Production profile characteristics of large dip angle coal reservoir and its impact on coalbed methane production: A case study on the Fukang west block, southern Junggar Basin, China

Abstract

Xinjiang, China has abundant coalbed methane (CBM) resources, where most of the CBM exists in large dip angle (LDA) reservoirs (over 50°) and the production profile characteristics and the impact on CBM production are rarely studied. This paper summarizes the production profile characteristics of CBM wells in LDA reservoirs based on field production data from the Fukang west block, southern Junggar Basin. The factors affecting CBM production were qualitatively and quantitatively analysed by scatter plot and Spearman rank correlation analysis. Eclipse numerical simulation software was used to simulate the gas production in LDA reservoirs. Using field production data and numerical simulation, we found that there are two peaks in the gas production profile of high-production wells (shallow part) and one peak in moderate- (middle part) and low-production (deep part) wells in the LDA reservoir. For high-production wells, large dip angles cause large differences in the depth over a small area, resulting in significant differences in the physical parameters of the upper and lower reservoirs, especially reservoir pressure, permeability and gas content, which lead to the phenomenon of unsynchronized gas production in the upper and lower reservoirs, resulting in two gas production peaks. For moderate- and low-production wells, the reservoir pressure is high and permeability is low due to the deeper burial depth, causing the time required to reach the critical desorption pressure to be longer and the second peak in the gas production to be delayed. The scatter plots and Spearman rank correlation analysis showed that there is a clear correlation between the reservoir pressure and permeability and the gas production, which means that both parameters play a major role in controlling CBM production. We also found that reservoir pressure and permeability are directly controlled by the burial depth, which indicated large dip angles can have a significant impact on CBM production.