A New Evaluation Method for Drillability of Hot Dry Rock Considering In-Situ Temperature in Formation

Abstract

ABSTRACT: Traditional assessment approach of rock drillability is limited in deep geothermal drilling, especially in Hot Dry Rock (HDR), due to the ultra-high abrasive resistance and temperature conditions. This paper proposed a new evaluation method to assess the drillability of HDR. A series of drilling tests were conducted on Gonghe granites under in-situ high temperature conditions. The effect of five key parameters, including bit weight, RPM, torque, penetration depth and rock temperature on rate of penetration (ROP) was investigated by HDR drilling experiments. Then, an unsupervised clustering method was employed to classify the ROP and its correlated parameters reasonably. Finally, the HDR drillability levels correlated with ROP were specified. Results showed that the ROP increases with a growth in rock temperature, which is conductive to promote the bit penetration. HDR is assigned a rating from 1 to 4 taking into account the effects of rock temperature, with a higher rating corresponding greater ease of drilling. The ROP may be classified into four modes: slow (0.28-2.39mm/min), slow-medium (0.56-3.09mm/min), medium (3.31-8.93mm/min) and fast (5.63-15.93mm/min). Our works are expected to provide a theoretical basis for the optimization drilling performance in HDR. 1. INTRODUCTION Hot dry rock (HDR) possesses the attributes of being environmentally friendly, highly efficient, and stable, making it a significant alternative energy source for the future. Efficient exploitation of geothermal resources in HDR is of great significance to the global energy transition. Drilling is the dominant technology for development HDR resources, accounting for approximately 35% to 60% of the overall engineering investment. This technology necessitates drilling holes into the high-temperature reservoirs composed of exceptionally hard crystalline rock formations, such as granite. However, the mechanical properties of HDR, i.e., high temperature, high hardness and abrasive resistance, significantly decrease the rock drillability (Kelkar et al., 2016; Lu and Wang., 2015; Zeng, 2015), resulting in bit wear and low drilling efficiency in real operations (Si et al., 2019; Wang et al., 2019). Hence, it is essential to develop an evaluation method for drillability of HDR, which is respected to enhance the drilling performance and reduce the operation costs in HDR resource excavation.