Laboratory Study of Fracture Propagation Behaviors in Fractured-Cavity Carbonate Reservoirs in Shunbei Oilfield

In order to determine the interaction behaviors between artificial fracture and nature fracture-cavity, we performed a laboratory true tri-axial fracturing experiment on natural carbonate outcrops with Mico-fracture and pores, and the natural fracture-cavity are simulated by pre-existing fracture-cavity. The propagation of artificial fractures is investigated by changing the approach angle for fracture and maximum horizontal stress, and arrangement of pre-existing cavity. Moreover, re-injection experiments are used to evaluate the conductivity of fractures. The results indicate that the extension of fractures in pre-existing fractured-cavity carbonate rocks is random. The extension of artificial fracture is steering when it encounters pre-existing cavity. Three interaction modes occur between artificial fracture and pre-existing fracture, and the large approach angle allows the artificial fracture to pass through the pre-existing fracture. Artificial fracture is always captured by pre-existing fracture, and the tortuosity is greater in the process of propagation. The breakdown pressure of pre-existing fracture-cavity carbonate rocks is about 50% lower compared to that of tight carbonate rock. Pre-existing fracture-cavity make the conductivity greater of effective fracture, but the length of effective fracture is not conducive to the conductivity. This research may contribute to the field application of hydraulic fracturing in carbonate formation. 1. INTRODUCTION The increase of burial depth causes the gradual decrease of carbonate porosity, which is a past view on the lack of high-quality reservoirs in deep carbonate rocks (SCHMOKER J W et al., 1982; EHRENBERG S N et al., 2009). In recent years, 60 % of the world ‘s new oil and gas reserves come from deep strata, and the exploration potential is huge (Wei C et al., 2017; Sun K et al., 2022). The deep strata of oil and gas reserves and production of marine carbonate rocks in Tarim Basin, Sichuan Basin and Ordos Basin have been increasing rapidly year by year, which has attracted wide attention from the industry. In the proven deep strata of carbonate reservoirs, two-thirds belonging to the type of fractured-cavity reservoirs (Guo W et al., 2022). Complex spatial structure is the characteristic of fractured-cavity reservoir, mainly including matrix pores, karst cavity, and natural fractures with different development degrees (Durrani M Z A et al., 2021; Tian F et al.,2019; Sun, K et al., 2021). Due to the extreme heterogeneity and complex flow environment of facture-cavity carbonate formation, that is a challenge to increase oil and gas extraction rate (Tian F et al.,2016; Dmitriy A. Martyushev et al., 2022).