Study on the Influence of Injection Rate on Fracturing Initiation and Breakdown Pressures in Fracturing-Flooding Sandstone Reservoirs
Authors: Cao Hu , Zhang Guangqing , Li Shiyuan , Zhou Dawei , Yu Chunlei , Sun Qiang 
Published: September 2023

ABSTRACT
Because of the difficulties in producing energy supply, fracturing-flooding has been applied in water driven low-permeability oil reservoirs, in which water is injected near the breakdown pressure to produce a large number of microfractures and increase the sweeping area. In this work, the fracturing-flooding experiments and Brazilian split tests were carried out using sandstone outcrops to study the influence of injection rate on the damage characteristics. The results show that: (1) There are two key pressures during the generation of fracturing-flooding fractures: fracturing initiation pressure (FIP) and formation breakdown pressure (FBP). The fracturing-flooding injection pressure should be designed between FIP and FBP. (2) The loading rate is controlled by the injection rate. When the injection rate increases from 4 mL/min to 12 mL/min, the FIP increases by only 3%, meaning that the injection rate or loading rate has little effect on FIP. (3) With the increase of injection rate and loading rate, the tensile strength and FBP first decrease and then increase, and the critical loading rate is 0.09 MPa/s. In order to obtain a wider injection pressure range, the injection rate should be far away from the critical rate. This study is of significance for optimizing fracturing-flooding.

INTRODUCTION
Fracturing-flooding is a technology that injects water near breakdown pressure to produce a large number of microfractures and increase the swept area. Because of the difficulties of producing energy supplements in water drive low-permeability reservoirs, China's Daqing Oilfield, Shengli Oilfield, Jiangsu Oilfield, and other oilfields have successively carried out fracturing-flooding field experiments and achieved ideal results (He and Wang, 2018; Yang, 2020; Gao, 2022; Wang, 2022; Huang et al., 2022). However, as a new technology, fracturing-flooding still has problems, such as an unclear understanding of the mechanism of increasing injection and the law of pressure transmission (Huang et al., 2022; Guo et al., 2022), which need to be solved urgently.
Macro-fractures are generated by the connection of micro-fractures, meaning there should be a fracturing initiation point (FIP) before the formation breakdown point (FBP). For the triaxial compression test, FIP is the point where the axial strain curve of rock changes from a linear to nonlinear, which characterizes that the rock changes from linear elastic deformation stage to a stable crack extension stage (Liang et al., 2012; Peng et al., 2015; Guo et al., 2019; Zhang et al.,2020). For the FIP of tensile failure, Xu and Zhao (2008) and Chen Lei et al. (2021) monitored the crack initiation process with a three-point bending test and found a stable extension stage before the breakdown. By analyzing the field injectivity tests curve, Feng and Gray (2017) found that the FIP occurs when the pressure-time plot deviates from the linear relationship. With hydraulic fracturing laboratory experiments, Wu et al. (2020) pointed out FIP can be determined with the deviation point in the pressurization rate curve. With the increase in injection rate, the difference between FIP and FBP becomes larger. Aliabadian et al. (2019) and Shapeng et al. (2021) monitored the crack evolution process of the Brazilian splitting experiment by Digital Image Correlation (DIC) and acoustic emission (AE) and also confirmed that there is FIP in tensile failure.