The effect of wettability on waterflooding and relative permeability at quite water-wet conditions in chalk core

Abstract

The effect of performing waterflooding in chalk has been previously tested with a purpose of Enhanced Oil recovery. Regarding oil recovery processes, rock wettability is the key parameter determining transport of fluid flow through porous media. This work shows how the core wettability affect the relative permeability in carbonate reservoirs and hence impact the oil recovery. In this project, carbonate chalk material from Stevns Klint is used as a porous media to evaluate the effect of wettability on unsteady state relative permeability estimations at room temperature of 23°C. For the experiment, two chalk cores were prepared as reference water-wet cores and used as examples of the optimum cases after a completely successful waterfloods and two other cores were used with modified wettability. All the cores were saturated to 20% of initial water saturation, cores were saturated fully by synthetic model oil/crude oil to establish initial conditions. Cores were afterwards subjected to flooding by a formation water and flooded two artificial versions of seawater (SW0T and SW1/2T brines) to measure wettability by chromatography wettability method. The reference water-wet cores showed quite high oil recovery while mixed-wet cores showed insignificantly lower oil recovery during both forced and spontaneous imbibition tests. Pressure drop curve behave more or less the same for the reference water-wet cores SK-R1, SK-R2 and mixed-wet core SK-C3, while core SK-C6 behave slightly different during forced imbibition test. Production history and pressure drop from cores SK-R1, SK-R2 and SK-C3 were used to do the history matching of relative permeability curves by Sendra modeling tool. Spontaneous imbibition was introduced in order to gather information about cores’ wettability and forced imbibition was introduced to monitor behavior of fluid flow through core under a viscous force dominated environment, while pressure drop across all cores and oil production measurements were recorded. Collected data was combined with end-points of relative permeability curves to calculate the effect of wettability on relative permeabilities. As synthetic model oil used in this project contained the polar organic components (POC) responsible for wettability alteration towards an oil-wet state, it was used with a purpose of changing the carbonate cores’ wettability through flooding. POC is quantified by the acid and base numbers, AN and BN, which are measured in mg KOH/g. In carbonate reservoirs, the acidic POC plays a major role on determining wetting state. After spontaneous imbibition tests, wetting state of the cores was analyzed by chromatographic wettability test, through which we obtain the water-wet surface area of all chalk cores. The experimental measurements acquired during the flooding processes will contribute to increase our understanding of how significantly wettability changes affect relative permeabilities and oil/water saturations during oil production. The concept of wettability is of great importance to the industry and academia, specially to check for its effect on enhanced oil recovery, where diverse mechanisms have been proposed to increase recovery by changing the wetting state of reservoir rocks, as it impacts reserve volumes and flooding performance.