Geothermal Well Drilling Research Articles

Drilling mud contamination effect on wellbore cement strength: An experimental investigation

Cementing is an essential downhole operation during the drilling of oil, gas, and geothermal wells, with the primary objective of providing structural support and a seal in the wellbore. It is important to attain adequate cement strength to preserve well integrity, inhibit fluid movement, enhance zonal isolation, and hold the wellbore casing in place. However, a significant challenge arises from the potential contamination of the cement by drilling mud (DM) during the placement process. This study aims to assess the impact of both water-based drilling mud (WBDM) and oil-based drilling mud (OBDM) contamination on the strength of API Class G wellbore cement by comprehensively investigating the underlying causes and the mechanism of strength degradation. Following a systematic methodological approach and the guidelines provided by API and ASTM, cement slurries were prepared and contaminated with DM at 10 %, 20 %, and 30 % concentrations by volume. Subsequently, both neat and contaminated samples were cured for 1, 3, 7, and 14 days, after which a series of uniaxial compressive strength (UCS) tests were conducted, and the results were further reinforced by microstructural and petrophysical property analysis. The results demonstrate that both WBDM and OBDM contamination significantly reduce cement strength, irrespective of curing time. Though the effect of OBDM contamination appears to be more pronounced, the effect of WBDM contamination is also remarkable. After final curing of 14 days, WBDM-contaminated cement samples exhibited an average strength reduction of 45.06 %, while OBDM contamination led to a 66.32 % reduction in strength compared to the neat cement with 30 % contamination. Additionally, for the same curing time and contamination percentage, porosity was found to be increased by 71.18 % and 91.33 %, respectively. The mechanism of neat and contaminated cement hydration is explained with the support of SEM-EDS tests, which confirmed the presence of contamination and revealed how two different drilling muds generate dissimilar pore structures within the cement sheath, affecting the hydration and ultimately contributing to the observed reduction in strength. The findings quantify the negative impact of WBDM and OBDM contamination on wellbore cement, emphasizing the importance of mitigating mud contamination during cementing operations.

Comparative life cycle assessment of integrated renewable energy-based power systems

Integrated renewable-based power cycles should be employed to produce more sustainable electricity. This is a comparative life cycle assessment (LCA) of three combined power plants, encompassing: case 1 involving combined geothermal and wind, case 2 featuring combined geothermal and solar, and case 3 integrating wind and solar systems. The base case perovskite solar cell (PSC) modelling assumes a 3-year lifespan and a power conversion efficiency of 17 %. However, diverse scenarios are evaluated through a sensitivity assessment involving enhancements in lifetime and efficiency. The base case evaluation emphasizes that the phases with the most significant negative environmental effects which includes the drilling of geothermal wells, construction of wind plants, and manufacturing and installation of PSCs. The midpoint findings indicate that boosting the power conversion efficiency of PSC from 17 % to 35 % yields a notable decrease in environmental impact. Moreover, extending the lifetime from 3 to 15 years led to reduction in CO2 emissions from 0.0373 and 0.0185 kg CO2 eq/kWh to 0.026 and 0.0079 kg CO2 eq/kWh in cases 2 and 3, respectively. Assessing worst and best-case scenarios highlights significant declines in certain impact categories. In case 3, terrestrial ecotoxicity (TE), photochemical oxidant formation (POF), human toxicity (HT), marine ecotoxicity (ME), and marine eutrophication (MU) saw reductions exceeding 88 % compared to worst-case results. The environmental effects observed in cases 2 and 3 stem from toxicity and metal depletion, mainly linked to the PSC. Endpoint results revealed that when considering a PSC lifespan of 10 years or more, the detrimental ecosystem impacts of cases 2 and 3 become less severe than those of case 1. Uncertainty assessment has been done for different cases and impact categories. The study's results are also novel in which it evaluated the innovative PSC technology when integrated with other renewable resources, contrasting it with other integrated plants.

