The characteristics of recycling gas drilling technology

Pet. Sci.(2012)9:59-65DOI10.1007/s12182-012-0183-zThe characteristics of recycling gas drillingtechnologyYang Shunji, Liu Gonghui, and Li JunI Key Laboratory of Petroleum Engineering of the Ministry of Education, China University of Petroleum, Beijing 102249, China2 Beijing Union University, Beijing 100011, Chinag China University of Petroleum(Beijing) and Springer-Verlag Berlin Heidelberg 2012Abstract: Recycling gas drilling is a new drilling technology. This paper can be divided into three parts,with the purpose of introducing and analyzing the characteristics of this new technology. First, the majorequipment characteristic of this technology was introduced. Secondly, compared with conventional gasdrilling, Angel's model was used to analyze the wellbore flow characteristics. Due to the closed loopand the effect of back pressure caused by the equipment, the gas flow rate decreases dramatically duringdrilling. Apart from this, it is also found that the kinetic energy at the casing shoe is always smaller thanthat at the top of the collar. The proposing of the drilling limit concept points out the basic differencebetween the two gas drilling technologies. Lastly, according to the results of the theoretical analysis,gas supplement operations for the wellbore must be conducted. Thus, two gas supplement schemes arepresented in this paper, to provide some guidance for field operationsKey words: Recycling gas drilling technology, major equipment, flow characteristics, comparativeanalysis, gas supplement scheme1 Introductiongas returned from the wellbore can be recycled by groundequipment and injected back into the wellbore again, nitrogenDrilling oil and gas wells with air, natural gas, carbon gas drilling will have a better prospect( Lunan, 1994; Teichrob,dioxide, and nitrogen has become a mature technology 1994). The concept of recycling gas drilling is proposed insince the 1950s(Angel, 1957). This technology, called this paper to achieve this idea. In previous research and fieldunderbalanced drilling(UBD), has traditionally been used tessts, we have acquired some mature results of recycling gasfor increasing rate of penetration in hard formations (Lyons drilling technology(Wu et al, 2009; Liu et al, 2009; Li et al,et al, 2001; Nas et al, 2010). It is also utilized for solving 2010a; 2010b; Tao et al, 2010). However, the drilling designthe problem of loss of circulation in depleted reservoirs and operation scheme of drilling with return gas are quiteand naturally fractured reservoirs such as those reported by different from conventional gas drilling. Therefore, in thisAllan(1994)and Hou et al (2005). These advantages have paper, Angels model is used to analyze characteristics ofmade significant differences in several field cases( Chen wellbore flow of recycling gas drilling Based on the analyticalet al, 2006). Recently, this technology has been employed results, two gas supplement schemes are proposed which playto drill hydrocarbon-bearing formations to reduce fluid a positive role in guiding the field operationdamage(Lyons et al, 2008). These formations include low-ermeability gas reservoirs and coal bed methane pay zone 2 Introduction to recycling gas drilling(Cox et al, 1999). Meanwhile, there are also some problems technologywhich are not handled perfectly(Malloy et al, 2007; Zhu etal, 2010; Zhao et al, 2010)Recycling gas drilling technology is proposed andCompared with dry air and natural gas, nitrogen gas is developed on the basis of the conventional gas drilling. Theeferred in drilling as it is inert and non-inflammability main characteristics of this technology is that the gas returnedCowar and Jones, 1952; Zabetakis, 1964; Allan, 1994). from the wellbore is purified by a separation and filtrationHowever, the biggest shortcoming of employing nitrogen gas system, and then injected back into the wellbore instead ofas the drilling fluid is the high cost of manufacturing. In most being dischargcases, the return gas is still predominantly nitrogen unless Fig. 1 illYH中国煤化工 cycling gas drillingthe well is a prolific gas producer. Therefore, if the nitrogen system at sCNMH Ghe gas supplier canbe nitrogen gas from a small n, generator, natural gas,orcorrEspondingauthoremailyangshunjil23(@163.comtail gas. The work gas fed to the compressors and boosterReceived June 20. 