James P. Verdon

Comparison of geomechanical deformation induced by megatonne-scale CO2 storage at Sleipner, Weyburn, and In Salah

Significance The economic and political viability of carbon capture and sequestration (CCS) is dependent on the secure storage of CO2 in subsurface geologic reservoirs. A key leakage risk is that posed by geomechanical deformation generating fractures in otherwise sealing caprocks. This study examines this risk, comparing and contrasting deformation induced at three large-scale CCS sites—Sleipner (Norwegian North Sea), Weyburn (Canada), and In Salah (Algeria). These sites show very different geomechanical responses, highlighting the importance of systematic geomechanical appraisal prior to injection, and comprehensive, multifaceted monitoring during injection at any future large-scale CCS operations. Geological storage of CO2 that has been captured at large, point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions. However, this technology will only be viable if it can be guaranteed that injected CO2 will remain trapped in the subsurface for thousands of years or more. A significant issue for storage security is the geomechanical response of the reservoir. Concerns have been raised that geomechanical deformation induced by CO2 injection will create or reactivate fracture networks in the sealing caprocks, providing a pathway for CO2 leakage. In this paper, we examine three large-scale sites where CO2 is injected at rates of ∼1 megatonne/y or more: Sleipner, Weyburn, and In Salah. We compare and contrast the observed geomechanical behavior of each site, with particular focus on the risks to storage security posed by geomechanical deformation. At Sleipner, the large, high-permeability storage aquifer has experienced little pore pressure increase over 15 y of injection, implying little possibility of geomechanical deformation. At Weyburn, 45 y of oil production has depleted pore pressures before increases associated with CO2 injection. The long history of the field has led to complicated, sometimes nonintuitive geomechanical deformation. At In Salah, injection into the water leg of a gas reservoir has increased pore pressures, leading to uplift and substantial microseismic activity. The differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site.

The effect of microstructure and nonlinear stress on anisotropic seismic velocities

Recent work in hydrocarbon reservoir monitoring has focused on developing coupled geomechanical/fluid-flow simulations to allow production-related geomechanical effects, such as compaction and subsidence, to be included in reservoir models. To predict realistic time-lapse seismic signatures, generation of appropriate elastic models from geomechanical output is required. These elastic models should include not only the fluid saturation effects of intrinsic, shape-induced, and stress-induced anisotropy, but also should incorporate nonlinear stress-dependent elasticity. To model nonlinear elasticity, we use a microstructural effective-medium approach in which elasticity is considered as a function of mineral stiffness and additional compliance is caused by the presence of low-aspect ratio displacement discontinuities. By jointly inverting observed ultrasonic P- and S-wave velocities to determine the distribution of such discontinuities, we assessed the appropriateness of modeling them as simple, planar, penny-shaped features. By using this approximation, we developed a simple analytical approach to predict how seismic velocities will vary with stress. We tested our approach by analyzing the elasticity of various sandstone samples; from a United Kingdom continental shelf (UKCS) reservoir, some of which display significant anisotropy, as well as two data sets taken from the literature.

Passive seismic monitoring of carbon dioxide storage at Weyburn

Carbon capture and storage (CCS) is currently one of several candidate technologies for reducing the emission of industrial CO2 to the atmosphere. As plans for large-scale geological storage of CO2 are being considered, it is clear that monitoring programs will be required to demonstrate security of the CO2 within the storage complex. Numerous geophysical monitoring techniques are currently being tested for this purpose, including controlled-source time-lapse reflection seismology, satellite synthetic aperture radar interferometry, electromagnetic sounding, gravity, and others. Passive seismic monitoring is an additional technique under consideration that complements these other techniques, and has potential as a cost-effective method of demonstrating storage security. This is particularly true over longer periods of time, as passive seismic arrays cost relatively little to maintain. Of the large-scale CCS pilot projects currently operational, thus far only the IEA GHG Weyburn-Midale CO2 Monitoring and St...

