An experimental study of shear strength of gas-hydrate-bearing core samples

Pet Sci.(2011)8: 177.DOI101007/s1218201101322An experimental study of shear strength ofgas-hydrate-bearing core samplesZhang Weidong", Ma Qingtao, Wang Ruihe and Ren ShaoranSchool of Petroleum Engineering, China University of Petroleum, Dongying, Shandong 257061, Chinac China University of Petroleum(Beijing)and Springer-Verlag Berlin Heidelberg 2011Abstract: The shear strength of gas-hydrate-bearing reservoirs is one of the most important parametersused to study mechanical properties of gas-hydrate-bearing reservoirs. The shear strength of gas-hydrate-bearing reservoirs changes with filling and cementation of gas hydrates, which will affect the wellboreand reservoir stability. Traditional shear tests could not be conducted on gas-hydrate-bearing core samplesbecause the gas hydrates exist under a limited range of temperature and pressure conditions. This paperdescribes a novel shear apparatus for studying shear strength of gas-hydrate- bearing core samples underoriginal reservoir conditions. The preparation of gas-hydrate-bearing core samples and subsequent sheartests are done in the same cell. Cohesion and internal friction angle of the core samples with differentsaturations of gas hydrates were measured with the apparatus. The effect of gas hydrates on the sheartrength of reservoirs was quantitatively analyzed. This provides a foundation for studying wellbore andreservoir stability of gas-hydrate-bearing reservoirsKey words: Natural gas hydrate, reservoir, experimental apparatus, shear test, internal cohesion, intemalfriction angle1Introductionand quartz powders containing gas hydrates in the laboratoryMoreover they studied the cohesion and friction angle ofShear strength is one of the most important mechanical quartz powders when the saturations of gas hydrates wereparameters of reservoir rocks in analyzing wellbore stability. 50% and 0However, it is difficult to measure the shear strength of gas- In this study, we developed a new experimental setup tohydrate-bearing core samples with commonly-used shear investigate the properties of gas-hydrate-bearing core samplesapparatus due to the presence of gas hydrates, which form under high pressure and low terwhen natural gas molecules and water come into contact atlow temperature and high pressure (Tan et al, 2005)2 ExperimentalTo solve this problem, Wu et al(1997)developed a Hoektriaxial cell for gas hydrate-bearing deposits. This device 2.1 Experimental apparatuscould be used to measure mechanical properties and failureThe shear apparatus shown in Fig. I was used for gasmechanisms of sediments containing gas hydrates under hydrate formation followed by shear testing. In the same cell,gas hydrates are not evenly distributed in the artificial core then shear tests were conducted, avoiding inconveniencessamples, leading to large errors in shear strength. Clayton and involved in core preparation and installation in shear testsPriest(2005)built a resonant column apparatus to synthesize The main component of this device was the shear unitmethane hydrate-bearing sediments and to measure their installed in the pressure cell and the shear unit consistedshear strength based the original triaxial shear apparatusf the shear cell and piston mechanisms. The shear cellAfter conducting triaxial shear tests on original hydrate had two chambers, and the connection was sealed with andeposits from the Malik 2L-38 well, Mackenzie Delta, annular piston. The upper cell was equipped with a piston forNorthwest Territories, Winters et al(2004)proposed that theshear strength of sediments containing gas hydrates increased compacting and a water inlet, a transverse piston mechanismwas Iwith an increase in gas hydrate saturation. Researchers at the中国煤化工 upper cell. PistonmechaGeorgia Institute of Technology tested the Poisson ratio, shear The lowde of the lower cellCNMHater outlet. All pistonstrength, and other mechanical parameters of sands, clays, mechanisms were connected to the high pressure pump byhydraulic lines. The lower shear cell was fixed on the bottom ofCorrespondingauthor.email:wdzhang@upc.edu.cnthe high pressure cell, and the upper one was connected to theReceived October 9, 2010top of the high pressure cell. a circulating water bath with anPet, Sci(2011)8:177-18234567Fig. 1 Schematic of a shear apparatus for hydrate-bearing core samples(Yin et al, 20081-Upper shear cell; 2-Annular piston; 3-Lower shear cell; 4-Rigid rod; 5-High pressure pipeline,6-Piston cylinder; 7-Pressure cell; &-Valve, 9-Entrance to the shear cell; 10-High pressure pump:Il-Piston vessel; 12- Methane cylinder, 13-High pressure air container; 14-Back-pressure valve,15-Transverse piston; 16-Compacting piston: 17-Intake line: 18-High pressure lineinsulation layer was used to ensure the system remained at low 2.2 Test proceduretemperatures. Fig. 2 shows the equipment in the laboratoryMethane(99.99% purity), quartz sand of 20-40 mesldistilled water were used to prepare gas hydrate-bearingsamples. According to the data on core samples containinghydrates, the hydrate-bearing reservoir matrix had the samedensity as quartz sand (Tinivella, 1999)The test procedures are as followsPreparation of gas hydrates in porous medium 1)Washboth the shear cell and the quartz sand with distilled watertwice and air dry them before use. 2) Put the dry quartz sandinto the upper and lower shear cells. 