2011is injected into the well through the standpipe DrillingPet. Sci.(2012)959-65operation starts as soon as the gas volume and pressure in the reused, only a small amount of make-up gas is needed fromwell is high enough. The gas stream returned to the surface the gas supplier for the requirement of the wellbore extensionsolids and liquids. The purified gas is led to the suction end significantly reduce the cost of drilling by using less wa cangoes through three stages of separation( Fig. 2) to remove and possible gas leakage. The recycling gas drillingof the compressors and recycling. Because the return gas is from a small gas supplier.WellheadBoosterSeparatorsFig. 1 Sketch of the recycling gas drilling systema.The components of the recycling gas drilling system are output nitrogen stream is fed to next separator for furtherrther described as followsseparation1)Gas supplier. The gas supplier can be a small nitrogen 4)Cyclone separator. This is a second-stage separatorgenerator, natural gas pipeline, or CO2 pipeline. In most that removes cuttings of greater than 7 um and liquid usingapplications, a small-size nitrogen generator with on-site centrifugal force. The outlet nitrogen gas is fed next separatormembrane separation is feasible and cost-effective.for purification2)Compressors and boosters. These are standard 5) Precision filter. This is the last stage separation,equipment currently used in gas drilling operations. The comprised of a fiberglass filter element that removes allsuction end of the compressors can be modified to adapt to particles greater than l um by filtration and aggregationthe recycling gas drilling system6) Discharge system. This is a specially designed system3)Inertial separator. This is the first stage separation that that discharges cuttings and liquids with minimal loss of workremoves a great quantity of large drilling cuttings of greater gasthan 0. 1 mm and liquids by means of inertial force. The The fiberglass filter elements will eventually be blockedLaL中国煤化CNMHInertial and cyclone separatorsPrecision filterFig. 2 Three stages of separationPet. Sci.(20129:59-65by small cuttings, which will generate a small back pressure expressed in the discrete form( Guo et al, 1994; Gas researchat the surface(Fu et al, 2009). The back pressure includes the Institute, 1997)pressure drops of the blooie line, cyclone separators, fine filterand the inlet of compressors, which makes the calculationfine pi+g|(-h)very comp-Pi+Pc t Pr t PippP=Apt+p“hpP=△ypwhere P, is the back pressure, MPa; P, Pe, Pr and Pip are thepressure drops of the blooie line, cyclone separators, fine where Api is the pressure drop in infinitesimal segmentfilter and the compressor inlet, respectively, MPai in the annulus, Pa; fnnulus is the Fanning friction factor inHowever, according to the current technological design the annulus; vmix is the velocity of gas-solid mixtures in theand field tests, the maximum back pressure should not exceed annulus, m/s; p, is the average density of gas-solid mixtures0.05 MPa(Fu et al, 2008; 2009). In this paper the back in infinitesimal segment i in the annulus, kg/m, p is thepressure gradient is set at 0.01 MPa/100 mpressure at infinitesimal segment i, Pa; Ap/l is the pressuredrop in infinitesimal segment i in the drill pipe, Pa; foine is the3 Wellbore flow models for drilling with Fanning friction factor in the drill pipe, dimensionless; vgas isreturn gasthe velocity of gas in the drill pipe, m/s; p, is the gas densityin infinitesimal segment i in the drill pipe, kg/m; p.Pipis]theDuring gas drilling, the flow of compressible gas of a pressure in infinitesimal segment i in the drill pipe Pa; h, isfixed mass within a closed loop system is pipe flow and the well depth of infinitesimal segment Apit is the pressuredescribed by the laws of mass conservation and momentum drop in the drill bit, Pa; Dh is the well diameter, m; D, is theand temperature follows the equation of state of an ideal gas. of the drill pipe, m; g is the acceleration of gravy, -meterconservation. The relationship between the flow rate, pressure outside diameter of the drill pipe, m; d is the inside diarThe upward vertical flow in the annulus can be describedin the discrete form( Guo et al, 1994; Gas Research Institute, 4 The changes of flow parameters during gas1997)drillingThe drilling process at a depth of 2, 500-3, 000 m wasannulussimulated for both recycling gas and conventi2D.