Investigation of Taylor bubble dynamics in annular conduits with counter-current flow

This study numerically investigates counter-current slug flow by considering the motion of a Taylor bubble in annular conduits with downward-flowing liquids using the Volume-of-Fluid method implemented in the commercial computational fluid dynamics software ANSYS Fluent (Release 19.2). The translational velocity of a counter-current ascending or co-current descending Taylor bubble in vertical concentric annuli and the corresponding distribution parameter (C0) are analyzed. The latter is correlated in terms of Eötvös number (Eo) and inverse viscosity number (Nf) within the range of Eo between 40 and 400 and Nf between 40 and 320. The proposed correlation provides an accurate fit to the numerical data with an average error of 2.64%, and is successfully compared with published numerical findings. In general, the smooth and stable shape of the bubble is disrupted as the counter-current flow velocity (Frl) increases above a critical value, leading to the formation of surface waves and the displacement of the bubble tip away from the annular gap center and towards the outer pipe. C0 increases with Eo and Nf, plateauing at high values of Eo. The effects of annulus inclination (θ) and eccentricity (ε) on bubble rise velocity are examined within the common range of θ and ε encountered during the drilling of oil, gas, or geothermal wells, i.e. 0° ≤ θ ≤ 60° and 0 ≤ ε ≤ 0.7, and their impact on the C0. The increasing Frl and θ lead to a streamlined bubble with pointed nose and thus a reduction in the wrap angle (θwrap), ultimately leading to reduced drag compared to the vertical annulus case and a decrease in C0. As the ε increases, which is accompanied by an increase in the degree of bubble eccentricity, the corresponding C0 decreases. For a constant Eo = 100 and Nf = 160 with inclination angle of θ = 40°and eccentricity of ε = 0.5, the C0 < 1 is observed.

Thermal infrared remote sensing and soil gas radon for detecting blind geothermal area

Geothermal resources in the Wugongshan area are abundant and most closely associated with faults. Due to the dense vegetation covering the research area, locating the geothermal resources using conventional ground technologies is costly and challenging. This paper proposes and implements a novel method in the area under examination. More specifically, remote sensing information (Landsat 8) and known geological information are used to delineate the geothermal target area. On January 19, 2021, the land surface temperature in the study area ranged between 7.7 and 21.2 °C. Combining the geothermal geological features of the regional area, two areas, namely Tangjiashan and Sanjiang, are delineated as geothermal target areas. The probable location of faults is determined by measuring the soil gas radon concentration. The relative coefficient KQ of radon activity attained in profiles varies between 1.7 and 13.2. The average values of KQ between different areas are ranked as Wenjia > Tangjiashan > Hongjiang > Sanjiang. Generally, areas with higher KQ of radon activity have higher land surface temperature (LST). Thus, the KQ and LST can be used to evaluate the shallow geothermal potential. Finally, by integrating the interpretation of remote sensing and soil radon measurements, the fault location is identified, and the result is successfully used to guide the drilling of geothermal wells in the Tangjiashan area.

ВЛИЯНИЕ ТЕМПЕРАТУРНОГО ФАКТОРА НА ВЕЛИЧИНУ ГИДРОСТАТИЧЕСКОГО ДАВЛЕНИЯ ПРИ БУРЕНИИ ГЕОТЕРМАЛЬНЫХ СКВАЖИН

One of the most important tasks of modern natural science is energy saving and replacement of non-renewable energy sources with wind, solar, geothermal, wave and other energies. The main advantage of geothermal water is that it can be supplied in places where it is continuously regenerated and directly consumes heat, energy, more importantly, useful chemistry, has healing properties and abilities. The article discusses the influence of the temperature factor on the value of hydrostatic pressure while drilling of geothermal wells using chemical reagents based on water-soluble polymers in the process of drilling deep wells, which ensure the stability of drilling fluids at high temperatures.As mentioned earlier, the regulation of hydrostatic pressure by increasing the drilling mud densityat high temperatures is inappropriate. Therefore, it is necessary to take measures to improve the sedimentation stability of drilling muds.

