Evaluation of Microhole Drilling Technology for Geothermal Exploration, Assessment, And Monitoring

Abstract

One of the greatest barriers to geothermal energy expansion is the high cost of drilling during exploration, assessment, and monitoring. Microhole drilling technology?small- diameter 2?4 in. (~5.1?10.2 cm) boreholes?is one potential low-cost alternative for monitoring and evaluating bores. However, delivering high weight-on-bit (WOB), high torque rotational horsepower to a conventional drill bit does not scale down to the hole sizes needed to realize the cost savings. Coiled tube drilling technology is one solution, but these systems are limited by the torque resistance of the coil system, helical buckling in compression, and most of all, WOB management. The evaluation presented herein will: (i) evaluate the technical and economic feasibility of low WOB technologies (specifically a percussive hammer and a laser-mechanical system), (ii) develop downhole rotational solutions for low WOB drilling, (iii) provide specifications for a low WOB microhole drilling system, (iv) implement WOB control for low WOB drilling, and (v) evaluate and test low WOB drilling technologies. iv This page left blank. v EXECUTIVE SUMMARY Our evaluation was able to validate and establish a proof-of-concept low weight-on-bit (WOB) drilling technology for microhole drilling, taking us one step closer to a more affordable approach to geothermal energy development. One of the greatest barriers to widespread geothermal energy development is the high cost of drilling during exploration, assessment, and monitoring. Albright and Dreesen (2003) suggest that microhole drilling could reduce costs by up to 70% over conventional drilling. Furthermore, the literature revealed that microhole drilling can reduce costs by 40?60% for exploration wells and 25?40% for production and injection wells (Zhu et al., 1995) compared to conventional (large diameter) drilling techniques. However, the technology faces many technical challenges that our research aims to address. Previous microhole studies (Jeanloz and Stone, 2013; NETL, 2006) have focused on high WOB drilling technologies, which deliver high torque rotational horsepower to a conventional drill bit but unfortunately, do not easily scale down to smaller diameter boreholes. Problems with drill string loading ? such as buckling, friction, and twist ? become more severe as the borehole diameter decreases. Coiled tube drilling technology (CTD) is one possible solution, but these systems are limited by the torque resistance of the coil system, helical buckling in compression, and WOB management. The research presented herein represents a major departure from previous microhole drilling studies. Rather than miniaturizing high WOB conventional rotating drilling methods, our study focuses on low WOB drilling technologies. This approach helps mitigate the issues surrounding drill string torsion and buckling and provides more flexibility in the type of components that can be used on the ground surface and in the bottom hole assembly (BHA). The primary objective of our research is to validate and establish a proof-of-concept low WOB drilling technology for microhole drilling for geothermal energy development. To meet our main objective, we undertook the following tasks: ? Researched historical microhole drilling costs to establish a baseline for our study. vi ? Developed and tested low WOB downhole rotational mechanisms compatible with CTD technology. ? Developed specifications for a field-deployable low WOB microhole drilling system. ? Developed low WOB control systems compatible with CTD technology. ? Evaluated and tested low WOB drilling technologies, including percussive hammers and high-power laser-mechanical drilling systems. Our study included a market survey to evaluate the true costs of microhole drilling using current technologies (i.e., high WOB) to establish a baseline for our project as we develop low WOB techniques. Microhole drilling cost savings are realized through reduced material costs (e.g., fluid volume, piping, etc.); smaller drilling/workover rigs; and lower fuel consumption. We found limited data on actual microhole drilling costs and were unable to specify a typical cost per foot of borehole drilled, but we were able to normalize drilling costs with respect to hole size to define a relative cost factor associated with borehole size, borehole depth, and casing size. The goal of our program is to evaluate comparable drilling scenarios using the relative cost factor approach to be competitive with previous microhole coiled tubing drilling operations. Our partners at Geothermal Resources Group conducted computer simulations to determine the limiting flow rate as a function of borehole depth, borehole diameter, enthalpy, productivity index (PI), and flow rate. The simulations demonstrated that using conventional techniques, such as production and injection tests, to characterize reservoir performance has limitations for scaling microhole test results to production- size boreholes. In smalle