-D)+g(b-)drilling. Flow parameters in both gas drilling operations(2) were analyzed, and the concept of the drilling limit and theselection criterion of critical points for recycling gas drilling十were proposed. The calculated parameters are shown in Table1. The initial gas injection rates in both gas drilling operationsThe downward vertical flow in the drill pipe can be at the depth of 2, 500 m are 100 Nm/minTable 1 The parameters of Siod wellbore configuration三剩减小间曲DDC角2500-300097.180471.43752159244475114.3168.27517.784.1 The changes of gas flow rate and total gas mass 4) Calculate the total gas mass in the wellborein the wellbore during drillingThe total mass of the gas in the wellbore is estimaThe total mass of the gas in the wellbore is affected followsby many factors, mainly including the wellbore pressureprofile, gas stream temperature, gas circulation rate, holeP (Dh-D,)dh+44ad2d hfiguration and the drill string assembly. The calculationsteps are as follows:where M is the total mass of the gas in the wellbore, kg; Pgas is1)Calculate the temperature profiles in the annulus and the gas densitythe drill pipe:H中国煤化 Is the well depth, mIn conventmass of the gas in2)Calculate the pressure profiles in the annulus and the the wellboreCN MH Gsing depth(Fig 3)drill pipeHowever, due to the gas circulating in the closed loop system,3)Calculate the density profiles in the annulus and the the total mass of the gas in the wellbore maintains its originaldrill pipe;value in drilling with return gas. As a result, the gas flow ratedecreases linearly with depth in recycling gas drilling( Fig 4)Fig. 4 also shows a comparison of the gas flow rateunder back-pressure or not in recycling gas drilling. The gasflow rate changes more quickly when drilling at a specifiedback pressure than when drilling without building up back a 1.0pressure. Moreover, with an increase in the back pressure, 2drill pipe under back pressurein the annulus under back pressurethe gas flow rate changes significantly. This change canbe explained by the flow parameter distribution alongthe annulus under no back pressiwellbore. Figs. 5 and 6 show the impact of back pressurethe pressure and gas velocity profiles. Due to back pressure,the gas expansion and flow are inhibited, which furtheto an increase in the gas density in the wellbore. At thetime, the total mass of gas in the closed loop system is fixed,and then the gas velocity in the wellbore when drilling at back2000pressure is lower than that when no back pressure is appliedon the drilling fluid based on the law of mass conservationMoreover, as the gas flow rate reduces, the total decrease inthe gas column pressure and the friction loss is higher thanthe back pressure, which means the standpipe pressure whendrilling at back pressure is lower than that when no backpressure is applied on the drilling fluidRecycling gas drilling under back pressure-H Recycling gas driling under no back pressure四Depth, mFig. 6 The gas velocity distribution alongthe wellbore for recycling gas drilling2700280029004.2 The changes of kinetic energy at critical pointsduring drillingFig. 3 The total mass of gas in the wellbore changing with well depthAn important aspect for successfully applying gasdrilling technology lies in the selection of the critical pointConventional gas drillingThe traditional selection criteria are the minimum kineticRecycling gas drilling under back pressureenergy method(Angel, 1957)and minimum velocity methodRecyding gas drilling under no back pressureJOhnson, 1995). This study adopted the minimum kineticenergy method. The critical point generally refers to aEEz30location in the annulus at which the cross-section of theannulus changes. If, at the critical points, the kinetic energydoes not meet the cuttings carrying capacity, a mud ring willbe formed. The critical points in this case are the casing shoeand the top of the collar. The kinetic energy at the criticalpoints must be greater than 214 J/mThe kinetic energy equation is中国煤化工ECNMHG2700Depth, mwhere E is the gas kinetic energy, J/m; v is the gas velocitym/sFig. 4 The flow rate in the wellbore changing with well depthOwing to the constant gas fow rate during conventionalPet. Sci.