Experimental investigation of injectivity loss: change in core permeability during re-injection of produced water

The oil industry receives a large amount of formation water along with hydrocarbons (gas and oil) during oil production, and in many cases the volumes of formation water exceed the share of hydrocarbons. Safe disposal of polluted water is a serious problem for the industry re-injecting the produced water into the formation via water injection wells is an attractive option. Typically, this includes mixing formation and sea water to provide the required volume of injected fluid and maintain hydrocarbon production regimes. Experience from Danish wells in the North Sea shows that injectivity decreases over several months when formation water is injected and the main challenges for reservoir injectivity during re-injection occur in the bottomhole zone of the well. The causes of injectivity loss are poorly identified but may include deposits of resin, bacteria, production chemicals, oil components and scale and corrosion products on the surface of the formation. The formation of a filter cake can be interpreted as a change in permeability and is studied using a combination of core injection experiments, chemical analysis and modelling of the injection process. The changes in the permeability of the rock and the composition of the fluids are presented.