CO2 will potentially cause formation damage when injected in sandstone formations, due to the precipitation of reaction products that are generated by the reaction between carbonic acid and different clays and feldspars, which often exists in sandstone formations.
Several parameters affect these interactions including pressure, temperature, brine composition, CO2 injection rate, and overall injection scheme. This paper addresses the effect of the temperature and injection scheme on the permeability reduction generated in the sandstone cores due to CO2 injection. A core flood study was conducted using Berea sandstone cores. CO2 was injected under supercritical conditions at a pressure of 1,300 psi, and at temperatures ranging from 70 to 250°F at injection flow rate of 5.0 cm3/min. Core effluent samples were collected and the concentrations of calcium, potassium, magnesium, aluminum, iron, and silicon ions were measured. Precipitated material collected in the effluent samples were analyzed using XRD and XRF. Core permeabilities were measured before and after the experiment to evaluate the damage generated.
A significant damage, between 35 and 55% loss in core permeability, was observed after CO2 injection. For shorter WAG injection the damage was higher, decreasing the brine volume injected per cycle the damage was less. At higher temperatures, 200 and 250°F, more damage was noted than at 70°F. Two mechanisms of damage were identified:
damage occurred due to the precipitation of the reaction products, and
damage due to the migration of clay particles, which were attached by the dissolved cementing materials.
Most sandstone formations are composed of quartz particles bonded together by cementing materials, carbonates, silica and clays (Economides and Nolte 2000). The chemical reactions between carbonic acid and formation rock are much simpler in carbonate rock than in sandstone formations. In sandstones, the surface reaction rates are slow and relatively uniform rock dissolution through the porous medium will be resulted (Wellman et al. 2003). In carbonates, the surface reaction rates are higher, leading to nonuniform dissolution patterns, wormhole channels will be created (Izgec et al. 2006).
Reduction in well injectivity, from 10 up to 100%, is always noted once CO2 is injected into the reservoir (Grigg and Svec 2003). Weyburn oil field in Canada is a sandstone reservoir where CO2 was used for EOR purpose. Monitoring of the produced brines showed an increase in Ca2+, Mg2+, K+, SO42-, HCO3-, and CO2 concentrations due to the dissolution of calcite, dolomite and K-feldspars (Raistrick et al. 2009).
The effect of the chemical reactions on the sandstone permeability during CO2 injection into sandstone formation has been studied by Sayegh et al. (1990). 5 wt% NaCl brine saturated with CO2 at 13.8 MPa was injected into sandstone cores from Pembina cardium reservoir at 45°C. A reduction in core permeability was noted due to the dissolution of calcite and siderite and migration of the fines, which was originally bonded to the rock by the carbonates cementing minerals.
Nightingale et al. (2009) analyzed a sample from the reservoir rock before and after CO2 injection, the analysis showed that a degradation of clay and feldspar grains, and a partial to complete removal of carbonate cements occurred, and residual clays were found in the rock sample after CO2 injection.
