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Journal of Natural Gas Science and Engineering 37 (2017) 551e559

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Journal of Natural Gas Science and Engineering

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A feasibility study of gas-lift drilling in unconventional tight oil andgas reservoirs

Boyun Guo a, *, Gao Li a, Jinze Song b, Jun Li c

a Southwest Petroleum University, Chinab University of Louisiana at Lafayette, USAc China University of Petroleum-Beijing, China

a r t i c l e i n f o

Article history:Received 9 July 2016Received in revised form23 November 2016Accepted 28 November 2016Available online 5 December 2016

Keywords:Gas-liftDrillingUnconventionalOilGasReservoirs

* Corresponding author.E-mail address: boyun.guo@louisiana.edu (B. Guo)

http://dx.doi.org/10.1016/j.jngse.2016.11.0571875-5100/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

Development of unconventional tight oil and gas reservoirs is an unsolved problem in the energy in-dustry due to the low productivity of oil and gas wells. The reason is the low permeability of reservoirrocks that are very vulnerable to the contamination of the water in the drilling and fracturing fluids.Although gas-drilling (drilling with gas) has shown to be promising to solve the problem, severalproblems hinder its application. These problems include formation water influx, wellbore collapse,excessive gas volume requirement, and hole cleaning in horizontal drilling. A new technique called gas-lift drilling has been proposed to solve these problems. A technical assessment of gas-lift drilling wascarried out in this study to determine the feasibility of the newly proposed drilling technique. It is foundthat, compared to conventional (positive circulation) gas drilling, gas-lift drilling can reduce gas injectionrate required for hole cleaning by at least 70%. The kick-off pressure for unloading the well depends onwater zone pressure and valve setting depth, and can be lowered by reducing valve spacing. The gasinjection pressure in gas-lift drilling will be in the same level as in the conventional gas drilling.Mathematical modeling shows that the temperature profiles in the annulus and inside the drill stringwill be significantly higher than the geo-temperature profile. The gas-lift valve can be designed to openand close automatically depending upon the water-induced pressure inside the drill string. The gas-liftvalve design experience gained from gas-lift operations in oil production can be employed in gas-liftdrilling. In this paper, it is concluded that gas-lift drilling has the potential to become a viable andfeasible technique for development of unconventional tight oil and gas reservoirs with improved per-formance and reduced cost.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

Tight reservoirs are generally recognized as oil reservoirs withpermeabilities less than 1 mD and gas reservoirs with permeabil-ities less than 0.01 mD. Tight sands and shale oil/gas reservoirs fallin this category. Producing oil and gas from tight reservoirs pre-sents a unique challenge to the energy industry due to the lowproductivity of oil and gas wells. This is attributed not only to thelow permeability of reservoir rocks but also to the fact that they arevery vulnerable to the contamination of water from the drilling andfracturing fluids. Li et al.’s (2012) study indicates that the produc-tivity of well can drop easily by 50% in hydraulically fractured wells

.

due to viscous-force-induced fluid filtration, even if the perme-ability damage due to capillary pressure is neglected. However, thepermeability damage due to capillary pressure is still the majorfactor dominating well productivity (Romero et al., 2003).

While water-free fracturing has been used for improving wellproductivity in unconventional reservoirs (Guo et al., 2014), gas-drilling (drilling with gas) has shown to be promising to solve theproblem (Li et al., 2014). But there are still several problems hin-dering the applications of gas-drilling. The first problem is theformation water influx during drilling large sections of wet for-mation intervals (Lyons et al., 2009). Extremely high gas injectionrate is required to remove water in large-hole drilling by the gasflow in the annulus, which is in many cases not feasible. This his-torical problem has become a bottleneck for gas-drilling applica-tions. The second problem is the wellbore collapse induced bywetting of formation rock in the upper-hole sections by the

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B. Guo et al. / Journal of Natural Gas Science and Engineering 37 (2017) 551e559552

produced formation water. Formation rocks such as shale containclays and thus absorb water, swell and create hole-wall stresses,resulting in borehole collapse (Lyons et al., 2001). The third prob-lem is the excessive gas volume required for removing drill cuttingsin the annulus drilled with drill bits larger than 10-inch diameter(Guo and Ghalambor, 2002), which is extremely costly. The fourthproblem is complications in horizontal drilling associated withformation of cuttings dome in the annulus. Typical drilling com-plications are associated with high-drag and torque of drill stringand induced pipe sticking (GRI, 1997).

