Session 1C: Drilling

So-called “unconventional” geothermal wells, also known as “EGS” (Enhanced Geothermal System) and “AGS” (Advanced Geothermal System) wells, are increasingly being drilled in large numbers. For understandable reasons, initial projects have been drilled at temperatures similar to those found in deeper oil and gas wells – in other words, initial EGS and AGS projects have been drilled at temperatures below 175C in order to take advantage of existing oil and gas drilling technologies developed to operate under those conditions.

However, because unconventional geothermal wells are drilled for heat, not hydrocarbons, an arbitrary temperature limit based on a specification appropriate for a different industry makes no sense. This is particularly true because, as will be demonstrated in the paper, there is a strong correlation between temperature of the reservoir and economic performance of an asset.

Further, although we cannot yet be certain which of the many proposed designs for EGS and/or AGS will ultimately prevail, almost without exception the pilots being drilled and the projects proposed rely on accurately drilled horizontal wells, often pairs or multiples of wells drilled closely together, sometimes at a given offset and sometimes even intersecting. This requires a high degree of accuracy and control when drilling and therefore strongly suggests that the technologies used for drilling horizontal and extended reach wells has a place in geothermal drilling – if only it could be made to work reliably at higher temperatures. Ideally the temperatures limits of those technologies could be increased to beyond 200C and ultimately as high as 300C.

Unfortunately, although the oil and gas industry has made great strides over the years in getting the operating temperature of downhole electronics from 120C to 175C, and, even on rare occasions, to 200C, the approaches taken in oil and gas tools do not lend themselves to getting to the even higher temperatures that unconventional geothermal will need.

This paper will describe a unique, blank sheet of paper, approach to the design and packaging of electronics and sensors, to create a brand-new tool that combines the functions of Measurement While Drilling (MWD) and Rotary Steerable Systems (RSS) and which will be able to break that 200C barrier.

It will describe the design decisions made and modelling done to ensure enhanced thermal performance without sacrificing any of the resistance to shock and vibration that oil and gas tools have acquired over the years – shock and vibration being significant in the hard, high friction basement formations that EGS and AGS will most likely inhabit.

It will show the results of thermal testing and lessons learned along the way and demonstrate the viability of a fully working integrated MWD/RSS system capable of operating in the previously “forbidden” region above 200C.

Drill pipe insulation is an effective method for downhole temperature management in high-temperature wells. Of late, the most popular pipe insulation method has been the application of a coating of low thermal conductivity to the inner wall of the drillpipe. When the coating is damaged, the insulation is also partially breached, leading to temperature management disruptions and higher downhole temperature.

Detection and prediction of coating damage is thus pivotal to successful downhole temperature management. There is currently no approach for the evaluation and detection of insulation coating damage. In this paper, we first propose a method to estimate drillpipe insulation integrity and then describe an automatic controller to account for the negative effects.

The insulation coating integrity evaluation workflow involves first using an advanced thermo-hydraulics model of the well to perform simulations of the planned drilling operations. From the simulation results, the relation between the BHCT (and the surface temperature) and the drillpipe’s average apparent thermal conductivity is established. We then use this relationship and compare it to the relationship between BHCT and surface temperature during actual drilling operations to determine damage (this is reflected as a change in the apparent thermal conductivity).

Next, to control the well’s bottomhole temperature, a feedforward proportional, integral, derivative (PID) controller is used. In this study, the surface inlet temperature and the pump flow rate are used as the control inputs. Data from high-temperature land wells in the US using insulated drillpipes were used for the study. The effect of peeling and thining of the insulation (the two prominent types of insulation damage) and its negative temperature effects were investigated.

This is the first study that demonstrates an approach to estimate the integrity of the drillpipe coating during actual drilling operation. In additon a controller is suggested to automatically control /recommend flow rate and inlet temperature change.

Interfacial Friction Angle (IFA) is the frictional resistance experienced at the interface between the PDC cutters and the formation. Mechanical specific energy (MSE) is the energy required to remove a unit volume of rock during drilling. IFA is affected by normal and drag forces, including depth of cut (DOC) and rate of penetration (ROP), which also impacts MSE. Accurate interpretation of IFA and MSE in hard rock is crucial and can be used to optimize hard rock and geothermal drilling. This study used an in-house rig to test two 3¾” PDC bits (4 and 5-blade designs) to characterize IFA and MSE under hydrostatic pressures of 0 psi, 1000 psi, and 2000 psi. Tests were conducted in Sierra White Granite (SWG) using water as the drilling fluid at constant rotational speeds of 80 and 150 RPM, and the weight on bit (WOB) was incrementally increased while ROP and bit torque (T) were recorded. Results showed that the impact of hydrostatic pressure on MSE becomes distinctly evident at WOB values greater than 4000 lbs for both bits. Beyond this threshold, the effect on MSE is decreased for the tests under confinement, suggesting a change in rock failure criteria from elastic to more plastic deformation. The MSE for the unconfined tests (0 psi) decreases substantially after 4000 lbs, whereas this decrease is not observed in the tests under confinement (1000 and 2000 psi). The unconfined tests showed a more apparent decrease in MSE in Phase 2 (the efficient drilling phase) with increasing WOB, where the most effective drilling is seen when MSE values approach the rock’s unconfined compressive strength (UCS). The values for MSE at a given confinement show the same trend for MSE but with a different low limit, indicating a distinctly different failure mode criterion. Results also showed that IFA becomes nearly constant in the efficient cutting phase of the drilling process. It was observed that increased confinement increases IFA during Phase 2 of the drilling process. During the inefficient phase of drilling (Phase 1), a higher IFA was observed due to insufficient WOB.

