Effects of Hydration during Drilling on Fracability of Shale Oil Formations: A Case Study of Da’anzhai Section Reservoir in Sichuan Basin, China

Abstract

Highlights: 1. How to quantitative evaluation of mechanical properties of shale in Da’anzhai section combined with triaxial mechanical experiments? The compressive strength of dry rock samples and soaked rock samples were tested by triaxial mechanical experiments. According to the stress-strain curves, the changes of compressive strength, elastic modulus and Poisson’s ratio before and after hydration were analyzed. 2. How to establish a mathematical model of shale fracability that comprehensively considers brittleness index, horizontal differential stress, and fracture toughness is proposed? Fracability index is proportional to brittleness index and horizontal differential stress, and inversely proportional to fracture toughness. Young ’s modulus, Poisson’s ratio and tensile strength in fracability index model are related to rock water saturation, so the fracturing index is also related to rock hydration. 3. What is the effect of shale hydration on fracability and fracture pressure? As the water saturation rate of the rock gradually decreases, the FI of the formation rock de-creases Combined with the variation law of water saturation rate, it shows that after the shale around the well is hydrated, the fracture pressure of the formation decreases Accurate evaluation of shale oil reservoir fracability helps avoid blind fracturing and ensures efficient fracturing. However, the current evaluation of the fracability index rarely considers the impact of hydration caused by drilling fluid invasion during drilling. The results of a rock triaxial mechanical test conducted to evaluate the mechanical properties of shale oil reservoirs are reported in this paper. Based on the results, we developed a comprehensive evaluation method of shale oil reservoir fracability that considers hydration; the effects of the brittleness index, horizontal difference stress, and fracture toughness; and the law of water phase intrusion into shale oil reservoirs. The research results show that the average compressive strength decreased by 37.99%, the average elastic modulus decreased by 53.36%, and the average Poisson’s ratio increased by 68.75% after being soaked for 48.00 h at 80 °C and 30.00 MPa. The water saturation rate at the borehole wall was the highest; with the extension to the stratum, it gradually decreases to the original water saturation rate of the formation, while the fluctuation radius gradually increases with time. The Young’s modulus and fracture toughness decrease, the Poisson’s ratio increases, and the fracability index reaches a maximum value at the wellbore (i.e., the highest water saturation rate), indicating that the strength of the hydrated rock decreases and it can be easily fractured. The case analysis shows that the optimal fracturing position of the Da’anzhai Section of Well NC2H is around 2600 m deep. After the hydration occurs, the fracture initiation pressure of the formation is reduced from the original value of 72.31 MPa to 66.80 MPa. This indicates that when hydration decreases, the formation fracture pressure also increases. The research presented in this paper can be used to optimize fracture location and set a reasonable fracturing pressure.