Impact of geomechanics in coal bed methane development and production, Barakar coals in central India

Abstract

Geomechanics plays a critical role in the different stages of coal bed methane (CBM) operations, starting from its development through production to its abandonment as in Sohagpur field of central India. In general, higher gas production has been observed from the coal seams in low-stress regimes compared to high-stress regimes; however, during the CBM development planning, stress information has been overlooked. The need for inclined and horizontal wellbores in coal seams has increased due to their many-fold rise in production when compared to vertical wells with a minimum surface footprint. However, these non-vertical wells have wellbore instability issues leading to high nonproductive time (NPT) during drilling and completion operations. Prolong challenges exist in achieving the desired half-length from hydraulic fracturing (HF) stimulation due to the limited understanding of the stress contrast between target coal and bounding lithology. The permeability of the coal reservoir varies greatly during depletion due to the combined effects of effective stress and matrix shrinkage. These effects directly impact productivity and hence, require a detailed understanding of how permeability varies during production life. In many instances, production from coal seams is complicated and unpredictable because of the influence of stress induced natural fractures. A geomechanical earth model is focused on understanding and mitigating the aforementioned challenges. In addition to coal thickness and gas content, stress-permeability maps are prepared to assist in the development program to prioritize wells in lower and moderate stress areas. Wellbore stability analysis has been carried out using calibrated stress and strength profiles along with other rock mechanical inputs to successfully drill high angle wells. This analysis indicates that with a slightly higher mud weight, high angle and horizontal wells can be drilled successfully. Hydraulic fracturing design incorporating the available stress contrast can help in achieving the desired fracture parameters without height growth. The coal permeability model suggests that during depletion, coal permeability is expected to increase by about 1.2–3.5 times with a positive impact on productivity. Critically stressed fracture analysis has been carried out on mapped fractures, which suggest that about 48% of the natural fractures are in a stressed state. Presence of these natural fractures has a positive (faster dewatering) as well as a negative (external water) impact on productivity at different places in the study area.