Describing flow behavior of polymer gel in fracture using rheology

Abstract

In many successful conformance controls to reduce excessive water production, large volumes of gels were extruded through fractures during placement. The pressure gradient for gel extrusion depends strongly on fracture width and gel composition. Extrusion experiments directly measure gel properties in fractures, but they are both expensive and time-consuming. The rheology behavior of the gels showed a strong parallel to the results obtained from previous gel extrusion experiments. The previous researches showed two discrepancies and one similarity between the rheology estimation and extrusion experiment. The two discrepancies are: the pressure gradient required to extrude a gel through a fracture substantially greater than anticipated based on rheology measurements; different relationships of pressure gradient versus fracture width. The similarity is the relationships between pressure gradient versus flow rate and shear stress versus shear rate are fairly close. This paper examined these discrepancies and the similarity, in hopes of understand and ultimately substituting the rheology measurements for extrusion experiments.Extensive experiments established that wall slip and first normal stress difference were not responsible for the pressure gradient discrepancy. Steady shear and oscillatory shear data were collected with a rheometer using both smooth and rough parallel-plate geometries and employing various gap heights. Wall-slip effects were present with smooth plates but negligible with rough plates. To explain the discrepancy, we noted that the aperture for gel flow (for mobile gel wormholing through concentrated immobile gel within the fracture) was much narrower than the width of the fracture.Considering the shear-thinning properties of the gels, two models were developed combining power-law model and shell momentum balances. The first model explained the discrepancy of the relationships between pressure gradient versus fracture width. The second model correlated pressure gradient, shear stress, flow rate, and shear rate to bridge the gap between gel rheology in fractures versus in a rheometer and explained the similarity between the rheology measurement and extrusion experiment.