Abstract
Previous studies have shown that seawater may alter thewettability in the direction of more water-wet conditions incarbonate reservoirs. The reason for this is that ions from thesalt (sulphat, magnesium, calsium, etc) can create a wettabilityalteration toward more water-wet conditions as salt is absorbed onthe rock. In order to initiate a more systematic study of this phenomenon a1-D mathematical model relevant for spontaneous imbibition isformulated. The model represents a core plug on laboratory scalewhere a general wettability alteration (WA) agent is included.Relative permeability and capillary pressure curves are obtainedvia interpolation between two sets of curves corresponding tooil-wet and water-wet conditions. This interpolation depends onthe adsorption isotherm in such a way that when no adsorption ofthe WA agent has taken place, oil-wet conditions prevail. However,as the adsorption of this agent takes place, gradually there is ashift towards more water-wet conditions. Hence, the basicmechanism that adsorption of the WA agent is responsible for thewettability alteration, is naturally captured by the model. Conservation of mass of oil, water, and the WA agent, combined withDarcy's law, yield a 2x2 system of coupled parabolicconvection-diffusion equations, one equation for the water phase andanother for the concentration of the WA agent. The model describesthe interactions between gravity and capillarity when initialoil-wet core experiences a wettability alteration towards morewater-wet conditions due to the spreading of the WA agent bymolecular diffusion. Basic properties of the model are studied byconsidering a discrete version. Numerical computations are performedto explore the role of molecular diffusion of the WA agent into thecore plug, the balance between gravity and capillary forces, anddynamic wettability alteration versus permanent wetting states. Inparticular, a new and characteristic oil-bank is observed. This isdue to incorporation of dynamic wettability alteration and cannot beseen for case with permanent wetting characteristics. Moreprecisely, the phenomenon is caused by a cross-diffusion termappearing in capillary diffusion term.
Highlights
Seawater has been injected into the naturally fractured Ekofisk chalk reservoir in the North Sea for nearly 20 years with great success
The left plot illustrates that enhancing molecular diffusion by increasing Dr will lead to a stronger transport effect of the wettability alteration (WA) agent into the core
This in turn speeds up the wettability alteration caused by the WA agent and the corresponding imbibition of seawater into the core, see right figure for the water distribution s
Summary
Seawater has been injected into the naturally fractured Ekofisk chalk reservoir in the North Sea for nearly 20 years with great success. It has been observed that the sulphate ions in seawater have a catalytic effect on wettability alteration processes [1, 2, 3, 4, 5, 6, 7, 8, 9]. The mechanism to explain the experimental results is suggested as follows: Sulphate can adsorb onto the chalk surface and make the surface less positively charged. Further studies show that calsium and magnesium may cooperate with sulphate ions to contribute to this wettability alteration process. In order to exploit fully the data that is generated through experiments there is a need for developing mathematical models which take into account dynamic wettability alteration
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