For some polymer gels applied in reservoirs to control water flow, a favorable disproportionate permeability reduction (DPR) occurs in which permeability to water is reduced to a much greater extent than to oil. Also, when the gels are subjected to an applied pressure gradient partial dehydration can occur, i.e., some solvent is forced out of the gel structure. In this study of a partially hydrolyzed polyacrylamide crosslinked with chromium acetate, the magnitudes of DPR and gel dehydration volumes were measured as a function of gel composition and the pressure gradients imposed on the gels following placement in unconsolidated sandpacks. For the range of parameters studied, the reduction of permeability to water and oil and the DPR were strong functions of gel composition and applied pressure gradients, although gel dehydration volumes varied by only a small amount.
High water production is a major concern in mature hydrocarbon reservoirs. Costs of handling and disposing of water produced from oil reservoirs often shorten the life of a production well. Disposal of the water is also an environmental concern. In order to reduce water production, polymer gels have been used to modify the mobility of water and oil in petroleum reservoirs.
When some gels are placed in a petroleum reservoir, permeability reduction occurs to a much greater extent for water than for oil. This phenomenon is known as favorable disproportionate permeability reduction (DPR). Reduced permeability to water leads to decreased production of water, and sometimes increased oil production, thereby prolonging the useful life of the reservoir. Results reported in the literature have shown that the application of several polymer gel systems can result in DPR. Mechanisms for DPR have been debated and the magnitude of the effect has been unpredictable from one application to another. Mechanisms for DPR that have been proposed and studied by several researchers are shown in Table 1.1–13
This paper presents the results of a study on the effects of gel composition, i.e. partially hydrolyzed polyacrylamide (HPAM) concentration, and applied pressure gradients on the magnitude of gel dehydration, and on the magnitude of residual resistance factors and DPR during flow through gel-treated sandpacks. Additionally, material balances on the phases and components in the sandpacks during flow experiments were conducted to give insights into mechanisms that are responsible for permeability reduction and DPR.
Flow experiments were conducted in sandpacks. Various compositions of Cr(III)-acetate-polyacrylamide gelant were injected into the sandpacks that contained brine at residual oil saturation of 13–17% and allowed to gel. Oil (n-dodecane) was then injected into the sandpacks at a constant pressure drop to dehydrate the gels. Brine (1% KCl solution) and oil floods were conducted following the dehydration to determine end-point permeabilities, residual resistance factors, and DPR. Figure 1 is a schematic of the experimental setup. Pumps were used to inject fluids into the sandpack. The effluent was collected in fractions. Pressure drops across the entire sandpack and each of the sections were monitored and recorded by pressure transducers and a data acquisition system. The experimental system was maintained at 30 °C. Table 2 summarizes the sequence of runs in each sandpack. For those sandpacks where weak gels (<3000 ppm polymer) were placed, Runs 14–18 were omitted. Following is a description of the preparation of the sandpacks and gelant and the experimental procedures. Experimental details can be found elsewhere.14
The sandpacks were made by packing silica sand (F110, US Silica Co.) in one-foot long holders. The sand holders were fabricated from acrylic tubes with an ID of 1.5 inches. End caps were attached to both ends of the holder body and sealed with O-rings. The end caps had fittings at the center to allow fluid to flow in and out. Grooves cut on the inner face of the end caps provided uniform distribution of fluid across the entrance and exit faces of the sandpacks.