A new technique called gas-lift drilling has been proposed tosolve the problems associated with the conventional gas-drilling.The gas-lift drilling will remove drill cuttings and produced for-mation water through the inside of the drill string rather thanthrough annulus. The feasibility of gas-lift drilling was investigatedin this study on the basis of theoretical analyses. Aspects consid-ered are pressure requirement, gas flow rate requirement, tem-perature profile, and design of gas-lift valves.

2. System description

Fig. 1 shows a sketch of the proposed gas-lift drilling system. Innormal drilling conditions, the gas provided by the gas compressorflows through valve V3, rotating head, down the annulus, throughbit where drill cuttings and produced formation water areentrained in the stream, up the inside of drill string, Kelly pipe,swivel, rotary hose, standpipe, valve V4, reaches blooey line, anddischarges to the pit. Whenever the pressure inside the drill stringis significantly high due to the excessive water inside the pipe, thegas lift valve will open automatically so that the water column in-side the pipe is gas-lifted to the surface. The high water columnusually occurs during startup of drilling after a pipe joint connec-tion. After the inside of the pipe is unloaded, the low pressure willcause the gas lift valve to close automatically, resulting in gas flowthrough only drill bit.

The design of the gas lift valve is illustrated in Fig. 2. The balancebetween the forces created by the annular pressure Pc, innerpressure Pt, dome pressure Pd, and string constant St determines theopening and closing of the valve. An increase in the inner pressurePt due to water column inside the drill string will open the valve togas-lift the water. The valve will close when the water is dischargedat surface, leading all gas stream to flow through the drill bit.

Fig. 1. A sketch of gas-lift drilling system.

3. Technical assessment

Major problems encountered in conventional gas-drilling aredrilling complications due to a) formation water influx, b) wellborecollapse, c) excessive gas volume requirement, and d) hole cleaningin horizontal drilling. The gas-lift drilling technique will not reduceformation water influx but it will remove the formation waterefficiently through the high efficiency of two-phase flow in drillstring and gas-lift effect in reverse circulation.

The wellbore collapse problem in conventional gas-drilling ismainly due to the swelling and sloughing of shale intervals in theupper section of open hole after being wetted by the formationwater produced from the lower section of open hole (Guo and Liu,2011). With the gas-lift drilling technique, the formation fluidproduced in the lower section of open hole is removed through theinside of drill sting so that the wellbore collapse problem due o theswelling shale can be totally avoided.

Excessive gas volume required by solid removl is a problemassociated with the upward flow of gas in the large annular space.This flow condition is eliminated in gas-lift drilling where reversecirculation is used. Since the inner area of the drill pipe is only 30%e40% of the area of annulus, it is expected that the gas volumerequirement can be cut by 60%e70% with gas-lift drilling. Morerigorous figures are given in the following assessment.

Drilling horizontal wells with conventional gas drill techniquepresents a special challenge due to drill cuttings accumulation inthe horizontal wellbore section. The formation of cuttings bed in thhorizontal annulus can cause pipe sticking and low quality bondingof cement. If gas-lift drilling is used, drill cuttings are removedthrough the inside of drill string, leaving clean annulus for smoothdrilling and cementing.

A technical assessment of the proposed gas-lift drilling tech-nique was carried out in this study. Feasibility of using the newdrilling technique was evaluated, considering gas kick-off and in-jection pressures, gas injection rate, temperature profile, and gaslift valve design for system dewatering.

