Determining Permeability in Coalbed Methane Reservoirs

ABSTRACT The major objective of this paper is to identify the most generally applicable method to analyze pressure transient tests in coalbed methane reservoirs. The desired methodology should allow us to analyze short term pressure tests and long term performance data consistently and in a way that can lead to an adequate reservoir description. An important part of this work was the evaluation of the methods currently proposed in the literature for estimating properties of coalbed methane reservoirs. The paper identifies the assumptions and data requirements for the conventional (i.e. Perrine-Martin) method and pseudopressure method of Kamal and Six along with their potential range of applicability in coalbed methane reservoirs. The paper compares the results using the analytical solution methods to the results obtained with a 2-phase, 3-dimensional, finite difference simulator. We show that oversimplifying or neglecting desorption of methane from the coalbed methane reservoir matrix as is done in the analytical methods can lead to inaccurate estimates of permeability. Thus, we concluded that the only generally trustworthy method of analysis of transient and longer-term production data is to use a coalbed methane simulator which includes diffusion and desorption. Another objective of the paper is to validate the method chosen for estimating permeability in a coalbed methane reservoir at a specific geologic location using actual field data. We developed a coalbed methane reservoir description by history matching actual production and pressure data. We compared the permeability values from the simulator to those estimated from the analytical solution methods and found that the analytical methods do not provide accurate estimates of permeability for this field example. The oversimplified assumptions used in the development of the conventional and pseudopressure methods have a significant influence on the permeability estimates and cause inaccurate results.