Abstract

Simulation results for three coal seam gas problems are compared for models developed by ICF Resources Inc. and ARCO Oil and Gas Company. Excellent agreement between the two simulators is obtained.

Introduction

As part of a well spacing study being conducted by the Gas Research Institute for a committee of coalbed methane operators in the San Juan Basin, ICF Resources was asked to compare its coalbed simulator with the general purpose black-oil simulator, as modified for coal seam gas. developed by ARCO Oil and Gas Company. The comparative problems were designed to 1) illustrate some of the unique features of methane recovery from coal seams, 2) utilize data typical of the Fruitland coal formation, and 3) be easily replicable with other models.

The ICF model was also benchmarked against the first SPE black-oil comparative problem as described in the appendix.

PROBLEM STATEMENT PROBLEM STATEMENT The three coal seam gas problems used the same areal simulation grid and locations for three vertical production wells as shown in Fig. 1. The grid in Fig. 1 represents 640 acres, so each block is 528 × 528 ft. A different layering scheme was used in each problem. For problem 1, communication was restricted to the wellbore which was completed in both coal layers and in the interbedded sandstone as shown in Fig. 2. In the second problem, the two no-flow barriers were not present and the layers were allowed to communicate with vertical permeabilities as given in Table 1. For problem 3, the sandstone layer was also removed, leaving two coal layers in communication.

Other differences among the problems in both reservoir and coal desorption properties are shown in Tables 1 and 2. Additional data for all problems are zero pore volume compressibility, no solution gas in water, zero capillary pressure, a natural fracture (cleat) spacing of 0.2 in., and wellbore radius of 0.3 ft. All wells were unstimulated in the sandstone layer, while wells 2 and 3 were provided with a negative skin (-4.5) to represent a hydraulic fracture in the coal seams, Well schedules and BHP's are given in Table 3, gas and water PVT data in Table 4, and gas-water relative permeabilities for the coal and sandstone in Table 5. Ten-year permeabilities for the coal and sandstone in Table 5. Ten-year simulations were made for all three problems.

DESCRIPTION OF SIMULATORS USED

ICF Resources Inc.

ICF used COMETPC 3-D, a three-dimensional (3D), two-phase, single or dual porosity simulator for modeling gas and water production from coal seams, devonian shales, and conventional production from coal seams, devonian shales, and conventional reservoirs. The model can simulate black-oil problems for gas-oil systems, The coalbed methane formulation is based on the nonequilibrium, pseudo-steady state approach discussed by King et al. Options are available to model stress-sensitive permeability, matrix shrinkage, and gas readsorption to coal. The finite-difference formulation is fully implicit, and an implicit wellbore algorithm adds stability and preserves user-specified rate and pressure constraints. Each well may be vertical, horizontal or deviated, with appropriate productivity indices internally calculated for either induced fracture or unstimulated conditions. The matrix equations are solved in 3D by a combined direct (D4) - slice SOR method. Additional details are given by Sawyer et al.

ARCO Oil and Gas Company

ARCO used a general purpose black-oil simulator developed initially for modelling naturally fractured reservoirs. An early version of the simulator is described by Dean and Lo. The simulator can perform fully-implicit, sequential, or IMPES calculations for 3D perform fully-implicit, sequential, or IMPES calculations for 3D systems, and models mufti-phase flow for single or dual porosity reservoirs. In dual porosity mode, the simulator can model the production of water, oil, and gas from naturally fractured production of water, oil, and gas from naturally fractured reservoirs, and the production of water and gas from coal seams.

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