Interview Questions for Casing Inspection Problem Solving

Casing inspection employs various techniques to assess the integrity of well casings, crucial for preventing environmental contamination and ensuring operational safety. These techniques can be broadly categorized into:

• Logging Tools: These tools are run down the wellbore inside the casing to gather data. Examples include:
• Caliper Logs: Measure the internal diameter of the casing, revealing areas of deformation or collapse.
• Acoustic Logs: Utilize sound waves to detect casing wall thinning, corrosion, or fractures. They can differentiate between various types of metal loss.
• Magnetic Flux Leakage (MFL) Tools: Detect metal loss, pitting, and cracks by measuring changes in the magnetic field around the casing. These are particularly effective for detecting corrosion.
• Ultrasonic Tools: Employ high-frequency sound waves to detect wall thinning and other defects, providing high resolution.
• Pressure Testing: This involves pressurizing the casing to a specified level and monitoring for pressure drops, which may indicate leaks or failures.
• Visual Inspection (for accessible sections): While limited to surface or near-surface sections, visual inspection can detect obvious signs of damage like dents, bends, or corrosion.
• Downhole Cameras: Used in some cases to provide visual images of the casing’s internal and external condition, especially valuable for detecting unusual debris or obstructions.

The choice of technique depends on factors like well depth, casing material, the type of anticipated damage, and budget constraints. Often, a combination of techniques is employed for a comprehensive assessment.

Interpreting casing inspection logs requires careful analysis of the data acquired from various tools. It’s a multi-step process:

1. Data Review and Quality Control: Initial checks ensure data accuracy and completeness. This might involve verifying tool calibration and accounting for environmental effects.
2. Identification of Anomalies: This step involves comparing the acquired data (e.g., caliper readings, acoustic signals) with baseline data or manufacturer specifications to identify deviations. For example, a significant decrease in caliper reading suggests possible collapse or corrosion.
3. Defect Characterization: Once anomalies are identified, their nature and severity are characterized. For instance, an MFL log might pinpoint the location and depth of corrosion pits, allowing for accurate sizing.
4. Correlation with other Data: Integrating data from different logs (e.g., caliper, acoustic, MFL) provides a more complete picture. For example, a thinning detected by acoustic logs may be further characterized as corrosion by MFL logs.
5. Report Generation and Recommendation: The findings are summarized in a comprehensive report, including descriptions of identified defects, their location, severity, and potential impact on well integrity. Recommendations for repair or mitigation are also provided.

Think of it like a detective investigating a crime scene. Each log provides a piece of evidence, and the interpreter must assemble these pieces to understand the whole story – the condition of the casing.

Corrosion is a significant threat to casing integrity. Identifying and assessing corrosion damage involves the following:

1. Visual Inspection (if accessible): Obvious signs like pitting, scaling, or general rusting can be identified visually. However, this is limited to accessible casing sections.
2. Magnetic Flux Leakage (MFL) Logging: MFL tools are particularly effective at detecting corrosion. They measure changes in the magnetic field caused by metal loss, providing precise location and depth of corrosion pits.
3. Electromagnetic Acoustic Transducers (EMAT): These non-contact transducers can measure wall thickness and identify areas of corrosion without direct contact with the casing.
4. Ultrasonic Logging: Although less sensitive to pitting than MFL, ultrasonic logging can measure wall thickness accurately and provide a comprehensive profile of casing condition.
5. Data Analysis and Interpretation: Data from MFL, EMAT, or ultrasonic logs are analyzed to determine the extent, depth, and severity of corrosion. This may involve comparing measured wall thickness to original specifications or using corrosion rate models.

A combination of techniques typically offers the most reliable assessment. For example, MFL can pinpoint corrosion locations, while ultrasonic can measure the depth of the damage. Understanding the type of corrosion (uniform, pitting, etc.) is essential to predict future behavior and plan appropriate mitigation strategies.