3.1. Kick-off pressure (KOP)

Kick-off pressure is defined as the maximum gas injectionpressure necessary for initiation of lifting water inside the drillstring. It is essentially the hydrostatic pressure of the water columnabove the gas-lift valve, which depends on the pore pressure of thewater producing formation. Fig. 3 illustrates the kick-off of forma-tion water in gas-lift drilling. Fig. 3(a) shows water column beforekicking-off, and Fig. 3(b) depicts the water columnwhen the lift gasenters the drill string. Before kicking-off, the column height Hw ofwater influx at depth Dw is expressed as:

Hw ¼ pw0:433Sw (1)

Where pw is the pressure in the water-bearing zone and Sw is waterspecific gravity. The static liquid level is at depth ðDw � HwÞ. Duringkicking-off, the liquid level is pushed to the valve depth. It will dropby Lw ¼ Dv � ðDw � HwÞ. During the same period, the liquid levelinside the drill string will rise by hw, which is determined by massbalance:

hw ¼ AaAp ½Dv � ðDw � HwÞ�: (2)

The maximum expected gas injection pressure during kicking-off is approximately expressed as:

Fig. 2. A design of gas-lift valve.

Fig. 3. Kick-off of formation water in gas-lift drilling. (a) Water column before kicking-off; (b) water column when the lift gas enters the drill string.

Table 1A data set for a typical gas drilling condition.

Open hole diameter 7.875 in.Drill pipe outer diameter 5 in.Drill pipe inner diameter 4.25 in.Depth of water zone 5000 ftDepth of gas lift valve 4700 ftWater specific gravity 1 air ¼ 1

B. Guo et al. / Journal of Natural Gas Science and Engineering 37 (2017) 551e559 553

pKO ¼ 0:433SwðLw þ hwÞ: (3)Substituting Eqs. (1) and (2) into Eq. (3) and rearranging the

latter give:

pKO ¼ 0:433Sw�1þ Aa

Ap

��Dv �

�Dw � pw0:433Sw

��: (4)

This equation indicates that the required kick-off pressure isproportional to the pressure in the water zone. Table 1 presents adata set for evaluation of a gas-lift drilling design. The parametervalues reflect a typical condition in gas drilling operations (Lyonset al., 2009). Calculated required kick-off pressures are shown inFig. 4. It indicates that, if a single gas lift valve is installed at 4700 ft

(300 ft above the water zone), gas-lifting the water from a 800 psiawater zone will require a compressor with an injection pressure of2000 psia which is beyond the pressure capacity of most availablecompressors used in the gas-drilling industry.

Fig. 4. Calculated required kick-off pressures for a single gas lift valve.

B. Guo et al. / Journal of Natural Gas Science and Engineering 37 (2017) 551e559554

The solution to reduce the required kick-off pressure is to usemultiple gas lift valves. Assuming the gas flow frictional pressureloss is negligible; the kick-off pressure with multiple valves iscontrolled by the valve spacing through

pKO ¼ 0:433SwSV : (5)

where SV is valve spacing in feet. Fig. 5 provides a relation betweenthe minimum required valve spacing and the kick-off pressure. Fora given available kick-off pressure the chart gives a correspondingminimum required valve spacing. Using low-spacing valves canreduce the kick-off pressure requirement. For example, if multiplegas-lift valves are installed with 300 ft valve spacing, a kick-offpressure of 400 psia will be needed to unload the well. Of course,valves with any spacing values less than 300 ft will all unload thewell when a kick-off pressure of 400 psia is available.

