Productivity Evaluation of Coalbed Methane Well with Geophysical Logging-Derived Tectonically Deformed Coal

Abstract

Tectonically deformed coal (TDC) has a significant influence on coalbed methane recovery. Well-logging is a reliable and efficient way to predict the development of TDCs for understanding the role of TDC on the productivity of coalbed methane (CBM) wells. However, there has been insufficient research regarding both the static physical properties and dynamic invasion of mud-filtrate related to TDCs. Therefore, a new TDC-detecting approach using two indicators of the relative physical properties and fluid attributes was proposed. Through the data normalization and optimization of correlation coefficients and factor analysis, five logs were chosen to construct the two indicators, and three types of TDC were recognized: I—undeformed or cataclastic; II—granulated; and III—mylonitized. It was found out that the identification error rate decreased from 30% to 15%. Furthermore, the thickness ratio of a well-preserved coal layer derived from TDC interpretation was adopted to correlate the gas production of a coal seam. An application in the Hancheng block demonstrated that the thickness ratio of 60% is an explicit threshold value to distinguish between high-yield well (>1000 m3/d) and low-yield one (<750 m3/d). The development of granulated and mylonitized coals mainly exerts negative influence on CBM well production.