Experimental and Numerical Study on a Two-stage Coring Method for Stress Measurement: Application to Deep and High-Temperature Geothermal Wells

Numerical study on impact energy transfer and rock damage mechanism in percussive drilling based on high temperature hard rocks

ABSTRACT: This paper presents the results of numerical studies aiming to assess the effects of cement hydration on the casing integrity in superhot well conditions. Casing integrity challenges for high and super-high temperature geothermal wells were first discussed. A simplified well cement material model was implemented in the stand-alone code, Casinteg, to simulate cement behaviour in field conditions. Numerical simulations were performed, based on the IDDP-2 well scenarios, to investigate the effect of cement retarders, cement hydration heat and cement shrinkage on the casing stresses. The obtained results have shown that by engineering proper cement solutions, the risks of casing failure in geothermal wells can be minimized to sustainably harvesting green power from superhot geothermal reservoirs. 1. INTRODUCTION To answer the growing need for energy and at the same time fighting against the global climate change, development of renewable energy sources is indispensable. Geothermal energy is today recognized as one of the potential renewable sources, contributing to the "green shift" from fossil fuels toward more sustainable energy. Currently, most commercial geothermal power plants operate at the temperature range of 150° – 300° C. However, exploring the huge potential from/of? super-high temperature reservoirs (up to 400° – 500 °C) may be needed to increase the power production output, thereby lowering levelized cost of electricity (LCOE) of geothermal projects (Reinsch et al., 2017). For sustainable exploitation of geothermal energy, robust and cost-efficient casing systems are an absolute demand. However, the harsh operation conditions encountered in the high-temperature (HT) geothermal reservoirs are very challenging and are detrimental for the well integrity. Improper material selection and design principles may lead to serious failure events, and loss of the production well, as reported in many HT geothermal well projects (Ford et al., 2017). Recent examples are experiences from the Iceland Deep Drilling Project (IDDP). The two first IDDP wells are themselves pilot experiments and great learning sources for both researchers and industry. However, they could not be used for power production due to well integrity problems. Both wells experienced severe casing damage/failure (casing collapse and corrosion), which were the main technical obstacle to tap geothermal power from these ultra-high temperature production wells (Kruszewski and Wittig, 2018).

In high-temperature geothermal wells, the formation usually has extremely high abrasiveness, hardness, and temperature, which pose severe challenges to drilling tools. Among them, the interaction between the cutter of the drill bit and the rock is the key factor determining the rock-breaking efficiency of PDC (Polycrystalline Diamond Composite) drill bits. To further explore the rock-breaking mechanism of cutters on granite, this study adopts a combination of experimental and simulation methods to conduct systematic research. The results indicate that the specific crushing work increases and then decreases with rising temperature, reaching a minimum of 0.388 J/mm3 at 200 °C. In the temperature range of 300 °C to 500 °C, the specific crushing work is 15% lower than at room temperature. The specific crushing work during instant cooling is 12–25% lower than that during self-cooling, with instant cooling showing higher rock-breaking efficiency. As the rake angle increases, the specific crushing work initially decreases and then increases. The smallest specific crushing work, 0.383 J/mm3, occurs at a rake angle of 10°, where the number of debris and particle size are maximized. With deeper cutting depths, the specific crushing work gradually decreases, resulting in more debris, larger particle sizes, and higher cutter surface temperatures. These findings clarify the variation laws of rock load, cutting tooth distribution, and rock fragmentation state when the PDC bit breaks rocks. A rake angle of 10° can be used as the selection of cutting tooth inclination angle for PDC bit design, providing a theoretical basis for the design and application of PDC bits in high-temperature geothermal drilling and holding significant guiding importance. Considering that increasing the depth of penetration can cause uneven wear of the cutter, the drilling parameters can be controlled under certain conditions to achieve a penetration depth of 2 mm, thereby improving the rock-breaking efficiency and working life of the PDC bit.

Interest in harnessing geothermal energy from superhot or supercritical resources has grown significantly globally, leading to the launch of multiple projects worldwide. Most of these initiatives involve drilling deeper than conventional high-temperature geothermal wells (>3 km) to access superhot geothermal fluids (>350 °C) with higher enthalpy (>2.086 MJ kg−1) to obtain higher power output per well. But construction and production of these wells face significant challenges, particularly corrosion, due to high temperatures and the presence of corrosive non-condensable gases like CO2, H2S, and HCl. To assess the durability of commonly used geothermal well casing materials under such harsh conditions, material testing is required in demanding conditions at high temperatures and pressures (>350°C, >100 bar) with corrosive gases. This study investigates the corrosion resistance of API L80 grade carbon steel (CS) and 13Cr stainless steel (SS) casing materials under simulated superhot geothermal conditions using a high-temperature, high-pressure (HTHP) autoclave. The materials were tested at 450 °C and 167 bar in H2O vapor containing H2S and CO2 gases for 168 hours. Both materials had very low uniform corrosion rates (<0.02 mm/year). The L80-13Cr SS had lower CR than CS showing better corrosion resistance than the L80-CS as well as no pits were detected as for the CS. The high superheat and low density of the steam likely contributed to the low CR of both materials as well as the surface layer formed from reaction products during the HTHP corrosion test. Specifically, the formation of a continuous and protective surface film with chromite (Fe2Cr2O4) and iron silicates on the L80-13Cr SS is considered to have contributed to better surface protection than the surface film formed on the L80-CS.

