Session 2C: Drilling

Drilling typically represents 30%-50% of the cost of a geothermal well, and decreasing drilling cost with polycrystalline diamond bits is a high priority for the geothermal industry. Efficient drilling is especially important in enhanced geothermal systems where costly directional drilling tools are required. Therefore, it is important to characterize the mechanical behavior of rock in high-pressure, high-temperature environments and understand how it interacts with diamond drilling tools.

In this paper, we utilize laboratory triaxial compression tests up to 200° C to characterize rock from the Utah FORGE project. Two rocks were studied: A monzonite from well 16A(78)-32 at a depth of approximately 5900 ft MD, and a gneiss from well 78B-32 at a depth of approximately 8500 ft MD. Triaxial compression tests were performed on these samples at several confining pressures up to 6,000 psi, and at temperatures up to 200° C.

After laboratory testing, a discrete element model was developed using the PFC3D software package. The model was tuned to match rock properties through a series of triaxial compressive simulations. Then, single cutter scraping experiments were modeled. Cutting forces from the model were compared to cutting forces from single cutter rock scraping experiments.

This paper demonstrates the effectiveness of thermally activated materials (TAMs) in managing severe to total fluid losses during geothermal (GT) well construction. The objective is to address lost circulation challenges in scenarios where conventional methods, such as the use of lost circulation pills and cement squeezes, fail to provide a satisfactory solution.

During GT drilling, severe lost circulation events frequently occur when the drilling fluid infiltrates fractured or vugular formations. The lost fluid experiences a temperature increase in the fractures and vugs due to the exposure to higher in-situ formation temperature with invasion depth. This temperature rise can be strategically utilized to seal the fractures or vugs. If TAMs are inherently present in the lost fluid, they can alter the viscosity, increasing it and making the fluid harder to displace, thereby preventing further fluid losses. This study examines the inclusion of TAMs in fluid formulations for use in targeted cement squeeze jobs.

Several types of TAMs have been identified and evaluated in different fluid formulations. To validate the effectiveness of TAMs, fracture plugging tests were conducted. Fluid formulations with TAMs were circulated through the sample, and their plugging behavior was observed and quantified. The resulting samples were then visualized using CT scans. These tests demonstrated that TAMs effectively sealed the fractures, providing robust evidence of their ability to mitigate fluid losses and the efficacy of the thermal activation mechanism. TAMs are novel materials that can be engineered to mitigate severe losses in GT wells, where lost circulation is one of the crucial challenges.

Lost circulation is one of the major problems encountered in drilling geothermal and HTHP (high temperature and high pressure) formations. A part of these problems can be attributed to the high temperature and presence of hard and fractured rocks. Consequences and treatment of these lost circulation events can be extremely costly. Underbalanced drilling and LCMs (lost circulation materials) are the two most common methods to address the lost circulation problem. However, some LCMs have proved to be unsuccessful due to the unpredictable nature of fractures, complicated dynamics of LCM, as well as the change of mechanical properties of LCMs under high-temperature conditions. Long-distance circulations in high-temperature environments usually lead to the decline of size, strength, and friction coefficient of LCMs in parallel to the reduction of drilling fluid viscosity, and eventually failure in sealing fractures. In this paper, we developed a coupled CFD-DEM by combining computational fluid dynamics with discrete element methods to simulate the fracture sealing process by LCMs. The viscosity of drilling fluid, particle size, Young’s Modulus, Poisson's ratio, and friction coefficient of LCMs were determined to represent possible variations under geothermal conditions. A series of sensitivity studies were conducted to further understand the fracture sealing efficiency of LCMs at elevated temperatures. The results show that a reduction in particle size, friction coefficient, and Young’s modulus lowers bridging’s probability, and slows down bridging initiation, but deepens the sealing depth, and tighter but the result is an unstable sealing zone. We noticed that the bridging mechanism changes from single-particle bridging to dual-particle bridging as particle size reduces. Also, the reduction of drilling fluid viscosity makes the sealing zone form faster and shallower.

