Abstract:

In order to study the effects of caves and flow rates on hydraulic fractures propagation and initiation pressure, two sets of cube hydraulic fracturing specimens—A and B（the size is 200 mm×200 mm×200 mm）—with two same specimens in each group are made by the mixture of cement, fine sand and water. The specimens in group A are intact, while that in group B is with Karst cave（the PVC pipe is pre-buried during pouring, and then, when the cube is formed, the pipe is removed to simulate natural caverns）. Fracturing experiments are carried out for two specimens in each group when the displacement was 7.5 and 10.0 mL/min respectively. The results indicate that for intact rock mass, the initial pressure will rise up with the increase of flow rate, and the induced hydraulically fractures will be more straightness; for the rock mass with caves, the caves will affect the extended path of hydraulic fractures and exert a kind of “attraction” on hydraulic fractures, which will make the hydraulic fractures extend towards caves, and the effect of “attraction” will decrease with the increase of the displacement. The initial pressure of fracturing in intact rock mass is higher than that in rock mass with caves under the same conditions. Caves are the main storage area for petroleum resources. The key to enhance oil and gas recovery in fractured-vuggy reservoirs is to set up fracturing operation parameters reasonably and produce hydraulic fractures that communicate caverns and boreholes. This study has certain guiding significance for optimizing the fracturing operation parameters.

Key words: hydraulic fracturing, fractured-vuggy reservoir, fracture propagation, caves, pump injection displacement