Numerical Investigation on Fracture Initiation Law and Breakdown Pressure in Perforated Horizontal Well in Mahu Conglemorate Reservoir

Abstract The Mahu Oilfield has discovered 1.24 billion tons of tertiary petroleum geological reserves. The full exploitation of its oil and gas resources has irreplaceable significance for China and the world. However, the Mahu reservoir has strong heterogeneity, and there is currently a problem of unclear understanding of the mechanism of fracture initiation and unknown factors affecting fracture initiation. This article first proposes a gravel generation algorithm based on gradual expansion and boundary coupling to characterize the heterogeneity of conglomerate reservoirs. Subsequently, a hydraulic fracture initiation and propagation particle discrete element model based on the flow tube network is established. Finally, numerical simulations of the hydraulic fracturing process are carried out considering different gravel properties under perforated conditions in horizontal wells. The mechanism, laws, and main controlling factors of fracture initiation in conglomerate reservoirs are revealed. The results indicate that hydraulic fracturing in conglomerate reservoirs can be categorized into three types based on the media surrounding the perforation: matrix initiation, gravel-bypassing initiation, and gravel-crossing initiation. The internal strength of gravel is higher than matrix, and the corresponding breakdown pressure for gravel-crossing initiation is the highest among the three types of fracture initiation. The increase in gravel content and size will lead to an increase in the proportion of gravel-crossing initiation fractures, and the corresponding breakdown pressure will go up by an average of 6.4 MPa and 4.4 MPa compared to the breakdown pressure of the matrix initiation condition. The increase in gravel strength makes it difficult for fractures to form a crossing path when encountering gravel, resulting in an increase in the proportion of fractures around the gravel and an average increase of 8.1 MPa in breakdown pressure compared to the matrix initiation condition. This article provides a powerful tool for simulating the initiation and propagation of hydraulic fractures in conglomerate reservoirs.