(2012)9:59-65gas drilling, the gas velocity and density remain constant in 5 Gas supplement schemehe certain depth of wellbore. Therefore, the kinetic energy atthe casing shoe is constant in gas drilling(Fig. 7). However, Owing to the drilling limit, the total mass of gas in thethe depth of the collar top increases gradually during drilling, wellbore at the ultimate depth is inadequate to carry cuttingsso the kinetic energy at this point decreases continuously. Therefore, a gas supplement operation must be conductedFurther studies show that the selection of critical points is before drilling to this depth. This special operation willdeeply affected by wellbore configuration. With the extension make the total mass of gas in the wellbore sufficient agaiof the wellbore, the critical point changes from the casing achieve normal drillingshoe to the top of the collarLikewise, controlling the total mass of gas in the wellboreis also very important. If the total mass of gas in the wellboreKinetic energy ( the collar topis excessive, the compressor will be choked and then stopKinetic energy ( the casing shoeworking. However, if the total mass of gas in the wellboreMinimun kinetic energy of carrying drilling cuttingsis inadequate, compressor surge will occur. To solve thisproblem, the authors proposed two gas supplement schemesOne is the stage gas supplement scheme in which the gassupplement operation must be conducted before drilling tothe ultimate depth. The other is the real- time gas supplementscheme in which the gas supplement operation should beconducted at all times in drilling in order to maintain the totalmass of gas consistently sufficient5.1 The stage gas supplement schemeTo ensure safe drilling, the gas supplement operation is00 2900 3000 conducted each100 m Compressor surge willDepth msupplied gas is excessive. Therefore, with the compressorFig. 7 Kinetic energy at the critical points changingunder its normal working conditions, the supplementwith well depth for conventional gas drillingoperation should be terminated when the circulating gas flowrate reaches the capacity of compressors. At this time, theDue to the decreasing gas flow rate and back pressure whole operation will run into the next recycling gas drillinginhibiting gas expansion during recycling gas drilling, the modekinetic energy at the casing shoe decreases linearly with well As shown in Fig. 9, the recycling gas drilling is operateddepth, and its value is consistently lower than the kinetic at 2, 500 m. Since the total mass of gas in the wellboreenergy at the top of the collar(Fig. 8). Therefore, in this remains fixed (562 kg), the gas flow rate decreases. Then, thestudy, the casing shoe is the critical point. When drilling to gas supplement operation is conducted at a depth of 2, 600the depth of 2, 830 m, the kinetic energy at the casing shoe m and is terminated when the gas mass increases to 592 kgcannot meet the capacity of carrying drilling cuttings and the At this time, the gas flow rate recovers to 100 Nm/min.Thegas supply must be increased. Deeper than this depth, the whole drilling process was repeated for each 100 m like thistotal mass of gas in the wellbore cannot maintain the normal The total mass of gas in the wellbore demonstrates a steppedtransport of drilling cuttings without supplementation. This increase( Fig. 10)depth is called drilling limit of recycling gas drillingH- Kinetic energy ( the collar topKinetic energy the casing shoe-Minimum kinetic energy of carrying driling cuttingsEz≥=0中国煤化工26002700CNMHG 2900Fig. 8 Kinetic energy at the critical points changingFig. 9 The gas flow rate changing with wellwith well depth for recycling gas drillingdepth in the stage gas supplement schemePet Sci(20129:59.650.100g0.095a000270029003000Depth, mFig. 11 The gas supplement rate versus well deptFig. 