Casing failures can have multiple causes, often stemming from a combination of factors:

• Corrosion: Chemical reactions between the casing material and the surrounding environment (e.g., sour gas, produced water) can lead to significant metal loss and weakening.
• Mechanical Damage: During drilling, completion, or production, the casing can be subjected to mechanical stresses causing dents, bends, or collapse. External forces from shifting ground conditions can also contribute.
• Cyclic Loading: Repeated pressure changes (e.g., during well testing or production) can lead to fatigue and eventual failure, even without significant corrosion.
• Cementing Issues: Poor cementing can leave gaps between the casing and the formation, creating pathways for corrosion or fluid migration that can weaken the casing.
• Temperature and Pressure Variations: Extreme temperature or pressure changes can induce stresses exceeding the casing’s yield strength, causing failure.
• Improper Casing Design: Inadequate casing design, with insufficient wall thickness or inadequate consideration for anticipated stresses, significantly increases the risk of failure.

Investigating a casing failure requires a thorough analysis of all these potential contributing factors, often involving detailed metallurgical examination and analysis of well logs and operational history.

Casing pressure testing is a crucial technique for verifying the integrity of the well casing. It involves pressurizing the casing to a specified pressure and monitoring for pressure drops over a set time period.

Its significance lies in:

• Leak Detection: Pressure drops indicate leaks or perforations in the casing, allowing early detection of potential problems before they escalate.
• Assessment of Overall Integrity: By observing how the casing responds to pressure, its overall strength and condition can be indirectly assessed.
• Prevention of Environmental Contamination: Detecting and repairing leaks prevents the escape of potentially harmful fluids from the wellbore into the surrounding environment.
• Confirmation of Repair Success: Pressure testing is used after repairs to verify the effectiveness of the work in restoring casing integrity.

The test pressure and duration depend on factors like casing strength, well depth, and potential environmental risks. Pressure testing provides a valuable check on the overall health of the well and helps prevent more serious and costly problems down the line.

Determining appropriate casing design parameters is critical for ensuring well integrity and operational safety. This is a complex process, involving various factors, and often requires the expertise of specialized engineers.

Key parameters to be determined include:

• Casing Grade and Wall Thickness: The choice depends on the anticipated stresses (pressure, temperature, and external loads), the expected lifespan of the well, and the corrosive nature of the well fluids.
• Casing Length: Determined based on the required depth of isolation and the anticipated geological conditions.
• Connection Type: The type of casing connections (e.g., premium threads, welded connections) influences the strength and integrity of the casing string.
• Cementing Design: The design of the cementing job ensures proper annular sealing and protects the casing from external forces and corrosion.

Specialized software and engineering calculations are typically employed to account for all the relevant stresses and determine the necessary casing parameters. Safety factors are included to provide additional margin against unforeseen events.

A poorly designed casing can lead to premature failure, resulting in costly repairs, downtime, and potential environmental consequences. Hence, a rigorous design process is essential.

While casing inspection methods provide invaluable insights into well integrity, they do have limitations:

• Limited Reach: Some tools, such as visual inspection, are limited to accessible sections of the casing. Even advanced tools like MFL have limitations regarding the depth of penetration.
• Tool Resolution and Sensitivity: Different tools have different resolutions and sensitivities. For example, MFL might be excellent at detecting corrosion but less sensitive to small cracks.
• Environmental Influences: Factors like temperature, pressure, and the presence of well fluids can influence the accuracy and reliability of some tools.
• Interpretation Challenges: Analyzing the data from various tools can be complex and requires expertise. Subjective interpretation can lead to differences in assessment.
• Cost and Accessibility: Advanced inspection techniques can be expensive, especially in deep or remote wells. Accessing the casing itself may present logistical challenges.

It’s crucial to be aware of these limitations when planning and interpreting inspection results. Employing multiple techniques and critically evaluating the data helps to mitigate these limitations and obtain a more comprehensive understanding of casing condition.

Caliper logs measure the diameter of the wellbore at various depths, providing crucial information about the casing condition. Imagine a tailor measuring your body – the caliper is like a sophisticated measuring tape for the well. By analyzing the caliper log, we can identify areas of casing deformation, such as ovalization (where the casing becomes elliptical) or collapse (where the casing diameter significantly reduces). These deformations often indicate stress on the casing, possibly from external pressure or internal corrosion. For example, a consistent decrease in diameter across a section might suggest a buckling problem, while localized reductions could indicate corrosion or external pitting. We look for deviations from the nominal casing diameter – any significant variations beyond the manufacturer’s tolerances signal a potential integrity issue requiring further investigation.

