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.
Highlights
In China, coalbed methane (CBM) recovery faces many challenges due to highly diversified and complicated geologic conditions [1]
The relative physical properties indicator (RPPI) and relative fluid indicator (RFI) for each layer with a thickness of 12.5 cm were extracted as quantitative identification indicators of Tectonically-deformed coal (TDC)
Type I coal tended to have less than 50% of non-overlapping area, while type II had 50%–70%, and type III had greater than 70%, corresponding to shallow, medium, and deep invasion, respectively
Summary
In China, coalbed methane (CBM) recovery faces many challenges due to highly diversified and complicated geologic conditions [1]. Tectonically-deformed coal (TDC) is one of the crucial factors leading to low gas productivity [2]. Getting a high production of gas from tectonically deformed coal (TDC) is very difficult. The occurrence of TDC has caused many problems, such as poor permeability [3,4,5,6], coal fine blockage and rapid productivity degradation [7,8]. The accurate interpretation of TDC zones is of importance in the productivity evaluation of CBM wells. TDC is known as the deformation of the texture and sedimentary structure of coal mass in the course of tectonic history evolution. TDC can be classified into three types, cataclastic, granulated, and mylonitized [9]. Due to extremely rare samples of CBM exploitation wells, extracting explicit information about TDCs using geophysical logging has become a key issue
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