The optimization of a potential geothermal reservoir using abandoned wells: a case study for the forest reserve field in Trinidad

The worldwide dependence on non-renewable energy sources continues as existing energy systems have been built on these supplies. There is an established link between these conventional energy resources, greenhouse gas (GHG) emissions and climate change and its associated negative effects. As a signatory to the Paris Agreement, Trinidad and Tobago (TT) has been exploring strategies to reduce GHG emissions and the use of geothermal energy is one potential option. Through enhanced geothermal systems, TT, even without extensive volcanic heat reservoirs, can still develop this renewable source by utilizing oil reservoirs. This study evaluates the possibility of designing and evaluating geothermal systems using wells from the Forest Reserve fields in South-western Trinidad as a case study. The Forest Reserve fields have a high number of abandoned oil and gas wells which can be re-used for tapping the required heat and reducing the requirement for drilling of new injection and production geothermal wells. Key information and data from these wells and reservoirs were used as input for CMG software to model, design and evaluate the feasibility of the geothermal reservoir and wells as being possible and viable for geothermal production. A base model was constructed in the CMG software which was subjected to three essential manual sensitivity analyses (well distance, stream temperature, and injection pressure) to obtain an optimized model which was then subjected to hydraulic fracturing. The optimal model of the retrofitted geothermal systems demonstrated to be the best case scenario due to the shallow nature of the reservoir in the area of interest. Six retrofitted geothermal systems (3 injector wells and 3 producer wells) showed that capable capacity of 3.3721 MWe for a 25-year period with an Internal Rate of Return of 190% and an Net Present Value of US$1,431,263,840.00 utilizing a Minimum Acceptable Rate of Return of 10%, Capital Expenditure of US$12MM, Operating Expenditure of US$2MM and a cost of geothermal electricity at US$0.05 per kWh. These results were then used in Crystal Ball to apply Monte Carlo simulations where it confirmed that the project is 100% economically feasible. The cumulative carbon dioxide reduction after the 25-year period was 50,062,500 tons of CO2.

Gel Pills for Downhole Pressure Control during Oil and Gas Well Drilling

During drilling of petroleum or geothermal wells, unforeseen circumstances occasionally happen that require suspension of the operation. When the drilling fluid is left in a static condition, solid material like barite may settle out of the fluid. Consequently, the induced hydrostatic pressure that the fluid exerts onto the formation will be reduced, possibly leading to collapse of the borehole or influx of liquid or gas. A possible mitigation action is placement of a gel pill. This gel pill should preferably be able to let settled barite rest on top of it and still transmit the hydrostatic pressure to the well bottom. A bentonite-based gel pill is developed, preventing flow of higher density drilling fluid placed above it to bypass the gel pill. Its rheological behavior was characterized prior to functional testing. The designed gel pill develops sufficient gel structure to accommodate the settled barite. The performance of the gel was tested at vertical and 40° inclination from vertical. Both conventional settling and the Boycott effect were observed. The gel pill provided its intended functionality while barite was settling out of the drilling fluid on top of this gel pill. The barite was then resting on top of the gel pill. It is demonstrated that a purely viscous pill should not be used for separating a high density fluid from a lighter fluid underneath. However, a bentonite or laponite gel pill can be placed into a well for temporary prevention of such intermixing.

