New Developments in Testing Procedures for Coalbed Methane Wells in Australia
Authors: Badri M. , Clare R. 
Published: December 2007

In recent years, testing of coalbed methane wells has been conducted in many exploration areas in the Bowen, Sydney, and Galilee Basins of Australia (Fig. 1) to locate suitable areas for development. The primary objective of the testing has been to locate low-stress areas with high permeability and adequate methane gas content that would warrant commercial development.

To comply with economic constraints, most of the exploratory, cored holes have been drilled using mineral rigs. The wells ranged in depth from 300 m (984.3 ft) to 1300 m (4265.3 ft) with hole sizes ranging from 76 mm (2.99 in.) for a PQ size hole to 101 mm (3.976 in.) for a CHD hole.

In the Australian market the expense incurred from the testing represents a substantial part of the total well cost. This is due to the number of intervals that must be tested in each cored hole to collect sufficient data on which to base an assessment of a well's potential production performance. To minimise test time and properly isolate the different seams, resettable slimhole packer systems have been developed for different sized holes.

This paper will discuss (1) the early testing practices, such as the slug testing approach, which have been discontinued because of its limited capability to obtain investigation criteria, (2) the injection/fall-off test method, which has become the method of choice because of the non-flowing characteristics of the exploration wells and the broad scope of information that it can secure; and (3) the use of microfracturing to relate coal basin stress fields to geological structure and lithology and to develop a stress profile with depth for fracture-design purposes.

The following specifics will be addressed:–Difficulties in openhole testing because of unstable hole conditions and the compounding of the problem by the need for conducting multiple testing procedures in the same well.–The precautionary drilling practices and tools that are needed to avoid potential problems, such as difficulty in reaching bottom or sticking of test tools that create the need to jar loose when the test is completed.–The test tools and the quality control methods used to ensure proper functioning of these tools to help eliminate unnecessary downtime.–Use of newly developed, resettable slimhole inflatable packer systems that have been used for the testing of the different sized core holes to minimise test time and properly isolate the different seams.–The test design and interpretation, which can be of value to many operators.

Introduction

Water and gas production rates from a coal seam reservoir are controlled by the cleat system permeability, initial reservoir pressure, coal thickness, relative permeability and gas content.

The selection of well locations for commercial production is made by performing quantitative formation evaluation to estimate the parameters required for reservoir characterisation, well performance forecasting and stimulation design. Pressure transient testing provides the formation flow capacity (kh), the reservoir pressure values, and the skin factor, and if carried out after a stimulation treatment it can serve as a diagnostics tool, yielding some insights into the effectiveness of the induced fractures. Stress testing is performed to estimate the general stress distribution and magnitudes in the vicinity of the subject well. Vertical stress profiling performed by testing multiple coal horizons in a well can be used to properly model the fracturing mechanisms through the use of a 3D fracture simulator and optimise any future stimulation treatments.

Due to the complex nature of coal reservoirs, e.g., dual porosity, methane desorbtion and two-phase flow, simple flow equations to model the production in this medium over the life of the well are unavailable.

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