force. This prevents the unwanted hazards.
To wall the hole with impermeable filter cake: In the absence of casing, drilling fluid gives the support to the wall of the borehole to prevent it from collapsing.
Types of drilling fluid: There are two types of drilling fluids:
Water Based Fluids (WBFs)
Non-Aqueous Drilling Fluids (NADFs)
Water Based Fluids (WBFs): WBFs consist of water mixed with additives like bentonite clay to control mud density. Other additives like anionic polymers and starch are added to achieve different operational properties like thinning and filtration control respectively.http://ars.els-cdn.com/content/image/1-s2.0-S0146638004001780-gr7.jpg
Fig.2: Generalized distribution of components in water-based drilling
Non-Aqueous Drilling Fluid s (NADFs): NADFs are a mixture of Non-Aqueous Base Fluid (NABF) with water and other chemicals. In NADFs additives are used as well to control its properties. NADFs are of three types: (James et al., 2007)7:
Group I: High-aromatic content fluids. These fluids include crude oil, diesel and conventional mineral oils. These fluids contain 5-35% of aromatics.
Group II: Medium-aromatic content fluids. These fluids include products produced from crude oil with 0.5 and 5% of aromatics.
Group III: Low -aromatic content fluids. These fluids are produced by chemical reaction and include highly refined mineral oils.
Fig.3 Generalized distributions of components in oil based mudhttps://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSMe8CL02-5MvLL45nCqKzgkfjAt0Hig9pPRmTG8VSD9cTeDYKE
Many drilling problems are due to conditions or situations that occur after drilling begins and for which the drilling fluid was not designed. The most common changes is the mud weight or density. Weighting material is added when high-pressure formations are expected.
Some of the problems are8:
1. Lost circulation
Lost circulation can occur in several types of formations, including high permeable formations, fractured formations and cavernous zones.
Lost circulation materials can be added to the mud to bridge or deposit a mat where the drilling fluid being lost to the formation. These materials include cane and wood fibers, cellophane flakes and even padi husks were used in oil drilling in Sumatra.
Fig.4 Lost Circulation
2. Stuck pipe
Stuck pipe can occur after drilling has been halted for a rig breakdown, while running a directional survey or when conducting other non-drilling operation.
The drill pipe may stick to the wall of the hole due to the formation of filter cake or a layer of wet mud solids on the wall of the hole in the formation.
Fig.5 some mechanical occurrances that causing stuck pipehttp://agushoe.files.wordpress.com/2011/06/stuck-pipe.png
Heaving or sloughing hole
This occurs when shales enter the well bore after the section has been penetrated by the bit. To solve this problem, drilling is suspended the hole is conditioned (by letting the mud in circulation for a period of time).
4. Bottomhole temperatures often exceed 400 degrees F (204 degrees C). Most wells encounter high-volume flows of calcium-magnesium chloride brines, and significant carbonate influxes. These combine to present unique problems for drilling fluids systems. Present unique problems for drilling fluids systems. The deleterious effects of temperatures above 300 degrees F (149 degrees C) upon drilling fluid systems have been widely documented, especially when calcium is present in lignosulfonate systems.
When Mobil drilled the 76-1 discovery well and subsequent wells in the Mary Ann field (1979-1982), they encountered bottomhole temperatures over 400 degrees F (204 degrees C) nearing the productive Norphlett formation. They also drilled productive Norphlett formation. They also drilled into calcium and sodium chloride brine flows which seriously affected the mud systems, especially at such temperatures. Hydrogen sulfide and carbonate contamination occurred as well (Collins et al., 1986)11
Drilling hazards8: The following are some of the most common hazards in drilling and can be overcome by proper control of the mud properties.
Salt section hole enlargement
Salt section can be eroded by the drilling fluid and causes hole enlargement. These enlargement will require larger mud volume to fill the system and in case of casing the hole, larger cement volume is required. To avoid these problems a salt saturated mud system is prepared prior to drilling the salt bed.
Fig.6 A numerical simulation of hole enlargement while drillinghttp://www.drillingcontractor.org/wp-content/uploads/2011/05/spe140145-fig5_fmt.jpeg
Heaving shale problems
Areas with shale sections containing bentonite or other hydratable clays will continually absorb water, swell and slough into the hole. Such beds are referred to as heaving shales and constitute a severe drilling hazard when encountered. Pipe sticking, excessive solid buildup in the mud and hole bridging are typical problems. Various treatments of the mud are sometimes successful, such as
Changing mud system to high calcium content by adding lime, gypsum etc which reduces the tendency of the mud to hydrate water sensitive clays.
Increasing circulation rate for more rapid removal of particles.
Increasing mud density for greater wall support
Decreasing water loss mud
Changing to oil emulsion mud
Changing to oil-based mud.http://www.utahgeology.com/picts_rg/road_hw24_clip_image036.jpg
Fig.7 The Arapien Shale because of complex heaving
Blowout is the most spectacular, expensive and highly feared hazard of drilling. This occurs when encountered formation pressure exceed the mud column pressure which allows the formation fluids to blow out of the hole.
Mud density or the mud weight is the principal factor in controlling this hazard. In drilling a blowout preventer (BOP) stack is always attached at the top of the conductor pipe. In case of a gas kick (a sign that may lead to a blowout) the BOP stack can close the annular space between the drilling pipe and the conductor pipe or casing or shut the whole hole (with a blind ram of the BOP).
Fig.8 BPâ€™sÂ Deepwater Horizon oil blow-out in North Sea. Source: telegraph.co.ukhttp://i.telegraph.co.uk/multimedia/archive/01708/oil_1708772c.jpg
Lost circulation means the loss of substantial amount of drilling mud to an encountered formation. Lost circulation materials are commonly circulated in the mud system both as a cure and a continuous preventive. These materials are the fibrous materials such as the hay, sawdust or padi husk and lamellated (flat and platy) materials such as mica, cellophane.