1、Air Drilling(空气钻井）空气钻井）1 Hole Cleaning2 Equipment3 Drilling Practices4 Problems5 Gas Type6 Mist DrillingSection 2 Page 1“Dry air is labeled as such here to recognize its use as one of the most common types of UB fluids.However,most of the info here applies to any gas used as the UB fluid as well as
2、to air.”Gas BehaviorWhere T is expressed in degrees R.P1 x V1 T1=P2 x V2 T21 ft3 of gas 10,000 ft TVDBHP=5200 psi2 ft3 whencirculated to 5000 ftUnload condition!Length ofFluid Column=Length of GasGas Rising inAn Open WellboreIf the injection rate of N2 is 900 scf/minute at 700 F and an injection pre
3、ssure of 600 psi.What is the volume flowrate of this gas downhole at conditions of 5200 psi and a temperature of 2500 F.V2=(14.7 x 900 x 710)/(5200 x 520)=3.47 ft3/mA standard cubic foot has a pressure of 14.7 psi at a temperature of 600 FTo convert Fahrenheit to Rankin add 460Section 2-Page 2Gas La
4、wHole CleaningHole cleaning with air is a complicated subject.In simple terms,the upward air flow exerts a drag force on the cutting.If the drag force is greater than the gravitational force,the cutting will move upward and out of the hole.Cuttings removed on most air drilling operations tend to be
5、very fine.The term“dusting”is often used to describe the removal process.The bit generates cuttings comparable in size to overbalance drilling;however,they tend to remain in the hole until ground down to a small enough size for the air flow to lift from the well.The pressure of the air increases wit
6、h depth,causing the density of air to increase as well.The increased pressure reduces the air velocity;however,the increase in density assists with the cuttings transport.Increased AirVelocity at LessAir PressureLess EffectiveHole CleaningAs Depth IncreasesDrag ForceDue to Air VelocityGravitationalS
7、ettling ForceCuttingResultingCutting VelocitySection 2-Page 3Air/Gas Injection RequirementsAir injection required for adequate hole cleaning is very difficult to calculate.Angel(1957)suggested that effective cuttings transport is related to the kinetic energy of the air(gas)striking the cutting.KE=D
8、 v2D density of air(gas)v annular velocity of air(ft/sec)Through testing and experiments on shallow blast drilling it was determined that 3000 ft/min proved to be an effective minimum air velocity for cuttings transport.Section 2-Page 4Cutting Carrying Capacity of a FluidLarger cuttings are more eas
9、ily transported in a viscous fluid.In Air Drilling cuttings are ground down before transporting.Hole angle has the same effect unselective of fluid type.In a gaseous fluid the liquid phase is the main transporting medium.Annular velocity is too low for gas.Dependent upon:Fluid“Viscosity”Fluid Veloci
10、tyCutting SizeHole AngleThere is an indirect relationship between the viscosity and the velocity.Without dealing with Rheology:the greater the viscosity the less the velocity.Air Drilling requires 3000 ft/min.Some Drilling Muds require 100 ft/min.Section 2-Page 5Factors Effecting Hole CleaningAnnula
11、r GeometryHoleAngleGas InflowsAnnular GeometryBHA members cause an increase in air annular velocity due to the reduced flow area.The resulting loss of velocity at BHA tops make it a likely area for larger sized cuttings to gather and settle.Annular velocity is proportional to the square of the annul
12、ar diameter.Hole AngleIn normal mud drilling,high angle holes are much more difficult to clean than vertical holes.The same is true with air drilling.Tests have shown that the critical air velocity increases with increasing hole angle.Gas InflowsInflowing gas into the annulus creates a restriction o
13、r back pressure on the air stream below the influx.This reduces the air velocity and thusly reducing hole cleaning effectiveness.Section 2-Page 6Choking VelocityAnnulus Pressure LossAnnular Air VelocityCritical Air VelocityMinimumPressure LossChokingVelocityAt high air flow rates the cuttings move a
14、long nearly at the same velocity as the air column and a“steady-state”condition exists with a constant ROP.By varying air injection rate,one can determine the minimum annulus pressure loss associated with the steady state condition.The velocity associated with this minimum loss is called the critica
15、l air velocity.If the air injection rate was decreased,cuttings would accumulate in the annulus resulting in an increasing annulus pressure loss and increasing BHP.Continuing to decrease the air rate will eventually cause the cuttings to build-up and“pack-off”the annulus.This is known as the choking
16、 effect and the minimum air velocity causing this is known as the choking velocity.Section 2-Page 7Effect of Penetration Rates on BHP020004000600080001000012000102003040506070020004000600080001000012000500100150200Depth(ft)Bottomhole Pressure(psi)Depth(ft)Air(0 ft/hr)Air(90 ft/hr)4 Inch Hole and 2 7
17、/8 inch Pipe11 Inch Hole and 5 inch PipeAir(0 ft/hr)Air(90 ft/hr)These are based on predictions made using Angels kinetic energy model for injection rates equivalent to 3000 ft/min standard air velocity.Actual BHP measurements confirm that these estimates are accurate with only a few psi difference
