Hydraulic fracturing has been a key technology in the development of coalbed methane (CBM) resources worldwide. Obtaining adequate fracture length and conductivity has limited the ability to obtain adequate productivity improvement to further develop many small seams or stacked-seam reservoirs. Fracture complexity and growth out of the interval have frequently been cited as limiting factors in achieving optimal length in these types of intervals; however, the diagnostics to evaluate and model these effects have been limited. Finally, many of the past studies of hydraulic fracturing mechanics in coal have been focused on North American examples where normal faulting stress states are present, unlike many of the coal-producing basins worldwide and particularly in Eastern Australia.
Using examples from the Scotia Field, we describe how past and present stress framework analyses and post-frac treatment diagnostics were integrated to better describe the in-situ stress state. Through analyses of these examples, we qualify the inter-relationships of productivity to in-situ stress, pre-existing fractures, and observations from the induced hydraulic fractures. Finally, we describe cases where the hydraulic fracturing complexity and in-situ stress conditions lead to wellbore complications and observable rock-mechanical failures. The end result is a more predictive model, which is being used to develop this CBM resource.
Introduction
Initially, most CBM projects focus on identifying contiguous coal sections with sufficient gas in place to support commercial production. Most often, basic geologic mapping and coal quality information are used to delineate and high-grade prospective areas. Ultimately, the reserves potential of these areas must be proven by direct measurement of key reservoir properties and production testing.
Experience from analyses of coalbed methane projects worldwide indicate the more important reservoir parameters controlling gas productivity include:permeability (fracture/cleat system),gas content,well spacing,initial gas and water saturation in the fracture/cleat system, andthe relationship between reservoir pressure and flowing well pressure.1
Of these, permeability is the most important since it controls the flow rate of gas and water and can be affected by in-situ stress.In CBM projects in-situ stress can also affect the benefit derived from reservoir stimulation; often the well productivity and economics of a project can become highly dependent on the degree of stimulation effectiveness.2
Inter-relationship of Permeability to Stress.
Since coals are a complex, fractured, dual-porosity system, a bulk permeability value is used to describe the system of low-permeability organic matrix components surrounded by cleats or natural fractures.3Because of the inherent variability in permeability and productivity, CBM exploration and development projects tend to be statistical plays, where large multi-well projects are used to explore and develop an area of reservoir.Ultimately, the average well productivity and the required well spacing to maximize gas recovery will govern the overall economics of the project.
In many areas, proper characterization of the in-situ stress regime has played a pivotal role in the successful commercialization of CBM. Firstly, the aperture of fractures and cleats and hence their contribution to permeability tends to be extremely stress-sensitive.Sparks et al., identified how the permeability of fractures and cleats vary as a log function of in-situ stress (Fig. 1).4 Thus, low mean stress regimes support open and more conductive natural fractures.
Inter-relationship of Permeability to Stress.
Since coals are a complex, fractured, dual-porosity system, a bulk permeability value is used to describe the system of low-permeability organic matrix components surrounded by cleats or natural fractures.3Because of the inherent variability in permeability and productivity, CBM exploration and development projects tend to be statistical plays, where large multi-well projects are used to explore and develop an area of reservoir.Ultimately, the average well productivity and the required well spacing to maximize gas recovery will govern the overall economics of the project.
In many areas, proper characterization of the in-situ stress regime has played a pivotal role in the successful commercialization of CBM. Firstly, the aperture of fractures and cleats and hence their contribution to permeability tends to be extremely stress-sensitive.Sparks et al., identified how the permeability of fractures and cleats vary as a log function of in-situ stress (Fig. 1).4 Thus, low mean stress regimes support open and more conductive natural fractures.
