Research on Hydraulic Fracture Propagation Law of Carbonate Reservoir Considering the Influence of Vugs
Authors: Zhou Jun , Yao Yiming , Zhang Baoping , Wu Zhiying , Sheng Mao 
Published: September 2023

ABSTRACT
The size and shape of the vugs in the fracture-cavity carbonate reservoir are uncertain, and the heterogeneity is very strong, which has obvious influence on the effect of acid fracturing. A large-scale physical simulation test with different vug sizes, different distribution characteristics and ground stress differences was carried out to analyze the characteristics of fracture penetration and migration. At low horizontal stress difference (K ≤0.15), hydro fractures will be non-plane propagation when they encounter vugs, and the fracture morphology becomes more complex. When there is a high horizontal in-situ stress difference (K ≥0.36), the horizontal in-situ stress difference will overcome the stress concentration of the hole body and expand, and will directly pass through the vug and continue to expand. The experimental results can provide a theoretical basis and guidance for the design and construction parameter optimization of the acid fracturing scheme for fracture-cave carbonate rocks.

INTRODUCTION
The reservoirs of Ordovician Yingshan Formation and Yijianfang Formation in Tarim Basin are mainly composed of carbonate karst erosion pores, caverns and fractures, and the reservoirs are distributed in fracture-vuggy zone as the basic unit. The development scale and morphology of fracture-vuggy are highly uncertain, and the heterogeneity is extremely strong. The development of fracture-vuggy carbonate reservoir often requires acidizing fracturing. However, due to the influence of karst cave development on local stress, the extension path of artificial fractures also changes obviously.
By means of finite element method or element splitting method, the physical model and mathematical model considering the propagation of artificial fractures by kart cave were established, and the interaction mechanism between artificial fractures and solution under different conditions was analyzed(Pan et al., 2012, Li et al., 2009, Wang, 2017, Du, 2015). Some researchers have explored the communication between artificial fracture morphology and holes by prefabricating large size holes in large object molds (Li et al., 2020, Zhou, 2016, Liu et al., 2019, Guo et al., 2020).However, there are some differences in the understanding of different scholars. Some believe that artificial fracturess will expand in the direction of the karst cave and eventually communicate with the karst cave, and some believe that the interference of the stress concentration of the karst cave will offset the karst cave during the expansion of artificial fractures (Weng et al., 2015, Luo et al., 2020, Wang et al., 2020). Therefore, in order to clarify the propagation law of artificial fractures in fracture-cavity carbonate reservoirs, the true triaxial compression fracture propagation experiment of carbonate rocks considering the effect of pores is carried out, and the influence law of pore size, horizontal stress difference and pore location on fracture propagation is explored.