Exploration drilling is key to answering critical resource questions. Projects can appear uneconomic before a resource is understood if sunk exploration costs are too high, thus low-cost exploration is key to growth. The importance of reducing early exposure is becoming more relevant as the data set from extensive drilling campaigns in the 1970’s and 1980’s has been consumed. Available levers to reduce exploration cost typically require a compromise in scope.

The utilization of small diameter drilling, primarily using mining technology has been attempted in the past by multiple operators where this technique has traditionally failed to significantly reduce cost while delivering adequate scope. A recent drilling campaign has demonstrated an 85% campaign cost reduction utilizing wireline coring on a mining rig, resulting in significant reduction in total exploration spend. This campaign has reached a technical limit with high reliability and low non-productive time (NPT) forcing the team to investigate more novel ways of working to drive further gains. Sufficient subsurface information has been obtained to de-risk prospects while reducing environmental impact. Key methods to achieve these savings are discussed and are comprised of the following elements. Firstly, a robust performance improvement process that started with focusing on building reliable repeatable operations, followed by a plan/monitor/review/feedback loop to continuously learn. Secondly, a project-based way of working was utilized to empower team members, push decision making to the appropriate level and set the project up for scaling by ensuring a common group was working each prospect. Thirdly, project scope was reduced to the lowest practical level. Lastly, the overall operating model leveraged expert knowledge with a focus on learning together and building vendor relationships. This paper summarizes the success of the campaign with a learning curve showing dramatic well on well reductions that are comparable to learning curves published from the Oil and Gas (O&G) industry. Additionally, the impact of moving from field to field is highlighted showing that most of the learnings were transferable.

The geothermal industry is witnessing a growing number of initiatives that necessitate the boring of extended horizontal passages through rigid rock strata. Pioneering efforts in this domain include the construction of inclined or horizontal wells, which serve as subterranean heat exchangers or part of horizontal Enhanced Geothermal Systems (EGS). However, these operations often encounter difficulties such as inadequate weight transfer onto the drill bit, complications from drill string buckling, and the occurrence of stick-slip motion. To address these issues, a novel downhole anchoring technology has been developed to boost the efficiency of drilling operations, particularly in deeper wells with more challenging rock formations and extensive lateral sections. This technology employs a mechanism that latches onto the wellbore walls, effectively mitigating shock and vibration. This leads to a reduction in damage to both the wellbore and downhole tools. The anchoring system incorporates intelligent controls, actuation mechanisms, and sensor-based feedback. These features allow for autonomous regulation of the drilling process directly at the drill bit level instead of being managed from the surface. Additionally, the technology can exert both pushing and pulling axial forces to apply and dynamically adjust the additional weight on the drill bit (WOB). This enhancement significantly improves the lifespan and efficiency of both the drill bit and the drill motor. The initial model of this anchoring tool underwent testing in the second quarter of 2023 within a shallow test well operated by a leading rig contractor in Houston, Texas. This paper presents a comprehensive overview of the development and subsequent testing phases.

This paper presents a comprehensive analysis of the application of Polycrystalline Diamond Compact (PDC) bits in geothermal drilling operations, aiming to enhance overall drilling efficiency and geothermal wellbore performance when drilling hard and abrasive granite. Additionally, it explores the unique challenges associated with geothermal drilling in granite and evaluates the suitability of PDC bits in addressing these challenges.
Key aspects covered in the paper include the engineering, design, and optimization of PDC drill bits tailored for hard and abrasive rock in geothermal environments, with a focus on factors such as cutter geometry, cutting structure design and bit body material composition.
This paper provides insights into the implementation of PDC bits in geothermal projects that drill hard and abrasive granite lateral intervals. Learnings from traditional oil and gas drilling methods shed light on the potential advantages, such as increased penetration rates, increased run lengths and reduced drilling costs.
The findings of this study contribute valuable knowledge to the geothermal drilling community, offering a roadmap for optimizing drilling practices through the utilization of advanced PDC bit technology. This paper concludes with recommendations for future research directions and the continued advancement of geothermal drilling techniques to support the sustainable development of clean, renewable geothermal energy resources.
Unlocking geothermal energy from some of the most challenging geologic production zones below the Earth’s surface utilizing the latest technologies and drilling techniques has proven to demonstrate the economic viability of this type of clean energy extraction. These innovations can be used as building blocks for future innovations and technologies to unlock the Earth’s geothermal potential even more efficiently.