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1、Unit 6 Well ControlDefinitionProcess and Equipment for kick detection and controlWell control Well control refers to the control of downhole formation pressures penetrated by the well. There are three distinct well control levels that may occur during drilling operations. Primary control. Primary co
2、ntrol is given by maintaining the density of the drilling fluid such that hydrostatic pressure at all depths where formations are exposed exceeds formation pore pressures. Mud hydrostatic pressureformation pore pressurePrimary controlThere are four main reasons, in general, why primary control might
3、 be lost during drilling operations:1.The well penetrates an overpressured zone with a higher formation pressure than mud hydrostatic.2. A weak downhole formation allows sufficient mud to leave the wellbore so that the level of mud in the annulus drops.3. Failure to fill the hole properly when pulli
4、ng out of the hole.4. Swabbing. If the drillstring is pulled up with sufficient speed, the reduction in pressure at the bottom can be enough to allow formation fluids to enter the well. BOP stackThe primary function of the BOP is to form a rapid and reliable seal around the drillstring or across the
5、 empty hole (if no pipe is in the hole) to contain downhole pressures. There are currently two types of preventers available that allow this seal to be formed. Most BOPs contain at least one of each type as the different characteristics of each are useful for different operations. Annular preventer/
6、bag type preventerFixed pipe ramVariable bore pipe ramBlind-shear ramDetection equipmentPit-volume totalizerFlow indicatorBag type preventer (annular preventer)Functions:a.Sealing on irregular shaped tubularsb.Allowing pipes move in or out of the well under pressure (stripping)Ram type preventer a.F
7、ixed pipe ram. The seal is sized to fit one outside diameter only Variable bore pipe ram The seal element can accommodate a narrow range of diameters, for example 3-1/2 to 7. It can only seal on round pipe, not square or hexagonal shapesBlind rams Designed to seal on the open hole. Blind-shear rams
8、have blades incorporated that can cut through drillpipe (though not through drill collars or casing) BOP stacka BOP stack would have at least one bag preventer and two ram preventers. BOP stacks for deeper wells might have up to four ram preventers and two Annular preventers. The ram preventers gene
9、rally have a higher pressure rating and are always installed below the bag preventers. If only two ram preventers are used, the bottom set will normally be blind-shear rams and the upper set will be pipe rams. One reason for placing the blind rams on the bottom is that if the pipe rams leak, it is p
10、ossible to close the blind rams below and safely open up the upper preventer to change the rams. BOP control systemsA hydraulic control system is used to move BOP units using hydraulic fluid under pressure. BOP control systems1. A reserve hydraulic fluid tank holding fluid at atmospheric pressure.2.
11、 A set of bottles holding fluid under high pressure (usually 3,000 psi) with pressurized nitrogen.3. A high pressure manifold connected to the bottle system.4. A low pressure manifold that contains fluid at the working pressure of the ram preventers (usually 1,500 psi).5. A pressure regulator that f
12、eeds fluid from the high pressure manifold to the low pressure manifold, which reduces the unregulated pressure to the working pressure. 6.A set of valves attached to the low pressure manifold that can direct working pressure fluid to the rams (to open or to close them) and that directs exhaust flui
13、d back to the reserve hydraulic fluid tank.BOP control systems7. A valve that controls the opening and closing of the bag preventer.8. A pressure regulator that feeds fluid from the low pressure manifold to the bag preventer control valve. Bag preventers operate at a lower pressure than ram prevente
14、rs, but as the seal element gets worn, higher pressure is required to make the bag seal around the pipe. This regulator is also used to lower the closing pressure to the bag preventer if it is desired to strip pipe in through the preventer or to increase the closing pressure on a worn element.9. Two
15、 sets of pumps to maintain system pressure. One set is driven by compressed air from the rig air system, the other is powered by electricity This provides some backup in the event of either the rigs air or electrical system failing. The air driven pumps give a higher flow rate and pressure the syste
16、m to around 2,800 psi. The electric pump tops the system to the full pressure of 3,000 psi at a lower flow rate. Kick Detection Equipment1. The pit-volume totalizer provides a readout showing the total volume of drilling fluid held on the surface in the active system tanks. If this total increases,
17、and if the increase is not due to the mud engineer adding chemicals or fresh mud to the system, then a kick is occurring.2. The flow indicator is an instrument attached to a paddle that sits in the flowline from the annulus. This paddle is pushed up by the returning mud stream-the amount it is pushe
18、d depends on the flow rate, among other things. If the flow rate should increase an alarm will sound. If the flow rate out increases, but the mud pump speed has not been increased, it is possible that the extra flow out is due to an influx entering the wellbore. Generally, the flow indicator will gi
19、ve the first positive indication of a kick. However, the paddle type flow indicator is prone to false alarms because of cuttings and other debris that may stick to the paddle or build up underneath it.Signs of a kickDrilling breakMud pit gainFlowing wellShows of gas, oil or salt waterDecrease in cir
20、culating pressure (flow rate increase)Pump stroke increaseImproper hole fill-upString weight change (buoyancy effect decreases, and string becomes heavier)Killing the wellIf the surface instruments indicate that a kick is in progress while drilling, normally the driller will stop drilling, pick up t
21、he drillstring so that the bit is above the bottom of the hole and stop the pumps. A visual check is then made by looking down through the rotary table, into the bell nipple, at the level of mud in the annulus. If the well is in fact kicking, then the mud in the annulus will still be moving upwards
22、even though the pumps are shut down. Having confirmed a kick, the driller will close the BOP as quickly as possible and notify the toolpusher and drilling supervisor in charge of the rig.If the permeability of the flowing formation is high, the kick can develop very quickly. A larger drilled hole wi
23、ll also allow influx to flow in faster but this is compensated for (to an extent) because the capacity of the hole is greater in a larger hole. It is also possible that if permeability is very low, little or no influx enters the wellbore even though mud hydrostatic is less than the formation pore pr
24、essure.Well control proceduresShut well inPull kelly upOpen hydraulic closing ram(HCR)valve but not superchoke or choke manifold.Close annular preventerRecord pressureRecord shut-in drill pipe pressure(SIDPP)Record shut-in casing pressure(SICP).Kill methodsDrillers methodWait-and-weight method (engineers method)Circulate and weight method
