Core Scale Modeling of Polymer Gel Dehydration by Spontaneous Imbibition

Capillary spontaneous imbibition (SI) of solvent (water bound in gel) from formed polymer gel into an adjacent, oil-saturated porous medium was recently observed in laboratory experiments. Loss of water from the gel by SI might influence the blocking capacity of the gel residing in a fracture, by decreasing its volume, and might contribute to gel failure, often observed in water-wet oil fields. This work presents an original modeling approach to simulate and interpret spontaneous imbibition of water from Cr(III)-acetate-hydrolyzed-polyacrylamide (HPAM) gel into adjacent oil-saturated rock matrix. Simulations were compared to experiments on the core scale, using two different boundary conditions: all faces open (AFO) and two-ends-open free spontaneous imbibition (TEOFSI). Capillary forces enable water (used as gel solvent) to enter the rock matrix. The gel particle network itself is, however, inhibited from entering because of its structure, and remains on the surface of the rock matrix. We developed a theory that describes the gel as a compressible porous medium and describes the flow of water through gel. The polymer structure of the gel is proposed to constitute a gel matrix of constant solid volume. Gel porosity, defined by the volume fraction of solvent, is modeled as a function of pore pressure and gel compressibility. Gel permeability is modeled as function of gel porosity using a Kozeny-Carman approach. The flow equations for the gel and core domains were solved simultaneously by implementing the proposed description into the core-scale simulator IORCoreSim. The gel surrounding the core was discretized and included as a part of the total grid. The simulated flow of water through and from the gel occurred in a transient manner, driven by the coupled gradients in gel fluid pressure and gel porosity. Gel porosity initially decreased in a layer close to the core surface because of reduced aqueous pressure, and continued to decrease in layers away from the core surface. The propagation rate was controlled by two main gel parameters: First, gel compressibility controlled the pressure gradient within the gel network, and the amount of water transported from the outer part of the gel toward the core surface to balance the pore pressure; and, second, gel permeability limited how fast water could flow within the gel at a given pressure gradient, thus increasing the time scale of the overall imbibition process.