The Influence of Tensile Strength Difference with Variable Gravel Sizes on the Hydraulic Fracture Propagation in the Conglomerate Reservoir

Abstract

Conglomerate reservoir is a representative reservoir of unconventional oil and gas resources, and hydraulic fracturing is also used to increase its production. However, the tensile strength of conglomerates with different sizes is various, which has an important impact on the hydraulic fracture propagation in the conglomerate reservoir. To study the tensile strength difference caused by the size of conglomerate gravel, some Brazilian splitting tests were conducted. To find the influence of tensile strength difference on the hydraulic fracture propagation, some true triaxial hydraulic fracturing experiments for layered formations were carried out. The concept of the fracture element in conglomerate formations was proposed. The results showed the following: (i) the tensile strength of conglomerate rock decreased with the increase of the gravel size, and the average tensile strengths of specimens with gravel diameters of 2.0–6.0 mm and 6.0–20.0 mm were 65.5% and 43.6% of that with 0.1–2.0 mm diameter, respectively; (ii) hydraulic fractures easily deflected at the interface of fine-grained conglomerate and medium-grained conglomerate; (iii) hydraulic fractures were prone to propagate to the layers with larger gravel sizes, and the smaller the gravel size, the lower the penetration probability. It is helpful to increase the understanding of the hydraulic fracture propagation under different gravel sizes in the conglomerate reservoir, and it can provide a reference for well distribution and hydraulic fracturing design.

Fracture Propagation Mechanism of Tight Conglomerate Reservoirs in Mahu Sag

“…Thus, research on the fracture propagation law of natural glutenite reservoirs need to be further conducted. In this paper, the glutenite reservoir in the Mahu sag was taken as the research object, and a Brazil fracturing test was conducted to measure the tensile strengths of conglomerate specimens with different gravel particle sizes and cementation degrees [14].…”

“…In combination with CT scanning, it was found that the initiation fracture was at th edge of the fine gravel, which indicated that the small pressure drop during the initia fracturing stage corresponded to the fracture of the cementation interface of the gravels The fracture propagated at the edge of the fine gravel and frequently interacted with th gravel during the extension, as was reflected by the high frequency of pressure fluctua tions in the curve, indicating that the subsequent pressure fluctuation corresponded to fracture propagation at the interface of the small-grain gravel cement around the well which meant that multi-peak pressure was exerted at the beginning of the fracture prop agation stage. This phenomenon was caused by the positive correlation between the frac ture toughness of the gravel and the grain size [14]. After the fracture of the small-grain gravel around the well, the fracture tended to pass around the gravel with a more tortuou morphology, and the gravel had an influence on fracture roughness, resulting in decrease in local fracture width (the average width of the "gravel-penetrating" fractures was only the general width of the "gravel-bypassing" fractures).…”

“…Thus, research on the fracture propagation law of natural glutenite reservoirs need to be further conducted. In this paper, the glutenite reservoir in the Mahu sag was taken as the research object, and a Brazil fracturing test was conducted to measure the tensile strengths of conglomerate specimens with different gravel particle sizes and cementation degrees [14]. Then, a fracture propagation simulation test of natural glutenite outcrop samples was carried out using a small-scale true tri-axial hydraulic fracturing experimental system [15].…”