As exploitation of unconventional resources such as coalbed methane (CBM) reservoirs becomes increasingly essential, there is a growing need to develop hydraulic fracture treatment design for these reservoirs with complex geology and stress conditions. Conventional methodologies have failed to address the difficulties involved in the design and execution of fracture treatments in these complex conditions. These include unrestricted height growth breaking through the roof and floor of the coal seam, massive fluid loss as a consequence of the high leak-off formation and poroelastic effects. This paper presents an integrated approach to optimise hydraulic fracture treatments and addresses the associated problems encountered during the hydraulic fracturing process.
A 3D poroelastic, finite element based numerical design tool has been developed to describe the fracture geometry for given reservoir and operating conditions. This is coupled with a production model appropriately quantifying the post-frac productivity of a CBM reservoir. Finally, cost analysis is carried out to optimise design parameters against different production scenarios using a hybrid genetic-evolutionary optimisation tool.
Our study has shown a significant improvement in understanding the fracture containment mechanisms and impact of poroelastic effects in stimulating CBM reservoirs. Furthermore, results of this study demonstrate that use of an integrated approach in the design of hydraulic fracture treatments results in a higher yield and cost-effective exploitation of CBM prospects.
Exploitation of unconventional resources such as coal bed methane reservoirs is motivated by the growing demand for hydrocarbon supply. This calls for a cost-effective stimulation methodology to maximise profit derived from these resources. Due to their characteristic difference with the conventional oil and gas reservoirs, largely accepted norms in the hydraulic fracturing literature are in many ways inadequate to address problems associated with hydraulic fracture stimulation in CBM reservoirs. In particular, the complexity in stimulating CBM reservoirs stems from the highly heterogeneous nature of their in-situ stress distribution and their naturally fractured and high leak-off characteristics. Furthermore, their production mechanism is significantly different from conventional gas wells since dewatering is required to lower the reservoir pressure and allow gas to desorb. When fracturing CBM reservoirs, abnormally high treating pressures often occur. In addition, stress barriers restricting fracture height growth may be inexistent.