Coalbed Methane Pilots: Timing, Design, and Analysis

Abstract

Abstract Four distinct, sequential phases comprise a recommended process for coalbed methane (CBM) prospect assessment: initial screening, reconnaissance, pilot testing, and final appraisal. A stepwise approach through these four phases provides for 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 covering each of these phases in some degree, this paper focuses on the third, the pilot testing phase – that which normally takes place after reconnaissance, but before final appraisal. A step-wise, phased CBM prospect assessment process allows us to: gain local knowledge early at low cost; progressively acquire appropriate data to systematically assess the geological situation and reservoir conditions; define and attempt to fill knowledge gaps that represent risk and uncertainty; increasingly understand the distributions of key parameters that control reserves, deliverability, and value, and; stage expenditures and provide multiple decision points through the process. In the coalbed methane 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 to be a discovery and be appraised for development. Only by drilling, completing and producing a cluster of wells in a CBM pilot test, short of proceeding directly to a partial development, are we 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 appraisal and pilot tests allow 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 demonstrating whether the subject coal reservoir will desorb and produce consequential gas, and to gather the data critical to risk and evaluate the prospect at the next, perhaps most critical, decision point. These objectives must be met in a timely manner. To do this, the specifications for the pilot are often not those which will be used for an optimized well or optimized development pattern in terms of costs or production. This is intentional. The goals of piloting are different than that of development, so the recommended design is different. The design recommendations focus on collecting superior data for the parameters key for interpretation and simulation of the reservoir, retaining flexibility in the face of the level of uncertainty remaining after the reconnaissance phase, and getting to a definitive answer on the coal reservoir's viability in an acceptable time frame. Detailed data analysis methods for coalbed methane are not covered here as most are well covered in the literature. Rather, discussions of the importance, use and potential pitfalls of data collected at the various phases of the assessment process are included. Simple examples and field examples alike 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 and beyond, is formulated and presented to guide the engineer.