Coalbed-Methane Pilots—Timing, Design, and Analysis

Abstract

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...

Impact of Various Parameters on the Production of Coalbed Methane

“…The increasing peak rate with increasing swelling capacity illustrates that the desorptioninduced permeability increase is playing an increasingly important role in gas production. A similar phenomenon has been observed by other researchers, even when modeled after different permeability models or different coal geometry models (Robertson and Christiansen 2007;Wang et al 2011).…”

“…Many studies have been conducted, but it is primarily assumed that gas transport in a matrix system is a diffusion process dominated by the gas desorption process (Barenblatt 1960;Palmer and Mansoori 1998;Pekot and Reeves 2002;Cui and Bustin 2005;Gu and Chalaturnyk 2005;Yi et al 2009). Coal desorption models are classified into two main groups: The equilibrium desorption model ignores the kinetics of the process in gas-flow transport formulations by assuming the desorption time to be zero, whereas the nonequilibrium model considers desorption as dynamic and applies Fick's diffusion law to simulate the diffusion process (Roadifer and Moore 2009;Ziarani et al 2011). However, such studies accommodate the flow response as overlapping continua and only consider the desorption process in matrix and Darcy's flow in fractures; the real connections between adjacent matrix blocks are not considered.…”

Reservoir Engineering for Unconventional Gas Reservoirs: What Do We Have to Consider?

“…If there are too many inputs in the model that cannot be defined correctly because of lack of/poor quality data, then you may not be further ahead than using a simpler method/tool for forecasting, particularly for single-well scenarios. For example, CBM reservoirs are inherently complex reservoirs, but in some scenarios, where dual porosity effects can be ignored (sorption times are small), simple tank-type simulators may be adequate to capture the salient features of single-well performance, as illustrated in Clarkson and McGovern (2005), and more recently in Roadifer et al (2009). For shale gas reservoirs, there have been a number of "hybrid" single-well forecasting approaches introduced, that combine analytical transient flow modeling (often linear flow) with Arps' decline forecasts once boundaries are reached (ex.…”

Stochastic Modeling of Two-Phase Flowback of Multi-Fractured Horizontal Wells to Estimate Hydraulic Fracture Properties and Forecast Production

“…· Gas desorption is often assumed to be near-instantaneous, due to the fine fracture spacing and/or high diffusion coefficient (Roadifer et al, 2009), and may be treated as a single porosity system. · Water mobility in the matrix is often assumed to be negligible, hence flow in the matrix to the fractures is single phase.…”