This paper reviews the fracturing history in the Cooper Basin and summarizes the results of over 650 fracturing treatments characterised by high tectonic stresses, high fracturing pressures, high reservoir temperatures, and stacked reservoir lithologies of sands, shales, and coals. Initial treatments targeted multiple moderately high permeability (1–10 md) formations, while multi-staging operations are now targeting single, less extensive, lower quality reservoirs. The paper discusses the techniques used for predicting and designing for natural fracture leakoff, high near wellbore pressure losses and high fracture gradients. It shows the changes in fracturing ideologies and how they have altered the completion strategies, predicted fracture geometries, fracturing materials, treatment schedules, and post-frac production.
Introduction
Hydraulic fracturing began in the Cooper Basin in 1968 and has been a critical technology in the development of its gas and oil reserves. Although fracturing has been used extensively in other regions of Australia such as the small to medium sized oil fields of the Eromanga Basin in Central Australia and the Coalbed regions of Eastern Australia, this paper is focused on the fracturing experience in the more extreme conditions of the Cooper Basin.
Cooper Basin Description. The basin is a Late Carboniferous to Middle Triassic, non-marine sedimentary environment, which underlies the desert region of Eastern-Central Australia (Figure 1). It is characterised as fluvio-lacustrine, with fining upward sandstones, siltstones, interbedded shales and coals. Deposition varies between braided and meandering fluvial sands & alluvial fans, distributary channels, and crevasse splays1.
Figure 1 - Location of Cooper Basin (Blue) and Overlying Eromanga Basin (Green)
The basin is the primary on-shore producing area in Australia for natural gas, while also producing significant oil and LPG. It extends over a region of 50,000 sq. miles (130,000 km2) and contains over 120 separate gas fields and 10 oil fields. Production from Cooper Basin reservoirs is presently 600 MMscf/day from 700 gas wells and 2,500 bopd from 50 oil wells. Estimates for recoverable oil and gas reserves are 43.9 MMstb and 8.2 tcf, with remaining reserves (as of 1998) of 14.8 MMstb and 3.6 tcf1. Reservoir depletion effects on fracturing can be significant and will be presented later in the paper.
A graph of the basement depth structure is presented in Figure 2 and shows the major basin details and fractured well locations. The primary features are the Nappamerri Trough that is located in the middle of the basin and the various ridges that surround the Trough. Most of the significant fields are located on the North-West, South, East, and North-East ridges. Previous publications have been written concerning the Tirrawarra field2–3 which is located in the North-West ridge and the Kurunda Field4 which is located in the South-East ridge. Other publications describe general basin reservoir characteristics and/or fracturing behaviour5–12.
