Percolation Capacity in Deep and Ancient Karst Gas Reservoirs with Extra-Low Pressure Gradient

Most ancient karst gas reservoirs with deep burial depth, with developed fracture and cave, belong to multi-media ones and are characterized by low permeability and porosity as well as strong heterogeneity. In the process of development programming, how much production pressure different available for not only efficient exploration and development of reserves but also EOR has long been a major challenge. In common experiments, the simulated pressure difference is often more than 1.0 MPa, resulting in the pressure gradient ranging from 10 MPa/m to 33.3 MPa/m far beyond the real one. This bigger gradient is inconsistent with that in actual flow, which may bring about making improper measures. So, it’s necessary to carry out special experiments on flow simulation at the low pressure gradient in order to made clear the main factors affecting percolation capacity and single-well deliverability under similar formation condition. In this study, some comparative experiments on percolation and depletion in the deep karst gas reservoirs were conducted at the extra-low pressure gradient (<1.0 MPa/m) by means of perfecting metering apparatus and methods. The necessity to guide reservoir development on the basis of percolation-law analysis was also confirmed. Results show that (1) with regard to cores from different reservoirs, there is obvious nonlinear flow, especially from certain area with extra-low pressure gradient; (2) as for a few cores with connate water, there is apparent threshold pressure effect because percolation channel is sealed by the connate water; (3) for vuggy cores, there is obvious initial pressure in their flow; and (4) for porous cores, their depletion flow takes the longest. Fast depletion rate may enlarge the production pressure different further to cause uneven exploitation more seriously or even insufficient gas play. Compared with common experiments, the recovery, which between about 30% to 55% obtained from the depletion experiments, comes closer to that in the actual deep karst gas reservoirs.