Constraining coalbed methane reservoir petrophysical and mechanical properties through a new coal structure index in the southern Qinshui Basin, northern China: Implications for hydraulic fracturing

Abstract

Structurally deformed coal (SDC) influences not only gas outbursts during coal mining but also coalbed methane (CBM) production. Different SDCs have different mechanical strengths and physical properties. The structurally constrained petrophysical and mechanical properties of a CBM reservoir from the southern Qinshui Basin (SQB), northern China, have been investigated by using a coal structure index (CSI). The CSI was established as a quantitative criterion for the deformation of SDCs based on surface conditions and blockiness using the data of 783 coring samples from 109 CBM wells. The results show that undeformed coal and slight brittle SDCs are dominant in the SQB, with ductile SDCs distributed locally. Both compressive strength and tensile strength decline exponentially with increasing CSI average (avg.). Coal structure is a crucial factor for coal seam fracturing; with an increasing CSI (avg.), the major fracture length will first increase to the maximum of 266 m at a CSI (avg.) of 45 and then decrease rapidly. However, branch fractures are independent of the CSI (avg.). The major fracture height attained a minimum of 4.39 m at a CSI (avg.) of 40 and then slowly increased. Therefore, slightly brittle SDCs should be beneficial for fracturing, at least for high-rank coals. The presence of variable SDCs can have a considerable impact on CBM reservoir petrophysical and mechanical properties. Finally, a CSI model that strives to establish the relation between structure deformation and CBM reservoir physical and mechanical properties in coal is expected to effectively guide hydraulic fracturing.