3.2. Operating pressure

Operating pressure is the gas surface injection in drilling con-dition where the formation water is continuously lifted inside thedrill string by the gas flowing through the drill bit. Under thiscondition all gas lift valves are closed. The mathematical models forpredicting the operating pressure have been described by severaldocuments including Angel (1957), Capes and Nakamura (1973),Supon and Adewumi (1991) and Guo and Liu (2011). Since thepressure calculation is a routine practice that is well known to gas-drilling engineers, details are not presented in this paper. Thepressure drop at drill bit is negligible due to the special design of bitfor reverse circulation drilling that has large-water-hole withoutnozzle installation. For an illustrative data set shown in Table 2,

Fig. 5. Calculated required minimum valve spacing as a function of kick-off pressure.

reflecting a typical condition in gas drilling operations (Lyons et al.,2009), Guo and Liu's (2011) model predicts pressure profiles forconventional gas drilling (positive circulation) and gas-lift drilling(reverse circulation) shown in Figs. 6 and 7. These two figuresindicate operating pressures of 150 psia and 140 psia for the con-ventional gas drilling and gas-lift drilling, respectively. The reasonwhy the latter is lower than the former is that the inner wall of thedrill string has lower roughness, and thus friction factor, than thatof the open-hole annulus. It can be shown that if the sameroughness values are assigned for both inner pipewall and annulus,the calculated injection pressure for gas-lift-drilling is slightlyhigher than that for the conventional gas drilling. Nevertheless, thegas injection pressure in gas-lift drilling will be in approximatelythe same level as in the conventional gas drilling.

3.3. Gas injection rate

The minimum gas injection rate required for hole cleaning canbe determined with the minimum velocity criterion and the min-imum kinetic energy criterion (Guo and Ghalambor, 2002). Thelatter has been adopted in most commercial software packages.Although the minimum kinetic energy criterionwas first presentedby Angel (1957) for lifting drill cuttings, Guo and Ghalambor's(2002) model is more accurate because it considers 4-phase (gas,oil, water, and solid) flow and can deal with liquid removal. Theyillustrated that the kinetic energy required for lifting water insideproduction tubing is 3.6 lbf-ft/ft3. This value is higher than the ki-netic energy of 3 lbf-ft/ft3 required for lifting drill cuttings sug-gested by the widely accepted Angle's (1957) method for liftingsolid. Therefore, a gas injection rate that is adequate for liftingwater will be sufficient for cleaning the hole. Guo and Liu (2011)used the concept of kinetic energy index (KEI) to describe thehole cleaning power of gas. The KEI is defined as the gas kineticenergy divided by 3. Thus a gas with kinetic energy 3 lbf-ft/ft3 has aKEI value 1 and that with kinetic energy 3.6 lbf-ft/ft3 has a KEI value1.2. Since the gas kinetic energy calculation is a routine practicewell known to gas-drilling engineers, details are not presented inthis paper. For the data set presented in Table 2, a gas injection rateof 2060 scfm for gas-drilling will give KEI¼ 1.2 in the annulus at theshoulder of drill collar. The calculated KEI profile with Guo and Liu's(2011) model is plotted in Fig. 8. For the same data set, a gas in-jection rate of 603 scfm for gas-lift drilling will give KEI¼ 1.2 insidethe drill string above the drill collar. The calculated KEI profile withGuo and Liu's (2011) model is plotted in Fig. 9. A comparison ofthese two flow rates indicates that the minimum required gas in-jection rate for hole cleaning in gas-lift drilling is about 30% of thatin conventional gas drilling.

3.4. Operating temperature

Designing the dome pressure pd of the gas-lift valve requires thetemperature in the annulus at the valve depth. Analytical modelsdeveloped in gas-lift oil production engineering were evaluated forthis application by Jiang (2016) who compared her newmodel withHasan and kabir (2012) model and Gilbertson et al.’s (2013) modelusing the temperature data measured in actual wells. Fig. 10 pre-sents a comparison of results given by Jiang's (2016) model andHasan and kabir (2012) model using the basic well data presentedby Hasan and kabir (2012) paper. It indicates that Hasan and kabir(2012) model under-estimates fluid temperature by 4 �C in the pipeat surface and over-predict the temperature by 3 �C at bottom hole,while Jiang's (2016) model over-estimates the temperature by 1 �Cin the pipe at surface and under-predicts the temperature by 0.5 �Cat bottom hole. This is explained as that in Hasan and kabir (2012)model, the Joule-Thomson cooling effect is accounted using the

Table 2Well and drill string data for a typical gas drilling condition.