Casing-to-casing resistance study performed at Reykjanes geothermal field in Iceland to estimate fracture connectivity

Successful drilling of geothermal wells into fractured hot rock through which geothermal fluid flows is critical to ensure efficient operation of geothermal power plants. This study focuses on investigating a casing-to-casing resistance method for estimating fracture connectivity between wells at Reykjanes geothermal reservoir in Iceland to better understand the fluid-flow in the reservoir. Conductive well casings were successfully used as electrodes to transfer the electric current deeper into the fractured reservoir in an electrical resistance survey. The low resistance measured between wells implies that the wells are connected to the main production zone at Reykjanes and that the current is travelling through fractures filled with conductive geothermal brine from one well casing to another rather than going through a less conductive rock. The measured resistance is compared to an analytical study showing the promising potential the casing-to-casing resistance method has for gaining valuable information about the fracture connectivity in geothermal reservoirs.

Developing a conceptual model and power capacity estimates for a low-temperature geothermal prospect with two chemically and thermally distinct reservoir compartments, Hawthorne, Nevada, USA

The Hawthorne area in the Basin and Range province in Nevada in the western USA has been the focus of geothermal investigations for over 40 years, with initial discovery of blind resources via anomalously-warm water wells. Subsequent studies and drilling of temperature gradient holes and geothermal wells identified three separate blind geothermal prospects in the Hawthorne part of the Walker Lake basin. In this study, we conducted a detailed review of all existing geoscience data acquired at the site to date to develop a quantitative estimate of geothermal resource potential for one of the Hawthorne geothermal prospects (prospect A — along the southwest side of the basin). This included review of substantial well data from water wells and geothermal exploration wells (downhole temperature logs, lithology, water chemistry, borehole televiewer, and alteration mineralogy), detailed geological and structural mapping information, geophysical data (gravity, magnetic, and seismic reflection), 2-meter temperature data, and an existing 3D geological model of the basin. We find that the thermal anomalies associated with prospect A reflect the influence of two geothermal fluids in close proximity that are chemically-distinct, with different temperatures and spatial extent (lateral and vertical). One fluid represents a deeper resource, hosted in altered, fractured Mesozoic granitic basement along a segment of the Wassuk Range-front fault system, and characterized by equilibrated, alkali-chloride fluids, with ∼4000 ppm total dissolved solids (TDS) and a maximum measured temperature of ∼115 °C at ∼1500 m depth. A second fluid is hosted in Neogene basinal sediments at <400 m depth, with maximum measured temperatures of ∼100 °C, TDS of ∼1000 ppm, and a sodium-sulfate fluid chemistry. The outflow of this shallow resource can be tracked down gradient into the basin using well temperature data, which map a vertically-constrained plume that cools with distance from the inferred upflow location. The data suggest that the deeper resource is conductively transferring heat to the shallow resource, and structural and/or stratigraphic compartmentalization is preventing direct interaction and fluid mixing. Through our development of a new conceptual model of prospect A (with P10, P50, and P90 scenarios), and application of the power density method, we estimate the deep and shallow systems may have resource potential of 7 MWe (P50) and 1.6 MWe (P50) respectively.

Geological structures controlling the placement and geometry of heat sources within the Menengai geothermal field, Kenya as evidenced by gravity study

Menengai volcano is one of the late Quaternary caldera volcanoes formed on a massive shield in the inner-trough of the Kenya rift valley, associated with a high thermal gradient resulting from shallow magmatic intrusion. While drilling of geothermal wells in Menengai area proved the existence of exploitable steam, the complexity of its geological setup has led to various technical setbacks where sited well targets turned out to be unproductive. These challenges were attributed partly to the lack of adequate knowledge to delineate the depth and lateral extent of heat sources. The present work attempts to understand the geological structures that appear control the placement and geometry of heat sources within the Menengai geothermal field. This study utilizes the sensitivity of gravity data to deep structures resolution to determine a three-dimensional (3-D) inversion model. A total of 1823 gravity points were used in generating the Bouguer anomaly, using a Bouguer density of 2.23 g/cm3. The model was constrained using estimated densities of drill hole cutting recovered from the geothermal wells. The resultant model was then interpreted together with surface manifestations, geology, and geological structures. Lineament structures were extracted from satellite images and used to enrich the geological structures. The model showed a close relationship between faults and inferred geometry of subsurface volcanic complexes displayed as discrete dike-like bodies in the near-surface environment. The structural system controlling the extent of the dense trachytic body of Olrongai is distinct, but for the caldera, some inference can be made from the results of this study. These faults are interpreted as the feeder dikes of the dense syenitic intrusives, that are hosted within the trachytic formation, and believed to be the heat source for the geothermal system. These structures act as conduits of magma to shallow levels, supplying the much-needed heat to the geothermal reservoir.

