Abstract
The unconventional gas reservoirs usually have extremely low permeability, small average pore sizes, and often absolute water-wet surface character. As a result, the residual water saturation and the critical capillary pressure needed to mobilize water from natural porous systems might also be unexpectedly high. Therefore, a detailed laboratory studies have been carried out with the aim at determining the detrimental effects of pore structure, wettability, capillary and imbibition forces, and permeability in tight sand gas and BCGA reservoirs. Based on experimental results, it was proved that the water was a natural blocking phase. Namely, the water may cause serious formation damaging hard to cure when the reservoirs were ever contacted with water. Hence, the operators definitely face with difficulties when water-based fluids are used in drilling, well completion, fracturing, and production technologies. Consequently, the physico-chemical considerations suggest that application of water-free fluids is recommended in any phases of field operations. Similar difficulties are arising in respect to gas transport in tight media. Since the mass transport is usually diffusion, the driving force of gas production should be treated with weaponry of physico-chemical approach and thermodynamic calculations. The paper to be presented will also focus on critical PVT problems of fluids saturating the reservoir space. It will be shown that gas/liquid equilibrium of some fluids (e.g. water and heavy hydrocarbons) and supercritical phase equilibrium of light hydrocarbons, CO2 and H2S may have significant influence on rate and composition of gas production. Thus, new paradigms and theoretical approach are necessary in the future, and further fundamental and applied research seems to be indispensable to develop novel recovery technologies for gas recovery from unconventional gas reservoirs.
