The Effect of Rock Heterogeneity on Fracture Initiation During Supercritical CO2 Fracturing

Abstract

ABSTRACT To solve the issues during large-scale hydraulic fracturing such as massive consumption of water, formation damage and environmental contamination, etc., supercritical CO2 (SC-CO2) has been introduced as an alternative anhydrous-fracturing fluid. However, the heterogeneity of formation would have significant impact on fracture initiation. In this study, a series of tight sandstones was collected to perform SC-CO2 fracturing experiment. The effects of bedding-plane angle (BPA) and permeability of sandstone on fracture initiation are investigated. The results indicate that the initiation pressure of rock shows a fluctuating trend, it goes up firstly, then slightly goes down with the increasing of BPA. And the fracture morphology is significantly affected by the heterogeneity of rock, which are more inclined to distribute along the bedding planes. Moreover, the rocks with lower permeability have higher initiation pressure, and more complicated fractures would be induced. With the increasing of the permeability, the initiation pressure of rock reduce significantly, also the fractures morphology are gradually simplified. When the permeability reaches to 0.5 mD, the rock cannot be broken by SC-CO2 in the experimental pumping rate, but it can be fractured by water. It means that SC-CO2 as fracturing fluid is more appropriate to implement in low permeability reservoirs since its low viscosity and strong penetration. This research explores the formation adaptability and provides theoretical guidance for field application of SC-CO2 fracturing. INTRODUCTION Tight gas as typically unconventional reservoir, is widely distributed in China, which will play a significant role in the supplement of conventional hydrocarbon. According to the fourth oil and gas resource evaluation of CNPC, China's total onshore tight gas resources are 21.85 × 1012 m3, with 60% of the total resources in the Ordos Basin (Jia et al. 2022). Tight sandstone gas resource is usually with porosity less than 10%, in-situ permeability less than 0.1 mD, pore throat diameter less than 1 μm, and gas saturation less than 60% (Jia et al. 2012; Gong et al. 2016). These reservoirs cannot be produced directly without artificial stimulation method (Scanlon et al. 2014; Ma et al. 2015).