Speaker
Description
The Tarim ultra-deep, fractured, low-porosity sandstone gas reservoir is deeply buried and is characterized by high temperature, high pressure, high in-situ stress, multi-scale fracture development, and strong edge and bottom water drive, which together result in a highly complex system. At present, the gas reservoir is experiencing severe problems such as water invasion and a rapid decline in gas production. Therefore, this study independently developed a multi-field coupled physical experimental platform capable of replicating the high-temperature, high-pressure, and high-stress conditions. In addition, a preparation method for three-dimensional large-scale rock samples (260 mm × 260 mm × 260 mm) was established to accurately characterize the multi-scale features of the “large fracture, small fracture, and matrix pore” in the reservoir. On this basis, four types of physical experiments were conducted: single-phase gas depletion production, constant-volume bottom-water depletion production, depletion production under different production pressure differences, and gas production with drainage to enhance recovery. The experimental results show that, in the early stage of single-phase gas depletion production, gas stored in the large fracture is produced first, followed by gas supplied from the matrix block surrounding the large fracture. Subsequently, small fracture and the entire matrix block are progressively activated, and the slope of the cumulative gas production curve increases until a pseudo-steady state is reached within a relatively short time. This behavior confirms the flow characteristics of sequential utilization and coupled superposition among large fracture, small fracture, and matrix block. It was also found that the matrix gas-supply capacity decreases as gas reservoir pressure declines, further exacerbating the imbalance between supply and production. During constant-volume bottom-water depletion production, the experimental process can be divided into three stages based on the pressure-evolution characteristics. Gas supply from the matrix and fractures stage, water sealing stage, and unsealing stage. When the water content in the fracture system exceeds a critical threshold, the matrix gas-supply capacity drops sharply, leading to gas water sealing effect, a mechanism revealed for the first time in this study for bottom-water fractured gas reservoirs. Continuous drainage and pressure reduction in the fracture system can partially alleviate water sealing and restore intermittent gas-supply capacity from the matrix block, however, the associated production enhancement is limited. A smaller bottom-hole pressure drop corresponds to a higher natural gas recovery factor. Experimental results under different production pressure differences indicate that faster gas-production rates and larger water volume multiple lead to lower cumulative gas production before water sealing, higher abandonment pressures of production well, greater difficulty in unsealing the reservoir after water sealing, and correspondingly lower recovery factors. Under different drainage volumes, gas production with drainage water also exhibits three stages, the gas supply from the matrix block and fracture, a drainage-well discharge stage, and production resumption stage of production well. Drainage wells can, to some extent, improve the recovery of such gas reservoir. The study provides a basis for rational development and enhanced recovery strategies of the ultra-deep, fractured, low-porosity sandstone gas reservoir.
| Country | China |
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