Session 5F: Downhole Logging

The exploration and exploitation of vapor dominated geothermal reservoirs present unique challenges due to their complex subsurface dynamics and elusive fluid behaviors. This abstract proposes a novel approach of utilizing and testing of advanced high temperature & high pressure sub-surface wireline conveyed tooling to enhance the characterization and understanding of such reservoirs and the wellbore environment. Traditional techniques often struggle to accurately capture these properties, leading to suboptimal resource utilization and operational inefficiencies. In response, this testing study advocates for the integration of cutting-edge sub-surface tool testing methodologies, leveraging state-of-the-art technologies to provide comprehensive insights into reservoir conditions.

Through the development of advanced downhole tools equipped with high-resolution sensors and high-temperature mitigation techniques, new approaches to sub-surface wireline conveyed cased hole and open hole logging aim to capture data including temperature gradients, pressure differentials, steam flow profiles, feed-zone identifications, and wellbore integrity. By obtaining and analyzing this data in conjunction with existing reservoir models and geological data, geologists, reservoir engineers, and seismologists can optimize feed-zone characterization models, refine placement strategies, and improve reservoir management practices. Furthermore, the continuous monitoring enabled by these tools facilitates early detection of reservoir and wellbore anomalies and enhances the efficiency of reservoir surveillance programs.

The continued development of this technology offers significant potential for the geothermal industry by enabling more informed decision-making processes, reducing operational risks, limiting operational non-productive times, and maximizing resource recovery. Furthermore, the insights gained from these tests can contribute to the advancement of geothermal energy as a sustainable and reliable source of clean energy, thus supporting global efforts towards mitigating climate change and transitioning towards a low-carbon future.

Drilling cost-efficient, safe, and deep wellbores is key to scaling geothermal energy. However, the elevated costs associated with high non-productive and invisible loss times (NPT/ILT) are taxing on the operators, acting as a severe bottleneck in utilising geothermal energy. In deep geothermal wells, the leading contributors to NPT/ILT are the failure of downhole tools and/or absence of their measurements.

Continuous downhole parameter measurements are often hindered by mud circulation interruptions or downhole tool failures caused by high operating temperatures. During drilling operations, the mud temperature fluctuates due to its thermal interaction with the surrounding environment, which is typically hotter. This variability is particularly pronounced during activities such as connection and tripping, when mud circulation and downhole measurements are unavailable. Estimating temperature distribution along the wellbore before deploying costly downhole tools is critical to mitigate the risk of failure. Furthermore, developing an effective strategy to cool the mud relies on factors including circulation rate, duration, and inlet mud temperature.

In this study, we introduce an approach that combines physics-based modelling and machine learning techniques for real-time prediction of mud temperature distribution. This method aims to enable precise and continuous estimate of mud temperature along the wellbore, facilitating more efficient thermal well management. An automated calibration system tailored for adjusting parameters within the physics-based hydrothermal model was also developed. This calibration process is designed to enhance the accuracy of the model and, further improve the performance of the monitoring system. The calibration system is smartly triggered by Discrete Event Simulation (DES) technique as and when operational conditions change on the rig. The advisory system synergises with the monitoring system to suggest optimal mud circulation durations and flow rates. These recommendations are tailored to the inlet mud temperature, thereby ensuring precise control over the targeted temperature profile within the wellbore.

Our approach has been validated using select cases from the Utah Forge dataset for which wellbore temperature measurements were available. Our real-time monitoring system was able to estimate the mud temperature profile along the wellbore after prolonged stagnation of mud inside the wellbore due to other operations like connection and tripping when bottom hole mud temperature measurements were absent. The automatic calibration technique proved efficient in updating uncertain model parameters to provide improved predictions. Ultimately this approach can drastically improve temperature monitoring and management during the construction of a geothermal well and reduce tool failure.

Geothermal well test analysis is an important tool for understanding reservoir behaviour and determining the parameters of fluid flow dynamics such as reservoir permeability, skin effect, wellbore storage and reservoir boundaries. Traditionally, only the pressure transients are analysed, while temperature transients are often overlooked despite their sensitivity to the changes in the wellbore and reservoir conditions. This work presents the temperature and pressure transient analysis conducted during the pre and post-deflagration testing of well BR064 in the Ohaaki geothermal field, New Zealand. The study uses a novel numerical framework to investigate both temperature and pressure data resulting from the well-testing and matching them with their respective derivatives (temperature and pressure). Analysing temperature gives additional information about the reservoir and enhances the conventional pressure transient analysis. Through the temperature derivatives, it is possible to understand the complex heat transfer phenomena in the wellbore and obtain further information from existing well test data that are normally not considered.

This paper presents the ongoing development of a chloride-based wireline tool designed to detect and quantify inflows from feed zones in geothermal wells. The tool aims to characterize stimulation events in EGS wells at Utah FORGE (Frontier Observatory for Research in Geothermal Energy) and other EGS sites. Successful development of the chloride tool would greatly improve production monitoring of the fractures and enable proactive prescription of additional stimulations over the life of the field, thus helping to improve EGS commercial feasibility.

