Introduction of Blowout in Oil Drilling - Wikipedia

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Blowout (well drilling) - Wikipedia, the free encyclopedia Page 1 of 3 Blowout (well drilling) From Wikipedia, the free encyclopedia A blowout is the uncontrolled release of formation fluid from a well being drilled, typically for petroleum production. A blowout is caused when a combination of well control systems fail — primarily drilling mud hydrostatics and blow-out preventers (BOPs) — and formation pore pressure is greater than the wellbore pressure at depth. When such an incident occurs,
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  Blowout (well drilling) From Wikipedia, the free encyclopedia A blowout is the uncontrolled release of formation fluid from a well being drilled, typically forpetroleum production. A blowout is caused when a combination of well control systems fail — primarilydrilling mud hydrostatics and blow-out preventers (BOPs) — and formation pore pressure is greater thanthe wellbore pressure at depth. When such an incident occurs, formation fluids begin to flow into thewellbore and up the annulus and/or inside the drill pipe, and is commonly called a kick . If the well is notshut in, a kick can quickly escalate into a blowout when the formation fluids reach the surface,especially when the fluid is a gas, which rapidly expands as it flows up the wellbore and accelerates tonear the speed of sound. Blowouts can cause significant damage to drilling rigs, and injuries or fatalitiesto rig personnel.Prior to the development of blow-out preventers, blowouts were common and -- assuming they producedoil -- were referred to as oil gushers. Formation kick A kick can be the result of improper mud density control, an unexpected overpressured (shallow) gaspocket, or may be a result of the loss of drilling fluids to a formation called a thief zone . If the well is adevelopment well (and not a wildcat), these thief zones should already be known to the driller and theproper loss control materials would have been used. However, unexpected fluid losses can occur if aformation is fractured somewhere in the open-hole section, causing rapid loss of hydrostatic pressureand possibly allowing flow of formation fluids into the wellbore. (See Underground Blowout discussion in next section.) Shallow overpressured gas pockets are generally unpredictable and usuallycause the more violent kicks because of rapid gas expansion almost immediately.The primary means of detecting a kick is a relative change in the circulation rate back up to the surfaceinto the mud pits. The drilling crew or mud engineer keeps track of the level in the mud pits, and aincrease in this level would indicate that a higher pressure zone has been encountered at the bit.Conversely, a drop in this level would indicate lost circulation to a formation (which might allow influxof formation fluids from other zones if the hydrostatic head at depth is reduced from less than a fullcolumn of mud). The rate of mud returns also can be closely monitored to match the rate that it is beingpumped down the drill pipe. If the rate of returns is slower than expected, it means that a certain amountof the mud is being lost to a thief zone, but this is not necessarily yet a kick (and may never becomeone). In the case of a higher pressure zone, an increase in mud returns would be noticed as the formationinflux pushes the drilling mud toward the surface at a higher rate.The first response to detecting a kick would be to isolate the wellbore from the surface by activating theBOPs and closing in the well. Then the drilling crew would attempt to circulate in a heavier kill fluid  toincrease the hydrostatic pressure (sometimes with the assistance of a well control company). In theprocess, the influx fluids will be slowly circulated out in a controlled manner, taking care not to allowany gas to accelerate up the wellbore too quickly by controlling casing pressure with chokes on apredetermined schedule. In a simple kill, once the kill-weight mud has reached the bit the casingpressure is manipulated to keep drill pipe pressure constant (assuming a constant pumping rate); this willensure holding a constant adequate bottomhole pressure. The casing pressure will gradually increase asthe contaminant slug approaches the surface if the influx is gas, which will be expanding as it moves upPage 1 of 3Blowout (well drilling) - Wikipedia, the free encyclopedia8/3/2009  This page was last modified on 3 August 2009 at 02:16. ■ the annulus and overall pressure at its depth is gradually decreasing. This effect will be minor if theinflux fluid is mainly salt water. And with an oil-based drilling fluid it can be masked in the early stagesof controlling a kick because gas influx may dissolve into the oil under pressure at depth, only to comeout of solution and expand rather rapidly as the influx nears the surface. Once all the contaminant hasbeen circulated out, the casing pressure should have reached zero.Sometimes, however, companies drill underbalanced for better, faster penetration rates and thus they drill for kicks as it is economically sounder to take time to kill a kick than to drill overbalanced (whichcauses slower penetration rates). Under these circumstances, always with qualified personnel on the rig,calling in a well control specialist may not be necessary. Blowout When all the controls described above fail, a blowout occurs. Blowouts are dangerous since they caneject the drill string out of the well, and the force of the escaping fluid can be strong enough to damagethe drilling rig. Blowouts often ignite due to the presence of an ignition source, from sparks from rocksbeing ejected along with flammable fluids, or simply from heat generated by friction. (Rarely theflowing gas will contain poisonous hydrogen sulfide and the oil operator might decide to ignite thestream to convert this to less hazardous substances.) A well control company will then need toextinguish the well fire and/or cap the well, and replace the casing head and hangars.Sometimes, blowouts can be so forceful that they cannot be directly brought under control from thesurface, particularly if there is so much energy in the flowing zone that it does not deplete significantlyover the course of a blowout. In such cases, other wells (called relief wells) may be drilled to intersectthe well or pocket, in order to allow kill-weight fluids to be introduced at depth. (Contrary to what mightbe inferred from the term, such wells generally are not used to help relieve pressure using multipleoutlets from the blowout zone.)An underground blowout is a special situation where fluids from high pressure zones flowuncontrolled to lower pressure zones within the open-hole portion of the wellbore. Usually they come upthe wellbore to shallower formations (typically near the last casing shoe) that have been fractured fromthe overall effect of hydrostatic mud head plus casing pressure imposed at the time of the initial kick.Underground blowouts can be very difficult to bring under control although there is no outward flow atthe drill site itself. However, if left unchecked, in time the fluids may find their way to the surfaceelsewhere in the vicinity (possibly cratering the rig), or may pressurize other zones, leading toproblems when drilling subsequent wells. See also Drilling mud ■ Oil rig ■ Oil well ■ Underbalanced drilling ■ Retrieved from Categories: Drilling technology | productionPetroleumHidden categories: lacking sources from February 2008Articles | sourcesAll articles lacking | Articlesmerged from November 2008to be | mergedAll articles to bePage 2 of 3Blowout (well drilling) - Wikipedia, the free encyclopedia8/3/2009  Text is available under the Creative Commons Attribution/Share-Alike License; additional termsmay apply. See Terms of Use for details.Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profitorganization. ■ Page 3 of 3Blowout (well drilling) - Wikipedia, the free encyclopedia8/3/2009
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