Film editings remotion: During boring there are little stone fragments that break from the wall of the borehole into the well. These fragments can do jobs like stuck pipe which can even take to abandon the undertaking. So the boring fluid helps to go around these little film editing to the top where they are removed from the fluid.
Pressure control: Inside the borehole, there are high force per unit areas of fluids beneath the surface which push upwards and can do runawaies. Boring fluids controls this force per unit area by using downward force which is greater than the upward force. This prevents the unwanted jeopardies.
To palisade the hole with impermeable filter bar: In the absence of shell, boring fluid gives the support to the wall of the borehole to forestall it from fall ining.
Types of boring fluid: There are two types of boring fluids:
Water Based Fluids ( WBFs )
Non-Aqueous Drilling Fluids ( NADFs )
Water Based Fluids ( WBFs ) : WBFs consist of H2O assorted with additives like bentonite clay to command mud denseness. Other additives like anionic polymers and amylum are added to accomplish different operational belongingss like thinning and filtration control respectively.http: //ars.els-cdn.com/content/image/1-s2.0-S0146638004001780-gr7.jpg
Fig.2: Generalized distribution of constituents in water-based boring
Non-Aqueous Drilling Fluid s ( NADFs ) : NADFs are a mixture of Non-Aqueous Base Fluid ( NABF ) with H2O and other chemicals. In NADFs additives are used every bit good to command its belongingss. NADFs are of three types: ( James et al. , 2007 ) 7:
Group I: High-aromatic content fluids. These fluids include rough oil, Diesel and conventional mineral oils. These fluids contain 5-35 % of aromatics.
Group II: Medium-aromatic content fluids. These fluids include merchandises produced from rough oil with 0.5 and 5 % of aromatics.
Group III: Low -aromatic content fluids. These fluids are produced by chemical reaction and include extremely refined mineral oils.
Fig.3 Generalized distributions of constituents in oil based mudhttps: //encrypted-tbn0.gstatic.com/images? q=tbn: ANd9GcSMe8CL02-5MvLL45nCqKzgkfjAt0Hig9pPRmTG8VSD9cTeDYKE
Many boring jobs are due to conditions or state of affairss that occur after boring Begins and for which the boring fluid was non designed. The most common alterations is the mud weight or denseness. Burdening stuff is added when hard-hitting formations are expected.
Some of the jobs are8:
1. Lost circulation
Lost circulation can happen in several types of formations, including high permeable formations, fractured formations and cavernous zones.
Lost circulation stuffs can be added to the clay to bridge or lodge a mat where the boring fluid being lost to the formation. These stuffs include cane and wood fibres, cellophane flakes and even padi chaffs were used in oil boring in Sumatra.
hypertext transfer protocol: //gpcpetro.com/images/lost_circ_front.png
Fig.4 Lost Circulation
2. Stuck pipe
Stuck pipe can happen after boring has been halted for a rig dislocation, while running a directional study or when carry oning other non-drilling operation.
The drill pipe may lodge to the wall of the hole due to the formation of filter bar or a bed of wet clay solids on the wall of the hole in the formation.
Fig.5 some mechanical occurrances that doing stuck pipehttp: //agushoe.files.wordpress.com/2011/06/stuck-pipe.png
Heaving or sheding hole
This occurs when shales enter the well bore after the subdivision has been penetrated by the spot. To work out this job, boring is suspended the hole is conditioned ( by allowing the clay in circulation for a period of clip ) .
4. Bottomhole temperatures frequently exceed 400 grades F ( 204 grades C ) . Most Wellss encounter high-volume flows of calcium-magnesium chloride seawaters, and important carbonate inflows. These combine to show alone jobs for boring fluids systems. Present alone jobs for boring fluids systems. The hurtful effects of temperatures above 300 grades F ( 149 grades C ) upon boring unstable systems have been widely documented, particularly when Ca is present in lignosulfonate systems.
When Mobil drilled the 76-1 find good and subsequent Wellss in the Mary Ann field ( 1979-1982 ) , they encountered bottomhole temperatures over 400 grades F ( 204 grades C ) approaching the productive Norphlett formation. They besides drilled productive Norphlett formation. They besides drilled into Ca and Na chloride seawater flows which earnestly affected the clay systems, particularly at such temperatures. Hydrogen sulphide and carbonate taint occurred every bit good ( Collins et al. , 1986 ) 11
Boring hazards8: The followers are some of the most common jeopardies in boring and can be overcome by proper control of the clay belongingss.
Salt subdivision hole expansion
Salt subdivision can be eroded by the boring fluid and causes hole expansion. These expansion will necessitate larger clay volume to make full the system and in instance of casing the hole, larger cement volume is required. To avoid these jobs a salt saturated clay system is prepared prior to boring the salt bed.
Fig.6 A numerical simulation of hole expansion while drillinghttp: //www.drillingcontractor.org/wp-content/uploads/2011/05/spe140145-fig5_fmt.jpeg
Heaving shale jobs
Areas with shale subdivisions incorporating bentonite or other hydratable clays will continually absorb H2O, crestless wave and gangrene into the hole. Such beds are referred to as heaving shales and represent a terrible boring jeopardy when encountered. Pipe sticking, inordinate solid buildup in the clay and hole bridging are typical jobs. Assorted interventions of the clay are sometimes successful, such as
Changing clay system to high Ca content by adding calcium hydroxide, gypsum etc which reduces the inclination of the clay to hydrate H2O sensitive clays.
Increasing circulation rate for more rapid remotion of atoms.
Increasing clay denseness for greater wall support
Decreasing H2O loss clay
Changing to oil emulsion clay
Changing to oil-based mud.http: //www.utahgeology.com/picts_rg/road_hw24_clip_image036.jpg
Fig.7 The Arapien Shale because of complex heave
Runaway is the most dramatic, expensive and extremely feared jeopardy of boring. This occurs when encountered formation force per unit area exceed the mud column force per unit area which allows the formation fluids to blow out of the hole.
Mud denseness or the clay weight is the chief factor in commanding this jeopardy. In boring a runaway preventer ( BOP ) stack is ever attached at the top of the music director pipe. In instance of a gas boot ( a mark that may take to a runaway ) the BOP stack can shut the annulate infinite between the boring pipe and the music director pipe or shell or close the whole hole ( with a unsighted random-access memory of the BOP ) .
Fig.8 BP & A ; acirc ; ˆ™s & A ; Acirc ; Deepwater Horizon oil blow-out in North Sea. Beginning: telegraph.co.ukhttp: //i.telegraph.co.uk/multimedia/archive/01708/oil_1708772c.jpg
Lost circulation means the loss of significant sum of boring clay to an encountered formation. Lost circulation stuffs are normally circulated in the clay system both as a remedy and a uninterrupted preventative. These stuffs are the hempen stuffs such as the hay, sawdust or padi chaff and lamellated ( level and Platypoecilus maculatus ) stuffs such as isinglass, cellophane.