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The Extended Leak Off Test (XLOT) is a sophisticated formation integrity test that can be performed during drilling, recompletion, or at the well abandonment stage. The test is usually characterized by multiple cycles, creating and manipulating a fracture that can extend several meters away from the wellbore. The test can provide more data (both formation stress and fracture mechanics) compared to traditional leak-off tests. This data is used extensively both for determination of the in-situ formation stress for well barrier integrity assessment and for more general rock mechanical work such as quantifying fracture gradient for use in wellbore stability programs for drilling and completion operations. The interpretation is performed by analysis of the surface pressure and, often with downhole data from memory gauges (or, increasingly, with real-time data from wired pipe) at different stages of the XLOT test. The typical XLOT pressure analysis chart is shown below (see Fig.1). The key determined parameters are:–Leak Off Pressure (LOP)–Fracture Initiation Pressure (FIP)–Formation Break Down Pressure (FBR)–Formation Propagation Pressure (FPP)–Instantaneous Shut-In Pressure (ISIP)–Formation Closure Pressure (FCP)–Fracture Reopening Pressure (FRP)Figure 1The traditional XLOT interpretation plot. A key requirement of the test is to ensure hydraulic connectivity to the targeted formation only. This can be achieved in the case where annulus barriers are in place and perform well. Unintentional communication to non-targeted zones may result in abnormal behavior, more complex interpretation of obtained data, larger uncertainty in the meaning of the results and ultimately failure of the XLOT test. To verify the well barriers integrity prior to the XLOT different techniques can be utilized. The main one is cement bond logging across the cemented barriers. This indicates the condition of the cement behind the first casing and increases the level of confidence the test will be conducted successfully. "However, recent case studies have shown that an indication of good bond above and/or below the target formation from a cement bond log cannot guarantee the isolation required to sufficiently hold the applied pressure [Maxim Volkov]." The paper demonstrates an approach taken by Equinor in a special application where XLOT testing was advanced by adding downhole monitoring during the test. This targeted the following parameters to evaluate the new essential components of XLOT interpretation: –depth and capacity of opened and re-opened fractures,–actual sealing of the cement barriers above and below the targeted zone,–failure investigation in case the FBP cannot be achieved.
The Extended Leak Off Test (XLOT) is a sophisticated formation integrity test that can be performed during drilling, recompletion, or at the well abandonment stage. The test is usually characterized by multiple cycles, creating and manipulating a fracture that can extend several meters away from the wellbore. The test can provide more data (both formation stress and fracture mechanics) compared to traditional leak-off tests. This data is used extensively both for determination of the in-situ formation stress for well barrier integrity assessment and for more general rock mechanical work such as quantifying fracture gradient for use in wellbore stability programs for drilling and completion operations. The interpretation is performed by analysis of the surface pressure and, often with downhole data from memory gauges (or, increasingly, with real-time data from wired pipe) at different stages of the XLOT test. The typical XLOT pressure analysis chart is shown below (see Fig.1). The key determined parameters are:–Leak Off Pressure (LOP)–Fracture Initiation Pressure (FIP)–Formation Break Down Pressure (FBR)–Formation Propagation Pressure (FPP)–Instantaneous Shut-In Pressure (ISIP)–Formation Closure Pressure (FCP)–Fracture Reopening Pressure (FRP)Figure 1The traditional XLOT interpretation plot. A key requirement of the test is to ensure hydraulic connectivity to the targeted formation only. This can be achieved in the case where annulus barriers are in place and perform well. Unintentional communication to non-targeted zones may result in abnormal behavior, more complex interpretation of obtained data, larger uncertainty in the meaning of the results and ultimately failure of the XLOT test. To verify the well barriers integrity prior to the XLOT different techniques can be utilized. The main one is cement bond logging across the cemented barriers. This indicates the condition of the cement behind the first casing and increases the level of confidence the test will be conducted successfully. "However, recent case studies have shown that an indication of good bond above and/or below the target formation from a cement bond log cannot guarantee the isolation required to sufficiently hold the applied pressure [Maxim Volkov]." The paper demonstrates an approach taken by Equinor in a special application where XLOT testing was advanced by adding downhole monitoring during the test. This targeted the following parameters to evaluate the new essential components of XLOT interpretation: –depth and capacity of opened and re-opened fractures,–actual sealing of the cement barriers above and below the targeted zone,–failure investigation in case the FBP cannot be achieved.
Planning and execution of the well plug and abandonment(P&A) requires detailed knowledge of the downhole barrier's integrity status and position of its downhole completion elements.[Maxim Volkov] This input is utilized for the determination of zones for permanent plugs, casing cuts and retrieval procedures. The last one can be significantly optimized if conducted across the zones with minimal total wall thickness. Cutting the casing across casing collars, fins or casing decentralization may extend the operation requiring more time and resources to achieve success. However, the detailed completion information is not always available due to several well handover. This may result in casing cut performed blindly, based on the position of the first casing collars revealed by caliper or ultrasound surveys. This may result in an increase of the RIG operation timing from few hours to few days. The paper shows results of downhole scanning during three subsea wells P&A campaign at the stage of environmental plug deployment. This included cutting and pulling out of casing wellhead elements (casing stumps) after milling works, before setting environmental cement plug in seabed zone. Casing wellhead design may include the first 13 3/8" and 20" casing joint welded fins to centralize casings. Typical casing joint with fins (rigid type) is shown in figure 1. The abandonment program included milling of these casings and the cutting through the minimal or nominal wall thicknesses with avoiding fins and collars of the casings. The depth of the cutting window initially was set from the available well diagrams, which were not available in all the wells. Thus, the main role of the scanning was to verify and optimize the casings cut window depth to perform the process with minimal issues. Figure 1 Casing joint with rigid centralizers-fins.
Maintaining well integrity is critical to sustaining production from mature and aging fields. Disposable fibre optic technology has been deployed in wells in the North Sea to locate known tubing leaks in the completion. The disposable fibre optic intervention system releases a probe into the well to enable the deployment of bare fibre optic line. The fibres are released from the probe as it descends into the well. In the presented case study, the probe contained both single-mode and multi-mode fibre optic lines to enable simultaneous Distributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS) surveys to be performed. Once deployed in the well, pressure manipulation programs were performed to activate any tubing or casing leaks while acquiring DTS and DAS data. As a result of the exceptional sensitivity of the bare fibres and the effective coupling of the fibre with the tubing wall the technology is shown to be highly effective in detecting leaks and confirming barrier integrity. In the presented example a leak was located along with the direction and rate of the fluid movement in the ‘B’ annulus. The simplicity of the system and highly efficient operations greatly reduced survey times in comparison to conventional intervention techniques thereby greatly reducing the cost of intervention. It can be demonstrated that the disposable fibre optic deployment system provides a game changing and cost-effective solution for both leak detection and determining liquid levels in the wells. The disposable fibre solution is a unique deployment method which provides an alternative to conventional well surveys, reducing the complexity, time and cost to acquire valuable distributed well data. This is the first case history published for this technology in leak detection application.
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