10 The total mass of gas changing withwell depth in the stage gas supplement scheme5.2 The real-time gas supplement schemeThe starting point of this scheme is that the total mass ofgas in the wellbore should be always adequate This means $70that, the total mass of gas in the wellbore for recyclingdrilling should be the same as that for the conventional gas566drilling under the same drilling conditions. This scheme not 640only can avoid drilling limit, but also brings the advantagesof recycling gas drilling into full play. However, the real-time 9make the gas flow rate meet the requirement of capacity of 2 600pgas supplement rate must be controlled at all time in order tothe compressor. Its discrete formula is as followsDThe relationship between well depth and time isFig. 12 The total mass of gas versus well depthin the real-time gas supplement schemeH -H,=rope(7)6 Conclusionswhere t, is the time; M, is the total mass of gas in the1) The flow characteristics of recycling gas drilling werewellbore at time t, kg: Po is the gas density under atmospheric analyzed quantitatively. Because the total mass of gas in thepressure,kg/m; Lu(t,+i)is the gas supplement rate at time t,, wellbore is fixed, the gas flow rate decreases during drilling,Nm/min; H;+ is the well depth at time t, m; rop is the rate of which has an impact on the drilling cuttings transportpenetration, m/h2) Through analyzing the profiles of flow parametersThe relationship between the gas supplement rate and well the impact of the back pressure on the gas flow rate wasdepth can be obtained by substituting Eq (7)into Eq (6). The explained. The gas flow rate declines significantly withrecycling gas drilling is operated from 2, 500 m. At the same increases in the back pressure and the well depth. Moreovertime, the gas supplement operation is conducted to ensure the total decrease in the gas column pressure and the frictionthat the total mass of gas in the wellbore during recycling gas loss is greater than the back pressure, which makes thedrilling is always consistent with that during conventional gas standpipe pressure during drilling at back pressure is lowerdrilling. Due to the increasing bottom hole pressure and the than that when no back pressure is applied on the drillingcompressibility of gas, the gas density of unit depth increment fluid.increase, which will further lead to the gas mass of unit depth3)The seleautf-rcycling gas drillingincrement increasing. Therefore, the gas supplement rate is different fi中国煤化 drilling. MoreoverFig. I1.Simultaneously, the total mass of gas in the wellbore that at the top of the collar in recycling gas drilling but naincreases quasi-linearly with the depth extension as shown in the kineticCN MH Galways lower thain recycling gas drilling conforms to that in conventional gas conventional gas drillingdrilling(Fig. 124)The concept of drilling limit is proposed in recyclingPet. Sci.(2012)959-65gas drilling. At this depth the total mass of gas cannot meetapplication study. Paper SPE 93725 presented at the SPE Middlethe capacity of carrying drilling cuttings, and additional gasEast Oil and Gas Show and Conference, March 12-15, 2005must be addedKingdom of Bahrain5)Two gas supplement schemes are proposed, namely Johnson PW. Design techniques in air and gas drilling: Cleathe stage gas supplement scheme and the real- time gascriteria and minimum flowing pressure gradients. Journal of CaPetroleum Technology. 1995. 34(5): 18-26supplement scheme. In actual operations, a combination LiJ. Liu GH and Han L X. Study on the gas circulation drilling systemof both schemes can be used to achieve optimal drilling Drilling Production Technology. 2010. 3(3 ): 48-50(in Chinese)pertormanceLi J, LiuG H and Wang X Z. Open loop field test of nitrogen circulationAcknowledgementsdrilling system. Oil Drilling& Production Technology. 2010. 32(39-12(in Chinese)The authors are grateful for the financial support from theLiu G H, Tao Q and Li J Gas volume control techniques for circular gasNational Natural Science Foundation of China(50974021)drilling Oil Drilling Production Technology. 2009. 31(4): 32-35(inChinese)and Major Project of Chinese National Programs for Lunan B Underbalanced drilling-surface control systems. Journal ofFundamental Research and Devclopment(973 ProgramCanada Petroleum Technology. 1995. 34(7): 29-352010CB226704)Lyons W C, Guo B and Seidel F A. Air and Gas Drilling Manual. 2001McGraw-Hill. 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