A novel experimental system for accurate gas sorption and its application to various shale rocks

Molecular Simulation of Adsorption in Deep Marine Shale Gas Reservoirs

Deep marine shale gas reservoirs are extremely rich in the Sichuan basin in China. However, due to the in situ conditions with high temperature and high pressure (HTHP), in particular reservoir pressure being usually much higher than the test pressure, it is difficult to accurately clarify the adsorption behavior, as seepage theory plays an important role in shale gas reserves evaluation. Therefore, three kinds of sorbent, including illite, quartz and kerogen, and two simulation methods, containing the grand canonical ensemble Monte Carlo method and molecular dynamics method, are synthetically used to determine the methane adsorption behavior under HTHP. The results show that both absolute adsorption and excess adsorption decrease with the increase of temperature. When the pressure increases, the absolute adsorption increases quickly and then slowly, and the excess adsorption first increases and then decreases. The superposition of wall potential energy is strongest in a circular hole, second in a square hole, and weakest in a narrow slit. The effect of pore size increases with the decrease of the pore diameter. Under HTHP, multi-layer adsorption can occur in shale, but the timing and number of layers are related to the sorbent type.

Novel Model for Rate Transient Analysis in Stress-Sensitive Shale Gas Reservoirs

Technical advances in hydraulic fracturing and horizontal drilling technologies enable shale to be commercially exploited. Due to the technical and economic limitations of well testing in shale gas plays, rate transient analysis has become a more attractive option. After hydraulic fracturing, flow mechanisms in multiple scaled pores of shale become extraordinarily complicated: adsorption in nanopores, diffusion in micropores, and non-Darcy flow in macropores. Moreover, shale gas reservoirs are stress-sensitive because of ultralow permeability and diffusivity in a matrix. Furthermore, the porosity and permeability of natural fractures are stress-dependent as well. Accounting for all of these complex flow mechanisms, especially the aforementioned stress-sensitive parameters, a semianalytical production solution of a multiple fractured horizontal well (MFHW) can rapidly predict the entire production behavior. Scholars have done much work on the complex flow mechanisms of shale. Most models regarded permeability as a stress-sensitive parameter while diffusivity and porosity were considered to be a constant. However, diffusivity and porosity were proved to be stress-sensitive as experimental science developed. In this study, we present a novel semianalytical model for rate transient analysis of MFHW, which simultaneously incorporates multiple stress-sensitive parameters into flow mechanisms. Substituting stress-dependent parameters (diffusivity, porosity, and permeability) into governing equations resulted in strong nonlinearities, which was solved by employing the perturbation method. Production behaviors with only stress-sensitive permeability were compared with multiple stress-dependent parameters. The new model with multiple stress-sensitive parameters declined slower than the permeability-sensitive model, and the new model matched better with the field data. In addition, the effects of major stress-sensitive parameters on production decline curves were analyzed by the proposed model. The sensitivity analysis indicated that different parameters had their own degree of sensitivity intensity and influence on the production period. Finally, 1001 wells from the Marcellus shale play were divided into three well groups. Estimated inversion values of reservoir parameters from the three well groups and relevant single wells were consistent with the field data. The inverted values of single wells fluctuate within the inversion values of well groups, which indicates that the production behavior of well groups could be a guide for rate transient analysis of a single well in shale gas reservoirs.