Experimental Investigation of the Role of Relative Permeability and Wettability in Geological CO 2 Storage in Saline Aquifer

Abstract

The first task for carbon capture and storage is to find suitable locations to store CO2 safely and permanently. Saline aquifers are considered to be one of the best options for CO2 storage. This study can be divided into two sections. In the first part, sequestration of carbon dioxide in a saline aquifer which is located below Longyearbyen in Svalbard was evaluated. In the second part, the wettability behavior of CO2 at geological storage conditions on different minerals was investigated.

A detailed study of petrophysical properties was performed on cores from shallow marine formation of late Triassic-mid Jurassic age. Experimental and numerical studies have been performed to evaluate CO2 storage capacity. A total of 52 samples of core material from one well (Dh4) were collected and tested to find the potential units for CO2 injection. Analysis of the results showed very poor porosity and permeability. This poses a serious challenge with respect to achieving viable levels of injectivity and injection pressure. For further investigation, two long (32 and 35 cm) sandstone samples were selected for laboratory core flooding experiments at reservoir conditions. Experimental protocol and detailed CO2-brine drainage and imbibition relative permeability data for these two samples of rock were provided. Capillary pressure measurements and simulation of the transient process was used to support the interpolation of the experimental flooding data.

The results showed that CO2 can displace 30-40% of the brine in the core. CO2 dissolution in brine and formation of weak carbonic acid resulted in dissolution of calcite cement, reaction with iron minerals and induction of new micro fractures in the core. Severe hysteresis effects on one sample most likely resulted from changes in the rock composition. The relative permeability characteristics on these two low permeable samples showed that the presence of very small amounts of residual gas saturation in the core reduced the permeability to brine significantly.

The capillary-sealing efficiency of the caprock is one of the major factors that control the safety of geological CO2 storage. Possible changes in wettability due to physical geochemical processes could possibly decrease the capillary entrance pressure and reduce the sealing integrity of the caprock. Changes in wettability have therefore been measured by measuring the CO2 contact angle on some selected minerals typical for reservoir rocks in the presence of brine at reservoir conditions.

We present a set of CO2 contact angle data on the minerals representing reservoir and seal rock at pressure, temperature and salinity conditions, representative of a CO2 storage operation. Among these minerals, quartz, feldspar and calcite are strongly water wet with non-significant change in contact angle while the water wettability of muscovite mica changed from strongly water-wet to intermediate water-wet. Also wettability alteration of the CO2 as a function of time for calcite and muscovite mica at constant conditions was investigated. The water wettability of the calcite mineral did not change significantly at the time scale of the laboratory experiment while the wettability of mica definitely changed from strongly to intermediate water wet.