Influences on High-Voltage Electro Pulse Boring in Granite

As the exploration and drilling of oil, natural gas and geothermal wells are expanding continuously, research into high-efficiency rock drilling technology is imperative. High-voltage electro pulse boring (EPB) has the advantages of high rock breaking efficiency and good wall quality, and is a new and efficient potential method of rock breaking. The design of electrode drill bits and the selection of drilling process parameters are the main obstacles restricting the commercialization of EPB. Accordingly, it is necessary to determine the influences on high-voltage EPB. In this study, based on the equivalent circuit of high-voltage electro pulse breakdown, a mathematical model of high-voltage electro pulse discharge in rock is established. Meanwhile, a numerical simulation model of high-voltage EPB of hard granite is established based on a coaxial cylindrical electrode structure, which is often used for electrode drill bits. The simulation analysis software Comsol Multiphysics (Comsol Multiphysics®5.3a, COMSOL Co., Ltd., Stockholm, Sweden) is used to study the influences of granite composition, electrode spacing and electrode shape on the high-voltage EPB process. In addition, the influences of electrical parameters on high-voltage EPB are calculated according to a model of high-voltage electro pulse discharge in rock. Finally, it is demonstrated that high-voltage EPB is influenced by granite composition, electrical parameters, electrode spacing, and electrode shape, and the relationships between these factors are obtained. This study is of guiding significance for improving rock breaking efficiency, reducing energy loss, designing electrode drill bits and selecting drilling process parameters.

Borehole damaging under thermo-mechanical loading in the\xa0RN-15/IDDP-2 deep well: towards validation of numerical modeling using logging images

A wider exploitation of deep geothermal reservoir requires the development of Enhanced Geothermal System technology. In this context, drilling and stimulation of high-enthalpy geothermal wells raise technical challenges. Understanding and predicting the rock behavior near a deep geothermal wellbore are decisive to implement stimulation strategies to reach the couple temperature/flowrate target. Numerical modeling can contribute to enhanced stimulation processes thanks to a better understanding of impact of stress release, pressure changes and rock cooling in the near-wellbore area. In this paper, we use Discrete Element Method (code PFC2D, © Itasca Consulting Group), and more specifically bonded-particle model to capture the thermo-mechanical processes at metric scale. The application case corresponds to the beginning of thermal stimulation at Reykjanes in well RN-15/IDDP-2 (Iceland, IDDP-2 project and H2020 project DEEPEGS). A cold fluid is injected at a depth of 4.5 km where the rock temperature is above 430 °C and the well pressure is around 34 MPa. Since we have site-specific data and logging images after drilling, we attempt to link the simulations with the reality. The numerical results are confronted with incipient interpretation of logging images and with analytical solution to go towards validation of the modeling approach. Numerical results show breakouts and thermally and/or mechanically induced fractures consistent with the analytical solutions. Moreover, the sensitivity analysis on uncertain parameters yields important clues regarding some logging features as, for example, asymmetric damaging or caving.

