Determination of the degree of coal deformation and its effects on gas production in the southern Qinshui Basin, North China

Abstract

Coal deformation significantly impacts coalbed methane (CBM) productivity. However, existing methods are incapable of quantitatively evaluating coal deformation on a high accuracy when there is limited core recovery from a CBM well. This study proposes a well logging coal structure index (WCSI) to quantify the degree of coal deformation. The WCSI of targets is calculated based on the data from well logs and coal cores, and it can also be estimated from well logs when no coal cores are recovered from wells. Thus, the WCSI can be applied to the regional-scale evaluation of coal deformation degree, and its effectiveness has been validated by data for 245 samples from the No. 3 coal seam in 55 exploration wells in the southern Qinshui Basin (SQB). Results indicate that the reservoir permeability and hydraulic fracturing effectiveness depend on the degree of coal seam deformation, which further affects the unique distribution of gas production in the known production regions in the SQB. Generally, the lowly deformed coal seam (WCSI (avg.) < 40) occurs mainly in the tectonically ‘stable’ regions (no faults or only small-scale faults development) where the micro-fractures/cleats in coal are not well developed. Low WCSI (avg.) commonly indicates low permeability (<0.06 mD) and poor hydraulic fracturing of the reservoir, resulting in relatively low gas production rate of wells in these regions. Secondly, the moderately deformed coal seam (40 < WCSI (avg.) < 60) is mainly found in areas close to moderate-scale faults, where coal seams have relatively high reservoir permeability (>0.06 mD) and hydraulic fracturing is easiest. This causes high gas production rate from wells in these regions. Thirdly, the highly deformed coal seam (WCSI (avg.) > 60) is generally found close to large-scale faults, indicating very low reservoir permeability (<0.03 mD) and the worst effectiveness from hydraulic fracturing of the reservoir, resulting in the lowest gas production rate of wells in these regions. In summary, the production efficiency from CBM wells in the SQB can thus be described in terms of the WCSI (avg.) value, and hence this parameter can help with preliminary forecasting gas production.