1) Well Geometry:Total measured depth: 5000 ftBit diameter: 7.875 inDrill pipe OD: 5 inDrill pipe ID: 4.25 inDrill collar length: 300 ftDrill collar OD: 5.75 inDrill collar ID: 3 in2) Material Properties:Specific gravity of rock: 2.7 water ¼ 1Specific gravity of gas: 1 air ¼ 1Gas specific heat ratio: 1.25Specific gravity of misting fluid: 1 water ¼ 1Specific gravity of formation fluid: 1 water ¼ 1Pipe roughness: 0.0018 inBorehole roughness: 0.2 in3) Environment:Site elevation (above mean sea level): 0 ftAmbient pressure: 14.7 psiaAmbient temperature: 75 FRelative humidity: 0.8 fractionGeothermal gradient: 0.01 F/ftMinimum required velocity under standard conditions: 50 ft/sec4) Operating condition:Surface choke/flow line pressure: 15 psiaRate of penetration: 30 ft/hourRotary speed: 50 rpmMisting rate: 0 bbl/hourFormation fluid influx rate: 10 bbl/hourDe-watering efficiency: 0 fractionBit orifices: Orifice-1: 64 1/32nd in.

Orifice-2: 64 1/32nd in.Orifice-3: 64 1/32nd in.

Proposed gas injection rate: 1800 scfm

Fig. 6. Calculated pressures profile for conventional gas drilling.

B. Guo et al. / Journal of Natural Gas Science and Engineering 37 (2017) 551e559 555

theoretical approach where the mass fraction of annular fluid isneglected.

Fig. 11 illustrates a comparison of results given by Gilbertsonet al.'s (2013) model and Jiang's (2016) model using the basic welldata provided by Gilbertson et al.'s (2013) paper. It shows thatGilbertson et al.’s (2013) model underestimates tubing/drill pipetemperature at shallow depth and over-estimates tubing/drill pipetemperature at bottom hole by up to 5 �C. This is due to the fact thatGilbertson's (2013) model does not consider Joule-Thomson cool-ing effect. In addition, Gilbertson's model does not have the

capability of calculating the annular temperature, which limits itsapplications. In contrast, Jiang's (2016) model over-estimatestubing-temperature at shallow depth and underestimates tubing/drill pipe temperature at bottom hole by 2 �C. This is consistentwith the result shown in Fig. 9. The reason is that the Jiang's (2016)model considers sonic flow of fluid when the fluid enters the stringthrough a restriction (gas lift valve in this case), which “generates”the upper bound of Joule-Thomson cooling. This flow conditionmay not exist in the tested wells.

Jiang's (2016)mathematical model was adopted for temperature

Fig. 7. Calculated pressures profile for gas-lift drilling.

Fig. 8. Calculated KEI profile for gas drilling.

Fig. 9. Calculated KEI profile for gas-lift drilling.

B. Guo et al. / Journal of Natural Gas Science and Engineering 37 (2017) 551e559556

Fig. 10. A comparison between the new model and Hasan's model with measured data.

Fig. 11. A comparison between the new model and Gilbertson's (2013) model.

B. Guo et al. / Journal of Natural Gas Science and Engineering 37 (2017) 551e559 557

prediction in this study. The model was derived based on thefollowing assumptions: 1) single-phase gas flow in the annulusreaches steady state condition, 2) multiphase flow inside the drillstring establishes a constant mass flow rate, 3) thermal conduc-tivity of cement sheath controls heat transfer in the radial direction(heat-resistance of casing is negligible), and 4) thermal propertiesof fluids remain pressure-independent. Using the data in Table 2,Jiang's solution gives result shown in Fig. 12 under steady flowconditions. It indicates that the temperature profiles in the annulusand inside the drill pipe are both significantly higher than the geo-temperature profile. This is interpreted as that the hot fluid at thebottom hole flows upward with a convective velocity that is fastenough not to allow establishment of thermal equilibrium withannular fluid and formation rock at the same depth. The fluid insidethe pipe losses its heat to the annular fluid and annular fluid lossesits heat to the formation rock during the steady countercurrentflow.

