Coalbed-Methane Pilots—Timing, Design, and Analysis
Authors: Roadifer R. D. , Moore T. R. 
Published: October 2009

Four distinct sequential phases form a recommended process for coalbed-methane (CBM)-prospect assessment: initial screening, reconnaissance, pilot testing, and final appraisal. Stepping through these four phases provides a program of progressively ramping work and cost, while creating a series of discrete decision points at which analysis of results and risks can be assessed. While discussing each of these phases in some degree, this paper focuses on the third, the critically important pilot-testing phase.

This phased CBM-prospect assessment process allows us to

Gain local knowledge early at low cost Progressively acquire and compile appropriate data to assess the geological situation and reservoir conditions systematically Identify and attempt to fill the most important knowledge gaps that represent the greatest uncertainties and risks to the prospect Increasingly understand the distributions of key parameters that control reserves, deliverability, and value Stage expenditures and provide multiple decision points through the process Ultimately, produce a project with very low development risk

In the CBM-prospect assessment process, the pilot test serves the same function as a conventional exploration well. If it proves successful, then the prospect can be considered a discovery and can be appraised for development. By drilling, completing, and producing a cluster of wells in a CBM pilot test, short of proceeding directly to a partial development, we are able to locally dewater and depressurize the coal seam to be tested and, thereby, desorb and deliver measurable volumes of gas. If correctly implemented, the pilot test allows us to assess the local variability of key reservoir parameters, collect the information necessary to simulate the reservoir's producibility, and, thereby, estimate potential project reserves to a reasonable degree of accuracy.

This paper contains roughly 30 specific recommendations and the fundamental rationale behind each recommendation to help ensure that a CBM pilot will fulfill its primary objectives of (1) demonstrating whether the subject coal reservoir will desorb and produce consequential gas and (2) gathering the data critical to evaluate and risk the prospect at the next—often most critical—decision point. Importantly, these objectives must be met in a timely manner. To do this, the specifications for the pilot are often not those that will be used for an optimized well or field-development pattern in terms of costs or production. This is intentional. The goals of piloting are different from the goals of development. So, the recommended designs are different. The pilot design recommendations focus on collecting superior data that will quantify key parameters for interpretation and simulation of the reservoir, retaining flexibility in the face of the level of uncertainty remaining after the reconnaissance phase, and arriving at a definitive answer on the coal reservoir's viability in an acceptable time frame.

Detailed data-analysis methods for CBM are not discussed here—these are well covered in the literature. Rather, we focus on the importance, use, and potential pitfalls of data collected at the various phases of the assessment process. Examples are used to highlight the purpose and importance of various aspects of the data gathering and analysis. A general history-matching process—valid at the pilot-stage analysis and beyond—is presented as a guide.