No abstract

No abstract

Laboratory core flood and field scale tests have demonstrated that about 5 to 40% more oil can be released from sandstone reservoirs by injecting low salinity water, rather than high salinity fluids such as seawater or formation water. The effect has been explained by a change in wettability of the minerals that form the pore wall, as a result of the decrease in divalent cation concentration. Using X-ray photoelectron spectroscopy, we have demonstrated that even for solvent cleaned core samples, mineral surfaces retain a significant quantity of carbon containing material. Thus, pore wall wettability is more likely dominated by tightly adsorbed organic material than by the character of the underlying minerals. To test this hypothesis, we used the chemical force mapping (CFM) mode of atomic force microscopy (AFM) to directly measure adhesion forces on individual quartz grains that were plucked from core plugs. We functionalized AFM tips with model oil compounds so they would represent tiny oil droplets, and we measured their ability to adhere to surfaces as salinity changed. We examined grains from a sandstone core plug that had been cut into segments, which had been stored in kerosene or solvent cleaned. On all samples, surfaces were more oil wet (higher adhesion) in artificial seawater (ASW; 35,600 ppm) than in ASW diluted with ultrapure deionized water to ∼1,500 ppm. XPS demonstrated that solvent cleaned surfaces had less adsorbed organic material than the kerosene stored sample. AFM measurements showed that the low salinity effect, namely the change in adhesion caused by decreasing salinity, was twice as high on kerosene stored samples as on solvent cleaned surfaces. The organic material that is adsorbed on the pore surfaces in the preserved sandstone offer very sticky anchor points for adhering oil molecules. This suggests that in reservoirs, even hydrophilic minerals located at the pore-fluid interface have tightly adhering hydrocarbons and the low salinity response depends on the behavior of this adsorbed material.

No abstract

The long‐term mechanical strength of calcite‐bearing rocks is highly dependent on the presence and nature of pore fluids, and it has been suggested that the observed effects are due to changes in nanometer‐scale surface forces near fracture tips and grain contacts. In this letter, we present measurements of forces between two calcite surfaces in air and water‐glycol mixtures using the atomic force microscope. We show a time‐ and load‐dependent adhesion at low water concentrations and a strong repulsion in the presence of water, which is most likely due to hydration of the strongly hydrophilic calcite surfaces. We argue that this hydration repulsion can explain the commonly observed water‐induced decrease in strength in calcitic rocks and single calcite crystals. Furthermore, this relatively simple experimental setup may serve as a useful tool for analyzing surface forces in other mineral‐fluid combinations.