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Multi-scale Fractures and Water Invasion Dynamics in Buried-Hill Bedrock Gas Reservoirs of the Jianbei Slope, Qaidam Basin, China

<p>In this study, efforts are made to comprehensively analyze the multi-scale fracture development, the dynamic water intrusion, and the favorable remaining gas areas in the ancient buried-hill bedrock gas reservoir in the Jianbei slope of the Qaidam Basin, based on integrated analyses of thin sections, FMI imaging logging, three-dimensional seismic data, and dynamic production data. Fractures in the bedrock are demonstrated to be controlled by tectonic stress, magma intrusion, hydrothermal activity, and weathering and leaching. The complex fracture genesis leads to differences in the scale and type of bedrock fractures with multiple scales and high angles. The dip angle of the fracture gradually increases from the unconformity on the top of the bedrock downward inside the bedrock. Meanwhile, the dip angle may decrease from the center to both sides due to the difference in the fracture strength of the fault zone. Fractures are mostly near-east-west oriented and they normally intersect with the ground stress at an acute angle. Effective fractures can provide advantageous seepage channels for fluids. Formation water is demonstrated to rapidly channel through large-scale fractures while laterally invading through small-scale fractures in the gas reservoir. The western area of the reservoir is featured by intensive water invasion, rapid pressure drop, and fast production decline, while the eastern area is characterized by weak water invasion as well as relatively stable pressure and productivity. Three relatively independent water-seal gas systems with JB1-3, JBH1-3 and J3 Wells as the center are formed, and the remaining gas is enriched.</p>