For successful delivery of well integrity (WI), there needs to be an understanding of the risks that can cause undesirable events such as safety hazards or loss of containment. Performing a risk assessment (RA) on a well, or type of well, will help determine and rank the potential risks and provide information that allows limited resources to be applied in the most effective manner. The main objectives of performing a risk assessment include (a) following a formal process to assess risk consistently and to enable comparison between well-barrier failure-mode scenarios; (b) qualitatively assessing well-barrier failure risk for every segment of a well; (c) documenting suggestions that are offered by the riskassessment team for mitigating well-barrier failure risk; and (d) providing a report of the methodology, failure-mode scenarios, risk ranking, and potential mitigation actions for use as a reference tool for managing WI on a routine basis.Our WI/RA model follows a common qualitative risk-assessment process-a team-based, structured brainstorming format, using the "What-If Methodology" to identify potential hazards associated with well-barrier failure modes. In addition, the model has the following attributes:It incorporates a unique method to segment well barriers into discrete sections, successively "failing" each section for evaluation. The list of analyzed well-barrier failure modes, along with their risk ranking, becomes the risk register for the well or type of well.It is adaptable for assessing well-barrier failure modes on a single well, or a group of wells, having the same general design parameters. An entire well portfolio can be assessed quickly by analyzing types of wells rather than individual wells.It can be used to assess well-barrier failure risk for any type of well.The model can easily be modified to conform to any company's risk model.The WI/RA model has been proven to Successfully assess well-barrier failure risk for thousands of wells Focus specifically on well-barrier failure modes, and as a result be an effective tool that should be incorporated into a "bestin-class" WI program Be used as a management tool to provide guidance for how limited resources can be used effectively to continuously deliver WI.
When operators are faced with issues involving casing leaks, a typical course of action is to pull the tubing and make efforts to identify and locate the source of the leak by logging or other mechanical means. If the leak source can be successfully located, a mechanical method is generally employed to patch the leaking casing. This methodology is time consuming and expensive. Locating casing leaks with the tubing in place using conventional logging techniques has historically been difficult. Where some tools, such as temperature tools, may provide an indication of an anomaly in annuli, the data may be subjective or the leak may be too small to measure. When active, a leak will produce a spectrum of sonic frequencies that may be either audible, ultrasonic or both. Ultrasonic energy will pass through steel but travels relatively short distances. A tool developed around these principles has been successful in accurately locating casing leaks behind tubing. Pressure-activated sealants have been used for a number of years to cure a wide variety of leaks in casing, tubing, control lines, and well heads as well as micro-annulus leaks in cement. For the purpose of repairing a casing leak behind tubing, the liquid sealant may be pumped into the annulus and displaced to the leak site. The liquid sealant will not polymerize until it is exposed to the differential pressure through the leak site. Knowing the leak rate, pressure and precise location of the leak aids in the selection of the sealant formulation and deployment method. This helps to reduce overall repair cost as well as increase the probability of a successful repair. This paper will describe the ultrasonic method of leak detection and the method of curing leaks with pressure activated sealant with tubing in place. Case histories will be presented where these methods were employed to repair casing leaks without removing the tubing. Introduction Perhaps the most challenging well integrity issue with which operators deal with today are casing leaks. Not only are the methods to repair these types of leaks without pulling the tubing limited, but the detection of these leaks using conventional logging methods with the production tubing in place is practically impossible. A common diagnostic methodology is to rely on some fairly subjective logging data and pressure responses to determine where a pressure barrier is leaking. Following this, cement is pumped down the annulus or through punched tubing in an attempt to seal off the leak. This process, along with other hardening sealant methods, can be problematic. Additionally, using this method will also make other operations or future workovers difficult or impractical. Pressure activated sealants have been used on numerous occasions to repair casing leaks with the tubing in place. A major advantage in utilizing this technology is that the sealant will only solidify where the leak is active. In addition, the material is easily removed by mechanical means and will not add difficulty to future workover operations if required. As is true with other remediation methods, a complete understanding of the leak source is critical when planning a pressure activated sealant operation. This is especially true when dealing with leaks behind the tubing. Optimal sealant formulations may be selected along with deployment methods for maximum affect. While rate and differential can be determined by pressure and well bore data, a leak behind casing is more complex. Detection of casing leaks is difficult using conventional logging techniques. These leaks will produce no reading on spinners (for obvious reasons) and may not produce temperature changes that are of a magnitude to confirm a leak point. This is true even with fairly large leaks (>1gpm). Conventional noise logs can detect fluid or gas movement, but must be used in a stationary mode and distant noise sources may confuse interpretation. Tracer logs may be used but can also produce imprecise results. The ultrasonic leak detection method has been proven to be useful in detecting leaks behind casing with a high degree of accuracy. This suggests that it would be a useful tool in evaluating wells for repair using a pressure activated sealant method where accurate spotting of the treatment is critical.
