The Application of Formation Microimager and Wireline Formation Tester for Characterization of Coalbed Methane Reservoirs

Abstract

Logging wells drilled in coalbed methane reservoirs (CBM) is a common practice in the industry. As a rule of thumb, the logging suite comprised of standard methods is run to identify coal layers for subsequent degasification. However, given the increased popularity of the commercial methane production from CBM reservoirs, the petrophysical data from standard methods is not sufficient to evaluate the productivity of deposits and to assess the effectiveness of such projects. The permeability of the coal matrix is usually non-existent. The natural fractures are key controlling factor for gas migration and the performance of a CBM reservoirs depends largely on this parameter. In this study formation microimager was used to identify fractured intervals and based on these results the most promising intervals were selected for further testing. The well testing of low permeability formations can be extremely difficult due to the long build-up times to achieve different flow regimes. This step can be optimized by deploying wireline formation tester to estimate the permeability of the coal layers in open hole. Innovative extended spacing dual packer configuration of the wireline formation tester was deployed to selectively test coal layers ranging from 1 to 9 m deposited at shallow depths of 200-900 m. Fragile nature of the coal seams often resulted in washouts, which presented some challenges while inflating the packers to isolate the interval. Several cycles of fluid pumping were performed followed by pressure build-up periods until the radial flow regime was observed. Data processing and monitoring was carried out in real-time allowing to obtain high quality data and to optimize the operations. Data acquired by formation microimager was used to perform facial analysis to determine depositional environment and to further aid in mapping the coal layers and for future well placement considerations. The knowledge about the depositional environment allows to predict the regions where coal layers are formed confined by the flooding areas. The geological setting of the coal basin is made up of many coal layers of varying thickness deposited at depths of tens to hundreds of meters. The application of an advanced logging suite comprising of formation microimager and wireline formation tester allowed to selectively test the most promising coal layers. All the measurements were carried out in open hole at a drilling stage.