18、from predicted values.Values for drilling ROPs of 90 ft/hr and circulating only(0 ft/hr)are shown.Section 2-Page 8Air Injection and BHP EstimatesAir Drilling Equipmentq Air compression equipmentq Drillstring valvesq BOPS and control headsq Returns handling systemSection 2-Page 9Compressors and Boost
19、ersRequired to compress air at ambient conditions to a pressure suitable for air drillingUsually a combination of one or more compressors and a booster is used Reciprocating and rotary screw most commonly used to compress atmospheric air to 100 to 300 psigBoosters are positive displacement compresso
20、rs used to increase the delivery pressure.Section 2-Page 10Air Compression System LayoutThe discharge lines connecting the compressors and boosters to the air header and the standpipe are typically 4”with a pressure rating in excess of the rating of the discharge equipment.A check valve is typically
21、 installed just downstream of the compression equipment.All other valves are ball valve types.Mist/water and foam pumping units will also be connected into the air header and used to“mist-up”or“foam-up”when air drilling is no longer viable.The air header manifold is just upstream of the standpipe.Th
22、e air header connects to the standpipe and to the“blooie”line via primary and secondary jets.Primary and secondary jet lines are typically 2”diameter.CompressorsBoostersTo StandpipeFoam PumpWater/Mist PumpTo Secondary JetTo Primary JetHigh PressureVent LineAir Header Check ValveFrom PrimaryMud Pumps
23、Section 2-Page 11Drillstring ValvesA drillstring“spring”type float valve should always be run near the bottom of the drillstring to prevent backflow(solids and/or formation fluids)up the string.A string valve(flapper type)is generally run in the upper section to minimize the time required to bleed t
24、he string down during connections.The spring that closes the flapper is typically removed since the compressed fluid in the string will force it closed when required.This allows wireline tools to be run(with care)once the pressure has bled off and the flapper is hanging open.OpenClosedOpenClosedFlap
25、per Type FloatSpring Type FloatSection 2-Page 12Rotating Head and BOPSRotating HeadBlooie LineTypicalBOP StackAir returning to the surface must be diverted away from the rig floor and into the blooie line for safe discharge.Diverting equipment is usually either a rotating head or a rotating BOP.Eith
26、er unit allows for the string to be rotated while effecting and maintaining a pressure seal around the kelly or drillstring.The rotating head or rotating BOP uses a elastomer element for sealing.Operating pressure limits can range from 500 psi to as much as 2500 psi rotating and 5000 psi static.Kell
27、y drive drilling utilizes a“kelly driver”bushing to rotate the bowl assembly.A conventional BOP stack must still be installed to provide for complete well shut-in and for controlling emergency well control conditions.Bell NipplePressuredFluidRig FlowlineSection 2-Page 13SampleCatcherPropaneDegasserP
28、umpPilotLightPrimaryJetGasSamplerSecondaryJetRotatingHeadBOPsBlooie Line Rig-upReturning air flow is taken from the diverter to a flare pit through the blooie line.The blooie line should be oriented 450 away from prevailing wind direction and should be securely anchored.It should be of sufficient le
29、ngth so that the flared or un-ignited gas and cuttings are safely away from the rig.Length varies from 150 ft to 300 ft depending on location.The cross sectional area of the blooie line should be at least equal to the cross sectional annular area of the longest section of the wellbore.Section 2-Page
30、 14Blooie Line Jets2002”Union2”PipeBlooie LineSecondary Jet2”UnionPrimary JetAir fromStandpipeManifold4”x 2”Swedge”Plate4”Collar cut in half”to 1”TubingCovered w/rubber hoseCompressed air bypassed during connections should be flowed to the primary jet located inside the blooie line.The primary jet r
31、educes pressure in the line which helps pull produced gas flowing from the well into the blooie line and away from the rig floor.The primary jet should be located a distance of 4 times the blooie line diameter away from the exhaust end.The bleed-off line should be connected to the secondary jet,whic
32、h is normally located in the blooie line closer to the diverter.Section 2 Page 15Air FlowStandpipeValve 2Valve 1Valve 3Valve 4FromCompressorsBypass LineTo Primary JetBleed-off LineTo Secondary JetFrom MudPumpsValve 51.Valves 1 and 2 are normally open when air drilling and valves 3,4,and 5 closed.2.V
33、alve 2 is closed allowing the compressor to continue to operate by discharging output through the primary jet on the blooie line.3.Valve 5 is open and the string bled-off through the secondary jet on the blooie line.The standpipe manifold is configured to allow the compressed air to be bled from the