3)Adjust the shear cellto make sure the shear planes of the upper and lower cellscontact completely. 4) Lift the annular piston up to the properposition to seal the shear cells. 5)Install the high pressurecell and inject water into it. 6)Control the high-pressure celltemperature at 3C with the circulating water bath. 7)Use the中国煤化工8 Withdraw all air outof thep(evacuate the cell). 9)CNMH Gil water overflows fromthe top end. 10)Inject methane and collect water flowingfrom the shear cell and measure its volume V. I 1)Closethe bottom valve when a bubble forms and then inject theFig. 2 The experimental shear device for gas hydrate-bearing core samples remaining methane into the shear cell. 12) Inject distilledPet. Sci(2011)8:177-182water from the bottom of the shear cell until the pressureTable 1 Data acquired in the shearing processreaches 7 MPa. 13)The pressure in the high pressure cellPressure inwill decline as the reaction goes on. It is necessary to refill Time the shear cell pressure pressureTemperaturethe distilled water at frequent intervals to maintain pressureat 7 MPa. 14)Repeat Step 13 until the pressure is constant10.7The methane injected into the cell will completely react with10.7water to form hydratesShear test 1)apply the confining pressure of 7 MPa to108the shear cell, which equals the pressure in the high-pressure108cell. 2)Push the annular piston down to shear the coresample. 3)Record the pressure changes. The pressure will 20108rise slowly at the beginning and then decline sharply when 25 10.8the core sample is sheared. 4)Record the pressure P whenthe shear failure happens in the shear cell. The lateral pushingl1.3.9force F is equal to the product of the pressure P times the area 3510.711.2of the lateral piston A. The ratio of F to A is the shear strength10.8ll1of the hydrate-bearing2.3 Determination of gas hydrate saturation109l12The hydration reaction of methane is given by(Sk1998)Il1CH4(g)+ndH20←→CH4,ndH2O(s)where nmd is the hydration number, the molar ratio of water 5811.1reacting with methane, nhyd=5.75 in this case.After the temperature and pressure of the core cell are 60stabilized, all methane injected into the cell is assumed toreact completely with water. There are only gas hydrates and 64l10water remaining in the artificial core samples. Then the gasl1.0hydrate saturation can be calculated as follows(Ren et al,2010)6834.44411.06.32during 0-20 s, and the shear piston began to move whenwhere S is the gas hydrate saturation; VcH, and PcH, are the the pressure reached 6.32 MPa, i.e. starting pressure underinjection volume and pressure of methane, L; M is the molar experimental conditions. The shear piston began to movemass of methane hydrates, g/mol; R is the gas constant, upwards to push the upper shear cell at the time of 54 s, andR=8.31 J/(mol- K); Pore is the pore volume, L; Z is the the shear failure occurred during 54-62 s for gas-hydrate-gas compressibility factor; Ph is the gas hydrates density, bearing core samples. The maximum shear pressure wasP=0.91g/cm6.9 MPa. It was at the pressure release stage after 62 S, andthe pressure was back to 6. 32 MPa at the end. So the shear2. 4 Analysis of shearing processstrength was 0.58 MPa(6.90-6. 32)under the test conditionsIn the process of gas hydrate formation, the injectionpressure of methane was kept at 3. 7 MPa and the methaneinjection volume was 2 L. The gas hydrate saturation wascalculated to be 55% from Eq (2). In the shearing processthe back pressure was kept at 11 MPa to insure the confiningpressure higher than 11 MPa. At the early stage, the shear 9piston did not contact with the shear cell and the data werrecorded every 5 seconds. At the later stage, the piston pushed中国煤化工down into the shear cell and the data were recorded every 2CNMHGthe pressure changed quickly. Theresults are shown in Table 12.4.1 Shear pump pressureTime,sAs shown in Fig 3, the shear pump was pressurizedFig. 3 Curve of pump pressure vs time in the shearing processesai(20118171822.4.2 Shear cell temperatures Fig. 4 shows, the temperature of the high-pressurecell increased slightly due to the interference of external 9temperature in the piston compression process. Thetemperature declined significantly at 58 s, eventually to 3. 