A critical desorption time method to improve the calculation accuracy of gas loss in shale gas content testing

During the conventional coring in shale gas reservoirs, a large amount of gas is lost, so it is difficult to estimate the lost gas content accurately. And consequently, the evaluation of shale gas content is deviated more from the real values. In this paper, the applicability of existing estimation methods for lost shale gas content calculation was firstly evaluated. Then, a critical desorption time method suitable for estimating the lost shale gas content was developed based on the gas diffusion principle in the process of core extraction. Afterwards, based on the measured desorption data of conventional coring samples of one shale gas well from the 7th Member of Yanchang Formation of Upper Triassic, Mesozoic in the Ordos Basin, the lost shale gas content was calculated by using the USBM method, polynomial function method and critical desorption time method, respectively. Finally, the calculation results were compared with the measured gas content of the pressure coring samples from the same horizon in this well. And the following research results were obtained. First, there is a linear relation between the cumulative desorbed gas and the square root of the diffusion time before the critical desorption time, and the slope of the trend line is the same. After this time point, there is a late non-equilibrium dif fusion stage, when the collected desorp - tion data present that the non-linear relation and the desorption curves are parallel to each other. Second, the shale gas content calculated by the critical desorption time method was closer to the measured shale gas content of pressure coring samples. In conclusion, the critical desorption time method is more applicable to shale gas content testing, and it can increase the calculation accuracy of shale gas content significantly. © 2019, Natural Gas Industry Journal Agency. All right reserved.