Step-rate injection tests for the determination of the critical pressure for reagent injection in aquifers systems

The determination of critical pressure through step rate injection tests plays a major role in environmental engineering (remediation of contaminated aquifer systems) as well as in oil and gas industry. Critical pressure is the pressure at which fracture is created in an un-fractured formation or it will widen an existing fracture in a formation. The determination of this value is crucial in oil engineering for better operation of water-floods and has extensively used to increase oil and gas recovery fromtight reservoirs. Injection above critical pressure, not only produce an early breakthrough, poor sweep, reduced oil and gas recovery butalso result in loss of costly injection fluids. On the other hand, injection far below the critical pressure decreases oil and gas recovery from reservoir. For groundwater remediation, the identification ofthe critical pressure is an important issue also for the design of field injection of reagents into the contaminated aquifers. Injection below the critical pressure results in permeation delivery of the reactants, and consequently fairly homogeneous distribution in the proximity of the injection point. In turn, injection above it causes fracturing of the porous medium, fluid delivery along preferential flow paths, even at high distances for the injection point, and a final non homogeneous distribution of the reactant. Permeation or fracturing injection can be preferred in specific cases. To enhance the usefulness of step rate injection tests, the current paper presents a methodology for the determination of the critical pressure for reagent injection in aquifer systems and the interpretation of real tests conducted in a shallow aquifer system in Belgium. Although step rate injection tests have been frequently performed for several years in oil and gas industry by using Newtonian fluids (usually water) in order to access formation properties and to determine the critical pressure, for the first time this technique is applied here to aquifers systems and by using non-Newtonian fluids. This work was conducted in the framework of the AQUAREHAB project (FP7 - G. A. Nr. 226565).