Drilling mud losses are the most unpredictable and challenging problem in geothermal drilling operations. High temperatures and a corrosive environment make the design of a geothermal drilling fluid a very complex task. The presence of high temperature exacerbates the problems due to the thermal degradation of drilling fluid additives. Furthermore, thermal degradation implicates mud rheology affecting the Equivalent Circulating Density (ECD). This condition could potentially lead to wellbore tensile failure. Lost Circulation Materials (LCM) are used for wellbore strengthening in these environments. However, the study of LCM performance in High Pressure-High Temperature (HTHP) conditions is limited.

Thus, an extensive laboratory study was performed on lost circulation materials in Water-Based Mud (WBM) applications. Laboratory tests were performed using an HTHP rheometer to measure drilling fluids' properties up to 260°C. 11 different LCMs were tested in a controlled environment for understanding properties that made those components prone to fail at high temperatures. The results show that coarsely granular, flaky, and fibrous materials tend to degrade at high temperatures. This condition is manifested in a viscosity increase up to 4 times the baseline when tested at 149°C. This condition leads to frictional losses increase leading to an undesirable ECD increment. An HPHT Permeability Plugging Tester (PPT) was used to measure the wellbore strengthening performance of LCMs. The results show that fine granular materials performed better in the high-temperature tests presenting the highest fracture sealing pressures. The results also show that particle size has repercussions in thermal stability and LCM effectiveness to increase wellbore tensile strength.

1. Introduction

The increase in energy demand worldwide has led to the rise in the need for new energy generation sources such as renewable energy. Geothermal power generation is one of the most important renewable energy resources. A geothermal power generation plant benefits from the thermal energy stored in the earth to generate clean power. Geothermal energy is renewable energy with the highest capacity factor. According to information from the US Energy Information Administration (EIA 2020), the geothermal energy capacity factor averages 72% in the last ten years (Fig. 1). It is not uncommon for geothermal plants to reach values well over 90% (Sanyal and Enedy 2011, Vivas et al. 2020). Because of its independence from seasonal factors, geothermal energy is one of the more efficient baseload power sources that can operate continuously to meet the minimum power demand 24/7.

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