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A Multidisciplinary and Multi-Scale Approach to Identify Hydrogeochemical Processes Altering Porosity-Permeability Properties of Reservoir Rocks


Several processes, such as oil degradation, seawater injection, and inflow of external water by seismic pumping, affect reservoir rock matrices which can be considered to be chemically reactive. Thus, complex hydrogeochemical reaction chains are established among minerals, formation water, oil-derived aqueous hydrocarbons, and gases. Such reactions can cause formation of minerals, and especially of expandable clay minerals. These processes may strongly reduce the number of large pore and the permeability, and consequently, are attributed to significant mechanisms of formation damage. However, hydrogeochemical reactions can induce mineral dissolution, and therefore, improve the reservoir properties. The oil-water contact (OWC) is a hot spot of such processes, where porosity-permeability changes obstructing oil production can be triggered. To evaluate such hydrogeochemical processes and their consequences on reservoir properties, it is necessary to consider that slightest decrease in porosity caused by mineral formation can induce massive permeability reduction. Thus, it is substantial for a successful reservoir engineering (1) to specify whether any mineral can form in specific environments, and, if so, which type and which amount of them can form, (2) to identify which mechanisms induce their formation, and (3) to plausibly predict the spatial and temporal distribution of their formation. Our approach combines a series of analytical methods working from mm-scale (XRD and optical microscopy) to nanometer-scale (SEM and HRTEM) to specify the rock alteration in the Siri oilfield (Danish North Sea). To identify the hydrogeochemical processes which triggered the rock matrix alteration and to specify the parameters controlling its intensity, we applied hydrogeochemical batch modeling by using the program PHREEQC. This modeling enables us to numerically reproduce the proven formation of berthierine, quartz and calcite, and, furthermore, to characterize the hydrogeochemical conditions for their precipitation. Berthierine (plus quartz and calcite) formation results from glauconite dissolution under strong reducing and pH-buffered conditions evolving at OWC. Additionally, we bridge the gap from results of such nanometer-scale investigation to their applications on the reservoir scale. Regarding the spatial and temporal distribution of rock matrix alteration, we upscale our approach by applying a 3D reactive mass transport modeling (using the USGS's PHAST program).