Characterization of hysteretic multiphase flow from the mm to m scale in heterogeneous rocks

Incorporating mm-m scale capillary pressure heterogeneity into upscaled numerical models is key to the successful prediction of low flow potential plume migration and trapping at the field scale. Under such conditions, the upscaled, equivalent relative permeability incorporating capillary pressure heterogeneity is far from that derived conventionally at the viscous limit, dependent on the heterogeneity structure and flow rate, i.e. dependent on the capillary number. Recent work at the SCA 2017 symposium (SCA2017-022) demonstrated how equivalent functions can be obtained on heterogeneous rock cores from the subsurface under drainage conditions; going beyond traditional SCAL. Experimental observations using medical CT scanning can be combined with numerical modelling so that heterogeneous subsurface rock cores can be directly characterized and used to populate field scale reservoir models. In this work, we extend this characterization approach by incorporating imbibition cycles into the methodology. We use a Bunter sandstone core with several experimental CO 2 -Brine core flood datasets at different flow rates (2x drainage, 1x imbibition and 2x trapping) to demonstrate the characterization of hysteretic multiphase flow functions in water-wet rocks. We show that mm-m scale experimental saturations and equivalent, low flow potential relative permeabilities can be predicted during drainage and imbibition, along with trapping characteristics. Equivalent imbibition relative permeabilities appear as a function of capillary number, as in the drainage cases. We also find that the form of capillary pressure function during imbibition has a large impact on the trapping characteristics, with local heterogeneity trapping reduced (or removed), if the capillary pressure drops to zero, or below at the residual saturation

Capillary trapping of CO2 in oil reservoirs: observations in a mixed-wet carbonate rock

Early deployment of carbon dioxide storage is likely to focus on injection into mature oil reservoirs, most of which occur in carbonate rock units. Observations and modeling have shown how capillary trapping leads to the immobilization of CO2 in saline aquifers, enhancing the security and capacity of storage. There are, however, no observations of trapping in rocks with a mixed-wet-state characteristic of hydrocarbon-bearing carbonate reservoirs. Here, we found that residual trapping of supercritical CO2 in a limestone altered to a mixed-wet state with oil was significantly less than trapping in the unaltered rock. In unaltered samples, the trapping of CO2 and N2 were indistinguishable, with a maximum residual saturation of 24%. After the alteration of the wetting state, the trapping of N2 was reduced, with a maximum residual saturation of 19%. The trapping of CO2 was reduced even further, with a maximum residual saturation of 15%. Best-fit Land-model constants shifted from C = 1.73 in the water-wet rock to C = 2.82 for N2 and C = 4.11 for the CO2 in the mixed-wet rock. The results indicate that plume migration will be less constrained by capillary trapping for CO2 storage projects using oil fields compared with those for saline aquifers

Analysis of viscous crossflow in polymer flooding

Polymer flooding improves oil recovery by improving flood front conformance compared with waterflooding as well as, in some cases, extracting more oil from lower permeability zones in the reservoir by viscous cross-flow. However viscous cross-flow of water from the low permeability zone may also adversely affect the polymer flood by causing the polymer slug to be diluted and possibly to lose its integrity. The extent to which viscous cross-flow improves or reduces recovery depends upon the permeability contrast between the low and high permeability zones, the viscosity ratios of the fluids (oil, water and polymer solution) and the geometry of the layers. This paper uses inspectional analysis to derive the minimum set of 6 dimensionless numbers that can be used to characterise a polymer flood in a two layered model. A series of finely gridded numerical simulations are then performed to determine the contribution of viscous crossflow to oil recovery from secondary and tertiary polymer flooding in this system. We show that viscous cross-flow will only make a positive impact on oil recovery from secondary polymer flooding when the viscosity ratio values of oil to polymer solution is less than 1 and permeability ratio between the layers is less than 50. Furthermore, we show that there is an inverse relationship between the permeability ratio between layers and the amount of degradation the polymer slug experiences due to viscous crossflow in the high permeability layer. As the permeability contrast between layers increases, the slug degradation decreases. Also, the results show that the desired positive impact from viscous crossflow is higher in secondary polymer foods when compared to tertiary polymer floods. Finally, the results can be used to make initial estimates of the contribution of both viscous cross-flow and mobility control in polymer flooding applications without the need to perform extensive and time consuming numerical simulations

