Turning a Negative Into a Positive: Shale Annular Barrier Identification for Plug and Abandonment
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Summary Well abandonment is one of the biggest challenges in the oil and gas industry, both in terms of cost and effort as well as the technical hurdles associated with wellbore sealing for an indefinite term. A mechanism that may be exploited to simplify well abandonments is using natural shale or salt formations for the creation of annular barriers. Currently, uncemented annuli often require casing cutting and pulling or milling before abandonment plugs can be set, which necessitates the use of a drilling rig. This is an expensive, time- and labor-intensive process, particularly offshore. However, shale or salt creep may naturally form a barrier behind uncemented casing sections. With a qualified annular shale barrier in place, the well may only require the setting of abandonment plugs within the existing casing string(s), a task that can often be done rigless and with significantly less effort. The work described in this paper presents the results of a rock mechanical investigation into the creep behavior of North Sea shales and their ability to form effective annular barriers. Field core from the Lark shale, a member of the Hordaland Group, was used to conduct dedicated, customized experiments that simulated the behavior of shale confined under downhole effective stress, pressure, and temperature conditions to fill in an annular space behind a simulated casing string. Full-scale triaxial rock mechanics equipment was used for testing cylindrical shale samples obtained from a well-preserved field core in a setup that mimicked an uncemented casing section of a well. The deformation behavior of the shale was monitored for days to weeks, and the formation of the annular barrier was characterized using dedicated strain measurements and pressure pulse decay probing of the annular space. The large-scale laboratory results clearly show that the Lark shale will form competent low permeability annular barriers when left uncemented, as confirmed using pressure-pulse decay measurements. They also show that experimental conditions influence the rate of barrier formation; higher effective stress, higher temperature, and beneficial manipulation of the annular fluid chemistry all have a significant effect. This then opens up the possibility of activating shale formations that do not naturally create barriers by themselves into forming them (e.g., by exposing them to low annular pressure, elevated temperature, different annular fluid chemistry, or a combination of these). The results are in very good agreement with field observations reported earlier by several North Sea operators.
Summary Well abandonment is one of the biggest challenges in the oil and gas industry, both in terms of cost and effort as well as the technical hurdles associated with wellbore sealing for an indefinite term. A mechanism that may be exploited to simplify well abandonments is using natural shale or salt formations for the creation of annular barriers. Currently, uncemented annuli often require casing cutting and pulling or milling before abandonment plugs can be set, which necessitates the use of a drilling rig. This is an expensive, time- and labor-intensive process, particularly offshore. However, shale or salt creep may naturally form a barrier behind uncemented casing sections. With a qualified annular shale barrier in place, the well may only require the setting of abandonment plugs within the existing casing string(s), a task that can often be done rigless and with significantly less effort. The work described in this paper presents the results of a rock mechanical investigation into the creep behavior of North Sea shales and their ability to form effective annular barriers. Field core from the Lark shale, a member of the Hordaland Group, was used to conduct dedicated, customized experiments that simulated the behavior of shale confined under downhole effective stress, pressure, and temperature conditions to fill in an annular space behind a simulated casing string. Full-scale triaxial rock mechanics equipment was used for testing cylindrical shale samples obtained from a well-preserved field core in a setup that mimicked an uncemented casing section of a well. The deformation behavior of the shale was monitored for days to weeks, and the formation of the annular barrier was characterized using dedicated strain measurements and pressure pulse decay probing of the annular space. The large-scale laboratory results clearly show that the Lark shale will form competent low permeability annular barriers when left uncemented, as confirmed using pressure-pulse decay measurements. They also show that experimental conditions influence the rate of barrier formation; higher effective stress, higher temperature, and beneficial manipulation of the annular fluid chemistry all have a significant effect. This then opens up the possibility of activating shale formations that do not naturally create barriers by themselves into forming them (e.g., by exposing them to low annular pressure, elevated temperature, different annular fluid chemistry, or a combination of these). The results are in very good agreement with field observations reported earlier by several North Sea operators.
Annular Creep Barrier Evaluation and Qualification Using Ultrasonic Measurements
Summary It is well-known that formations that exhibit active creep behavior under downhole conditions, such as reactive shales and mobile salts, can form annular barriers across uncemented or poorly cemented annular sections behind casing strings. Such creep barriers can simplify well abandonments, particularly in high-cost offshore environments. Evaluation and qualification of creep barriers in the field, however, have proven challenging and labor-intensive when casing is perforated, and annular rock material is pressure-tested to verify its sealing ability. This work seeks to eliminate the need for pressure testing by allowing the barrier to be qualified using only casedhole log measurements. Sophisticated rock mechanical laboratory experiments under realistic downhole conditions were conducted to investigate the formation of creep barriers by North Sea Lark shale. The experiments evaluated barrier formation while varying annular fluid chemistry and temperature. Measurement parameters included creep rate, pressure transmission across newly formed barriers, pressure breakthrough through the newly formed barriers as well as ultrasonic responses by the shale. It was found that the Lark/Horda shale has a distinct anisotropic ultrasonic wave velocity profile that uniquely characterizes it. This can be used to identify its presence in an annular space when contacting the casing. A main conclusion is that a Lark shale barrier can be qualified through casedhole sonic and ultrasonic logging alone without the need for pressure testing if (1) the magnitude of the wave propagation velocity of the shale behind casing can be confirmed (2077 m/s for Lark shale); (2) the characteristic velocity anisotropy profile, unique to the shale (~10.1% for Lark shale), can be verified; (3) good contact with/bonding to the casing is observed; and optionally (4) anisotropy in the time behavior of the shale contacting the pipe is observed when the barrier is stimulated artificially. If these conditions are met, then our experiments show that the barrier will have excellent hydraulic sealing ability, with a permeability of a few microdarcies at most and a breakthrough pressure that approaches the minimum horizontal effective stress value. Additional findings are that shale heating will accelerate barrier formation but may damage the shale formation in the process. Extraordinary fast annular closure and barrier formation with evident shale rehealing was observed by using a concentrated KCl solution as pore fluid, showing the merits of barrier stimulation by chemical means. This result can be explained by considering the effect of solutes on shale hydration forces.
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