Mechanism of Connecting Natural Caves and Wells Through Hydraulic Fracturing in Fracture-Cavity Reservoirs

In karst fracture-cavity reservoirs, the oil/gas is primarily distributed in natural fractures and caves. Therefore, hydraulic fracturing is frequently performed to connect the wells and natural karst caves to enhance the oil/gas production. In this work, the mechanisms of connecting the caves through hydraulic fracturing are examined through experimentation, numerical modeling, and field observations. The experimental and numerical results show that three main connection modes exist for natural caves, specifically connections through straight main fractures, crooked main fractures, and pre-existing fractures, corresponding to connection modes I, II, and III, respectively. In the experiments, 6%, 17%, and 77% caves were noted to be connected through connecting modes I, II, and III, respectively. Connection mode III was the most effective, exhibiting a maximum angle value of 225°. When the geostress difference is large, few natural fractures are active, and the connection mode I/II frequently occurs. When the geostress difference ≤ 4 MPa, injection rate ≤ 10 ml/min, fracturing fluid viscosity ≤ 10 mPa s, a great number of natural fractures are active, and the connection mode III frequently occurs. Therefore, pre-existing fractures must be exploited to enhance the production of oil/gas in karst fracture-cavity reservoirs. Field observations indicated that by gradually injecting an acid fluid, the activated number of pre-existing fractures can be increased, thereby enhancing the efficiency of hydraulic fracturing and increasing the average daily oil/gas production.