Classification method of shale gas analytic stages and its application

China is rich in shale gas resources, and improving the oil recovery of shale gas wells is one of the hot issues in the current research. The production of shale gas wells releases shale gas by reducing formation pressure by repeated fracturing. In this paper, the curvature of shale desorption efficiency curve is analyzed to study the pressure suitable for formation shale gas release. According to the inflection point and stagnation point of the curvature curve, the starting pressure, sensitive pressure and turning pressure are obtained. the curvature curve can be divided into four stages: sensitive desorption, rapid desorption, slow desorption and inefficient desorption. In actual exploitation, shale desorption cannot go through all four desorption stages, and can be determined according to the ratio of VL/PL. When the ratio is greater than 2.59, there are four integral stages in shale desorption, and the larger the ratio is, the longer the sensitive desorption stage is. At 1.0-2.59, there are three stages of desorption; at 0.54-1, there are only two stages; below 0.54, there are only low efficiency stage. The isothermal adsorption study on the core samples of the Upper Paleozoic in Yulin area of Ordos Basin shows that the shale of the Upper Paleozoic in the study area has more starting pressure and turning pressure, and less sensitive pressure, and the ratio of VL/PL is between 0.5 and 2.0. Generally, the isotherm adsorption curve can be divided into three stages: fast desorption, slow desorption and low efficiency desorption. The low efficiency desorption stage contributes the most to the productivity of Upper Paleozoic shale in this area, and the slow desorption is supplementary. According to the desorption stage and pressure node in shale mining, the formation pressure can be appropriately reduced to improve shale gas recovery efficiency.