Interpreting the Data: We carefully examine the log for:

• Consistent Diameter Reduction: Indicates potential buckling or external pressure.
• Localized Diameter Reductions: Suggests corrosion, pitting, or localized damage.
• Irregular Diameter Fluctuations: May point to damage, scale build-up, or cement problems.

By comparing the caliper log data with other well data (such as pressure tests and cement logs), we can build a comprehensive picture of the casing’s overall condition.

Acoustic and electromagnetic (EM) methods are non-invasive techniques used to assess casing integrity without requiring the removal of the casing. Think of them as sophisticated ‘medical imaging’ for oil wells.

Acoustic Inspection: Uses sound waves to detect flaws in the casing. These tools transmit acoustic waves through the casing and measure their travel time and amplitude. Changes in these parameters can indicate areas of damage, such as cracks, corrosion, or thinning. The method is particularly effective at detecting longitudinal cracks, which can be difficult to identify with other methods.

Electromagnetic Inspection: Uses electromagnetic signals to detect conductive materials within or adjacent to the casing. This is particularly useful for identifying corrosion, which often involves the formation of conductive compounds. It can also help to locate the presence of metallic debris or other conductive objects near the casing.

Example: A combination of both acoustic and EM scans can detect and differentiate between a crack (detected primarily by acoustic methods) and significant corrosion (detected primarily by EM methods).

My experience in well integrity management involves a holistic approach to ensuring the safety and operational efficiency of wells throughout their lifecycle. This includes proactive risk assessment, regular inspection planning, and a detailed understanding of various remediation techniques. I’ve been involved in projects ranging from the identification of minor casing issues to the management of significant integrity risks requiring immediate intervention.

Examples of my contributions:

• Developing and implementing well integrity management plans for numerous wells, incorporating risk-based inspection strategies.
• Interpreting data from various inspection technologies (caliper, acoustic, EM) to assess casing condition and identify potential risks.
• Collaborating with engineering teams to design and oversee effective remediation strategies for damaged casing sections.
• Maintaining detailed records and documentation for all well integrity activities, ensuring regulatory compliance.

I’m adept at using this data to guide decision-making regarding well operations, maintenance, and intervention strategies, consistently prioritizing safety and environmental protection.

Discrepancies between different inspection methods are common and often require careful consideration. Think of it as getting multiple medical opinions – each doctor may use different diagnostic tools, and the results might not always perfectly align. It’s crucial to thoroughly investigate these inconsistencies.

Handling Discrepancies:

• Re-evaluation of Data: Start by meticulously reviewing the data from each inspection method, checking for errors in data acquisition or interpretation.
• Comparison with other Data: Cross-reference the results with other available well data, such as pressure tests, production logs, and historical records. This helps to establish a clearer picture of the well’s condition.
• On-site Inspection: If uncertainties remain, further on-site inspections may be necessary. This could include deploying more advanced inspection tools or conducting a physical inspection (where feasible).
• Expert Consultation: Consult with experienced well integrity engineers to gain a second opinion and leverage their expertise in interpreting complex data sets.

The goal is not to choose one method over another but to develop a comprehensive understanding of the casing condition by considering all the available information.

Identifying and prioritizing casing integrity risks is a critical aspect of well integrity management. It’s akin to performing a risk assessment for a building – we prioritize the most urgent repairs first. My approach combines quantitative and qualitative data analysis to effectively prioritize risks.

My Approach:

• Risk Assessment: I begin by conducting a thorough risk assessment, considering factors like the age of the well, the well’s operating conditions, and the history of any previous interventions. I utilize standardized risk assessment matrices and incorporate quantitative data such as corrosion rates and pressure test results.
• Data Analysis: I analyze data from various sources (inspection logs, production data, pressure tests) to identify potential problem areas. Advanced data analysis techniques, such as statistical modeling and machine learning algorithms, can be employed to assist in this process.
• Prioritization: Risks are prioritized based on severity and likelihood of failure. This often involves using a risk matrix that considers the potential consequences of failure (e.g., environmental damage, production loss) and the probability of failure.
• Regular Monitoring: Continuous monitoring and periodic re-assessment are essential to track changes in risk levels and adapt the management strategy accordingly.

The ultimate goal is to develop a risk-based inspection and intervention plan, focusing resources on the highest-priority risks to ensure the long-term safety and efficiency of the well.

