Abstract:

To study the shale gas occurrence characteristics of the Longmaxi Formation in northeast Chongqing, the isothermal
adsorption data under high temperature and high pressure and porosity data under burden pressure of the well WX2 shale were analyzed. According to the principle of minimum error, we have chosen a suitable adsorption model for the study area. Based on the relationship between porosity and effective stress, the free gas model is established. In addition, comprehensive analysis of the characteristics of adsorption gas, free gas, and changes in total gas with the buried depth was conducted. The results show that excess adsorption quantity of well WX2 shale increased first and then decreased under different temperatures with an increasing pressure. The maximum adsorption capacity decreased gradually as the temperature increased. After correction, the absolute adsorption quantity shows a rapid increase first and then a slow increase with increasing pressures. As the absolute adsorption data can be fitted with D-A model with the minimum average error, it can represent the real adsorption process of shale in the study area. Pores and micro-cracks of shale samples will gradually close during pressurization, and reopen during depressurization. This process will cause irreversible damage by plastic deformation, but irreversible damage accounts for small proportion. Porosities of samples from different directions have a negative exponent relationship with effective stress. Parallel samples of lamina-rich shales have greater initial porosity and
stronger pore stress sensitivity than the vertical samples. Shale gas occurrence characteristic is comprehensively controlled by reservoir characteristics, adsorption capacity, reservoir temperature, and reservoir pressure. Among them, temperature has the negative effects on adsorption gas content and free gas content, and the reservoir pressure has the positive effect. The adsorption gas content, free gas content and total content increases first then decreases. Adsorption gas content and free gas content are mainly controlled by temperature and reservoir pressure, respectively. In addition, relative abundance of adsorption gas and free gas will change near the critical depth, and this will be of great importance for evaluating shale gas enrichment.

Key words: Well WX2, occurrence characteristics, gas content, critical depth, northeast Chongqing