Rationalizing the Influence of Crude Wetting on Reservoir Fluid Flow With Electrical Resistivity Behavior
Authors: Swanson B.F. 
Published: December 2007

In laboratory tests designed to simulate a natural distribution of crude oil surfactants in porous rock, waterflood behavior may exhibit oil-wet characteristics, whereas electrical resistivity remains essentially the same as for water-wet systems. These observations agree with a hypothesis that considers the influence of pore-wall roughness on the contact angle in a rock/water/oil system.

Introduction

The literature confronts us with apparent contradictions when considering the wetting state of crude oil reservoirs. There abounds laboratory evidence that many reservoir systems behave in a neutral to oil-wet fashion in fluid flow tests. But data also have been reported showing that the electrical resistivity of highly oil-saturated rocks is extremely high. Electric logs of natural systems seldom find reservoir resistivities approaching the extreme. We also see little evidence of inverted saturation profiles we might expect in reservoirs if fluid profiles we might expect in reservoirs if fluid positions are reversed from the water-wet case. Thus, positions are reversed from the water-wet case. Thus, observed resistivity data appears to refute laboratory flow data that indicate some reservoirs to be oil-wet.Fluid flow behavior, comparing water-wet with oil-wet, is exhibited in two ways:the relative permeability to water is higher at any saturation in permeability to water is higher at any saturation in the more oil-wet case, resulting in an equal relative permeability to water and oil at lower water permeability to water and oil at lower water saturation when oil-wet, andin the oil-wet case, oil continues to drain from the rock during waterflood after water breakthrough whereas, in the water-wet case, at least for favorable viscosity ratios, oil production ceases shortly after water breakthrough.

Experimenters have reported differences in electrical behavior in rock/water/oil systems of differing wettabilities. Their results can be summarized as follows. As oil saturation increases, the electrical resistivity increases far more dramatically in the more oil-wet systems than for water-wet systems. Some data have indicated that resistivity approaches infinity even at moderate water saturations. A common feature in these studies is that the oil-wet data were obtained on rocks or synthetic pore systems within which the entire internal surface had been coated with some surface active material, such as Dri-Film, to produce the oil-wet character. This results in an affinity of the total surface of the rock for oil. In other words, a complete reversal of fluid positions was achieved when compared to the water-wet case and, thus, resistivity became extremely high. But, if we consider the manner in which crude oil accumulates in a reservoir, it appears unlikely that the above process properly simulates how oil surfactants may deposit in real reservoir systems.

Natural Crude Accumulation

To describe the original distribution of fluids in an oil reservoir, two properties of the system must be explored.

JPT

P. 1459