Search citation statements
Paper Sections
Citation Types
Year Published
Publication Types
Relationship
Authors
Journals
Annular shale creep barriers which can guarantee long-term well integrity over the entire lifespan of the well can be stimulated by temperature elevation caused by artificial heating inside the wellbore. Prior work has shown that heating can significantly accelerate barrier formation, but may also damage the shale formation if certain temperatures are applied. This paper reports on the optimum thermal conditions for shale barrier formation based on extensive new laboratory as well as literature data. Thermally accelerated creep behavior was studied for the Lark and the Shetland North Sea shales. Large-scale triaxial equipment was used to study the behavior of shales under downhole stress and pressure conditions while varying temperature. In addition, an extensive literature study investigated the thermal effect of shales used for nuclear containment, such as the Boom Clay in Belgium, Cox Shale in France, and Opalinus Clay in Switzerland. The investigation focused on the impacts of temperature elevation on important shale properties such as creep rate, sealing and self-healing ability, and temperature-induced porosity, permeability, and mineralogical changes. Both the laboratory investigation and the literature study showed that there is an optimum range for artificial thermal stimulation of shale barriers, with an upper temperature of 200°C – 300°C that should not be exceeded. At lower temperatures, thermal pore fluid expansion may lead to effective stress reduction and shear failure on shale bedding planes. In the optimum range, fluid thermal expansion is effectively negated by thermally-induced shale consolidation, and barrier formation is optimally accelerated, which is of great practical value for field implementations. Above the optimum range, irreversible dehydration and metamorphosis of the clay constituent of the shale happen and the shale loses its ability to creep to form a barrier and self-heal. This important result shows that heating inside wellbores to improve/accelerate creep of shales needs to be a controlled, engineered process in order to yield a competent barrier. This favors the use of a temperature-controlled heater rather than a less-controllable exothermic reaction. Shale barriers seal annuli much better and more reliably than cement barriers. Moreover, their self-healing ability offers the ability to guarantee annular well integrity for an indefinite period, including the P&A phase. Thermal stimulation is preferred by operators to accelerate barrier formation without requiring annular access. The findings of this paper provide important theoretical and practical guidance on how to optimally stimulate shale barriers and avoid pitfalls associated with thermally-induced shale damage.
Annular shale creep barriers which can guarantee long-term well integrity over the entire lifespan of the well can be stimulated by temperature elevation caused by artificial heating inside the wellbore. Prior work has shown that heating can significantly accelerate barrier formation, but may also damage the shale formation if certain temperatures are applied. This paper reports on the optimum thermal conditions for shale barrier formation based on extensive new laboratory as well as literature data. Thermally accelerated creep behavior was studied for the Lark and the Shetland North Sea shales. Large-scale triaxial equipment was used to study the behavior of shales under downhole stress and pressure conditions while varying temperature. In addition, an extensive literature study investigated the thermal effect of shales used for nuclear containment, such as the Boom Clay in Belgium, Cox Shale in France, and Opalinus Clay in Switzerland. The investigation focused on the impacts of temperature elevation on important shale properties such as creep rate, sealing and self-healing ability, and temperature-induced porosity, permeability, and mineralogical changes. Both the laboratory investigation and the literature study showed that there is an optimum range for artificial thermal stimulation of shale barriers, with an upper temperature of 200°C – 300°C that should not be exceeded. At lower temperatures, thermal pore fluid expansion may lead to effective stress reduction and shear failure on shale bedding planes. In the optimum range, fluid thermal expansion is effectively negated by thermally-induced shale consolidation, and barrier formation is optimally accelerated, which is of great practical value for field implementations. Above the optimum range, irreversible dehydration and metamorphosis of the clay constituent of the shale happen and the shale loses its ability to creep to form a barrier and self-heal. This important result shows that heating inside wellbores to improve/accelerate creep of shales needs to be a controlled, engineered process in order to yield a competent barrier. This favors the use of a temperature-controlled heater rather than a less-controllable exothermic reaction. Shale barriers seal annuli much better and more reliably than cement barriers. Moreover, their self-healing ability offers the ability to guarantee annular well integrity for an indefinite period, including the P&A phase. Thermal stimulation is preferred by operators to accelerate barrier formation without requiring annular access. The findings of this paper provide important theoretical and practical guidance on how to optimally stimulate shale barriers and avoid pitfalls associated with thermally-induced shale damage.
As part of the energy transition and the aim to reduce greenhouse gas (GHG) emissions, more carbon in the form of CO2 will be captured and stored underground in wells intersecting suitable reservoirs for storage. The long-term integrity of such wells is a considerable concern, given that CO2 is a fluid that reacts with Portland cement and steel, which can erode well barriers over time. Moreover, low temperatures and temperature cycling in injection and storage wells can lead to cement cracking and debonding from casing, creating annular flow paths for CO2 to surface and allowing for CO2 to attack cement more severely. This paper reports on an investigation into using shale formations as alternative annular barrier that can guarantee integrity during CO2 injection and long-term storage. Building upon previous work done as part of our ongoing Shale-as-a-Barrier (SAAB) investigation, rock mechanical laboratory tests were conducted into the behavior of shale creep in wells experiencing CO2 injection. A special experimental setup was constructed to be able to establish an annular shale barrier at simulated field conditions (using either in-situ formation temperature or thermal stimulation) and then testing this barrier during simulated CO2 injection conditions. During CO2 injection, the well will experience a very significant reduction in temperature, which in conventional wells can lead to the debonding of cement from the casing and the formation of a micro-annulus that compromises the annular barrier. Note that temperature cycling in wells is a lead cause of the loss of annular isolation and flow to surface in oil and gas wells. In the experiments, shale barriers were first generated and verified at a variety of in-situ and elevated temperatures (which affect shale creep rate). The barriers were than subjected to a significant temperature reduction and temperature cycling with wellbore temperatures reaching a low value of -14 °C. In all cases, the shale barrier continued to function and maintain annular pressure integrity, indicating that well temperature reduction and cycling associated with CO2 injection will not negatively affect it. This is a very significant result and insight, because the same cannot be guaranteed for a Portland cement barrier. In addition, shale barriers are impervious to any chemical attack by CO2 and are expected to last for an indefinite time period, given that we are dealing with actual caprock material. Carbon storage wells pose new challenges to well construction. These include the low absolute temperatures and large cyclic temperature cycles during CO2 injection which could lead to cement debonding and micro-annulus formation providing a pathway for CO2 migration to surface, as well as chemical attack of cement and casing by CO2 during long-term storage. This work shows that creeping shale formations can deal with both challenges, and provide a superior annular isolation solution when compared to conventional Portland cement. The work could have large positive implications for how (barriers in) carbon storage wells will be constructed in future, and how permanent storage of CO2 underground can be guaranteed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.