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.
Highlights
IntroductionDriven by the economic development and world’s energy demand, the need for unconventional resource development is gradually growing [1,2,3,4,5,6,7,8], especially shale oil and gas and unconventional oil and gas [9,10,11]. e conglomerate reservoir is a representative unconventional reservoir formed by rapid deposition near sources [12,13,14], with the characteristics of low permeability [15], large lithological changes, and strong heterogeneity [16, 17]
Some true triaxial hydraulic fracturing experiments for layered formations were conducted, and the influence of tensile strength difference on hydraulic propagation was analyzed. It is helpful for understanding the hydraulic fracture propagation under different gravel sizes in the conglomerate reservoir, and it can provide a reference for the well distribution and hydraulic fracturing design
Hydraulic fracture propagation was closely related to the rock tensile strength
Summary
Driven by the economic development and world’s energy demand, the need for unconventional resource development is gradually growing [1,2,3,4,5,6,7,8], especially shale oil and gas and unconventional oil and gas [9,10,11]. e conglomerate reservoir is a representative unconventional reservoir formed by rapid deposition near sources [12,13,14], with the characteristics of low permeability [15], large lithological changes, and strong heterogeneity [16, 17]. The difference in tensile strength with variable gravel sizes and the distribution characteristics increase the difficulty in well designing and understanding the hydraulic fractures propagation directions [18]. E grain size was one of the most important parameters affecting the mechanical properties of rocks [19]. Some researchers studied the relationship between gravel size and rock strength. Hugman [20] established an inverse relationship between the rock strength (including the wield stress) and the mean grain size in carbonate rock. Olsson’s study [21] showed that the rock strength was inverse with the square root of the grain size for marble. Lakirouhani [23] concluded that there was a significant positive correlation between grain size and tensile strength
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