Characterizing Coal Bed Methane Using Pressure Buildup and Falloff Data Acquired with Wireline Formation Testers: Case Studies

Abstract

Over the past few years there has been a surge of interest in coal bed methane (CBM) resources in many parts of the world. Also known as coal seam gas (CSG), CBM has become an important source of energy because of increasing global demand for cleaner fuels. CBM is distinct from conventional hydrocarbon reservoirs as methane is stored within coals by adsorption. With matrix porosity generally lower than 4%, cleats and fractures are the main conduits for production from coals. Given differences in structure compared to conventional reservoirs, drilling into coal seams requires the use of minimum overbalance and nondamaging fluids. In addition, evaluation of CBM reservoirs has many technical challenges. One of the main challenges is to ascertain coal cleat behavior and estimate permeability and ultimately productivity of a target zone. Traditionally this has been done by production or injection tests using conventional testing techniques. In Australia, a wireline-deployed straddle packer configuration was used to address this challenge, with demonstrated benefits for determining permeability and productivity.Unlike traditional methods of conducting a closed chamber test across a large interval, this methodology uses a straddle packer with a downhole pump in a toolstring deployed on wireline. The packer spacing can be adjusted prior to deployment to suit the expected height of the coal bed to be tested. The tool is capable of both injection into and production from a coal bed interval with a much smaller storage volume compared to conventional test strings. Pressure is continuously monitored in real time ensuring that acceptable limits are not exceeded during either the injection or drawdown phases, to avoid excessive force on a coal seam while maintaining single-phase flow. The analysis of both drawdown-buildup and injection-falloff results reveals the strengths and limitations of the two techniques.

The Application of Well Test Deconvolution to Wireline Formation Tester Pressure Buildup and Falloff Data to Improve Coalbed Methane Reservoir Characterization

“…Several cases of combining pressure falloff and buildup to characterize CBM seams using wireline formation tester straddle packers have been described Harfoushian et al 2013). The tests were analysed using traditional pressure transient analysis techniques.…”