The recent developments of the chloride tool have focused on preparing for the upcoming field deployment at the Utah FORGE site. The field-scale tool assembly features a FORGE sensor package housing the Ion Selective Electrode (ISE) chloride sensors, a Mitco PTS sensor package for secondary downhole measurements, a wire guide component, and an electronics housing for data transmission through a 7-conductor feedthrough. A high-temperature logging tool has been developed and tested to capture and transmit data from the chemical sensors to the surface at 260°C. Numerical simulations at the field scale were conducted under downhole conditions (225°C and 5000 psia) and showed no significant change in fluid flow behavior compared to laboratory conditions. Simulations focused on the design of the field-scale tool housing to assess ten different positions relative to the feed zone height. The simulations suggested that the best signal recordings occurred at the beginning of the tool's Run in Hole (RIH) motion when the lower part of the housing met the feed zone. The tool will be deployed at the Utah FORGE site using a wireline truck in a vertical pilot well and directional production well.

Downhole logging tools embedded with various sensors are commonly lowered into multi-thousand-foot geothermal wells. One of the many challenges associated with evaluating geothermal wells is the formation temperatures can exceed 200 ˚C. A common technique for a logging tool is to use low temperature electronic components that are emplaced inside a special housing referred as a vacuum flask. These flasks are designed to mitigate the heat transfer to the components, which allow the tool to operate at the elevated temperatures for several hours. Unfortunately, when a logging tool is required to operate at 250 ˚C for long-term, the vacuum flask is not a viable solution. With a greater need for fracture formation detection and well integrity characterization of enhanced geothermal system (EGS), the tools are required to operate in the harsh environment for 24 hours to 6 months. The use a geothermal downhole tool for these extended periods of time while operating at temperatures above 200 ˚C requires all the components to be rated for those temperatures. Sandia National Laboratories is developing methods to improve the reliability of embedded electronic systems to operate to temperatures reaching 300 ˚C. Utilizing off-the-shelf HT integrated circuits (IC), the ICs will be either gold-tin bonded or wire bonded (depending on the device format) onto a ceramic printed circuit board (PCB). Ceramic PCBs do not degrade at 300 ˚C temperature, unlike other PCB materials. Along with these bonding methods, Sandia will also explore various epoxies with similar thermal coefficient expansion to conformal coat the ICs to further improve the reliability while under vibration and/or mechanical shock. Using the above techniques will allow for long-term operation of geothermal subsurface instrumentation while exposed to temperatures reaching 300 ˚C. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Innovative E-line Intervention Solutions for Geothermal Wells

E-line interventions play a crucial role in maximizing the productivity and lifespan of geothermal wells. Geothermal energy is an important renewable energy source with significant potential for global energy sustainability. However, geothermal wells face various challenges, including scaling, corrosion, mechanical issues, and reservoir decline, which necessitate periodic interventions to maintain or restore their optimal performance. E-line interventions offer a versatile and efficient method for diagnosing, evaluating, and resolving these issues.

The methodologies employed in e-line interventions can be categorized into two main approaches: logging and mechanical interventions. Logging interventions involve the deployment of various tools to acquire real-time data on well conditions to diagnose and evaluate well performance and assists in decision-making for subsequent interventions. Mechanical interventions, on the other hand, employ e-line conveyed tools to directly address issues in the wellbore such as wellbore cleanout, scale removal, casing repair, pipe recovery and fishing operations, thus contributing to enhanced well performance and prolonged lifespan. A description of the mechanical intervention solutions will be detailed in the paper with the objective to broaden the knowledge of the extensive applications.

Wellbore Cleanout and Scale Removal

During the geothermal wells’ life, obstruction of scale and other mineral build up often disrupt the wells performance, which in many cases can lead to well shut in. E-line wellbore cleanout solutions involve the deployment of specialized tools and equipment downhole to remove debris, scale, and other obstructions from the wellbore. By utilizing e-line technology, operators can perform precise and targeted cleanout operations, minimizing downtime and maximizing production.

Casing Repair and Fishing Operations

In the realm of casing repair, the intervention offerings available on e-line include solutions for compliance to damaged or deformed casings. Additionally, the company's robotic fishing tools have replaced conventional methods, offering enhanced precision and reduced downtime to fishing operations.

Advantages and Impact

This paper will present the comprehensive range of interventions provided by EIS and a pathway to enhanced operational efficiency in geothermal wells. Moreover, the novel solutions being presented have proven to be robust and highly effective in addressing various downhole wellbore challenges. EIS empowers operators to maximize production rates, minimize environmental impact, and ensure the long-term integrity of geothermal assets. Embracing these innovative solutions is essential for unlocking the full potential of geothermal energy and advancing the transition to a sustainable energy future.