Quantitative evaluation of adsorbed and free water in deep shales:a case study on the Wufeng-Longmaxi Formations from the Luzhou area,southern Sichuan Basin,China

Deep shale gas reservoirs commonly contain connate water, which affects the enrichment and migration of shale gas and has attracted the attention of many scholars. It is significant to quantitatively estimate the amounts of adsorbed and free water in shale matrix pores, considering the different impacts of pore water (adsorbed water and free water) on shale gas. In this paper, pore water in six deep shale samples from the Wufeng-Longmaxi Formations in the Luzhou area, southern Sichuan Basin, China, was quantitatively evaluated by saturation-centrifugation experiments. Further, the impact of shale material composition and microstructure on the pore water occurrence was analyzed. The results show that amounts of adsorbed and free water are respectively 1.7967–9.8218 mg/g (mean 6.4501 mg/g) and 9.5511–19.802 mg/g (mean 13.9541 mg/g) under the experimental conditions (30°C, distilled water). The ratio of adsorbed water to total water is 15.83%–42.61% (mean 30.45%). The amounts of adsorbed and free water are related to the pore microstructure and material compositions of shale. The specific surface area of shale controls the amount of adsorbed water, and the pore volume controls the amount of free water;organic pores developed in shale solid asphalt contribute specific surface area and pore volume, and inorganic pores developed in clay mineral contribute pore volume. Therefore, the pores of shale solid asphalt accumulate the adsorbed water and free water, and the pores of clay minerals mainly accumulate the free water.

CO_(2),N_(2),and CO_(2)/N_(2)mixed gas injection for enhanced shale gas recovery and CO_(2)geological storage

In this work,using fractured shale cores,isothermal adsorption experiments and core flooding tests were conducted to investigate the performance of injecting different gases to enhance shale gas recovery and CO_(2)geological storage efficiency under real reservoir conditions.The adsorption process of shale to different gases was in agreement with the extended-Langmuir model,and the adsorption capacity of CO_(2)was the largest,followed by CH_(4),and that of N_(2)was the smallest of the three pure gases.In addition,when the CO_(2)concentration in the mixed gas exceeded 50%,the adsorption capacity of the mixed gas was greater than that of CH4,and had a strong competitive adsorption effect.For the core flooding tests,pure gas injection showed that the breakthrough time of CO_(2)was longer than that of N_(2),and the CH_(4)recovery factor at the breakthrough time(Rch,)was also higher than that of N_(2).The RcH of CO_(2)gas injection was approximately 44.09%,while the RcH,of N_(2)was only 31.63%.For CO_(2)/N_(2)mixed gas injection,with the increase of CO_(2)concentration,the RcH,increased,and the RcH,for mixed gas CO_(2)/N_(2)=8:2 was close to that of pure CO_(2),about 40.24%.Moreover,the breakthrough time of N_(2)in mixed gas was not much different from that when pure N_(2)was injected,while the breakthrough time of CO_(2)was prolonged,which indicated that with the increase of N_(2)concentration in the mixed gas,the breakthrough time of CO_(2)could be extended.Furthermore,an abnormal surge of N_(2)concentration in the produced gas was observed after N_(2)breakthrough.In regards to CO_(2)storage efficiency(S_(Storage-CO_(2)),as the CO_(2)concentration increased,S storage-co_(2)also increased.The S storage-co_(2),of the pure CO_(2)gas injection was about 35.96%,while for mixed gas CO_(2)/N_(2)=8:2,S sorage-co,was about 32.28%.

Prediction of decline in shale gas well production using stable carbon isotope technique

Prediction of shale gas production is a challenging task because of the complex fracture-pore networks and gas flow mechanisms in shale reservoirs.Empirical methods,which are used in the industry to forecast the future production of shale gas,have not been assessed sufficiently to warrant high confidence in their results.Methane carbon isotopic signals have been used for producing gas wells,and are controlled by physical properties and physics-controlling production;they serve as a unique indicator of the gas production status.Here,a workable process,which is combined with a gas isotope interpretation tool(also known as a numerical simulator),has been implemented in Longrnaxi shale gas wells to predict the production decline curves.The numerical simulator,which takes into account a convection-diffu-sion-adsorption model for the matrix and a convection model for fractures in^(13)CH_(4) and ^(12)CH_(4) isotopologues,was used to stabilize the carbon isotope variation in the produced gas to elucidate gas recovery.Combined with the production rates of the four developing wells,the total reserves ranged from 1.72×10^(8) to 2.02×10^(8) m^(3),which were used to constrain the trend of two-segment produc-tion decline curves that exhibited a transition from a hyperbolic equation to an exponential one within 0.82-0.89 year.Two-segment production decline curves were used to forecast future production and estimate ultimate recovery.

Hydrocarbon accumulation and key exploration & development technologies of Changning-Weiyuan marine shale gas field, southern Sichuan

China is rich in shale gas resources and has broad prospects for development. However, the quality ofshale gas resources in China is different from that in the United States, so independent unique technologies are urgently required in China. In this regard, PetroChina has carried out theoretical andtechnical researches systematically on shale gas in Longmaxi Formation in southern Sichuan according toits geology-engineering characteristics. Guided by the “three-control” theory for marine shale gasenrichment and high yield (i.e. sedimentary diagenesis controls reservoir, preservation conditions control accumulation, and continuous thickness of Type I reservoir controls production), six key explorationand development technologies in southern China have been innovatively developed for marine shale gaswhich experienced multi-stage tectonic evolution. The technologies include comprehensive geologicalevaluation technology, development optimization technology, optimal and fast drilling technology forhorizontal well, volume fracturing technology for horizontal well, factory-like operation technology, andefficient clean production technology. These technologies have enabled the large-scale commercial recovery of shale gas. By the end of 2019, PetroChina had obtained proved geological reserves of1.06
1012 m3, had built production capacity of 10
109 m3, and had cumulative gas production of20
109 m3 in southern Sichuan. The remarkable results of application are of great significance forvigorously developing shale gas industry to reduce external dependence, ensure national energy security,and realize the strategy of "ecological priority and green development".