# Hydraulic Expansion Techniques for Fracture-Cavity Carbonate Rock with Field Applications
## 2 Proposed Method for Increased Production Using Hydraulic Expansion  
Hydraulic expansion involves injecting water into a formation under high pressure and high displacement. The injected water is used as the energy carrier to build a high-pressure environment in the bottom hole, reopen closed fractures around the wellbore, and break through the barrier (constant volume interface) of isolated fractures and holes around a well [11; 12]. According to the geological conditions of different types of wells and the distribution of adjacent wells, we devised various production-increasing processes, as described in Figure 2. In the face of wells around multiple sets of developed fracture-cavity bodies, we apply high pressure to prevent a nearby fracture network from opening the seam connecting a distant fracture-cavity reservoir. For a fault-controlled karst with clay filling, we break through the clay filling section of the natural fracture network to construct an injection production well pattern. As a well with constant volume and multiple rounds of water injection shows unfavorable production, we increase the injection rate to produce diversion and spread to remaining unswept fracture areas. For wells showing insufficient fluid supply after pre-acidification fracturing, we activate a closed or partially closed original high-permeability channel by applying high pressure. We also propose two composite processing methods, namely, using a composite blocking and expansion process to prevent the injected water from seeping down along large fault zones, and using an expansion and acidification/acid fracturing composite process to increase the production of these wells without an obvious communication between pre-acidification and fracturing. Figure 1: Section of a fracture-cavity carbonate reservoir. High Pressure to Prevent Nearby Fracture Network from Opening Seams Connecting Distant Fracture-Cavity Reservoirs  
In fracture-cavity carbonate rocks, a typical reservoir is characterized by the development of faults in the well area, and good reservoirs are located around the well. However, owing to insufficient formation energy, water injection in the adjacent well fails to replace oil, the pressure drops sharply, and the fractures close. For this type of reservoir, we propose hydraulic expansion using high pressure to prevent the nearby fracture network from opening the seam that connects the distant fracture-cavity reservoir [13]. As a representative example, consider expansion well 1 with a drilling completion depth of 5830 m, vertical depth of 5795.46 m, and average amplitude change rate between 0 ms and 40 ms. The well is located in a positive fold area and contains developed faults. The west and northwest directions indicate good reservoirs. However, regarding development, the well mainly uses the energy body of an adjacent well, and the water injection pressure is constant, leading to a production decline over the years [14]. Calculations indicate a formation extension pressure of approximately 85 MPa, while during construction an actual bottom hole pressure of 94 MPa, maximum pressure of 39 MPa, displacement of 60 m\({}^{3}\)/h, and water injection of 3534 m\({}^{3}\) were observed, reaching the formation opening pressure. Therefore, high pressure is used to hold down the nearby fracture network to open the seam, thus connecting the distant fracture-cavity reservoir. The construction curve in Figure 3 shows clear pressure fluctuations under the same displacement, indicating the connection of the reservoir around the well. In addition, the pressure is properly transmitted to the far end, demonstrating the feasibility of the proposed stimulation technique. Break through the Clay Filling Section of the Natural Fracture Network to Construct the Injection Production Well Pattern  
A cavity may appear next to another one in a fracture-cavity carbonate reservoir. In addition, the cavity has an adequate geological connection foundation, and all of the reservoirs are large fracture-cavity bodies [15; 16].