(432d) Applicable Insights of Micp-Derived Capillary Pressure on CO2 Injection for Wyoming Carbonsafe Project: When Does Capillary Pressure Affect the Pressure Front the Most?

Mercury injection capillary pressure (MICP) needs conversion with the IFT and contact angle at both laboratory and reservoir conditions to be applied to the reservoir simulation to better estimate storage capacity and area of review, where the data are often unavailable. In this work, these data were collected and studied for their sensitivity and the effect of MICP variance on storage capacity and pressure front, adopting different injection strategies for the Wyoming CarbonSAFE project. First, reservoir IFT and contact angle data of storage formations (Lower Cretaceous Lakota Sandstone, JurassicHulett Sandstone, and Pennsylvanian Minnelusa Formation) were collected and analyzed for their sensitivities as the conversion factor for MICP on the storage capacity and pressure front under bottom hole pressure (BHP) control. Then, optimal conversion factors were fixed to study the effect of the MICP variance of storage formations on the pressure front adopting rate control mode.

Brown (1951) discussed the MICP-saturation curve’s applicability to the water-gas capillary pressure-saturation curve using Purcell’s conversion factor (1950). The conversion factors depend on the contact angle of water-CO₂-rock and IFT between water and CO₂ at the reservoir conditions, where contact angle and IFT synthetically contribute to the reservoir capillary pressure derived from MICP and less water-wet (larger contact angle) rock surface and lower IFT contribute to higher storage capacity. However, while under BHP control in CO₂ injection simulation, the difference of the capillary pressure caused by the variance in conversion factors is considered insignificant (0.17 – 1.7 psi at the end points) compared to the pressure difference between BHP and initial reservoir pressure and thus results in minimum fluctuation towards actual injection rate and an almost identical CO₂ injection amount (Fig. 1a). This step suggests the negligible effect of the conversion factors based on comparable contact angle and IFT results on CO₂ storage capacity under BHP control.

Then, small conversion factors were selected and applied to MICPs. The heterogeneity of the samples regarding different MICPs was studied under the rate control mode. Rates were set up as 50 and 90 percent of the equilibrating rates under the BHP control mode. We expected that the influence of capillary pressure difference on the pressure fronts (with fixed USDW-based critical pressure) formed by 50% rates could be better visualized than 90% rates as the order of the magnitude of the pressures is closer.

There are two major findings. First, for CO₂ to enter the first grid cell, the wellbore pressure must conquer capillary pressure. Once CO₂ enters the cell and saturation accumulates, the cell (pore) pressure continues to build up in every direction; meanwhile, the gravity of CO₂ mass from the first cell lowers the needed entering pressure for the adjacent vertical cell. Thus, it is easier for the injected CO₂ to enter the adjacent vertical cell than the lateral cells. For the low capillary pressure case, increased pore pressure could effortlessly surpass the capillary pressure threshold. Then, CO₂ expands horizontally with a subtle delay (Fig. 1b) rather than in the high capillary pressure case, where CO₂ invades first vertically and thoroughly before moving horizontally (Fig. 1b). As a result, for both cases, although the injection amounts are the same due to same injection rates, the horizontal expansion of CO₂ in the low capillary pressure case is further and vertically shallower, which leads to higher pore pressure increments at the top layer cells. As the pressure front is delineated by thresholding the pressure increment based on the same critical pressure, the low capillary pressure case has a further pressure front compared to the high capillary pressure case, which is more obvious for the 50% injection rate scenario around a single well (i.e., well at pad 5 in Fig.1c). Second, for 90% injection rate case, it shows a minor difference between pressure fronts under high and low capillary pressures (Fig. 1d) as the influence of capillary pressure difference is considered insignificant under high pore pressures caused by high injection rates. Note that the pressure interference among the clustered wells (i.e., wells at pads 1 to 4 in Fig.1d) can overwhelm the influence of capillary pressure difference on the high injection rate scenario.

In this study, we have learned that from the injection simulation perspective, when only considering capillary pressure, the conversion factors (i.e., contact angle and IFT) to obtain water-CO₂ capillary pressure from MICP are much less influential than the rock samples’ petrophysical properties. Thus, statistically selecting the representative samples from the petrophysical perspective for MICP experiments is the key to better estimating the pressure front and achieving a thoughtful injection simulation for carbon storage projects.