Evolution and Application of Sealing Ability of Gypsum Caprocks under Temperature-Pressure Coupling:An Example of the ZS5 Well in the Tazhong Area of the Tarim Basin

Gypsum caprocks'sealing ability is affected by temperature-pressure coupling.Due to the limitations of experimental conditions,there is still a lack of triaxial stress-strain experiments that simultaneously consider changes in temperature and pressure conditions,which limits the accuracy of the comprehensive evaluation of the brittle plastic evolution and sealing ability of gypsum rocks using temperature pressure coupling.Triaxial stress-strain tests were utilized to investigate the differences in the evolution of the confinement capacity of gypsum rocks under coupled temperaturepressure action and isothermal-variable pressure action on the basis of sample feasibility analysis.According to research,the gypsum rock's peak and residual strengths decrease under simultaneous increases in temperature and pressure over isothermal pressurization experimental conditions,and it becomes more ductile.This reduces the amount of time it takes for the rock to transition from brittle to plastic.When temperature is taken into account,both the brittle–plastic transformation's depth limit and the lithological transformation of gypsum rocks become shallower,and the evolution of gypsum rocks under variable temperature and pressure conditions is more complicated than that under isothermal pressurization.The sealing ability under the temperature-pressure coupling is more in line with the actual geological context when the application results of the Well#ZS5 are compared.This provides a theoretical basis for precisely determining the process of hydrocarbon accumulation and explains why the early hydrocarbon were not well preserved.

Dynamics for multistage pool formation of Lunnan low uplift in Tarim Basin

Lunnan area in the Tarim Basin has become animportant onshore oil production base in China. Formationof the oil and gas pools in the low uplift of Lunnan has ex-perienced a comparatively complex process of dynamics.Based on the hydrocarbon generation period of source rocks,the formation period of cap rocks and traps, the analysis oforganic inclusion and the analysis of bitumen in the reservoir,this paper draws the conclusion that the low uplift area ofLunnan has experienced three pool formation periods: thePermian period, the Cretaceous-Early Tertiary period andthe Late Tertiary-Quaternary period and two oil and gasreservoir adjustment periods: the Late Permian period andthe Late Tertiary-Quaternary period. The comprehensivestudy indicates that the large-scale Ordovician buried hill,formed in Early Hercynian, became the reservoir during thePermian period, because the Cambrian-Lower Ordovicianoil was discharged laterally into the reservoir along the topof the Ordovician weathering crust from south to north. Thereservoir experienced a complicated process-reconstruc-tion in the end of Permian, adjustment in Cretaceous-EarlyTertiary and re-discharging process in Late Tertiary-Qua-ternary, leading to the early original heavy oil reservoir ofmarine facies and the late original light oil reservoir and gaspool. Carboniferous, Triassic and Jurassic oil and gas reser-voirs result from upward adjustment and re-distribution ofOrdovician oil and gas reservoirs. Of those results, the for-mation of Triassic-Jurassic oil and gas pools came under theinfluence of the northward-tilting structure. The oil and gassourcing from the different hydrocarbon source rock inter-vals vertically migrated into the base unconformity of Trias-sic system. Then the oil and gas migrated laterally fromnorth to south and accumulated into the reservoir.

Petroleum geological characteristics of Kela-2 gas field

The Kela-2 gas field is located in the center ofKelasu structural belt in Kuqa Depression. This trap is oneof a series of traps in the folded belts which are distributed ina string of pearls in the dual structure. The primary gas-bearing layers are sandstone of Lower Cretaceous K1bswhile the secondary layers are dolomite member and gluten-ite member of Lower Tertiary E1-2km and sandstone ofLower Cretaceous K1b. The main component of natural gasis methane whose content is higher than 97%. It is charac-terized by dry gas whose source rock is Jurassic coal meas-ure. The Kela-2 structural trap was formed during the Xiyuperiod and then became a reservoir in the late time. The res-ervoir formed late and the thick seal rock of Lower Tertiarygipsmantle are the avail reason why the giant Kela-2 gasfield has been well kept. The abnormal high pressure of theKela-2 gas field results from the strong structural compres-sion in the northern part during the Xiyu period.

A novel stimulation strategy for developing tight fractured gas reservoir

Conventional stimulation methods such as matrix acidizing,acid fracturing,or proppant fracturing have resulted in products that perform poorly and/or fail within months.Other options,such as water fracs with light sand,give better results but are prohibitively expensive.Mineral composition,brittleness index,stress regime,and petrophysical properties,which are favorable for creating complex fracture networks,can be obtained by geochemical and geomechanical analysis.The extended Reshaw and Pollard criterion shows that hydraulic fractures tend to be arrested by pre-existing natural fractures,and complex fracture networks would be created during fracturing.Additionally,the critical stressed faults theory indicates that the pre-existing natural fractures tend to slip with the shear mode as the pore fluid pressure increases.Rotating disk experiments and conductivity tests with artificial sheared plates have shown that flow channels can be etched at the location of scratches on fracture surfaces.Meanwhile,the carbonate cement in natural fractures can be chelated to form wormhole likely flow channels.Complex fracture networks with sufficient acid etched conductivity can be generated by water fracs with acid.A novel and economical volume stimulation strategy known as network acid fracturing has provided Tarim Oil Company the means to develope ultra-deep,ultra-high pressure,high temperature,and ultralow permeability but fractured gas reservoirs.Post-stimulation production performances of numerous wells with network acid fracturing are comparable to those with stimulated reservoir volumes.