Software and data analysis are integral to modern casing inspection. Imagine trying to assemble a complex jigsaw puzzle without a clear picture – the software provides that picture and helps us connect the pieces. Specialized software packages allow us to process and analyze large datasets from various inspection technologies.

Software Applications: We use software capable of:

• Data Visualization: Creating clear and insightful visualizations of inspection data, such as interactive 3D models of the wellbore.
• Data Analysis: Performing complex data analysis to identify trends, anomalies, and potential problems. This includes statistical analysis, signal processing, and machine learning techniques.
• Reporting and Documentation: Generating detailed reports and documentation for regulatory compliance and internal use. The software provides tools to document every aspect of the inspection process and its findings.
• Integration: Integrating data from multiple sources, such as pressure sensors, production logs, and previous inspection data to form a holistic understanding of the well’s state.

This software allows for a much more efficient and accurate analysis of casing integrity compared to manual methods, leading to improved decision-making and more effective risk management.

My experience encompasses a variety of casing repair and remediation techniques, chosen based on the specific nature of the damage and well conditions. Selecting the right technique is crucial – it’s like choosing the right tool for a specific job in a workshop.

Common Remediation Techniques:

• Cementing: Repairing damaged casing sections by injecting cement to provide external support and seal any leaks.
• Coiled Tubing Intervention: Using coiled tubing to deploy specialized tools for in-situ repairs, such as applying internal coatings or packers to seal leaks.
• Casing Patching: Installing patches over damaged sections to restore casing strength and integrity.
• Casing Replacement: In severe cases of damage, a section of the casing may need to be replaced entirely.

The choice of technique depends heavily on factors such as the type and severity of damage, the well’s operational status, the accessibility of the damaged section, and the overall well architecture. I have extensive experience in evaluating these factors to recommend and execute the most appropriate remediation strategy, ensuring minimal disruption to operations while maximizing well integrity and safety.

Evaluating the effectiveness of casing repair operations requires a multi-faceted approach, focusing on both immediate and long-term results. We start by comparing pre- and post-repair data from various inspection methods – such as caliper logs, acoustic logs, or video inspection – to quantify the improvement in casing condition. For example, a successful repair should significantly reduce the size and number of detected leaks or perforations.

Beyond immediate results, we also monitor the well’s performance over time. Has the repair prevented further deterioration? Have production rates improved or stabilized? A decrease in fluid losses or increases in pressure integrity would indicate a successful repair. We also analyze the longevity of the repair method used, taking into account the well’s operational parameters (temperature, pressure, etc.) to predict its lifespan and schedule future interventions as needed. Documentation, including thorough reports and images, is critical for accurate evaluation and future reference.

Managing and communicating casing inspection findings is crucial for informed decision-making. We employ a structured process. First, all data from different inspection tools (e.g., gamma ray logs, temperature logs, pressure tests) is integrated into a central database. This database allows for easy visualization and analysis, helping us pinpoint problem areas.

Then, a detailed report is generated, clearly highlighting all findings, including the location, severity, and type of any defects. We use visualizations like maps and cross-sections to make the data easily understandable for both technical and non-technical stakeholders. For critical findings, we issue immediate alerts to relevant personnel, using clear and concise language, avoiding jargon whenever possible. Regular meetings with the operations team and management are crucial to discuss findings, propose solutions, and ensure everyone is aligned on the next steps. Finally, the report is meticulously documented and stored according to company and regulatory requirements for future audits and reference.

Regulatory compliance is paramount in casing inspection. It ensures the safety of personnel, protects the environment, and prevents costly operational issues. Regulations vary by region but often focus on preventing wellbore leaks and ensuring the integrity of the casing string which is vital to preventing environmental contamination and maintaining safe operating conditions. Failure to comply can result in severe penalties, including fines, operational shutdowns, and reputational damage.

Our adherence to these regulations involves meticulous record-keeping, using certified inspection equipment, and employing qualified personnel. We stay updated on changes in regulations and best practices through continuous professional development and engagement with regulatory bodies. A strong safety culture, incorporating regular safety audits and training, is also critical in ensuring compliance and mitigating risks. For instance, regular inspection of the casing can prevent a blowout and mitigate potential environmental hazards that would trigger regulatory action.

Ensuring personnel safety during casing inspection operations is our top priority. This starts with thorough risk assessments before any operation begins, identifying potential hazards like high pressure, hazardous materials, confined spaces, and working at heights. Based on this assessment, we develop detailed safety plans outlining procedures, protective measures, and emergency response protocols.