Synthetic fluid inclusion logging to measure temperatures and sample fluids in the Kakkonda geothermal field, Japan

The shear behavior of shotcrete–rock interface same as the bond behavior plays a significant role on shotcrete behaviour in a tunnel. The shear strength of shotcrete–rock interface is noticeably affected by high temperature in high rock temperature tunnels. However, there are limited number of studies dedicated to investigate behaviour of shotcrete-rock interface in high temperatures. Therefore, a series of direct shear tests was performed, under the normal stress of 0.5, 1.0 or 1.5 MPa, to investigate the influence of high rock temperature on the shear characteristics of shotcrete–rock interface. Shotcrete–rock samples were cured under the temperature range of 20 to 100 °C and the relative humidity of 55 or 95 % before shear tests. The results show that curing temperature, relative humidity and normal stress have significant impacts on the peak shear strength, however, only the normal stress has the same impact on the residual shear strength. The peak shear strength as well as peak cohesion reaches its maximum value at 60 °C curing temperature with 55 % relative humidity level, and at 80 °C curing temperature when the relative humidity is 95 %. The shear stress-displacement curve presents various shapes in the section of post-shear due to the influence of high temperature and relative humidity.

The distinct characteristics of geothermal wells necessitate a unique and specialized approach to well design. This is critical for prolonging well lifespan and tackling the unique operational challenges present in mature geothermal fields, such as the case on a field in the Philippines. To enhance steam production, geothermal wells often intersect faults and complex geological formations, which result to partial to total fluid losses. These losses introduce significant uncertainty in cementing and compromise its quality. Such issues can lead to channeling, the formation of micro-annuli, and cement degradation — all of which undermine well integrity and reduce production efficiency. Furthermore, these cementing challenges, especially in the case of geothermal wells, may result to having trapped fluid in casing-to-casing annulus that can potentially compromise well integrity due to huge temperature swings that the wells encounter over their operating lives. Addressing these integrity concerns in a cost-effective manner is critical to maintaining safe, efficient, and economically viable well operations over time. To overcome these challenges, Welltec partnered with a geothermal operator in the Philippines to rehabilitate eight geothermal production wells affected by casing collapse and parted casing. A traditional workover involving bridge plugs was deemed impractical, as drilling out the plug risked further damage to liners exposed to corrosive, low-pH fluids. There was also a concern that trapped fluids could trigger additional casing failures. Therefore, a more controlled and precise cementing strategy was required to restore casing integrity. This paper introduces a tailored solution that integrates high-temperature Metal Expandable Packer technology and stage cementing collars. Case studies from a geothermal field in the Philippines illustrate how this approach restores well integrity without the need for a dedicated drill-out run. Building on experience from prior deployments, the improved MEP enables long-string tieback and casing relining to depths of approximately 1,300 meters — previously considered unattainable due to casing collapse, which had limited operations to around 700 meters. New well designs using CRA are also being optimized to benefit from such solution and withstand highly acidic environments. These wells require immediate relining, applying insights from earlier CRA wells in the same field. The MEP system eliminates the need to drill out plugs, thereby preventing damage to CRA casing. Integrating MEP technology into both drilling and workover operations offers a robust strategy to resolve integrity issues that typically lead to early well failure. This geothermal-specific technology enhances repeatability, safety, and operational control, ultimately enabling more reliable and extended utilization of geothermal resources.

Geothermal energy is a kind of green and sustainable energy and an essential alternative energy in the future. Improving the drilling speed of geothermal wells is helpful for the efficient exploitation of geothermal resources. Axial & Torsional Percussion Hammer (ATPH) was proposed in this paper for drilling enhancement. ATPH can convert the kinetic energy of the drilling fluid to percussion load, and improve the rock-breaking efficiency of the bit. Because of its mechanical seal design, it can operate efficiently at temperatures over 200 °C, which proves that it can be used in geothermal wells. The large eddy simulation (LES) method is chosen to calculate the percussion load characteristics of the ATPH oscillator, including its main frequency and amplitude. In the field drilling experiments of two high-temperature wells, ATPH showed better ROP enhancement ability and longer working life than PDM. In the well interval drilled by ATPH, the maximum temperature at the well bottom reached 202.54 °C.

