Charles M. Boyer II, Schlumberger
Resource and reserves estimation methodology for conventional oil and gas reservoirs is based in large part on the historic precedent of geologic and engineering evaluation over the last 150 years. Conversely, the methods used for shale gas reservoirs are recently developed and still evolving. Gas shale formations display complex reservoir characteristics, including free and adsorbed gas, natural fractures, and very low matrix permeability. These characteristics vary significantly vertically and laterally within shale reservoirs, controlled by both depositional setting and subsequent burial and tectonic history.
During early stages of a shale gas development program, critical data are required to fully assess the gas in place (resources) and the potential for economic recovery of that gas (reserves). These data, including formation geometry, porosity, organic content and composition, gas sorption, and reservoir pressure, are used by the geologist to begin to understand the static reservoir structure. Stochastic techniques are often employed to populate the static geologic model of the exploration area. Subsequent data obtained from completion and production operations begin to define the dynamic reservoir structure, including completed reservoir volume and flow dynamics. Combining traditional deterministic forecasting techniques (analog, volumetric, decline curve) with more specialized methods (shale gas specific analytic and simulation models) provides an initial understanding of reservoir performance and ultimate recovery. Awareness of the reservoir and reservoir property continuity is critical for assessing with reasonable certainty the extent and viability of this continuous play within and outside of the initial exploration area.
Assessing developed plays again relies on a combined stochastic / deterministic approach; however, in this more data-rich setting the geologist will rely on a more deterministic approach for evaluating the changing static and dynamic conditions within the reservoir. From this evaluation, production regions/compartments within the continuous deposit are defined. Production analysis and forecasting at this stage often uses a stochastic approach, in part because of the abundant production data and the known variability inherent in shale gas production. Understanding production variability in light of the static and dynamic reservoir conditions can further satisfy the reasonable certainty criteria required for reserves estimation.
AAPG Search and Discovery Article #90098©2009 AAPG Education Department, Houston, Texas 9-11 September 2009