Washout of Cr(III)-Acetate-HPAM Gels from Fractures

Abstract

Abstract This paper investigates washout of mature Cr(III)-acetate-HPAM gels from fractures. After gel placement, the pressure gradient for gel washout during brine or oil flow was similar to the pressure gradient observed during gel placement. The mechanism of gel failure involved the displacement of relatively mobile gel from wormholes. Generally, only a small fraction of the gel (<5%) was displaced during the washout process. Resistance to washout can be increased by injecting a more concentrated gel. However, this approach exhibits significantly higher pressure gradients during gel placement. The presence of a constriction in a fracture inhibited gel washout during the first pulse of brine flow after gel placement. However, during subsequent brine flow, gel erosion occurred upstream of the constriction to the same extent as downstream. The extrusion, leakoff, and washout behavior in fractures in strongly oil-wet polyethylene cores were similar to those in strongly water-wet Berea sandstone. Gel washout can be reduced by controlling gel placement rate. A Cr(III)-acetate-HPAM gel placed in a 0.04-in. wide fracture at 826 ft/d was about five times more resistant to washout than a gel placed at 4,130 ft/d. Gel washout can also be reduced using secondary crosslinking reactions. Post-placement reaction with Cr(III) acetate increased resistance to washout for a resorcinol-formaldehyde-HPAM gel by a factor from two to three. During steady state flow after first breaching the gel, a Cr(III)-acetate-HPAM gel reduced permeability to water (within the fracture) moderately more (2.5 to 4.7 times) than that to oil. Disproportionate permeability reduction in fractures was most evident at low flow rates. Introduction In many field applications, gel treatments were less effective than expected in reducing water production from fractured wells. Concern exists about the resistance of gels to washout after placement. This paper examines several issues regarding washout of mature Cr(III)-acetate-HPAM gels from fractures. In particular, we investigate how gel washout is affected by the applied pressure gradient, fracture width, gel composition, presence of a constriction in the fracture, nature of the fracture surfaces, and flow of oil versus brine. We also consider two promising new methods to reduce gel washout - rate control during gel placement and use of secondary reactions. Review of Gel Behavior in Fractures Gel compositions used for conformance control usually contain more than 90% water - and frequently more than 99% water. For example, much of our research focused on an aqueous gel that contained 0.5% Ciba Alcoflood 935™ HPAM (molecular weight ˜5×106 daltons; degree of hydrolysis 5% to 10%), 0.0417% Cr(III) acetate, 1% NaCl, and 0.1% CaCl2. The gelation time for this formulation is about 5 hours at 41°C. In this paper, this formulation (after aging 24 hours) is called our standard 1X Cr(III)-acetate-HPAM gel. Gels can be placed in fractures either as gelants1 or formed gels.2,3 Gelants (the fluid solution of crosslinker and polymer that exists before gelation) flow readily into fractures and medium to high permeability rock, exhibiting relatively low pressure gradients during placement. However, gelants often experience problems with gravity segregation in fractures.4 Also, when gelants contact reservoir fluids or rock minerals, compositional changes can occur that interfere with gelation.1,5 Alternatively, formed gels (i.e., the crosslinked product of gelation) can be extruded into fractures during the placement process. Formed gels have significantly fewer problems with gravity segregation and chemical interference than gelants. However, formed gels exhibit water loss and higher pressure gradients during placement that may affect their distance of propagation along a fracture or into a fracture system.2,3 During extrusion through fractures, formed gels dehydrate - i.e., water (or brine) leaves the gel and leaks off through the fracture faces into the porous rock.2,3 The crosslinked polymer remains behind in the fracture, becoming increasingly concentrated with time and gel throughput (i.e., crosslinked polymers do not flow through porous rock after gelation). Dehydrated gel usually becomes immobile at the point in the fracture where dehydration occurs. During placement, the only gel that generally moves through the fracture has basically the same composition, appearance, and properties as the injected gel.2,3 However, small bits of the dehydrated gel can erode to join the flowing gel.