3.5. Gas-lift valve

The gas-lift valve should be designed to open and close auto-matically depending upon the water-induced pressure inside thedrill string. The valve design experience gained from gas-lift in oilproduction operations can be utilized in gas-lift drilling. As shownin Fig. 2, the valve has a pressure-charged nitrogen-dome and anoptional spring loading element. While the forces from the domepressure and spring act to cause closing of the valve, the forces dueto annular and pipe pressures act to cause opening of the valve.When a valve is at its closed condition as shown in Fig. 2, forcebalance gives the minimum annular pressure required to open thevalve, called valve opening pressure, as follows (Brown, 1980):

Pvo ¼ 11� RPd þ St �R

1� RPt (7)

where

Pvo ¼ valve opening pressure, psig

Fig. 12. Temperature profiles given by Jiang's analytical solution under steady flow conditions.

Fig. 13. Sketch of a pipe-pressure sensitive gas-lift valve (after Brown, 1980).

B. Guo et al. / Journal of Natural Gas Science and Engineering 37 (2017) 551e559558

Pd ¼ pressure in the dome, psigSt ¼ equivalent pressure caused by spring tension, psigPt ¼ pipe pressure at valve depth when the valve opens, psiR ¼ area ratio Ap/AbAp ¼ valve seat area, in.2Ab ¼ total effective bellows area, in.2

Equation (7) implies that when the pressure inside the drillstring Pt is high due to water flow, the valve will remain in itsopening position, allowing gas to enter the drill string to lift water.With other parameters given, Eq (7) can be used for determiningthe required dome pressure at depth in valve design, i.e.,

Pd ¼ ð1� RÞPvo � St þ RPt : (8)When a valve is at its open condition, force balance yeilds that

the maximum pressure under the ball (assumed to be annularpressure) required to close the valve, called valve closing pressurePvc, is expressed as (Guo et al., 2006):

Pvc ¼ Pd þ Stð1� RÞ (9)An optional design of the gas-lift valve is shown in Fig. 13

(Brown, 1980). Its opening pressure is more sensitive to the watercolumn and thus pressure inside the drill pipe.

The opening pressure is defined as the pipe pressure required toopen the valve under actual operating conditions. Force balancegives (Guo et al., 2006):

Pvo ¼ 11� RPd þ St �R

1� RPc (10)

Equation (10) implies that when the pressure in the annulus Pc ishigh due to water flow, the valve will remain in its opening posi-tion, allowing gas to enter the drill string to lift water. This equationcan be used for determining required dome pressure at depth invalve selection, i.e.,

Pd ¼ ð1� RÞPvo � St þ RPc (11)

When a fluid valve is in its open position under operating con-ditions, the maximum pressure under the ball (assumed to be pipepressure) required to close the valve is called valve closing pressureand is expressed the same as Eq. (9).

4. Discussion

Previous sections provide a brief analysis of feasibility of thenewly proposed technique namely gas-lift drilling in the aspectspressure requirement, gas flow rate requirement, temperatureprofile, and design of gas-lift valves. The analysis was performed onthe basis of available mathematical models found in the literature.The kick-off pressure analysis was from simple hydraulics modelswithout considering pressure loss due to friction. The result can be

B. Guo et al. / Journal of Natural Gas Science and Engineering 37 (2017) 551e559 559

slightly optimistic. Safety factor should be applied to real gas-liftdrilling design. The result of analysis shows that multiple gas liftvalves are needed in order to reduce the kick-off pressure to apractical level.

