煤层气合层开发上部产层暴露的伤害机理

Damage mechanism of upper exposed producing layers during CBM multi-coal seam development

煤层气开采过程中,上部暴露产层伤害对煤层气井产能的影响短期内有可能被下部未暴露产层产气能力的提高所掩盖,因此未引起研究人员的重视。为此,依托于贵州西部土城区块煤层气勘探开发工程实践,结合15号煤启动压力梯度、气水两相渗流及应力敏感性测试,分析了上部产层暴露的储层伤害机理,探讨了合层开发煤层气井高产、稳产的排采控制措施。结果表明:1随着低渗透煤储层中游离气量增多、气泡变大,气水两相渗流产生的贾敏效应增强,导致水相渗流的启动压力梯度增大,水相渗透率快速下降;2合层开发煤层气井上部产层被动暴露后,套压持续回升导致气体"反侵"进入已经暴露的煤储层,井筒周围依次形成高含气带、液相滞留带、应力敏感带、高含水带,近井地带形成液相低渗区,使地层水、压裂液难以排出,将导致上部暴露产层产水、产气量快速衰减。结论认为,为了提高合层开发的效果,可在套压降至0.5 MPa后主动缓慢暴露上部产层,而在上部产层主动暴露后,应尽量避免套压的快速波动,杜绝套压的大幅回升,以免对近井地带煤储层造成永久性伤害。

In the process of CBM （coalbed methane） production, the effect of upper exposed producing layer damage on CBM well productivity may be concealed in a short term by the improved gas production capacity of lower unexposed producing layers, so not much attention is paid to it. Based on the engineering practice of CBM exploration and development in the Tucheng Block of western Guizhou, the reservoir damage mechanism of the upper exposed producing layers was analyzed by using the starting pressure gradient, the gas-water phase flow and stress sensitivity test of No. 15 coal seam. Then control measures for high-yield stable production of multi-coal seam development in CBM wells were discussed. It is shown that the Jamin effect gets stronger with the increase of free gas volume and the growth of gas bubbles in low-permeability coal reservoirs, resulting in the starting pressure gradient of water phase flow rising and the water permeability dropping fast. After the upper producing layers are exposed passively in CBM multi-coal seam development wells, the gas invades inversely into the exposed coal reservoirs due to the continuous buildup of casing pressure. The high gas-bearing belt, the liquid phase-detained belt, the stress-sengitive belt and the high water-bearing belt are formed successively around the borehole and the low-permeability liquid phase zone is formed near the borehole, so formation water and fracturing fluids cannot be drained out easily. As a result, the water and gas production rate of the upper exposed producing layers decline rapidly. It is concluded that for the improvement of multi-coal seam development effect, the upper producing layers should be actively exposed slowly when the casing pressure drops to 0.5 MPa. After the active exposition of upper producing layers, rapid fluctuation of casing pressure should be avoided and high-rate buildup of casing pressure should be eliminated, so as to prevent permanent damage in the coal reservoirs near the borehole.