Shale gas reservoirs: Theoretical, practical and research issues

Shale gas reservoirs are found all over the world. Their endowment worldwide is estimated at 10,000 tcf by the GFREE team in the Schulich School of Engineering at the University of Calgary. The shale gas work and production initiated successfully in the Unites States and extended to Canada will have application, with modifications, in several other countries in the future. The 'modifications' qualifier is important as each shale gas reservoir should be considered as a research project by itself to avoid fiascos and major financial losses. Shale gas reservoirs are best represented by at least quadruple porosity models. Some of the production obtained from shale reservoirs is dominated by diffusion flow. The approximate boundary between viscous and diffusion-like flow is estimated with Knudsen number. Viscous flow is present, for example, when the architecture of the rock is dominated by mega pore throat, macro pore throat, meso pore throat and sometimes micro pore throat. Diffusion flow on the other hand is observed at the nano pore throat level. The process speed concept has been used successfully in conventional reservoirs for several decades. However, the concept discussed in this paper for tight gas and shale gas reservoirs, with the support of core data, has been developed only recently, and permits differentiating between viscous and diffusion dominated flow. This is valuable, for example, in those cases where the formation to be developed is composed of alternating stacked layers of tight sands and shales, or where there are lateral variations due to facies changes. An approach to develop the concept of a super-giant shale gas reservoir is presented as well as a description of GFREE, a successful research program for tight formations. The paper closes with examples of detailed original gas-in-place (OGIP) calculations for 3 North American shale gas reservoirs including free gas in natural fractures and the porous network within the organic matter, gas in the non-organic matter, adsorbed gas, and estimates of free gas within fractures created during hydraulic fracturing jobs. The examples show that the amount of free gas in shale reservoirs, as a percent of the total OGIP, is probably larger than considered previously in the literature.