The operating pressure analysis was carried out using one of thewell-established hydraulics models for multiphase flow. No sig-nificant error is expected. The hydraulics model for pressurecalculation has been well-documented in the literature. Details areavailable from Guo and Liu (2011). Example calculations indicatethat the operating pressure in gas-lift drilling will be in the samemagnitude as in the conventional gas drilling.

The temperature model used in the analysis is a comprehensivemodel selected by Jiang (2016) considering single-phase gas flow inthe annulus and multiphase flow in the drill string with Joule-Thomson effect included. This is believed to be the best onedescribing the heat transfer process in gas-lift drilling. Model resultindicates that using geo-temperature can result in erroneousdesign of nitrogen dome pressure for gas lift valves. Steady stateflow is considered due to the fact that the gas-lift valve design usingthemodel is based on the normal drilling condition, not the start upor transient flow condition. Errors of the model may be from theassumption of pressure-independent thermal properties of fluids.

The design principle of gas-lift valves is directly transferablefrom the practice of oil production engineering with gas lift tech-niques. Various types of gas lift valves are available for automati-cally unloading the well. It will be not difficult to design new valvesfor gas lift drilling operations.

Removal of drill cuttings through drill bit should not be a majorconcern in gas-lift drilling because specially designed drill bits withlarge water holes near the center of the bits have been used inconventional gas drilling with reverse circulation. It has beenproven in the industry practice (Lyons et al., 2001) that the gasstream can efficiently entrain drill cuttings at bottomhole and carrythe cuttings through the drill bit.

However, there are other issues that need to be further evalu-ated for gas lift drilling. They are related to the erosional wear ofequipment in the flow loop, including gooseneck, rotary hose, pipeelbows, and valves. Anti-wear parts need to be specially designed.

5. Conclusions

A technical assessment of gas-lift drilling was carried out in thisstudy to determine the feasibility of the newly proposed drillingtechnique. The following are found based on the analyzed cases.

1. Compared to conventional (positive circulation) gas drilling,gas-lift drilling can reduce the gas injection rate required forhole cleaning by at least 70%. This will significantly cut down thecost of drilling operations.

2. The kick-off pressure for unloading the well (lifting water slugsinside the drill string) depends onwater zone pressure and valvesetting depth and can be lowered by reducing valve spacing.

3. The gas injection pressure in gas-lift drilling will be in the samelevel as in the conventional gas drilling.

4. Mathematical modeling shows that the temperature profiles inthe annulus and inside the drill string are significantly higherthan the geo-temperature profile. The temperatures frommodeling should be used in gas-lift valve dome design.

5. The gas-lift valve can be designed to open and close automati-cally depending upon the water-induced pressure inside the

drill string. The valve design experience gained from gas-liftoperations in oil production can be employed in gas-lift drilling.

In conclusion, the newly proposed gas-lift drilling is technicallyfeasible. It has a high potential of becoming a viable technique fordevelopment of tight oil and gas reservoirs.

Acknowledgements

This research was supported by the China National NaturalScience Foundation Founding No. 51274220, No. 51134004, No.51221003, 51274045, 51274221, and No. 51334003.

Nomenclature

Ab total effective bellows area, in.2

Ap valve seat area, in.2

Dw depth of water zone, ftHw column height of water influx, fthw raise of water level, ftPd pressure in the dome, psigPvo valve opening pressure, psigpw pressure in the water-bearing zone, psiaR area ratio Ap/AbSt equivalent pressure caused by spring tension, psigSV valve spacing, ftSw water specific gravity, 1 for fresh water

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A feasibility study of gas-lift drilling in unconventional tight oil and gas reservoirs1. Introduction2. System description3. Technical assessment3.1. Kick-off pressure (KOP)3.2. Operating pressure3.3. Gas injection rate3.4. Operating temperature3.5. Gas-lift valve

4. Discussion5. ConclusionsAcknowledgementsNomenclatureReferences