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Coalbed methane (CBM) reservoir performance is controlled by a complex set of reservoir, geologic, completion and operation parameters and the inter-relationships between those parameters. In order to identify, analyze and mitigate risks associated with any CBM prospect, one must first understand the relative importance of each of these parameters, how their relative importance changes under different constraints, and how they interactively affect CBM production. To date, no comprehensive parametric study on coalbed methane is known to have been conducted within the industry. The parametric studies that do exist in the literature have only considered a limited set of parameters and a limited range for those parameters while holding all the other parameters fixed, thus potentially skewing their results. This comprehensive parametric study has been conducted to enhance our understanding of CBM reservoir performance. In essence, it is an analysis of an extensive Monte Carlo simulation of more than 100,000 reservoir simulations in which the reservoir, geologic, completion and operational variables that impact the production performance of CBM reservoirs were varied. While it is stated that this was a coalbed methane parametric study, coalbed methane reservoirs typically occur in conjunction with sandstones of varying quality. For that reason, one-half of the reservoir simulations modeled the production from coalbed methane reservoirs in conjunction with sandstones of varying quality and degrees of wetness (fully water charged to various degrees of gas-charge). The study's goal was to establish the relative importance of the identified parameters and their inter-relationships, developing rank correlations between these parameters against several production end states (e.g., peak gas rates, dewatering times, cumulative gas, etc.). To avoid some of the potential problems of other similar studies, this study aimed to be as comprehensive as possible in the coverage of parameters varied with the tools available. In so doing, it provides a fuller understanding of the parameters involved in coalbed methane production and enables uncertainties and risk to be more accurately assessed for project economics, and will ultimately lead to the development of more accurate and efficient CBM screening models. Background Coalbed Methane is truly an unconventional gas resource. The most obvious difference between CBM and conventional gas reservoirs is in the gas storage mechanism. In conventional gas reservoirs, gas is stored as free gas in the pore spaces of the reservoir rock. In CBM reservoirs, the gas may be stored both as a free gas in the secondary porosity network (cleats or natural fractures) or the gas may be stored at almost liquid densities on the surface matrix of the coal by physical sorption. Sorption commonly accounts for over 99% of the gas-in-place in CBM reservoirs. To produce gas from a CBM reservoir, gas must first be desorbed from the coal. This is accomplished in practice by depressurizing the coals. Since most CBM reservoirs are 100% water saturated in the natural cleat fracture system, this requires water to be produced to depressurize the coal. Once gas has desorbed from the coal matrix, it diffuses through the coal matrix to the natural fracture network which provides the primary conduit to the production well.

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-th...

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.

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