Summary When operators are faced with well-integrity problems, a variety of methods may be used to detect the source of annular communication. Methods for detecting downhole leak points include spinners, temperature logs, downhole cameras, thermal-decay logs, and noise logs. However, many of these methods are ineffective when dealing with very small leaks and can result in collected data that require a significant amount of logging finesse to interpret. Ultrasonic listening devices have been used for a number of years to detect leak sources effectively in surface production equipment. Ultrasonic energy has some properties that, when compared to audible-frequency energy, make it ideal for accurate leak detection (Beranek 1972; Povey 1997; Evans and Bass 1972). Like audible-frequency energy, ultrasonic energy can pass through steel. However, ultrasonic energy propagates relatively short distances through fluids when compared to equal-energy audible-frequency sound. Thus, when an ultrasonic signal of this nature is detected, the detection tool will be in close proximity to the energy source. On this premise, an ultrasonic leak-detection tool was developed for downhole applications to take advantage of the unique properties of ultrasonic-energy propagation through various media. Data-acquisition equipment and filtering algorithms were developed to allow continuous logging conveyed on standard electric line at common logging speeds. Continuous logging has proved to be significantly more efficient in locating anomalies than static logging techniques commonly used in noise-logging operations. During development, the tool was shown to be effective in locating leaks as small as 0.026 gal/min with an accuracy of 3 ft in production tubing, casing, and other pressure-containing completion equipment. Leaks also have been detected through multiple strings of tubing and casing. The tool has proved to be effective in locating leaks that other diagnostic methods were unable to locate.
For successful delivery of Well Integrity, there needs to be an understanding of the risks that can cause undesirable events such as safety hazards or loss of containment. Performing a risk assessment on a well, or type of well, will help determine and rank the potential risks and provide information that allows limited resources to be applied in the most effective manner. The main objectives of performing a risk assessment include: Follow a formal process to assess risk consistently and to enable comparison between well barrier failure mode scenarios;Qualitatively assess well barrier failure risk for every segment of a well;Document suggestions that are offered by the risk assessment team for mitigating well barrier failure risk; andProvide a report of the methodology, failure mode scenarios, risk ranking, and potential mitigation actions for use as a reference tool for managing well integrity on a routine basis. The Well Integrity Risk Assessment Model follows a common qualitative risk assessment process; a team-based, structured brainstorming format, utilizing the What-If Methodology, to identify potential hazards associated with well barrier failure modes. In addition, the model has the following attributes: Incorporates a unique method to segment well barriers into discrete sections, successively "failing" each section for evaluation. The list of analyzed well barrier failure modes, along with their risk ranking, becomes the risk register for the well or type of well.Is adaptable for assessing well barrier failure modes on a single well, or a group of wells, having the same general design parameters. An entire well portfolio can be assessed quickly by analyzing types of wells rather than individual wells.Can be used to assess well barrier failure risk for any type of well.The model can easily be modified to conform to any company's risk model. The Well Integrity Risk Assessment Model has been proven to: Successfully assess well barrier failure risk for thousands of wells;Focus specifically on well barrier failure modes, and as a result is an effective tool that should be incorporated into a "Best in Class" Well Integrity Program;Be utilized as a management tool to provide guidance for how limited resources can be used effectively to continuously deliver well integrity.
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