Sedimentary rock is a common lithology we can encounter in geothermal reservoir drilling activity in Sumatra, Indonesia. This formation often challenges the drilling operation due to its mechanical behaviour, especially in the Ulubelu field, which often causes the wellbore stability issue. Based on the logs data in the UBL-M well, we discuss the characteristics of this sedimentary formation encountered during drilling. The sedimentary formation is a part of the extensive Tertiary Hulusimpang formation as a compacted siltstone unit deposited interlayered with volcanic products. Due to the overburden and the lithostatic pressure gradient, the sedimentary becomes overpressure, as the permeability and porosity are extremely low, where the pore pressure significantly exceeds the hydrostatic pressure. This conclusion is revealed through the compaction analysis derived from the available drilling parameter, geophysics log and mechanical failure evidence from the image log data. The normal compaction trendline (NCT) analysis from the sonic log indicates an abnormal pressure profile along the sedimentary interval up to 7000 psi. The image log shows a prominent borehole breakout feature which indicates that the formation was drilled in underbalanced conditions. The loading curve analysis shows that the overpressures were generated by fluid expansion or transfer processes, controlled by fractures and high-temperature geothermal fluid in the reservoir. Using this information, we managed to safely penetrate the thick sedimentary formation interval by increasing the bottom hole pressure to balance the overpressure in this formation. This study is critical to addressing the optimum drilling parameter once the well has to deal with the compacted sedimentary formation in geothermal reservoirs, especially in production or loss circulation zones, where the drilling parameter becomes more constrained to adjust. Moreover, the sedimentary unit can also contribute to the feedzone and become advantageous to drill.

Geothermal drilling operations face inherent challenges, with stuck-pipe incidents posing significant risks to both safety and project success. In this paper we introduce a groundbreaking stuck-pipe risk advisor (SPRA) that integrates causal artificial intelligence (AI) and semantic web technologies to provide explainable real-time decision support and enhance drilling performance.

Our causal-AI model uses advanced machine-learning algorithms to analyze historical drilling data, identifying complex relationships and potential risk factors leading to stuck-pipe incidents. By leveraging causal relationships, the SPRA can proactively assess the likelihood of a stuck-pipe occurrence based on current drilling conditions.

To enhance transparency and trust in the decision-making process, our system incorporates semantic web technologies to generate explainable outputs. The SPRA generates detailed explanations of its predictions, using semantic annotations to link specific risk factors and causal relationships. This not only aids drilling operators in understanding the basis of the system's reported risk levels, but also facilitates continuous learning and improvement of drilling practices.

Furthermore, the SPRA incorporates a flow-diagram-based interface for drilling operators, providing real-time visualizations of risk factors, potential mitigations, and interactive explanations. By empowering operators with actionable insights, the SPRA aims to support the wellsite team in reducing the frequency of stuck-pipe incidents, enhancing drilling efficiency, and ultimately contributing to the overall safety and success of geothermal drilling projects.

We present the development, validation, and practical application of the SPRA, demonstrating its potential to revolutionize geothermal drilling safety through causal-AI and semantic web technologies.

Geothermal power in Indonesia is an increasingly significant source of renewable energy as a result of its volcanic geology. A case study from Indonesia is presented on reducing drilling days on a geothermal well in North Sumtra using innovative hybrid polycrystalline diamond compact (PDC) and roller cone drill bit technology. Results of distance drilled and drilling penetration rates are compared to offset wells.

The primary obstacle in the Sorik Marapi field is formation drillability. The lithology is a mixture of multiple hard volcanic formations consisting of breccia, tuff and andesite. The coarse rock fragments cemented together can damage and wear down the cutters and inserts on drill bits. And the interbedded nature of the formations create non-homogenous drilling which can also damage drill bits. The operator began working closely with a service provider to re-imagine how a well can be put on production faster in order to return value for stakeholder quicker. The continuous improvement process led to a partnership in innovation for the 17.5-in and 12.25-in. sections

Consistent introduction of new drill bit technology has allowed this operator to complete wells faster by reducing drilling days. The solution was to deploy the hybrid drill bit which is more efficient, capable of producing higher penetration rates and smoother drilling. By combining the efficient shearing of PDCs and the crushing strength of roller cones the hybrid bit has the potential to maintain higher overall rate of penetration (ROP) for more footage than a roller cone or PDC bit could individually. The roller cone inserts pre-fracture the hard igneous lithologies, making it easier for the PDC cutters to plow away the formation. And the combined crushing and shearing cutting structure provides improved overall stability when drilling through interbedded formations. In addition the smooth drilling nature of the hybrid bit perfectly maintained the tangent well trajectory.

This paper describes the collaboration between the operator and bit service provider that led to the success in developing the optimal technology to deliver superior performance in igneous formations. The collaboration in technology development contributed to drilling wells more efficiently and more accurately and enabling the operator to reach TD reliably in the most challenging and complex geothermal drilling environments.