Evaluating Hydraulic-Fracture Effectiveness in a Coal-Seam-Gas Reservoir From Surface Tiltmeter and Microseismic Monitoring
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 133063, ’Evaluating Hydraulic-Fracture Effectiveness in a Coal-Seam-Gas Reservoir From Surface Tiltmeter and Microseismic Monitoring,’ by Raymond L. Johnson Jr., SPE, and Michael P. Scott, SPE, QGC; R.G. Jeffrey, SPE, and Zuorong Chen, SPE, CSIRO Earth Science and Resource Engineering; Les Bennett SPE, and Craig Vandenborn, SPE, Schlumberger; and Sergei Tcherkashnev, ASTO Geophysical Consulting, prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed.

In developing coalbed-methane or coal-seam-gas (CSG) fields or reservoirs, the effects of many parameters are important in understanding the success or potential improvement of hydraulic-fracturing treatments. Estimating fracture geometry relative to reservoir architecture is critical in understanding production variability. An outline for steps used to plan the monitoring program is presented along with a description of how geological data were integrated to better understand the results observed during the treatments. A logical framework was developed to evaluate and integrate these technologies for use in future CSG-well stimulation.  

Introduction

The wells in this study are in the Walloon coal measures of the Surat basin of eastern Queensland, Australia. The Walloon subgroup (WSG) is Middle Jurassic, comprising fluvial deposition dominated by definable transgressive events of peat-swamp deposition. It can be divided further into the Juandah and Taroom coal measures, which comprise 10 regionally correlatable coal packages with interbedded sandstone, silt-stone, and carbonaceous-shale intervals. Typically, the WSG is 1,000 to 1,200 ft thick, with net coal thickness ranging from 65 to 120 ft. There can be more than 50 plies of coal throughout these 10 coal packages, with average ply thickness of 1 to 2 ft and the maximum typically between 7 and 10 ft.

Generally, hydraulic fracturing resulted in low-productivity wells in the early pilot areas of the Surat basin and resulted in wellbore failures in the Undulla Nose area, which led to an underreamed, openhole completion as the primary completion method for WSG wells. As appraisal programs moved away from the more prolific Undulla Nose area, areas having variable and lower permeability were encountered. These areas required reevaluating the traditional WSG completion method and considering implementation of hydraulic fracturing to improve production.

On the basis of past wellbore failures, it was suspected that fracture complexity may be developing as a result of interaction of the created hydraulic fracture with the natural fractures and with the overall stress environment. When a program to incorporate hydraulic fracturing was considered, it was decided that an integrated diagnostics program would be essential in understanding the fracture geometry and overall stress environment to improve treatment designs and to help understand past-treatment results.