Coalbed Methane: Current Evaluation Methods, Future Technical Challenges

Abstract

Coalbed methane (CBM) produced from subsurface coal deposits, has been produced commercially now for over 30 years in North America, and relatively recently in Australia, China and India. Historical challenges to predicting CBM well performance and long-term production have included: accurate estimation of gas-in-place (including quantification of in-situ adsorbed gas storage); estimation of initial fluid saturations (in saturated reservoirs) and mobile-water-in-place, estimation of the degree of under-saturation (undersaturated coals produce mainly water above desorption pressure); estimation of initial absolute permeability (system); selection of appropriate relative permeability curves; estimation of absolute permeability changes as a function of depletion; prediction of produced gas composition changes as a function of depletion; accounting for multi-layer behavior, and accurate prediction of cavity or hydraulic fracture properties. These challenges have primarily been a result of the unique reservoir properties of CBM. Much progress has been made in the past decade to evaluate fundamental properties of coal reservoirs, but there is still work to be done to obtain accurate estimates of some basic reservoir properties.In recent years, horizontal wells and more complex well architectures and stimulation methodologies have been implemented to improve recovery of CBM. These more complex development options bring with them a new set of challenges for operators producing CBM. The exploitation of more geologically-complex coal with poorer reservoir quality will necessitate new and inventive ways to develop the existing natural gas resources and possibly combine this with new methods to extract energy from the coal in-situ. Development planning in these scenarios will become increasingly complex as will evaluation methods.The purpose of the current work is to review the state-of-the-art in CBM reservoir property and stimulation efficiency evaluation and speculate on possible CBM development scenarios for the future and the technical challenges they will bring. Current and future work required to meet these challenges will be discussed in the hope that industry, academia, and government bodies alike will be proactive in the development of solutions that will make future CBM recovery efficient, economic, and environmentally friendly.

Complex evolution of coal permeability during CO2 injection under variable temperatures

“…Volumetric strain associated with gas sorption can also be measured in terms of the adsorbed gas volume at standard pressure and temperature. Experiments on the volumetric strain associated with methane and carbon dioxide adsorption have shown that the sorption-induced volumetric strain is approximately proportional to the volume of adsorbed gas (Cui and Bustin, 2005;Clarkson and Bustin, 2010), which also has been confirmed by Day et al (2010). The relationship is described as…”

Effect of Nanoparticle Adsorption on the Pore Structure of a Coalbed Methane Reservoir: A Laboratory Experimental Study

“…According to the results of three experimental studies, the following important understanding of the influence of nanofluids on the pore structure of CBM reservoirs is obtained (1) The nanofluid treatment has a great impact on the SSA and pore diameter of the sample, especially on micropores, which is the key pore structure of CBM reservoirs. The maximum increase percentages of SSA, pore volume, and pore diameter are 228.12, 80.65, and 18.89%, respectively.…”

Unconventional gas reservoir evaluation: What do we have to consider?

“…Much of the historical research on sorbed gas storage has focused on coalbed methane reservoirs; a recent summary of CBM gas content and sorption isotherm measurements, along with a discussion of uncertainties, was provided by Clarkson and Bustin (2010). Unlike most CBM plays, however, free-gas storage is often significant in shale gas reservoirs, and must be included in gas-inplace calculations (Ambrose et al, 2010).…”