New Calculation Method and Application of Gas Loss from Coring Shale Gas Well

Gas content is one of the most important parameters of shale reservoir evaluation and productivity evaluation. In order to obtain gas content accurately, based on the first law of Fick and material balance equation, mathematical model of gas dispersion flow in shale reservoir is established, analytical solution is obtained, and evaluation method of gas dispersion in shale reservoir is formed. On the basis of this study, the onsite desorption gas measuring device and testing process for coring shale gas wells are designed, the time-varying shale desorption gas is obtained, and the residual gas of shale is measured by the crushing method. The calculation formula of shale gas content is obtained by fitting the test data, the shale gas loss, and total gas content are calculated, and then analyzed the influences of the shale gas-bearing properties and gas content on single well production and geological reserves by combining the data of shale absorbed gas. The results show that the gas content calculated by the new method is about 6.54% more than that of log interpretation, and about 7.57% on average more than that obtained by traditional empirical method. The gas content proportion of long Yi(1)subsegmental small layers 1 and 2 is smaller than that of long Yi(1)subsegmental small layers 3 and 4 and long Yi(2)subsegmental. It is considered that the amount of shale gas lost is large, because of the pressure release during the coring, and the comparative error of gas content obtained by several methods is within the acceptable range. So the new method can be used as an important mean to obtain shale gas content. The most direct factors affecting gas content are complex: Buried depth, porosity, total organic carbon content, water saturation, and formation temperature. Shale gas content is the material basis of single well production and geological reserves of shale gas, and it is also the decisive factor. Therefore, the accurate evaluation of shale gas content is one of the key techniques to evaluate shale gas well productivity and shale gas resources, which is of great significance.