We implement strict adherence to safety regulations and guidelines. This includes the use of appropriate Personal Protective Equipment (PPE), regular safety briefings and toolbox talks, and mandatory training for all personnel. We utilize specialized equipment and techniques to minimize risks, such as remotely operated vehicles (ROVs) for inspections in hazardous environments. Regular communication and close supervision are vital, especially during complex operations. Finally, regular safety audits and near-miss reporting systems help us identify areas for improvement and prevent future accidents. For example, a thorough pre-job briefing and adherence to strict procedures can help to prevent falls from elevated work platforms.

Integrating casing inspection data with other well data provides a holistic view of the well’s condition and performance. We use advanced software and databases to correlate information from various sources, including production data (flow rates, pressures), reservoir parameters, and other well logs (e.g., cement bond logs, temperature logs). This integrated approach is crucial for comprehensive well assessment.

For instance, by correlating casing inspection results with production data, we can identify leaks that may be contributing to production losses. Similarly, comparing casing integrity data with reservoir parameters can help assess the impact of reservoir conditions on casing performance. This integration allows us to make more informed decisions regarding well interventions, optimizing operations and minimizing risks. We often use visualization tools to represent this integrated data which may include cross-plots, 3D models or dashboards, allowing for easy identification of trends and anomalies which can aid in preventative maintenance and effective problem solving.

Casing inspection faces numerous challenges. One common problem is the difficulty of accessing and inspecting certain sections of the wellbore, especially in highly deviated or horizontal wells, making visual inspections or deployment of tools very challenging. Another significant challenge involves interpreting inspection data accurately, as data quality can vary based on the age of the well and the type of inspection method used. Difficult-to-interpret data can lead to inaccurate diagnoses of casing condition.

Environmental conditions (high temperatures, high pressures, corrosive fluids) also impact the effectiveness and longevity of inspection tools, increasing the risk of tool failure and requiring specialized and costly equipment. Furthermore, the high cost of advanced inspection techniques can limit their applicability, especially in older or marginal wells. Finally, regulatory requirements and safety protocols add complexity to operations, demanding careful planning and risk management.

I have extensive experience with various casing materials, including carbon steel, stainless steel, and various alloys. Carbon steel is the most common due to its strength and relatively low cost, however it is susceptible to corrosion, especially in aggressive environments. Stainless steel offers superior corrosion resistance, but at a higher cost. Various alloys, such as chromium-molybdenum (Cr-Mo) steels, are used in high-temperature and high-pressure wells due to their enhanced strength and creep resistance.

Understanding the properties of each material is crucial for selecting the right casing for a specific well. Factors to consider include yield strength, tensile strength, corrosion resistance, and susceptibility to stress corrosion cracking. For example, in a highly corrosive environment, selecting a corrosion-resistant alloy is essential to extend casing life and prevent failures. During inspection, this material knowledge informs the interpretation of inspection data and guides the selection of appropriate repair techniques. For example, the use of specialized corrosion-resistant coatings or downhole cementing can further extend the life of the casing and reduce operational costs.

Temperature and pressure significantly impact casing integrity. Think of a metal pipe under stress; extreme heat weakens the metal, making it more susceptible to deformation or failure, a phenomenon known as creep. Similarly, high internal pressure pushes against the casing walls, potentially exceeding their yield strength and leading to cracks or bursts. Conversely, very low temperatures can make the casing brittle, increasing its vulnerability to cracking under stress. The combined effect of temperature and pressure is particularly critical in deep wells or those in harsh environments. For instance, high-temperature, high-pressure (HTHP) wells often require specialized casing materials and designs to withstand these extreme conditions. We need to account for these factors when selecting casing materials and designing well integrity programs. The pressure-temperature envelope of the casing must be carefully considered and not exceeded to maintain long-term well integrity.

