Geological particularity and reservoir engineering response of deep coalbed methane

The successful pilot production of medium-deep and deep coalbed methane (CBM)in the eastern margin of O rdos Basin and Xinjiang has greatly boosted the progress of deep C B M exploration and development in China. To further promote the large-scale high production of deep CMB, it is necessary to systematically summarize and analyze the geological particularity and reservoir engineering response in deep layers by use of the current drilling results and development practices. Research results show that the depth effect on the critical parameters for C B M accumulation lies primarily in three aspects. (1) With the increase of burial depth, the stratigraphic environment parameters such as temperature field, pressure field and stress field tend to increase, the gradients tend to converge, the type of deep stress field changes, and the horizontal stress anisotropy weakens. (2) Highly saturated or oversalurated gas is generally developed in deep scams, thus greatly enhancing the industrial development value of free gas, but the higher-rank coal reservoirs are still dominated by adsorbed gas according to current mining depth. (3)Under deep condition of high stress. the coal structures arc weakened, in-situ permeability and mechanical properties tend to converge, and fluid production depends heavily on artificial seepage channels. Under the stratigraphic condition, the decrease in horizontal hydraulic fracture extension and the increase in ductile deformation easily lead to the formation of a complex network with high-angle and short fractures in deep coal scam. Low-output construction faces challenges in the limited fracture growth and poor sand-support so that fracturing opertitions should effort to maximize the full range of reservoir volume stimulation in the near-wellbore area, focusing on the change from being highly compressed to being crushed and supported. The gas-bearing characteristics of high to oversaturated conditions in deep seams can ensure the full utilization of strong reservoir energy and high permeability during initial production. However, the production capacity replacement by adsorbed gases can be only achieved when the reservoir pressure drops to a certain pressure. Research suggests that the integrated geological and engineering study on deep scams focus on C B M accumulation, migration and accumulation process as well as geological configuration for gas enrichment with different phases, explore optimization techniques and principles for reservoir stimulation, clarify the drainage and product ion-induced phase re-equilibrium process of coal reservoirs, property change patterns and gas production regularity, and seek for the best coupling of geological conditions, production enhancement measures, drainage and production control as well as reserve utilization. © 2023 Science Press. All rights reserved.