4oC. This is because the lower and upper shear cells weredisconnected and the gas hydrates absorbed heat and thenpartially dissociated into gas and water( Sun et al, 2002)3 Result analysesTime. sShear test data on gas-hydrate-bearing core samplesare shown in Table 2. The shear strength is the cohesion ofFig. 4 Curve of temperature vs time in the shearing processhydrate-bearing samples when the axial pressure is 0. The 3.1 Cohesioninternal friction angle of hydrate-bearing core samples wasalculated according to the Mohr-Coulomb criterion and Fig. 5 shows that the cohesion of artificial hydrate-bearingshear forces at different axial pressures(Huang et al, 1999). core samples increased quickly with increasing hydrateTable 2 Shear test data on core samples containing different contents of gas hydratesGas hydrate Gas hydrateNo. saturation formation timeGas hydrate formation Temperature Axial pressure Shear force1581.00.141.02.011.02.0113l101.020052H中国煤化工CNMH10Pet. Sci(20118:17182saturation when the saturation was less than 55%. this isbecause that gas hydrates were formed in the porous mediumin which water was present in excess. More water would fillthe remaining pores when the hydrate saturation was 8and then affected the gas hydrate cementation. Water in theemaining pores decreased as the hydrate saturation increased,therefore the gas hydrate cementation would increaseThe correlation between the cohesion angle and hydratesaturation for artificial core samples containing gas hydrates 2can be expressed as followsc=000002+0.0057S+0.0137R2=0991Hydrate saturation Sh,%Fig. 6 Relationship between intemal friction angle andpressure; is the internal friction angleThe shear failure curve of hydrate-bearing reservoirs ina-T coordinates can be obtained by c and 4, as Fig. 7 showsThe area on the inside of the shear failure curve is stableHydrate saturation S,%FIg. 5 Relationship between the cohesion and hydrate saturation3.2 Internal friction angleStability zoneFig. 6 shows that the internal friction angle of hydrate-bearing core samples increased with hydrate saturation whenthe saturation was less than 55%. the reason is due to waterfilled in the porous medium. Chen et al( 1998)determined theinternal friction angle of man- made permafrost to be 8.12with the triaxial test; Yue et al (1994)determined the angleof sea ice to be 39 with the lateral restraint shear strengthtest; and Masui et al (2007)performed triaxial tests on fournatural gas hydrate-bearing sediments drilled in the SouthFig. 7 Shear failure curve of hydrate-bearing reservoirsJapan Sea and several artificial hydrate-bearing sedimentsand determined the average internal friction angle to be 31at 50% hydrate saturation(Yun, 2005). It can be seen that 4 Conclusionsthe internal friction angle of hydrate-bearing sand packs is 1)a novel shear apparatus was built to study the shearbetween permafrost and sea icestrength of gas-hydrate-bearing core samples. In thisThe correlation between the internal friction angle and apparatus, the shear cell consists of upper and lower chambershydrate saturation for artificial core samples containing gas sealed with an annular piston. This allows the preparation ofhydrates can be expressed as followsthe gas hydrate-bearing core samples and the follow-up sheartesting to be conducted in the same shear cell.中=0.00352+0.0978S+15.941R2=0.9912)If the hydrate saturation was less than 55%, thecohesion of the hydrate-bearing core samples increased63 Applicationquickly when the hydrate saturation increased. Cementationbetween gas hydrates and quartz sand is responsible for theWe can use the Mohr-Coulomb criterion to evaluate the increase in cohesion. The intermal friction angle exhibited thewellbore stability during drilling through the hydrate-bearing same中国煤化工 hydrate-bearing corereservoirs(Hoek and Brown, 1980)sampleand sea ice3)CN MHGstrength and saturationr=c+(σ-P)恕g中was determined, but it can not reflect the variation of shearstrength during phase change of hydrates, because the shearwhere t is the shear strength; c is the cohesion of hydrate- strength will change when the water saturation of reservoirsbearing reservoirs; a is the normal stress; Pp is the porosity changes during hydrates phase changePet. Sci.(20118:177-8AcknowledgementsSloan Jr. E D. Clathrate hydrates of natural gases. Marcel Dekker IncNew York.1998.15-18The authors are grateful for financial support from Sun Z G, Fan SS, Guo K H, et al. Determination of dissociation heat of"Preliminary Research on natural gas hydrates production"atural gas hydrates. 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