Critical pressure analysis of the injection of shear thinning fluids in porous media

The determination of the critical pressure values through step-rate injection tests has always played a fundamental role in the oil and gas industry, and has started making the same in environmental engineering today. The critical pressure is the pressure value at which fractures open inside not fractured formations, or at which existing fractures come to be significantly enlarged within already fractured formations. The determination of those values has revealed to be crucial in petroleum engineering (i.e. as far as the design and the optimization of water-flooding operations are concerned), and as well is today extensively used in order to increase the hydrocarbon recovery from tight reservoirs. Fluid injection above the critical pressure produces early breakthrough phenomena, poor sweep efficiencies, reduced hydrocarbon recovery standards, and is very likely to cause significant losses of costly injection fluids. On the other hand, injection below the critical pressure values for sure decreases the hydrocarbon recovery values from reservoir. As far as applications to environmental engineering are concerned, the final groundwater remediation quality strongly depends on the correct identification of the critical pressure values, and directly affects the whole design of the field injection of reagents into the contaminated porous media. In all cases performing experimental analysis and applied research has shown to be indispensible and has been gone through, not only in order to characterize the correct critical pressure values. To date, step-rate injection tests have been performed for many years in the oil industry mainly by using Newtonian fluids (usually water, but not always) in order to get knowledge about formation features, and also in order to determine the above defined critical pressure values. Here for the first time the described technique is applied to aquifer systems, and by using non Newtonian (shear thinning) fluids. To enhance the usefulness of the step-rate injection tests, this paper also presents the implementation of a method for the determination of the critical pressure values referring to the injection of non Newtonian shear thinning fluids into porous media, and validates the interpretation of a field test performed in a contaminated aquifer in Belgium.