Evaluation Of Gas Supply Capability Of Tight Sandstone Gas Reservoirs With Simulated Downhole Production

Tight sandstone gas, as one of the most realistic unconventional resources in China, is the main force of gas reserves and output. According to the third resource assessment, tight sandstone gas resources in China are about20×1012m3. The percolation mechanism of tight sandstone have great difference between conventional reservoirs because of its tight lithology, slim throat, low porosity, low permeability, high content of clay mineral, and high capillary pressure. The economical and effective development of tight sandstone gas reservoir is of great significance to improve the stable gas production of wells and recovery of tight sandstone reserve.In this study, the research object is tight sandstone gas reservoir from Pernian in Upper Paleozoic in Ordos basin. The reservoir property characteristics and pore throat structure were analyzed by several technical methods such as X ray diffraction, scanning electron microscopy, cast thin section, and capillary pressure curves. The thesis researched the influence of pore structure, effective stress, water saturation and production pressure difference on gas supply capacity of tight sandstone.The thesis proposes the concept of limit pressure, which is able to weaken the slippage effect of tight medium and cleared its relationship with pore structures. The limit pressure of core samples with different permeability levels is determined by room experiments. The gas permeability tested at different pressure drops is almost a stable value whose range is less than3%when backpressure increases to the limit pressure. At this case, gas slip effect can be eliminated. The limit pressure of tight sandstone of Sulige gas field is confirmed as1.0MPa, which is a quarter of displacement pressure. Consequently, if the backpressure is exerted to limit pressure during experiment, the absolute permeability can be determined by testing only at a pressure value.A new method of measuring gas-water relative permeability curve is formed. The different water saturation is established by spontaneous imbibitions. The gas-water relative permeability curve of tight sandstone is tested by exerting limit pressure at outlet of the core samples. This new method can make up the deficiency of current methods, which can not establish lower water saturation and can not test the gas permeability under this condition. And the curves can reflect the in situ gas percolation characteristics veritably. Results indicated that the relative gas permeability curves of TK high core samples are shape of reversed S, the curves of TK middle are declined linearly with the increasing of water saturation, and the curves of TK low are concave upward shape. Research the influence of effective stress on the gas supply capability.The stress sensitivity of He8and Shan1interval of the Sulige gas field is from weak medium to medium-high. The in situ permeability is from1/3to1/4as the permeability of routine condition. Stress sensitivity increases with the decreasing of permeability. The experiment simulated gas percolation of altering pressure difference drops and analyzed the impact of pressure difference on yield per unit area and output increasing rate. Based on the test results, reasonable production pressure differential was determined.Considering synthetically geological characteristics and gas percolation, the gas supply capability index of tight sandstone reservoir is put forward. The thesis study the effect of physical property of reservoir, slippage effect, effective stress, water saturation diffution and production pressure difference on the gas supply capability of tight sandstone. Based on the influence factors of tight sandstone gas reservoir, the gas supply capability index (GDI) is introduced to analyze the impact of these factors on gas supply capability.