| Popis: |
In the 21st century the global gas demand will be met only by extensive production and utilization of unconventional gases. Unfortunately, these kind usually have extremely low permeability, small average pore sizes and strongly, 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 in reservoirs having D permeability. Using natural cores, the capillary pressure was calculated as a function of pore size distribution and wettability. Spontaneous imbibition tests were carried out to demonstrate the effect of aqueous and organic fluids. Based on experimental results, it was found that extremely high threshold pressure, sometimes several hundred bars, was needed to start water drainage from natural cores. Consequently, due to the unusual capillary forces, the water proved to be a natural blocking phase in such systems, and hence, the water may cause serious formation damaging hard to cure when the reservoirs is or were ever contacted with water. As a result, the operators definitely face with difficulties when water-based fluids are used in drilling, well completion, fracturing, and production technologies. Therefore, the physico-chemical considerations suggest that application of water-free fluids should be used in any phases of field operations. In addition, the presence of water in tight sand gas and BCGA reservoirs has detrimental effect on gas transport. Since the mass transport is usually diffusion, non-Darcy flow characterizes the gas rate, viz. the driving force of gas production should be treated with weaponry of physico-chemical approach and thermodynamic calculations. Thus, new paradigms and theoretical approach are necessary in the future, and further fundamental and applied research seems to be indispensable to develop novel technologies for gas recovery from unconventional gas reservoirs including tight sand, shale and BCGA formations. |
