The emergence of liquid-rich and gas shale reservoirs presents major strategic opportunities and challenges for the oil and gas industry. Accurate estimation of Stock Tank Oil and Gas Initially In Place (STOIIP & GIIP) is one of the priority tasks before defining the reserves. An accurate method is proposed to calculate hydrocarbon volumes using high-resolution geological models taking advantage of huge improvements made during last decade in the field of characterization and geological modeling of unconventional reservoirs. This exact method provides fluids in place in reservoir and surface conditions with an extended black-oil formulation including condensates. The physic including the equilibrium between gravity and eventually capillary forces and adsorbed gas is fully respected using, for each lithofacies, the most accurate available geological description with 3D porosity distributions, Langmuir isotherms (Langmuir, 1918), capillary pressure curves, and thermodynamic data. Adsorbed and liquid-rich gases are considered.
This method calculating hydrocarbons in place is the natural endpoint of any workflow devoted to the geological modeling of newly discovered reservoirs, particularly suited to heterogeneous reservoirs.
The knowledge generated by this calculation has significant impact on fracturation programs to increase the recovery rate and field development planning.
The characterization of volume of fluids in place in shale reservoir is a challenging task for many reasons, in particular because of the adsorbed gas which is a way of trapping gases in geological formations. The adsorption capacity of a gas on a rock depends on many parameters such as temperature, pressure, gas type and rock type. Shale gas is generated in source rocks, which are usually made of clay with high organic matter content. However, the organic content depends on the maturity of the organic matter from which they come from. Also, the maturity of the organic matter itself depends on the temperature history during the reservoir formation. Thus, in the same sedimentary basin, conventional reservoir can be formed from hydrocarbon expelled of unconventional reservoir. This is the case in the United States, where sedimentary basins that produce large quantities of unconventional hydrocarbons were already conventional oil and gas basins exploited for their conventional hydrocarbons. As for conventional reservoirs, there are large uncertainties on many parameters of the geological model. For shale reservoirs, another source of uncertainty is being added due to specific parameters related to them. Shale makes a wide variety of highly differing formations. All these formations differ from conventional reservoirs, and shale rock characteristics can differ from shale to shale, and even within the same shale.
