Transition from oil & gas drilling fluids to geothermal drilling fluids

Abstract

The harsh downhole conditions encountered in geothermal wells, specifically the high temperatures (HT) together with the toughness of the rock found in many geothermal formations, makes the drilling operation challenging. Drilling in such environments requires specialised drilling fluid formulations that have high thermal stability, good rheological properties, excellent lubricity and low formation damage. Given the wealth of experience in drilling wells in the oil industry, it is tempting to assume that the design of geothermal drilling fluids would be straightforward. However, is this the case? In this literature review, we have attempted to answer the question: “to what degree can developments in oil and gas drilling fluids be transferred to drilling fluids for geothermal wells?” To keep the scope of the review manageable, we have focused on two key aspects of drilling fluid design: rate of penetration (ROP) and HT fluid stability (and maintenance of the desired rheological properties of the fluid at high temperatures). The review has allowed the identification of gaps in both fundamental understanding and in existing technology. Rate of penetration is improved using low viscosity and low-density fluids, and we recommend that foams and aphron systems should be investigated to achieve this (depending on the application pressure). It should be noted, however, that such systems to date have only been studied at relatively low temperatures and the challenge of increasing the thermal stability of the formulation components needs to be addressed. Highly thermally stable polymer systems exist but these are both expensive and not widely available. A systematic study of the impact of copolymer molecular architecture on hydrolytic thermal stability is recommended. A promising solution to both maintaining good rheological properties at high temperature and providing fluid loss control is the use of particulates, especially those in the nano-size range. Additionally, nanocomposite systems show promise in this area and should be investigated. Particle stabilized foams and aphrons are a particularly interesting solution and we recommend that these are studied. It is also recommended to investigate the effect of drilling fluid on long term geothermal well performance.