Analysis of the Transfer Process and Dynamic Variations of Pore Pressure for the High Rank Coal in the Southern Qinshui Basin, China

Abstract

Abstract

The coalbed methane (CBM) resource in high rank coal reservoirs accounts for more than half of all of CBM resources in China, and the proved reserves of high rank coal account for approximately 85% of the national proved CBM resource. Determination of the dynamic characteristics of coal reservoir pressure is essential to evaluate the effective desorption range, interwell interference, and productivity. To reveal the dynamic laws of pore pressure for CBM reservoir during CBM production, the water and gas output from CBM wells in the southern Qinshui Basin were organized and analyzed firstly, and the process of CBM production were divided into four stages, namely, water output stage, gas-water output stage, stable gas output stage, and declining gas output stage. Then, the transfer process of pore pressure in different stages and different-scales fractures during the overall CBM production was analyzed, which shows that the reservoir pressure transmits through the fracture network called visible fractures system including exogenous joints, gas-expanding fractures and cleats. What is more, the basic transfer modes of reservoir pressure were proposed, which consist of water transfer and gas transfer. In addition, due to the close relationship between gas content and pore pressure, the overall variation principles of pore pressure and gas content for CBM reservoir were qualitatively and mathematically descripted respectively, and a field case in the southern Qinshui Basin was analyzed. The results show that there are similar variation principles for the reservoir pressure and gas content, both of which are sequentially slow, fast, and then slow again. The mathematical representation and field case validate the analysis results of the dynamic characteristics of reservoir pressure and gas content.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016