Fractured gas shale reservoirs in the Western Canadian Sedimentary Basin (WCSB) are often overlooked as a result of low initial production rates relating to vertical wellbores that have failed to intersect common vertical fracture sets. A recent study of the shale gas potential in WCSB has indicated there is a large resource potential for shale gas, with greater than 86 tcf of gas in place within Devonian through Cretaceous age shale formations. Furthermore, there is emerging evidence to suggest that shales have been contributing to conventional production.
This paper will describe the major challenges of developing a commercial scale shale gas project including:
Identifying geologic sweet spots, which meet the following criteria:
Organic rich
Thermally mature
Marine to transitional marine
Comparatively thick, and most importantly
Permeability enhanced by fracturing or higher
permeability interbedded facies
Acquiring a large lease base capable of placing large numbers of wells,
Designing and field-testing cost-effective drilling, completion, and stimulation procedures that are suited for the specific type of shale gas resources which are essential to the economic success of the project, and
Continuous technology improvement and innovation including the use of state-of-the-art fracturing technologies such as micro-seismic to improve well productivity.
Examples of a Canadian shale gas field pilot will be discussed to illustrate these key steps, which are critical to developing a successful commercial project.
Fractured shale reservoirs include some of the oldest known oil and gas production in North America. Natural gas was produced from the Devonian shales near Fredonia, NY as early as 1821, which pre-dates any known oil production by almost 40 years. By 1926, the Devonian Ohio shale of the Appalachian basin was in commercial production and was the largest known gas field in the world (1). This area currently contains the majority of shale gas wells in the US, with over 30,000 gas wells producing in the order of 120 bcf/year (2).
Fractured shales are differentiated from conventional reservoirs in that they are often both the source rock of the oil and gas, as well as the reservoir itself. The majority of the hydrocarbons are stored within the low permeability matrix rock of the shale reservoir, and a system of natural fractures provides the bulk of the transmissibility (permeability) within the reservoir. The size, extent, interconnectivity, and degree of cementation of the natural fractures typically dominate the overall productivity of the shale reservoir (3).
Gas shale reservoirs are classified as continuous type natural gas plays: that is accumulations that are pervasive throughout a large geographic area that offer long-life reserves with attractive finding and development costs (4). Most shale reservoirs have very low matrix permeability (nano to microdarcy level) and require the presence of the extensive natural fracture systems to provide and sustain commercial gas production rates.
Natural gas is stored in three ways: as free gas within the rock pores, adsorbed gas on the organic material, and as free gas within the system of natural fractures.
