Rectorite drilling fluid: high-temperature resistance for geothermal applications

Highlights

• Rectorite is used as the slurry-making clay of the high temperature resistant drilling fluid system.

• LP-organic modification greatly improves the rheological property and high-temperature resistance of the rectorite.

• H-LT drilling fluid system can resist up to 220 °C .

Abstract

This study aims to resolve some existing limitations of geothermal drilling and exploration of high-temperature drilling fluid systems, such as complex formulation, inconvenient preparation, inappropriate selection of additives, unstable high-temperature performance, and high costs, through theoretical, experimental, and microstructural analysis. The slurry-making ability of several relevant clay minerals were theoretically analyzed based on their slurry-making performance and rheological behavior under various high-temperature environments (i.e., 180, 200, and 220 °C). Rectorite was selected as the appropriate slurry-making clay mineral and was modified organically using a single colloidal material (LP). In addition, suitable high-temperature additives were selected to improve the performance of the drilling fluid system by reducing filtration loss and preventing borehole collapse during geothermal drilling. The proposed drilling fluid system exhibited a simple system structure, excellent rheological performance under high temperatures, reduced filtration loss, and inhibitory and anti-pollution characteristics. The microstructure of rectorite was analyzed using x-ray diffraction, infrared spectrometry, and environmental scanning electron microscopy. It was verified that the microstructural mechanisms improved the temperature resistance of the drilling fluid system, thus helping to solve the critical problems in high-temperature geothermal well drilling and other production processes.

Introduction

Researchers worldwide are currently engaged in the active development of geothermal resource applications (Esen et al., 2013). For example, a vertical ground-source heat pump with a U-tube borehole heat exchanger was used to redistribute the temperature in a road surface and bridge-deck heating system in order to melt ice and snow during the winter (Balbay and Esen, 2010, Balbay and Esen, 2013; Esen and Yuksel, 2013). Similarly, a compact ground heat exchanger was developed for a solar-assisted ground source heat pump system (Esen et al., 2017). However, to more efficiently exploit geothermal resources, geothermal energy utilization and related technologies need to be further developed (Kazemi et al., 2019). China is rich in geothermal resources, some of which are buried in deep and ultra-deep layers under high temperatures. Therefore, the exploration and development of geothermal technologies have gradually shifted to environments with higher temperatures in deeper formations, which has led to the emergence of several exceedingly high-temperature deep wells (Song et al., 2017; Yang et al., 2019). Temperature and pressure during the drilling of deeper wells increase owing to the variation in the geothermal gradient and the influence of pressure gradients; thus, the drilling fluid quickly loses its original performance (Wang et al., 2019).

The conventional method to improve the performance of drilling fluids is to add an external treatment agent to increase their high-temperature resistance. Moreover, the treatment agent is an indispensable component of a drilling fluid system, and its temperature resistance is directly related to the high-temperature stability of such systems (Li et al., 2020; Geng et al., 2019). Thus, several countries have experienced a long-term and rapid development in the field of high-temperature drilling fluid treatment agents. In the 1960s, salt-tolerant, calcium-tolerant, high temperature-resistant (170°C) iron–chromium salts were successfully developed and used as viscosity reducers (Saric-Coric et al., 2003). In the 1970s, RESINEX was successfully developed, which withstands a temperature of 230°C and can be regarded as a sulfonated phenolic resin and lignite compound (Zhang et al., 2010). Currently, RESINEX is widely used in China. In addition, a variety of new treatment agents and drilling fluid systems have been studied globally in the 21st century to solve rheological alterations at high temperatures (Elkatatny, 2019; Huang et al., 2019; Chu et al., 2020).

To date, several scholars have focused on the research and development of high-temperature treatment agents. However, these studies only expanded the types of treatment agents and focused on improving the performance of the treatment agents. Moreover, these studies have not conducted in-depth research on slurry-making clay minerals. Most scholars across the world have studied and used bentonite and sepiolite, which exhibit beneficial effects and are inexpensive. However, the slurry-making performance of these minerals is poor in high-temperature environments, which makes it challenging to meet the needs of geothermal drilling (Dong et al., 2019; Hou et al., 2014; Al-Malki et al., 2016).

Previous studies have reported that clay minerals undergo dispersion, coalescence, and passivation under high-temperature conditions. Specifically, the high-temperature coalescence of clay particles plays a leading role at temperatures higher than 180 °C. In addition, high-temperature dehydration decreases the clay particle coalescence stability and results in varying degrees of coalescence, which increases the size of the clay particles in the drilling fluid (Luo et al., 2017). Moreover, a reduction in the specific surface area has a significant impact on the rheological characteristics of the drilling fluid. Thus, to develop a high-temperature resistant drilling fluid system that can meet the requirements of high-temperature geothermal drilling and exploration applications, the problems of high-temperature (>180°C) dispersion, coalescence, and passivation of slurry-making clay must first be addressed. To this end, this study investigated high temperature-resistant clay minerals, analyzed the slurry ability of these clay minerals, and explored their slurry-making and rheological properties under various high-temperature environments (180, 200, and 220 °C). The rectorite clay mineral was selected as the slurry-making additive for the drilling fluid system owing to its excellent high-temperature resistance. Subsequently, the rectorite was modified and analyzed for slurry yield, rheological behavior, and high-temperature stability. Finally, the deficiencies of the initial slurry were eliminated through modifications of the rectorite; suitable additives and formulations were determined to develop a high temperature-resistant drilling fluid system with excellent performance, simple system structure, and environmentally sustainable characteristics. Furthermore, comprehensive performance indicators, such as rheological behavior, filtration loss, pollution control, and inhibitory characteristics at high temperatures, were evaluated for this system.