Speculative Volcanology

In 2009, exploratory drilling of geothermal wells in Iceland’s Krafla volcanic caldera unexpectedly struck magma. The fact that the encounter didn’t have catastrophic consequences has excited considerable interest - and an international research facility is now being set up to explore energy generation and other possibilities of closer engagement with magma. We take this event as an incitement to explore how the Earth-changing `violence’ of volcanic or igneous processes might be seen not simply as happening in time, but as both generative and destructive of time itself. We approach volcanism through the construct of a `speculative geology’ that draws on a recent return to metaphysical themes in philosophy as well as a growing interest in geological processes in the arts, humanities and popular culture. In this way, alongside cause-effect relations, we explore the more enigmatic processes through which subterranean geological forces offer an excessive potentiality from which humans and other life forms select and actualise a narrower range of creative or generative possibilities. The paper explores three significant volcanic episodes: a series of massive magma extrusions around 1.9 billion years ago linked to the ascendance of multicellular life, volcanism present in the East African Rift during pivotal phases of human evolution and the volcanic activity of the early-mid Holocene viewed as a contextual factor in the emergence of ancient practices of artisanal pyrotechnology. Our reading of the dynamic and violent interchange between the inner and outer Earth in these examples points to a non-self-identical planetary condition, on which the very structure of temporality emerges through a play of destruction and generativity. In this light, we circle back on the Krafla project to consider questions of risk, uncertainty and responsibility that attend the potential new interface with the underworld of magma.

Uncertainty analysis of geothermal well drilling and completion costs

The goal of this study was to characterize the uncertainty associated with the cost of drilling and completion of geothermal wells. Previous research and publications have produced correlations for the average cost of geothermal wells as a function of well depth. This project develops this concept further by using a probabilistic approach to evaluate the distribution of geothermal well costs for a range of well depths. The well cost uncertainty was characterized by identifying the main cost components of geothermal wells and quantifying the probability distributions of the key variables controlling these costs. These probability distributions were determined based on the detailed cost records of U.S. geothermal wells drilled or designed from 2009 to 2013 as well as cost data from drilling equipment manufacturers and vendors. Probability distributions of the key variables were examined to find statistically significant correlations between them. Lastly, the previously determined probability distributions of individual cost components and the correlations between them were input into WellCost Lite, a predictive geothermal drilling cost model, using the Monte Carlo method. This approach allowed us to generate the overall well cost probability distributions for 8000–15,000ft. (2400–4600m) geothermal wells. We have shown that the median geothermal well cost increases exponentially with depth. Deep wells typically have higher cost uncertainty and more positively-skewed cost probability distributions. The correlations presented in this paper can be used to determine the economic feasibility of geothermal energy systems, assess the project risk, and facilitate investment decisions.

Interpreting muon radiographic data in a fault zone: possible application to geothermal reservoir detection and monitoring

Abstract. Rainfall-triggered fluid flow in a mechanical fracture zone associated with a seismic fault has been estimated (Tanaka et al., 2011) using muon radiography by measuring the water position over time in response to rainfall events. In this report, the data taken by Tanaka et al. (2011) are reanalyzed to estimate the porosity distribution as a function of a distance from the fault gouge. The result shows a similar pattern of the porosity distribution as measured by borehole sampling at Nojima fault. There is a low porosity shear zone axis surrounded by porous damaged areas with density increasing with the distance from the fault gouge. The dynamic muon radiography (Tanaka et al., 2011) provides a new method to delineate both the recharge and discharge zones along the fault segment, an entire hydrothermal circulation system. This might dramatically raise the success rate for drilling of geothermal exploration wells, and it might open a new horizon in the geothermal exploration and monitoring.

Case Studies on High Angle Drilling of Geothermal Wells and Future Prospects at Takigami Field

Case Studies on High Angle Drilling of Geothermal Wells and Future Prospects at Takigami Field