Multidimensional Imaging of Density Driven Convection in a Porous Medium

Carbon dioxide (CO2) sequestration is a climate change mitigation technique which relies on residual and solubility trapping in injection locations with saline aquifers. The dissolution of CO2 into resident brines results in density-driven convection which further enhances the geological trapping potential. We report on the use of an analogue fluid pair to investigate density-driven convection in 3D in an unconsolidated bead pack. X-ray computed tomography (CT) is used to image density-driven convection in the opaque porous medium non-invasively. Two studies have been conducted that differ by the Rayleigh number (Ra) of the system, which in this study is changed by altering the maximum density difference of the fluid pair. We observe the same general mixing pattern in both studies. Initially, many high density fingers move downward through the bead pack and as time progresses these coalesce and form larger dominate flow paths. However, we also observe that a higher Rayleigh number leads to the denser plume moving faster towards the bottom of the system. Due to the finite size of the system, this in turn leads to early convective shut-down

What are the key processes of CO2 storage to represent in energy systems models? A dynamic model of CO2 storage in the UK Bunter Sandstone

Carbon capture and storage (CCS) is expected to play a key role in meeting targets set by the Paris Agreement and for meeting legally binding greenhouse gas emissions targets set within the UK [1]. Energy systems models have been essential in identifying the importance of CCS but they neglect to impose constraints on the availability and use of geologic CO2 storage reservoirs. In this work we analyze reservoir performance sensitivities to increasing average target injection rate, injection site location and varying CO2 storage demand for three sets of injection scenarios designed to encompass the UK\u27s future low carbon energy market. We use the ECLIPSE reservoir simulator and a model of the Southern North Sea Bunter Sandstone saline aquifer. We first find that increasing average target injection does not affect the ability to store CO2, but will be limited by the increase in bottomhole pressure at each site. We find that deeper injection sites will be the least limiting for injection as the near-site lithostatic pressure will be higher [2]. From the first set of varying injection scenarios we find that fluctuating amplitude and frequency of injection has little effect on reservoir pressure response and plume migration. Injectivity varies with site location due to variations in depth and regional permeability. In a second set of injection scenarios, we show that with envisioned UK storage demand levels for a large coal fired power plant, it makes no difference to reservoir response whether all injection sites are deployed upfront or gradually as demand increases. Meanwhile, there may be an advantage to deploying infrastructure in deep sites first in order to meet higher demand later. However, deep-site deployment will incur higher upfront cost than shallow-site deployment. In a third set of injection scenarios, we show that starting injection at a high rate with ramping down, a low rate with ramping up or at a constant rate makes little difference to the overall injectivity of the reservoir. Therefore such variability is not essential to represent CO2 storage in energy systems models resolving plume and pressure evolution over decadal timescales. [1] Future of Carbon Capture and Storage in the UK, UK Parliament House of Commons, Energy and Climate Change Committee, London: The Stationary Office Limited. [2] Agada S., J. S. (2017). The impact of energy systems demands in pressure limited CO2 storage in the Bunter Sandstone. International Journal of Greenhouse Gas Control, (in press)

Observations of 3-D transverse dispersion and dilution in natural consolidated rock by X-ray tomography

Recent studies have demonstrated the importance of transverse dispersion for dilution and mixing of solutes but most observations have remained limited to two-dimensional sand-box models. We present a new core-flood test to characterize solute transport in 3-D natural-rock media. A device consisting of three annular regions was used for fluid injection into a cylindrical rock core. Pure water was injected into the center and outer region and a NaI solution into the middle region. Steady state transverse dispersion of NaI was visualized with an X-ray medical CT-scanner for a range of Peclét numbers. Three methods were used to calculate Dt: (1) fitting an analytical solution, (2) analyzing the second-central moment, and (3) analyzing the dilution index and reactor ratio. Transverse dispersion decreased with distance due to flow focusing. Furthermore, Dt in the power-law regime showed sub-linear behavior. Overall, the reactor ratios were high confirming the homogeneity of Berea sandstone