Exploring trends in microcrack properties of sedimentary rocks: An audit of dry-core velocity-stress measurements

Rock-physics models are used increasingly to link fluid and mechanical deformation parameters for dynamic elastic modeling. We explore the input parameters of an analytical stress-dependent rock-physics model. To do this, we invert for the stressdependent microcrack parameters of more than 150 sedimentary rock velocity-stress core measurements taken from a literature survey.Theinversionschemeisbasedonamicrostructuraleffective-medium formulation defined by a second-rank crack-density tensor scalar crack model or by a second- and fourth-rank crack-density tensor joint inversion model. Then the inversion results are used to explore and predict the stress-dependent elastic behavior of various sedimentary rock lithologies using an analytical microstructural rock-physics model via the initial model inputparameters:initialcrackaspectratioandinitialcrackdensity. Estimates of initial crack aspect ratio are consistent among most lithologies with a mean of 0.0004, but for shales they differ uptoseveraltimesinmagnitudewithameanof0.001.Estimates of initial aspect ratio are relatively insensitive to the inversion method, although the scalar crack inversion becomes less reliableatlowvaluesofnormal-to-tangentialcrackcomplianceratio BN /BT.Initialcrackdensityissensitivetothedegreeofdamage as well as the inversion procedure. An important implication is thatthefourth-rankcrack-densitytermisnotnecessarilynegligible for most sedimentary rocks and evaluation of this term or BN /BT is necessary for accurate prediction of initial crack density. This is especially important because recent studies suggest thatBN /BTcanindicatefluidcontentincracks.

Significance for secure CO2 storage of earthquakes induced by fluid injection

The link between subsurface fluid injection and induced seismicity has gained recent significance with an increase in earthquakes associated with the disposal of oilfield waste fluids. There are obvious similarities between wastewater reinjection and proposed CO2 storage (CCS) operations. However, as well as the seismic hazard, induced seismicity during CCS operations poses additional risks, because an induced event located above the target reservoir could compromise the hydraulic integrity of the caprock. In this paper we re-examine case examples where earthquakes have been induced by wastewater injection into deep aquifers in the light of proposed future CCS operations. In particular we consider possible controls on event magnitudes, and look at the spatial distributions of events. We find that the majority of events are located below the target reservoirs. This is an encouraging observation from the perspective of caprock integrity, although it presents a challenge in terms of pre-injection characterization of deep-lying faults several kilometres below the target zone. We observe that 99% of events are found within 20 km of injection wells, suggesting a minimum radius for geomechanical characterization and monitoring. We conclude by making recommendations for modelling and monitoring strategies to be followed prior to and during commercial-scale deployment of CO2 storage projects.

Reservoir stress path characterization and its implications for fluid-flow production simulations

ABSTRACT The reduction of fluid pressure during reservoir production promotes changes in the effective and total stress distribution within the reservoir and the surrounding strata. This stress evolution is responsible for many problems encountered during production (e.g. fault reactivation, casing deformation). This work presents the results of an extensive series of 3D numerical hydro-mechanical coupled analyses that study the influence of reservoir geometry and material properties on the reservoir stress path. The stress path is defined in terms of parameters that quantify the amount of stress arching and stress anisotropy that occur during reservoir production. The coupled simulations are performed by explicitly coupling independent commercial geomechanical and flow simulators. It is shown that stress arching is important in reservoirs with low aspect ratios that are less stiff than the bounding material. In such cases, the stresses will not significantly evolve in the reservoir, and stress evolution occurs in the over- and sideburden. Stiff reservoirs, relative to the bounding rock, exhibit negligible stress arching regardless of the geometry. Stress anisotropy reduces with reduction of the Youngs modulus of the bounding material, especially for low aspect ratio reservoirs, but as the reservoir extends in either or both of the horizontal directions, the reservoir deforms uniaxially and the horizontal stress evolution is governed by the Poissons ratio of the reservoir. Furthermore, the effect of the stress path parameters is introduced in the calculation of pore volume multiplier tables to improve non-coupled simulations, which otherwise overestimate the average reservoir pore pressure drawdown when stress arching is taking place.