Behavioral design to model a reactive decision of an expert in geothermal wells

Software design based on agents represents a new perspective for computer science and more specifically, for Artificial Intelligence. It is a new theory that has innovated the analysis, design and implementation of system software. The design of agents poses problems related with: (1) autonomous decision-making process, (2) co-ordination, (3) negotiation, and (4) handling of mental states and communication. In a reactive multi-agent system, the group of agents is subject continually to local changes. These changes are designed by means of behavior rules whose results are influenced by the behavior of the rest of the agents. The design of these rules is inspired by the biological or cognitive sciences. Particularly, the design of cognitive rules corresponds with the principle of rationality; its perspective is focused on the interaction among the agents. One of the objectives of artificial intelligence refers to the development of systems that ease or increase the level of comfort in the daily life of humans. Such is the case for tasks with permanent focus on the input data in convergent methods or systems that help in the decision-making process involved in costly processes. In this paper we propose a design’s of the expert’s decision-making process trough the use of a cognitive model, and fuzzy sets to model the agents’ reactive deliberative process. Software system helps human expert in the estimation of the static formation temperatures. Furthermore, we will present an example based on a behavior developed from an expert in the field of geothermal sciences. The formulation of the human expert knowledge includes uncertainty, which is expressed in terms of fuzzy rules. An attempt to estimate formation temperatures from logged temperatures was solved whit this methodology based on reactive decision model. Thus, mathematically speaking an inverse problem is solved in this way. This paper describes and discusses the first experiences that form part of an incremental project whose final objective is to develop an expert system that allows the prediction of the degree of success of the drilling of geothermal wells.

TEMLOPI: a thermal simulator for estimation of drilling mud and formation temperatures during drilling of geothermal wells

This paper describes the development and application of the numerical code TEMLOPI v1.0, a useful tool for estimating the temperature distribution of the fluids employed for drilling geothermal wells. The simulator also allows estimation of the thermal disturbance of the surrounding rock caused by fluid circulation and well shut-in. TEMLOPI v1.0 is based on a mathematical model which considers the main heat transfer mechanisms and the heat exchange between the circulating fluid and the surrounding rock formation that occur during drilling of geothermal wells. The simulator was written in Fortran 77 using modular (block) programming. It runs on most IBM compatible personal computers and can be used in-situ. Input data includes the well geometry, the fluid and flow characteristics and the initial (undisturbed) formation temperature. Output files contain the transient temperature distribution (temperature vs depth) in the fluid flowing down the drill pipe and the annulus, the well inner face and the radial distribution in the surrounding rock. The software code model, architecture, input and output files and the solution algorithm are described in detail. Results obtained were validated by comparison with data published in the specialized literature and with data from well Az-29 from the Los Azufres Mexican geothermal field.

Development of New PDC Bits for Drilling of Geothermal Wells—Part 1: Laboratory Testing

Rock cutting, drilling and durability tests were conducted in order to obtain data to design polycrystalline diamond compact (PDC) bits for geothermal well drilling. Both conventional and new PDC bits with different rake angles were tested. The rock cutting tests revealed that cutting forces were minimized at −10 deg rake angle independent of rock type. In drilling and durability tests, a bit with backrake and siderake angles of −10 or −15 deg showed better performance concerning the penetration rate and the cutter strength. The new PDC bit exhibited better performance as compared to the conventional one, especially in hard rock drilling. Furthermore, a new PDC core bit (98.4 mm o. d., 66 mm i. d.) with eight cutters could be successfully applied to granite drilling equally as well as a bit with twelve cutters.

Development of New PDC Bits for Drilling of Geothermal Wells—Part 2: Field Testing

In order to develop polycrystalline diamond compact (PDC) bits of about 8-1/2 in. in diameter which are able to drill geothermal wells, we have conducted the investigation with respect to the structure of a PDC cutter and rake angles, etc., by means of fundamental laboratory and field tests. New PDC core bits of 8-15/32 in. diameter were designed and manufactured based on the results of these tests. Then, field tests using them were carried out in geothermal wells at Hijiori in Yamagata prefecture, Hot Dry Rock project site in Japan, on September 1989 and October 1990. It became clear that the new PDC core bit can be sufficiently applied to the drilling of heterogeneous hot-hard rock formations from the tests.