Wettability affects other properties related to multiphase fluids distribution and flow in porous media. To characterize reservoir wettability, low invasion preserved cores may be taken and characterized, but a common practice is to clean the samples cored conventionally and then age in crude oil to restore reservoir wettability. The cleaning process requires that the core samples are restored to their original strongly water wetness before oil migration. This is much more challenging than volumetric clenaing in routine core analysis for measurement of pore volume and porosity. Pore surface cleaning may not have much effects on the volumetric porosity measurement since it only takes a miniscule amount of polar hydrocarbon to alter the surface properties of a large surface area.
We investigate how clean does a core sample need to be for proper reservoir wettability restoration, by using both carbonate and clastic rock samples. Testing fluids include brines with different salinities and two crude oils with different characteristics in terms of resins and asphaltenes. Solvents used for core cleaning included toluene, xylene, and methanol. Core cleaning methods tested are the routinely used Soxhlet cleaning, as well as the forced flow-through cleaning. Various ways for assessing core cleanliness by analyzing solvent effluent were evaluated including tests of turbidity and UV light for crude oil cleaning and 10% silver nitrate titration for salts removal.
Results show that flow-through is much more effective than Soxhlet for pore surface cleaning, for both carbonate and clastic rocks.
Pore structure and wettability are two parameteres that affect petrophysical static and dynamic properties of reservoir rocks. Though they are basic, their proper characterization is still a challenge. Common methods include using mercury injection capillary pressure measurements for pore similarity analysis (Ma and Morrow, 1993) and evaluating fluid displacement efficiencies for wettability characterizaiton (Amott, 1959; Donaldson et al., 1969; Ma et al., 1999a).