• Energy Research

Capturing and storing carbon dioxide (CO2) underground for thousands of years is one way to reduce atmospheric greenhouse gases, often associated with global warming. Leakage through wells is one of the major issues when storing CO2 in depleted oil or gas reservoirs. CO2-injection candidates may be new wells, or old wells that are active, closed or abandoned. In all cases, it is critical to ensure that the long-term integrity of the storage wells is not compromised. The loss of well integrity may often be explained by the geochemical alteration of hydrated cement that is used to isolate the annulus across the producing/injection intervals in CO2-related wells. However, even before any chemical degradation, changes in downhole conditions due to supercritical CO2 injections can also be responsible for cement debonding from the casing and/or from the formation, leading to rapid CO2 leakage. A new cement with better CO2 resistance is compared with conventional cement using experimental procedure and methodology simulating the interaction of set cement with injected, supercritical CO2 under downhole conditions. Geochemical experimental data and a mechanical modeling approach are presented. The use of adding expanding property to this new cement to avoid microannulus development during the CO2 injection is discussed.

AbstractThe concept of CO2 storage relies on the long-term sealing properties of both the geological trap and the wells needed to inject and monitor CO2. Well integrity, a classical topic in the oil and gas industry, is thus critical for the performance of any CO2 storage complex in terms of containment. Thanks to the very low permeability of cement (typically less than 0.1 mDarcy); a properly cemented well ensures hydraulic isolation between reservoirs layers and shallow aquifers. Moreover, such low matrix permeability limits the cement/ CO2 interactions over the active period of a storage complex (of the order of 100 years) to a few meters. Leaks from a cased and cemented well, if any, are known to occur only through defects: mud-channel (in case of poor cement placement), cracks within cement and more importantly micro-annulus at the casing/cement or/and cement/formation interfaces. This last category of defects can lead to substantial leakage rate. Its importance has been recognized by the oil and gas industry since the 1960’s leading to the study of cement “bonding” properties. In the scope of CO2 storage, the understanding, modeling and monitoring of the occurrence of micro-annulus becomes of prime importance. We analyze the complete loading history of a cemented completion from cement placement to routine well operations. Further to classical failure type assessment used in the oil and gas industry (i.e. fail/no fail, good cement/bad cement), we aim at quantifying the vertical extent, azimuthal coverage and width of the created defects to adequately transform failure types into leakage pathways. Such a prediction of connected defects/leakage pathways along a cemented well imposes to consistently integrate the effects of lithology, geomechanics, cement placement (fluid loss, hydration), completion design and knowledge of pressure and thermal variation during the life of the well.The modeling of such a problem can be made tractable by recognizing the intrinsic hierarchy of lengthscales of a cemented well (i.e. the cement annulus is much thinner than the well dimension). The original three-dimensional problem is reduced to a much simpler two-dimensional one, which in turn can even be further reduced to a one-dimensional configuration in a lot of practical cases.Typical cases of interface debonding due to well de-pressurization and thermal cooling taking place after cement placement are carefully analyzed. Furthermore, we specially focus on injectors. Despite the use of all current best practices during well construction, the injection in itself can lead to the propagation of a debonding crack between cement and casing or cement and formation due to the high pressure generated at the perforations level. Such a problem has already been reported in hydraulic fracturing operations, and is a reasonable explanation of observed well leaks for injectors. A consistent model predicting the initiation and propagation of interface debonding during injection operations is then compared to carefully designed laboratory experiments. Such experiments also confirm that the azimuthal coverage of the interface debonding is only partial (i.e. less than 360°), an observation consistent with cement evaluation logs acquired on CO2 injectors. Finally, best practices to achieve and retain well integrity of CO2 injectors are highlighted from a careful examination of the results of both the model and the experiment.

AbstractThis reservoir geomechanical study assesses the impact on top and fault seals integrity of fluid pressure changes associated with carbon dioxide (CO2) storage in a saline formation. The case studied is the CO2SINK experiment at in Ketzin, Germany, where up to 60 ktons of CO2 are being injected. Injection commenced in June 2008.A 3-dimensional (3D) geomechanical model of the site is built through integrated analyses of geologic, seismic, logging, drilling, and laboratory test data. First, the grid is expanded from a reservoir model up to surface, down to basement and laterally by about 3 times the pressure perturbation dimensions, while honouring all available structural, stratigraphic and lithological data. The grid cells are populated with density, poroelastic and strength properties upscaled from a 1-dimensional (1D) mechanical model built and validated along the Ktzi 201/2007 CO2 injector well. Cells cut by faults are considered an equivalent medium representative of a jointed rock mass.The 3D geomechanical model is then dynamically linked to the reservoir model. Static equilibrium prior to injection is achieved by applying initial fluid pressure and gravity loads, as well as stress boundary conditions chosen so as to match in situ stress measurements. Stress path and rock deformation associated with CO2 injection are then simulated. Pressure change data is passed from the flow simulator to the geomechanical simulator at selected time steps. Calculated stress path and strains are then used to investigate the possible occurrence and location of caprock failure and fault reactivation. Other results, such as ground surface elevation changes and sources of uncertainties are also highlighted. No failure is observed in the caprock and faults remain stable during CO2 injection operations. Limited vertical displacement (maximum 5 mm) is predicted at surface.