The effect of CO2-enriched brine injection on the mechanical properties of calcite bearing sandstone

The mechanical and fluid-flow response of subsurface geological reservoirs due to injection of CO2 is of critical importance for the safe management and storage of anthropogenic carbon emissions. Although the time-lapse seismic method has proven to be an effective tool to remotely monitor changes in underground fluid saturations, variations in reservoir properties caused by geochemical interactions can also influence the seismic response. This can lead to ambiguity and uncertainty in monitoring the movement of injected CO2 and hence determination of reservoir seal integrity. Geochemical interactions can also modify the mechanical strength of the reservoir and therefore threaten its integrity. We conducted experiments to assess how the velocity and rock strength of a calcite-bearing sandstone are affected by flooding with CO2 saturated brine. The results indicate that both seismic velocity and rock strength are significantly reduced due to minor calcite dissolution. The implications at the reservoir scale for CO2 storage are twofold. Firstly, modifications in velocity can complicate seismic monitoring operations and lead to interpretation errors. This can be accounted for if shear wave velocity variations are used to detect fluid-rock interactions. Secondly, reduction in rock strength, caused by calcite dissolution, can threaten reservoir and wellbore integrity under stress conditions typically found in potential carbon repositories

Microseismic waveforms and velocity heterogeneity: Towards full-waveform location algorithm

Seismic forward modeling is an integral component of microseismic location algorithms, yet there is generally no one correct approach, but rather a range of acceptable approaches that can be used. Since microseismic signals are band limited, the length scale of heterogeneities can significantly influence the seismic wavefronts and waveforms. This can be especially important when subsurface heterogeneity is strong and/or vary on scales lengths equivalent of less than the dominant source wavelength. In this paper, we show that ray-based approaches are not ubiquitously suitable for all microseismic applications and hence we argue that for advanced imaging techniques in unconventional reservoir settings, ray-based algorithms may not be suitably accurate. Here we focus on exploring the feasibility of using one-way wave equations as forward propagators for full waveform event location techniques. One-way wave equations are capable of modeling the evolution of important and observable wave phenomena and could represent efficient full waveform modeling tools. As a feasibility study, we focus on the acoustic wave equation to explore efficiencies and compare traveltime and amplitude errors. However, the results have implications for one-way wave equations for elastic and anisotropic media

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

Rock physics models are being used increasingly to link fluid and mechanical defor- mation parameters for dynamic elastic modelling. In this paper, we explore the input parameters of an analytic stress dependent rock physics model. To do this, we invert for the stress dependent microcrack parameters of over 150 sedimentary rock velocity– stress core measurements taken from a literature survey. The inversion scheme is based on a microstructural effective medium formulation defined by either a second–rank crack density tensor (scalar crack model) or both a second– and fourth–rank crack density tensor (joint inversion model). The inversion results are then used to explore and predict the stress dependent elastic behavior of various sedimentary rock litholo- gies using an analytic microstructural rock physics model via the initial model input parameters: initial crack aspect ratio and initial crack density. Estimates of initial crack aspect ratio are consistent between most lithologies with a mean of 0.0004, but for shales differ by up to several times in magnitude with a mean of 0.001. Estimates of initial aspect ratio are relatively insensitive to the inversion method, though the scalar crack inversion become less reliable at low values of normal to tangential crack compliance ratio (BN/BT ). Initial crack density is sensitive to the degree of damage as well as the inversion procedure. An important implication of this paper is that the fourth–rank crack density term is not necessarily negligible 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 since recent studies have suggested that the ratio BN/BT can be used as an indicator of crack fluid content

Passive seismic monitoring of carbon dioxide storage at Weyburn

arbon 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 Storage Project has included passive seismic monitoring. Here we present the results from five years of passive seismic monitoring at Weyburn, and discuss the lessons learnt that can be applied when deploying passive seismics to monitor future CCS operations

Time-lapse seismic waveform modeling and seismic attribute analysis using hydro-mechanical models for a deep reservoir undergoing depletion

Extraction of fluids from subsurface reservoirs induces changes in pore pressure, leading not only to geomechanical changes, but also perturbations in seismic velocities and hence observable seismic attributes. Time-lapse seismic analysis can be used to estimate changes in subsurface hydromechanical properties and thus act as a monitoring tool for geological reservoirs. The ability to observe and quantify changes in fluid, stress and strain using seismic techniques has important implications for monitoring risk not only for petroleum applications but also for geological storage of CO2 and nuclear waste scenarios. In this paper, we integrate hydromechanical simulation results with rock physics models and full-waveform seismic modelling to assess time-lapse seismic attribute resolution for dynamic reservoir characterization and hydromechanical model calibration. The time-lapse seismic simulations use a dynamic elastic reservoir model based on a North Sea deep reservoir undergoing large pressure changes. The time-lapse seismic traveltime shifts and time strains calculated from the modelled and processed synthetic data sets (i.e. pre-stack and post-stack data) are in a reasonable agreement with the true earth models, indicating the feasibility of using 1-D strain rock physics transform and time-lapse seismic processing methodology. Estimated vertical traveltime shifts for the overburden and the majority of the reservoir are within ±1 ms of the true earth model values, indicating that the time-lapse technique is sufficiently accurate for predicting overburden velocity changes and hence geomechanical effects. Characterization of deeper structure below the overburden becomes less accurate, where more advanced time-lapse seismic processing and migration is needed to handle the complex geometry and strong lateral induced velocity changes. Nevertheless, both migrated full-offset pre-stack and near-offset post-stack data image the general features of both the overburden and reservoir units. More importantly, the results from this study indicate that integrated seismic and hydromechanical modelling can help constrain time-lapse uncertainty and hence reduce risk due to fluid extraction and injection