The robustness of seismic moment and magnitudes estimated using spectral analysis

We present an assessment of how microseismic moment magnitude, MW ,e stimates vary with the method and parameters used to calculate seismic moment. This is an important topic for operators and regulators who require good magnitude estimates when monitoring induced seismicity. It is therefore imperative that these parties know and understand what errors exist in given magnitude values, something that is poorly reported. This study concentrates on spectral analysis techniques and compares MW computed in the time and frequency domains. Using recordings of MW > −1. 5e vents at Cotton Valley, east Texas, the maximum discrepancy between MW estimated using the different methods is 0.6 units, a significant variation. By adjusting parameters in the MW calculation we find that the radiation pattern correction term can have the most significant effect on M W , generally up to 0.8 units. Following this investigation we make a series of recommendations for estimating microseismic M W using spectral methods. Noise should be estimated and removed from recordings and an attenuation correction should be applied. The spectral level can be measured by spectral fitting or taken from the low frequency level. Significant factors in obtaining reliable microseismic MW estimates include using at least four receivers recording at ≥1000 Hz and making radiation pattern corrections based on focal mechanism solutions, not average values.

Gravity-driven reacting flows in a confined porous aquifer

We develop a model for the dynamics of a reactive gravity-driven flow in a porous layer of finite depth, accounting for the change in permeability and density across the dissolution front. We identify that the two controlling parameters are the mobility ratio across the reaction front and the ratio of the buoyancy-driven flow to the fluid injection rate. We present some numerical solutions for the evolution of a two-dimensional dissolution front, and develop an approximate analytic solution for the limit of large injection rate compared to the buoyancy-driven flow. The model predictions are compared with some new analogue laboratory experiments in which fresh water displaces a saturated aqueous solution initially confined within a two-dimensional reactive permeable matrix composed of salt powder and glass ballotini. We also present self-similar solutions for an axisymmetric gravity-driven reactive current moving through a porous layer of finite depth. The solutions illustrate how the reaction front becomes progressively wider as the ratio of the buoyancy-driven flow to the injection rate increases, and also as the mobility contrast across the front increases.

A comparison of passive seismic monitoring of fracture stimulation from water and CO2 injection

Hydraulic fracturing is used to create pathways for fluid migrationandtostimulateproduction.Usually,wateristheinjected fluid, although alternative fluids such as carbon dioxide CO2 have been used recently. The amount of fracturing that CO2 can induce is also of interest for the security of carbon capture and storage. Hydraulic fracturing is usually monitored using passive seismic arrays, detecting microseismic events generated by the fracturing. It is of interest to compare the amount of seismicity that CO2 injection can generate in comparison with water. With this in mind, we have analyzed a passive seismic data set monitoring the injection of water and supercritical CO2 under very similar conditions, allowing us to make a direct comparison between the fluids.We examined event locations and event magnitudes, and we used shear-wave splitting to image the fractures that are generated. For both fluids,the event locations map the formation of fractures moving away from the injection well with normalsparalleltotheminimumprincipalstress.Theeventsduring water injection are limited to the injection depth, while during CO2 injection, activity migrates above the injection depth. Eventmagnitudesaresimilarinbothcases,andlargereventmagnitudes appear to correlate with higher injection pressures. Shear-wave splitting suggests that water injection generates more fractures, though the data quality is not good enough to make a robust conclusion about this.The comparability between water and CO2 injection means that lessons can be learned from theabundantexperienceofconventionalwaterinjection.

A comparison of passive seismic monitoring of fracture stimulation due to water versus CO2 injection

The principal observation from passive seismic monitoring at the Weyburn CCS/EOR project has been a very low rate of microseismicity. This has lead to the suggestion that, as CO2 has a higher compressibility, when injected it will have a lower seismic eff