Rate-transient analysis of 2-phase (gas + water) CBM wells

Abstract

In recent work, the authors (Clarkson et al., 2008, 2007; Jordan et al., 2006) demonstrated how modern production data analysis (PDA) methods, such as flowing material balance (FMB) and production type-curves, may be adapted to account for the unique reservoir characteristics of coalbed methane (CBM) reservoirs through the appropriate use of material balance and time transforms. Reservoir characteristics related to storage and fluid flow that were addressed included: adsorbed and free-gas storage; single-phase flow of water above desorption pressure (undersaturated coals); 2-phase flow of gas and water below desorption pressure (saturated coals); non-static absolute permeability during depletion; and multi-layer behavior. Example (field) applications of the new PDA methods were limited to vertical wells that were either openhole completed, or slightly stimulated with hydraulic fracturing methods.In this work, new workflows and analytical approaches are provided for analyzing vertical, hydraulically-fractured and horizontal CBM wells. The analysis and methodology for 2-phase flow reservoirs is complex, requiring modifications to account for desorption and changes in effective permeability. The proposed workflow for 2-phase CBM wells includes the transformation of the well production and flowing pressure data into dimensionless type-curve and straight line (ex. flowing material balance) coordinates using certain outputs (krg, pR) from the simulator used in turn to history-match the production data. Transient straight-line (pressure-transient analysis analog) techniques are applied for the first time to 2-phase CBM well production data. The type-curve and straight-line matches to actual production data are then used to retrieve reservoir properties (e.g. absolute permeability) and stimulation conditions (e.g. skin), which in turn are compared to reservoir simulation input as a consistency check. Both simulated and field cases are analyzed to illustrate the new procedures and analytical techniques.The primary contribution of the current work is the application of modern production analysis methods to 2-phase CBM reservoirs. These methods have been modified for CBM reservoir behavior and combined with analytical (or numerical) modeling to extract quantitative reservoir information from CBM reservoirs which exhibit a wide-range in production characteristics, and are completed in a variety of styles. The modifications proposed in this work to enable the use of single-phase flow techniques must be regarded as practical approximations. The methods rely heavily on late-time data because of the poor quality of water production and flowing pressure data that typically exists. The methods are expected to be used as a pre-cursor to or in parallel with field reservoir simulation, to assist with CBM development decisions.