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 isolation for an indefinite term. A mechanism that may be exploited to simplify well abandonments is using natural shale formations for the creation of annular barriers. Currently, uncemented annuli often require casing milling and pulling 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 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-Horda shale 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 tri-axial rock mechanics equipment was used for testing cylindrical shale samples obtained from well-preserved field core in a set-up 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 lab results clearly show that the Lark-Horda shales 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. The results are in very good agreement with field observations reported earlier by several North Sea operators.

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Oil and gas wells produce hydrocarbons for a limited number of years, and at the end of their production life they need to be plugged and abandoned. This process has to be done in a safe and economic way. Creep deformation of shale rock in uncemented casing sections may simplify well abandonments considerably. Creep can close the annular gap between a shale formation and an uncemented section of a casing string, generating a barrier that prevents hydrocarbons from flowing to the surface on the annular side. Wells with such a "shale-as-a-barrier" generated by creep now only require abandonment plugs on the inside of the casing, without the need for installation of additional annular barriers. This may eliminate such operations as casing milling and casing pulling, thereby allowing e.g. offshore abandonments to be done rigless, at significantly reduced cost. This paper presents the first results of an experimental investigation and numerical modeling study into the nature of the "shale-as-a-barrier" phenomenon. Specifically, we focus on laboratory and field scale numerical simulation of creep behavior of North Sea Lark shale rock for oil and gas well plug and abandonment purposes. In our Finite Element simulations of the shale creep phenomenon, we have used the time-hardening creep model, which assumes a non-linear relationship between creep strain and stress, temperature and time. The model parameters were obtained from a curve fit of laboratory experimental results conducted for a creeping shale. Then, using the experimentally-derived parameters, numerical simulation was performed for a laboratory scale model and result was validated against laboratory results. Once this validation had taken place, the model size was extended to the field scale for prediction of annular closure time and barrier formation. Simulations show a strong correlation between rock stiffness and annular gap closure time, as expected; hence, the success of any "shale-as-a-barrier" project is a distinct function of shale rock stiffness. Lowering near-wellbore stiffness artificially may accelerate annular barrier creation of slowly creeping shale formations.

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 cased-hole log measurements. Sophisticated rock mechanical lab 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 cased-hole 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/sec 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 formed / 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 micro-Darcy 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. Extra-ordinary fast annular closure and barrier formation with evident shale re-healing 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.

Chasing new plays often make operators reach to complex and non-traditional reservoirs. Especially, at offshore hydrocarbon discoveries it becomes critically important to characterize the fluid, assess pressure profile and understand the flow behavior. This work is a case history of successfully implemented reservoir surveillance techniques at HTHP offshore green field.The key challenges included presence of high pressure pockets/ panic zones, high cost of well testing with additional slickline & logistics, uncertainties of fluid phase and flow behavior at really tough HTHP conditions. These challenges were addressed by utilizing a flexible mix of modern day technologies. This successful case on reservoir surveillance is elaborated in the present abstract.1. Heavy duty dual packer Modular Dynamic testing: The dual packer formation evaluation tester could withstand 400F and 13500 psi formation pressures; while keeping drawdown in ultra low permeability sandy siltstone. 2. HTHP sour gas Permanent Down-hole Gauges (PDG): The presence of CO2 (nearly 10%) and H2S (110 ppm) with steam fractions was fatal for the wellbore integrity. Use of sour-proof Permanent downhole gauges to actively monitor the BHP & BHT as close to perforations. 3. Extreme performer Production Logging Techniques (Extreme PLT) for Ͼ400F zones: At high temperature he fluid phase behaviour, pay-zone resistivity and thin transition zone is hardly predictable. To overcome the uncertainties of fluid type, hold and thief zones the extreme PLT tools played the judging role.This paper shows insights of learning and how it was utilized as a solution. Huge saving was achieved on accounts of i. Fluid characterization and facility selection, ii. Recognized panic/ pressure inversion zones for future drilling campaign, iii. Saved cost of repeated slick-line operations for well testing by real-time well performance monitoring with PDG.The learning phase was relatively longer but the learned decision added significant value to the project. Use of tools and tailor made combinations made Reservoir Surveillance much easier, cost effective and ultimately reduced the uncertainties. The field has been put on production within relatively shorter timeframe.