Fracture Propagation Behaviors of High-Temperature Granites During Liquid Nitrogen Fracturing with Multi-Branch Wells

Abstract

ABSTRACT We propose a low temperature medium fracturing mode in combination with a multi-branch well to overcome the challenges of difficult initiation, communication, and control in the traditional hot dry rock (HDR) fracturing. Fracturing experiments on multi-branch wells subjected to various fluid conditions were conducted to analyze and compare fracture behavior. The method of pressure curve fitting analysis and the method of permeability test by re-injection fracturing fluid were used to quantitatively characterize the rock fracture morphology and evaluate the effect of liquid nitrogen (LN2) fracturing. The characteristic of injection pressure of HDR with different numbers of branch wells under LN2 fracturing are compared, and the characteristics of fracture morphology of HDR are evaluated. The experimental results show that: ① compared with hydraulic fracturing, multi-branch well can reduce the breakdown pressure in a certain range (About 25 to 61 %), and breakdown pressure can be further decreased under LN2 stimulation; ② with the increase of the number of branched boreholes, the extension of fractures is longer and fracture interconnection is better; ③ The HDR after multi-branch well fracturing has better conductivity than that of hydraulic fracturing. According to the experimental results, the multi-branch wells can enhance the thermal shock areas of LN2 in HDR, and has a great potential in creating complex fracture network. This technique is expected to provide a new insight into better developing HDR geothermal. INTRODUCTION Geothermal energy is an important part of the world's energy plan for the 21 centuries. It has many benefits over traditional energy sources, including being clean, efficient, and having large reserves (Lu et al,2018). The type of geothermal energy we’re talking about is found deep underground in certain rocks, like granite and gneiss. There are many ways to get this energy out of the ground, but the most common one is called an enhanced geothermal system. This involves creating fractures in the rock using a special fluid, so that hot water can be pumped through the rock and used to make electricity. Scientists are working to find ways to make this process more effective and cheaper.