Capillary Heterogeneity in Sandstone Rocks During CO2/Water Core-flooding Experiments

AbstractWe have successfully applied a novel experimental technique to measure drainage capillary pressure curves in reservoir rocks with representative reservoir fluids at high temperatures and pressures. The method consists of carrying out 100% CO2 flooding experiments at increasingly higher flow rates on a core that is initially saturated with water and requires that the wetting-phase pressure is continuous across the outlet face of the sample. Experiments have been carried out on a Berea Sandstone core at 25 and 50°C and at 9MPa pore pressure, while keeping the confining pressure at 12MPa. Measurements are in good agreement with data from mercury intrusion porosimetry. The technique possesses a great potential of applicability due to the following reasons: (a) it can be applied in conjunction with steady-state relative permeability measurements, as it shares a very similar experimental configuration; (b) it is faster than traditional (porous-plate) techniques used for measuring capillary pressure on rock cores with reservoir fluids; (c) by comparison with results from mercury porosimetry, it allows for the estimation of the interfacial and wetting properties of the CO2/water system, the latter being unknown for most rocks; (d) by combination with X-ray CT scanning, the method allows for the observation of capillary pressure–saturation relationships on mm-scale subsets of the rock core. The latter are of high relevance as they directly and non- destructively measure capillary pressure curve heterogeneity in sandstone rocks

Multidimensional observations of dissolution-driven convection in simple porous media using X-ray CT scanning

We present an experimental study of dissolution-driven convection in a three-dimensional porous medium formed from a dense random packing of glass beads. Measurements are conducted using the model fluid system MEG/water in the regime of Rayleigh numbers, Ra=2000−5000. X-ray computed tomography is applied to image the spatial and temporal evolution of the solute plume non-invasively. The tomograms are used to compute macroscopic quantities including the rate of dissolution and horizontally averaged concentration profiles, and enable the visualisation of the flow patterns that arise upon mixing at a spatial resolution of about (2×2×2)mm3. The latter highlights that under this Ra regime convection becomes truly three-dimensional with the emergence of characteristic patterns that closely resemble the dynamical flow structures produced by high-resolution numerical simulations reported in the literature. We observe that the mixing process evolves systematically through three stages, starting from pure diffusion, followed by convection-dominated and shutdown. A modified diffusion equation is applied to model the convective process with an onset time of convection that compares favourably with the literature data and an effective diffusion coefficient that is almost two orders of magnitude larger than the molecular diffusivity of the solute. The comparison of the experimental observations of convective mixing against their numerical counterparts of the purely diffusive scenario enables the estimation of a non-dimensional convective mass flux in terms of the Sherwood number, Sh=0.025Ra. We observe that the latter scales linearly with Ra, in agreement with both experimental and numerical studies on thermal convection over the same Ra regime

Global geologic carbon storage requirements of climate change mitigation scenarios

Integrated assessment models have identified carbon capture and storage (CCS) as an important technology for limiting climate change. To achieve 2 °C climate targets, many scenarios require tens of gigatons of CO2 stored per year by mid-century. These scenarios are often unconstrained by growth rates, and uncertainty in global geologic storage assessments limits resource-based constraints. Here we show how logistic growth models, a common tool in resource assessment, provide a mathematical framework for stakeholders to monitor short-term CCS deployment progress and long-term resource requirements in the context of climate change mitigation targets. Growth rate analysis, constrained by historic commercial CO2 storage rates, indicates sufficient growth to achieve several of the 2100 storage targets identified in the assessment reports of the Intergovernmental Panel on Climate Change. A maximum global discovered storage capacity of approximately 2700 Gt is needed to meet the most aggressive targets, with this ceiling growing if CCS deployment is delayed