Abstract
The Mahu Oilfield in Xinjiang, China, is the world’s largest conglomerate oilfield with massive geological reserves. In the multi-cluster staged of horizontal well fracturing, the breakdown pressure is high, and the fracture is difficult to initiate. Therefore, lowering the breakdown pressure is the key to hydraulic fracturing safely and efficiently. In this paper, we first develop a new water jetting experimental apparatus that can adjust perforation parameters to simulate spiral perforation in the oil field. It consists of perforation angle, position, and depth. The researchers went to the Mahu area for geological investigation and excavated conglomerate outcrops from the Baikouquan formation to conduct true triaxial hydraulic fracturing physical simulation experiments. The research results indicate that water jetting perforation can reduce the breakdown pressure by 6 MPa under immutable in-situ stress magnitude and distribution. It is easier to initiate when the angle between the perforation tunnel and the maximum horizontal principal stress is 0°, with the lowest breakdown pressure. If natural fractures are developed near the perforation hole, the communication between the perforation tunnel and the natural fractures prominently reduces the difficulty of fracture initiation. After fracturing, it is easy to form complex multi-fractures with high tortuosity and surface roughness, and the breakdown pressure declines sharply. The research results provide theoretical support for reducing the breakdown pressure in conglomerate reservoirs with strong heterogeneity.