Damage and failure mechanism of concentric perforated granite under static pressure after alternating action of high temperature and water at different temperatures

New concepts are required to accelerate the global development of high-temperature geothermal resources in a cost effective and environmentally safe way. Keeping the geothermal system in operation for several decades is challenging - yet key to economic success. Real-time monitoring of temperature and strain along the well bore is a powerful tool to evaluate the performance and integrity of installed subsurface components. A fiber optic cable was installed behind the casing of a high temperature geothermal well in Iceland (IDDP2). A second cable was installed in a low temperature well in Germany. During cementation, distributed strain data (Rayleigh backscatter) as well as distributed temperature sensing data (Raman backscatter) was acquired. In a subsequent measurement campaign, data shall be acquired in these wells during load changes (production/injection). In combination with concurrent laboratory investigations and conventional borehole logging data, this study will identify processes influencing the long-term integrity of casing and cement in high-temperature geothermal environments.

Within this study, a fiber optic cable was installed behind the anchor casing of a high temperature geothermal well in Iceland. The aim was to study the integrity of the cemented annulus during load changes, i.e. temperature and pressure changes during fluid production. Fiber optic distributed temperature data were acquired during installation/cementation, flow testing as well as during a subsequent shut-in period. Temperature data from the cementation were used to analyse the cement quality. During the onset of a flow test, measured temperatures in the annulus increased by more than 200°C. From the data a process influencing the well integrity could be identified.

The results are reported of a one-year program designed to investigate the feasibility of optical techniques applied to well-logging, to extend measurement capabilities in high-temperature geothermal boreholes. The basic concept is shown schematically. It makes use of a special armored cable containing fiber optic wave guides, connected to passive, downhole optical transducers. The latter modulate an optical carrier in response to borehole parameters. The optical carrier is a beam of infrared light transmitted from an optical source at the surface over an optical fiber. The modulated beam from the transducer is then returned to the surface over a second fiber, where conventional optical communications techniques are used to detect and decode the down hole information. (MHR)

A research program to develop an improved cement for use in high-temperature geothermal wells was carried out. The work involved in the selection and evaluation of an aluminum hydroxide-cured cement from the SwRI family of hydrothermal cements for this use are described. The physical testing program is described; the topics discussed include placement ability, compressive and bond strengths, permeability to water, compatibility to drilling muds, corrosion properties, and thermal properties.

Statistical study of analyses of water from 43 samples from geothermal wells, three groundwater wells, and one sample of local rainwater along with rainwater data from the Global Network of Isotopes in Precipitation has been used to identify the origin and evolution of geothermal water in the Niutuozhen (牛驼镇) geothermal field and estimate the renewability rate of its geothermal resource. The results show that the geothermal waters of the Jixianian Wumishanian dolomite reservoir and the Ordovician limestone reservoir are of Cl-Na type, the geothermal water of the Pliocene Minghuazhen (明化镇) Formation sandstone reservoir are Cl-Na type and HCO3-Na type and the groundwater of the Quaternary aquifer is HCO3-Na and HCO3-Na·Mg·Ca type. A linear relationship between silica concentration and temperature indicates that higher temperature probably enhances concentration of silica in Jixianian geothermal water. δ18O shift in Wumishanian geothermal water averaged 1.57‰, and was less than 1‰ in the other geothermal waters. The minimum and maximum 14C ages of Wumishanian geothermal water are 17 000 and 33 000 years from north to the south of the Niutuozhen geothermal field. Geothermal water and Quaternary groundwater belong to different groundwater systems with no hydraulic connections. Although the geothermal field receives some recharge from the Yanshan and Taihang mountains outside the northern and western boundaries of the geothermal field respectively, the renewability rate of geothermal water is on the scale of 10 000 years.

Geothermal energy is a type of green energy and it has been highly valued in recent years. According to the division of geothermal fields in Shandong Province, this area is a Yutai geothermal prospective area. However, few researchers conducted in-depth research in this area. This study is based on the project “Investigation and Evaluation of Geothermal Resources in Yutai Sag, Shandong Province”. Through data collection, ground investigation, geothermal field measurement, geophysical exploration, geothermal well drilling, pumping test, water quality analysis, and other means, the occurrence conditions of geothermal resources, thermal reservoir characteristics, and geothermal fluid chemical characteristics in this area are analyzed and studied by using the relevant data of geothermal exploration wells. The depth of the constant temperature zone is 20 m, the temperature of the constant temperature layer is 16°C, the Cretaceous geothermal gradient is about 1.26 °C/100 m, the geothermal gradient is about 2.94 °C/100 m, and the Ordovician geothermal gradient is about 3.98 °C/100 m. The limestone roof depth of the geothermal well is 2,089 m, the well depth is 2,309 m, the bottom-hole geophysical temperature measurement is 74.5 °C, the wellhead temperature is 68.5 °C, the maximum water inflow is 552 m3/d, the radius of influence is 852 m, the water chemical type is Cl-Na type, and the salinity is 26454.54 mg/L. It is a hot water geothermal resource in low-temperature geothermal resources. Geothermal water contains fluorine, bromine, strontium, and iron concentrations that met the standards of named mineral water. The contents of lithium and boron can reach the mineral water concentration, with a high physiotherapy value. This study provides a geological basis for the exploration and utilization of geothermal resources in this area and related areas.