Numerical Simulation Study on Multi-Directional Etching Connection in Acid Fracturing
Authors: Wang Xiaoyang , Qin Qiuping , Zhou Linbo , Huang Xin , Zhang Meizhu 
Published: August 2024

ABSTRACT:
With the development of the carbonate reservoir in Tahe Oilfield, the main target has shifted from the "large fracture cavities" around the wellbore to the "small fracture cavities". The existing acid fracturing technology can no longer meet the connection needs of the new target, and it is urgent to establish an efficient communication and stimulation technology system for the small fracture cavities around the well in the weathering crust karst area, to improve the connection ability of the acid fracturing process. Focusing on the above engineering problems, the Petrel and AiFrac-COMSOL data coupling method was developed. Using the "geology-engineering" coupling platform, the geological modeling software Petrel, and the high-precision numerical simulation software AiFrac were integrated. Then, the AiFrac-TOUGH numerical simulation program was used to perform hydraulic fracturing of the reservoir and construct artificial fractures. Finally, COMSOL Multiphysics was used for acid injection simulation to study the communication mechanism of multi-directional etching of acid liquids. The influence of in-situ stress and natural fracture occurrence on multi-directional etching of acid liquids was clarified.

1. INTRODUCTION
Multi-stage geological processes and structural movement have caused the Ordovician carbonate reservoir space in Tahe Oilfield diverse forms, size with large difference, and non-uniform distribution. With the development deeply, the development object has changed from " large fracture-cavity " around the well to " small fracture-cavity group. " The existing acid fracturing is difficult to meet the needs of connection in new objects. It is urgent to establish an efficient connection and increasing production technology system for small fracture-cavity around the well in karst area of weathering crust, so as to improve the connection ability of acid fracturing process.
Extensive research has been conducted by both domestic and international scholars on the fracturing of fractured-vuggy carbonate reservoirs, encompassing experimental and simulation studies. Liu et al. (2014) explored the impact of natural fracture development on hydraulic fracture propagation using large-scale laboratory fracturing experiments. They discovered that the propagation of hydraulic fractures is primarily influenced by the differential in-situ stress and the orientation of the fracture relative to the maximum principal stress direction. Contrary to being strictly aligned with the maximum principal stress direction, hydraulic fractures tend to propagate according to the principles of least resistance, giving priority to the path of maximum propagation and the shortest propagation route. Through extensive laboratory testing, Li et al. (2015) investigated the propagation characteristics of acid fracturing in carbonate reservoirs, revealing that the sequential injection of varied acid types enhances fracture flow capacity. Liu et al. (2019) conducted a true triaxial hydraulic fracturing experiment incorporating karst caverns, identifying that the size of these caverns and the differential in-situ stress significantly affect hydraulic fracture behavior, with larger cavern volumes impeding fracture propagation and resulting in notable pressure drops within the fracturing curve. Utilizing ABAQUS, Lin (2019) examined the effects of in-situ stress and the presence of karst caverns on hydraulic fracturing in a two-dimensional context, finding that elevated pressures within karst caverns facilitate the interconnection between fractures and caverns. Further laboratory experiments indicated that faults exert an attractive influence on hydraulic fractures. Building on these findings, Guo et al. (2020) analyzed how varying fracturing parameters, including fluid type, viscosity, and confining pressure, influence fracture propagation in carbonate reservoirs, proposing a novel fracturing approach that integrates the initial use of gelled acid followed by slick water. Guo et al. (2021) embarked on fracturing experiments utilizing liquid CO2 in conjunction with CT scanning techniques to elucidate the mechanisms of fracture propagation during CO2 fracturing in carbonate rocks.