34、 drillstring prior to making a connection.After the connection,valve 2 is opened first and then valves 3 and 5 closed.Air Manifold&Making ConnectionsSection 2-Page 16Making Connections1.It is a good practice to get“bottoms up”prior to shutting down for the connection.2.With the air off,the cuttings
35、in the annulus will fall around the BHA.If the pipe is raised the cuttings will be jammed and packed making stuck pipe more likely.3.Always wait until returns are established prior to picking the pipe up off the slips.4.An increase in standpipe pressure when breaking circulation will indicate cuttin
36、gs packing.Section 2-Page 17Tripping ConsiderationsIf the well is making gas,the string must be pulled through the rotating head element.The gas will be flowed away from the rig via the blooie line.The rubber element will need be removed to pull the BHA.When doing so,the primary jet should be used t
37、o remove gas from the surface of the well.If the well is not making gas,the rubber elements can be pulled prior to POOH to minimize wear on the element.Dont drop surveys into the well because of the speed of fall in air(damage will occur to the tool).After running in the hole,it is a good practice t
38、o start air injection and monitor returns prior to washing to bottom.This can help indicate if water is in the well.If water is present it must be unloaded and the well dried up prior to drilling ahead with air.Section 2-Page 18Functions of“Air”as a Drilling FluidDeliver energy to bitClean the drill
39、ing faceCool and lubricatePower downhole toolsTransport cuttingsSuspend cuttingsTransmit data to surfaceStabilize bore hole Create an impermeable barrierHydrostatic control over influxRelease cuttings at surfaceSection 2-Page 19Density of Air=0.0764 lb/scfAir and GasAir,natural gas,and nitrogen can
40、be used as the circulating fluid for UBD.Air drilling is the most common because it is cheap and readily available.However,water flows can lead to mud rings and the potential for downhole fires.Using natural gas will prevent the formation of a flammable gas mixture downhole when a hydrocarbon zone i
41、s penetrated.However,it will almost invariably form a combustible mixture when released into the atmosphere.Nitrogen offers the advantage of not being flammable when mixed with hydrocarbon gases(provided the oxygen level is sufficiently suppressed).5001000150020002500%O2 Required for Flammable MixPr
42、essure(psi)89101112Section 2-Page 20Downhole Fires(Air Drilling)Formation of a mud ring.This causes a restriction in the air flow which reduces air flowrate and increases the temperature of the air/gas below the mud ring.12With the air flow restricted even slight hydrocarbon inflows can lead to the
43、development of a combustible mixture(5 to 15%at atmospheric pressure increasing to 30%at 300 psi).Fires occur when a mixture of oil or natural gas and air,with the hydrocarbon concentration in a combustible level,is exposed to an ignition source.3Ignition SourcesThe compression itself(natural flash
44、point similar to a diesel engine)Sparks caused by drillstring in hard rock.Friction heating caused by pinhole leaks in the string.Downhole fires are very difficult to detect.They usually melt the drillstring at the fire site and make the resulting fish difficult to retrieve.They typically lead to a
45、sidetrack.Glazing of the reservoir due to the extreme downhole heat can reduce permeability of a productive zone.Section 2-Page 21Sonic Flow(Air Drilling)Usually,there are no jets installed in the bit when air drilling.The resulting pressure loss can cause sonic flow through the bit jets which would
46、 make pressure changes in the annulus impossible to detect.Sonic flow occurs when the speed of the expanding air flowing through a bit nozzle reaches the speed of sound(sonic).At this point the air cannot expand any faster and the upstream pressure becomes independent of the pressure into which the
47、jet is discharging.Sonic conditions exist when upstream pressure is 1.89 times the annulus pressure beneath the bit.The main consequence of sonic flow conditions is that any downhole problems that affect the annular pressure loss will be impossible to detect via the standpipe gauge.It is important t
48、o determine the drilling parameters that will cause sonic conditions.Section 2-Page 22Drying Out the Hole(Air Drilling)Mist should be circulated to assist with the water removal before attempting to circulate with dry air.A slug(4 5 bbls)of stiff foam can be used to remove as much remaining water as
49、 well.After as much water as possible is removed the well is then circulated for to 1 hour with dry air.When the well quits making water at the blooie line it is ready to dry-out.New hole should be drilled and the cuttings generated will absorb the remaining moisture in the well.Do this by drilling
50、ahead in 5 to 10 ft intervals circulating off bottom for a short period between intervals.Work the pipe to prevent mud ring formation from sticking the string.The well should start dusting before drilling 30 ft of new formation.If a small amount of water is being produced it make take 2 to 3 joints
