Strategies in Developing Coalbed Methane Prospects in Australia: An Example from the Galilee Basin of Central Queensland M.J. Zebrowitz, SPE, and M.R. Herrington, SPE, Enron Exploration Australia Pty Ltd; M.E. Blauch, SPE, Halliburton Energy Services; and J.M. Coughlin II, SPE, Dominion Energy Inc.

Abstract

This paper describes Enron Exploration Australia's methodology and strategy for designing hydraulic stimulations used as the completion technique on two pilot coalbed methane wells in the Galilee Basin in central Queensland, Australia. This process involved coal-seam characterization technologies based on the acquisition and petrologic analysis of slim-hole core data. Results to date illustrate the significance of coal-seam characterization, the difficulty in predicting gas/water production behavior based on theoretical modeling, and the stimulation process as a means of gathering critical production data for future development strategies.

Introduction

Although extensive coalbed methane reserves have been identified in Australia, the primary challenge facing the coalbed methane industry here is to prove the reserves' producibility potential. Enron Exploration Australia is currently facing this challenge in the Galilee Basin in central Queensland. Two areas containing 25 to 35 m of gas-bearing coals and 17 to 23 Bm3 of gas-in-place have been identified as potential gas discoveries.

The strategy for this Galilee Basin development (ATP 529P), an area of approximately 48,000 km2, has evolved since it was leased by Enron in November 1992 (Figure 1). Geological evaluation involved the interpretation and integration of 40,000 km2 of space-borne imagery, 10,000 line kilometers of seismic data, aeromagnetic and gravity surveys, a large vitrinite reflectance database, and stratigraphic cross sections/isopach maps. Wellsites were selected after performing an extensive study of lateral and vertical coal continuity and thickness, modeling lateral and vertical thermal maturity, and implementing a state-of-the-art fracture detection program for locating fracture permeability.

The Galilee Basin consists of two structural regions, the Koburra Trough and the Maneroo Platform ramp. Exploratory wells were strategically placed to evaluate each region (Figure 2). Exploratory drilling was conducted with slim-hole equipment, allowing extensive collection of detailed reservoir data: 1.5 km of core were recovered, 88 gas-content measurements were analyzed, and 34 drill-stem tests (DSTs) were performed.

During the 1993–94 exploration program, the Crossmore and Rodney Creek Anticlines were identified as potential significant gas discoveries. The Crossmore area contains 24 m of fully gas-charged coal and an estimated 17 Bm3 gas-in-place. The Rodney Creek area is underlain by 30 to 35 m of fully gas-charged coal and an estimated 23 Bm3 gas-in-place.

Using the extensive reservoir data set and developing the two structures on 260-ha spacing, engineers conducted a reservoir simulation study to forecast gas rates and cumulative gas recovery. After an accurate conceptual understanding of the coal seams being considered for production was developed, hydraulic stimulation was used as the completion technique on the two pilot wells. Two production wells were drilled and completed in late 1995 to establish baseline gas production rates. The results demonstrate the difficulty in predicting gas/water production behavior based on theoretical modeling.

Exploration Activities and Results

All available geological and geophysical data were used to adapt a comprehensive exploration approach to locate the areas that would be underlain with a highly fractured, gassy, thick coal section. The fracture-detection program centered on a fracture-kinetics model in which open fractures are most likely to form from the draping and warping of sediments over basement features. This model was developed as an analog to the broad platform of continuous permeability present in the San Juan Basin coalbed methane production "Fairway."

The primary basis for wellsite selection was the interpretation and integration of 40,000 km2 of landsat thematic mapper imagery, aeromagnetic and gravity coverage, and approximately 10,000 line kilometers of seismic data that detect subtle basement features with fracture-related permeability.

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