Gel Treatments in the Matrix

This paper describes an experimental investigation of the effects of rock permeability and lithology on the performance of several gels, including those formed from resorcinol-formaldehyde, colloidal silica, Cr³⁺(chloride)-xanthan, and Cr³⁺(acetate)-polyacrylamide. During these experiments, particular attention was paid to (1) the importance of pH to gelation, (2) gel performance as a function of fluid velocity, and (3) the use of tracers to assess the fraction of the pore space that was occupied by gel.

Aqueous solutions of xanthan gum are used to reduce water mobility in oilfields and are known to undergo rheological changes when crosslinked with Cr³⁺ ions. The objective of this study is to investigate how Cr³⁺ affects the flow behavior of xanthan solutions in porous media, particularly with regard to apparent viscosity and flow resistance. Laboratory experiments were performed using core plugs of varying permeabilities, and the flow characteristics were measured with and without the presence of Cr³⁺. The results show that Cr³⁺ addition significantly increases the resistance factor and apparent viscosity, indicating stronger retention and shear thickening at low flow rates. The implications for in-situ gelation and profile modification treatments are discussed.

Resorcinol/formaldehyde gels are used to show that gel performance in porous rocks depends critically on the pH at which gelation occurs. The gels generally reduced the permeability of low-permeability sandstone more than in high-permeability sandstone. However, residual resistance factors can be greater in sandstones than in less permeable carbonate cores. A simple mathematical model is used to assess whether pH effects can be exploited to optimize gel placement in injection wells.

This work investigates dehydration of polymer gel by capillary imbibition of water bound in gel into a strongly water-wet matrix. Polymer gel is a crosslinked-polymer solution of high water content, where water can leave the gel and propagate through porous media, whereas the large 3D polymer-gel structures cannot. In fractured reservoirs, polymer gel can be used for conformance control by reducing fracture conductivity. Dehydration of polymer gel by spontaneous imbibition (SI) contributes to shrinkage of the gel, which may open parts of the initially gel-filled fracture to flow and significantly reduce the pressure resistance of the gel treatment. SI of water bound in aged Cr(III)-acetate-hydrolyzed-polyacrylamide (HPAM) gel was observed and quantified. Oil-saturated chalk-core plugs were submerged in gel, and the rate of SI was measured. Two boundary conditions were tested: all faces open (AFO) and two-end-open oil-water (TEO-OW), where one end was in contact with the imbibing fluid (gel or brine) and the other was in contact with oil. The rate of SI was significantly slower in gel compared with brine, and was highly sensitive to the ratio of matrix volume to surface open to flow, decreasing with increasing ratios. The presence of a dehydrated gel layer on the core surface lowered the rate of imbibition; continuous loss of water to the core increased the gel layer concentration and thus the barrier to flow between the core and fresh gel. Severe gel dehydration and shrinkage up to 99% were observed in the experiments, suggesting that gel treatments may lose efficiency over time in field applications where a potential for SI exists. The implications of gel dehydration by SI, and its relevance in field applications, are discussed for both gel and gelant field treatments.