Inspection data is the cornerstone of a robust well integrity management plan. My approach involves a multi-step process. First, I thoroughly analyze the data from various inspection tools (e.g., caliper logs, acoustic logs, gamma ray logs) to identify potential anomalies, such as corrosion, pitting, or fractures. Secondly, I assess the severity and extent of these defects using established industry standards and guidelines, considering the well’s operational history and the surrounding geological environment. Next, I integrate this information into a risk assessment matrix, prioritizing potential failure modes based on their likelihood and consequences. This assessment forms the basis of the well integrity management plan, which outlines mitigation strategies such as repairs, enhanced monitoring, or production adjustments. For example, if the inspection data reveals significant corrosion in a specific casing section, the plan might include deploying a specialized liner to reinforce that area or more frequent monitoring. Regular review and updates to the plan are crucial as new data becomes available or well conditions change.

My experience encompasses a wide range of non-destructive testing (NDT) methods for casing inspection. I’ve extensively used acoustic techniques, such as acoustic logging and cement bond logging, to evaluate the condition of the casing and cement sheath. Acoustic logs can detect casing defects by measuring the speed of sound waves passing through the casing. Cement bond logging helps assess the quality of the cement bond between the casing and the formation, identifying potential gaps that could compromise well integrity. I’m also proficient in using electromagnetic tools, particularly for detecting corrosion. These tools measure the electromagnetic properties of the casing to identify areas with reduced thickness due to corrosion. Finally, I have experience with specialized techniques, such as inline inspection tools deployed in the casing to provide detailed imaging and precise measurements of defects. The choice of NDT method depends on the specific objectives of the inspection, the type of casing, and the well’s characteristics.

Assessing the risk of casing failure requires a holistic approach that considers various factors specific to the well environment. These include the geological conditions (e.g., presence of corrosive fluids, tectonic activity), the well’s operational history (e.g., previous interventions, production profiles), and the casing design and material properties. I use a risk assessment matrix to systematically evaluate these factors, assigning probabilities and consequences to different failure scenarios. For example, a well located in a highly corrosive environment will have a higher risk of corrosion-related failure than a well in a less aggressive environment. Similarly, a well with a history of high-pressure events will have a higher risk of pressure-induced failures. This risk assessment guides the selection of appropriate inspection techniques, mitigation strategies, and monitoring plans, ensuring proactive management of well integrity.

Selecting the right casing inspection tools and techniques involves a careful consideration of multiple factors. First, I define the inspection objectives, determining which aspects of casing integrity need to be evaluated (e.g., corrosion, cracks, cement bond). Next, I consider the well’s specific characteristics, such as depth, diameter, temperature, and pressure, to ensure compatibility with the available tools. I also analyze the accessibility of the well and the available time and budget. For instance, if corrosion is a major concern, electromagnetic tools might be chosen, whereas for evaluating the cement bond, acoustic logging would be more appropriate. Specialized tools, such as inline inspection tools, may be necessary for detailed defect assessment in complex wells, though they may have higher costs. The selection process often involves collaboration with experts, tool vendors, and operators to optimize the inspection strategy and achieve the desired results. The goal is to select the most cost-effective and efficient tools and techniques that meet the inspection objectives and comply with regulatory requirements.

Effective budget and timeline management is crucial for successful casing inspection projects. My approach starts with a detailed cost estimate that includes all aspects, from tool rentals and personnel costs to data analysis and report preparation. This estimate is then refined through discussions with stakeholders and tool vendors to identify cost-saving opportunities while maintaining the integrity of the inspection. Similarly, the project timeline is meticulously planned, considering the time required for tool mobilization, operation, data acquisition, and analysis. Regular progress monitoring and communication with stakeholders are essential to ensure the project stays on track and within budget. I utilize project management software to track expenses, monitor progress, and identify potential delays. Contingency plans are incorporated to address unforeseen challenges or delays that could impact the project schedule or budget.

The field of casing inspection is constantly evolving with several exciting advancements. One significant trend is the increased use of advanced data analytics and machine learning to improve the interpretation of inspection data and enhance defect detection. This includes automating data analysis, identifying subtle anomalies that might be missed by human analysts, and developing predictive models to forecast potential failures. Another key trend is the development of more sophisticated inspection tools, such as high-resolution inline inspection tools that provide detailed, three-dimensional images of the casing, enabling better assessment of defects. Furthermore, the integration of different data sources, including production data and geological information, provides a more comprehensive picture of well integrity, improving our ability to predict and prevent failures. The use of robotic and autonomous technologies is also gaining traction, improving efficiency and potentially reducing the risks associated with well intervention. These advancements are continuously enhancing our ability to monitor and maintain the integrity of oil and gas wells.