No abstract

No abstract

Significant strides have been made in recent years in gaining better understanding of the role of fluid compositions on reservoir wettability. However, our knowledge of the effects that the solid surface characteristics have on establishing and altering wettability is quite limited. This study examines the effect of rock mineralogy and surface roughness on wettability in rock-brine-hydrocarbon systems. The wettability is characterized by using two different techniques. The Wilhelmy plate technique has been used to obtain dynamic (advancing and receding) contact angles averaged over the surface area of the solid substrate used. These results are compared with point-values of dynamic contact angles measured using the dual-drop dual-crystal (DDDC) technique for both smooth and rough solid surfaces of different mineralogy and roughness. The surfaces have been characterized using an optical profilometer and Scanning Electron Microscopy. Fluids from the Yates reservoir in West Texas have been used. While the Wilhelmy Plate Technique displayed insensitivity, the DDDC Technique showed significant effects of mineralogy and roughness on dynamic contact angles. Introduction One of the common criticisms of contact angle measurements is that they use smooth crystal surfaces in place of reservoir rocks, which are, not only rough but are mineralogically heterogenous. The main purpose of studying the effects of surface roughness on contact angles has been to investigate the possibility of relating contact angles measured on smooth surfaces to those likely to be operative on rough surfaces within porous media. Although the importance of surface roughness to wetting behavior is recognized, difficulties have been reported in obtaining a definitive account of roughness effects. This study is an attempt to address these concerns. Wenzel(1) studied the effect of roughness on contact angle and proposed a theory, which is used to derive the relationship between the angle observed on a smooth surface, ? E, and the advancing angle on a rough surface, ?A:Equation 1 Where r is the roughness factor, defined as the ratio of the surface area of the rough surface to that given if the solid were microscopically smooth. If the surface is rough enough with a large number of asperities in it, the angle measured with the horizontal will actually be the apparent contact angle because the asperity on the surface might not be horizontal as illustrated by Adamson(2). The true contact angle can be much larger than what we measure with the horizontal. Cassie and Baxter(3) in their study on waterproofing fabrics used the example of bird feathers to show the difference between true and apparent contact angles. They reported an apparent contact angle of 150° as opposed to a true contact angle of 100°. Oliver et al(4) and Mason(5) in their theoretical study of the influence of surface roughness on spreading and wettability showed by calculation of the equilibrium shape of the liquid drop resting on a rough surface the relation between the true equilibrium contact angle at the three phase contact line and the apparent contact angle observed microscopically at the geometrical contour plane of the solid. In their attempt to clarify the Wenzel's relation between the apparent and true contact angles they considered surfaces having random roughness to derive a statistical relation between the angles which introduced a factor for surface texture in addition to surface roughness.

The interfacial phenomena of spreading and adhesion of fluids on rock surfaces have serious implications because of their impact on production strategy and oil recovery. The present study reports new experimental data on the effect of brine dilution and surfactant addition on spreading and adhesion behavior of Yates crude oil on dolomite surfaces. Spreading and adhesion have been characterized through measurements of oil-water interfacial tension (IFT) and dynamic (water-advancing and receding) contact angles. The interfacial tension was measured using Computerized Axisymmetric Drop Shape Analysis (CADSA) Technique, which was calibrated against the well-known du Nuoy Ring Technique. The Dual-Drop-Dual-Crystal (DDDC) Technique has been used to measure dynamic contact angles. In order to study the effect of brine dilution, Yates reservoir brine was mixed with deionized water (DIW) in various proportions. For each diluted brine, crude oil-water IFT, water-advancing and receding contact angles were measured. The oil-water IFT initially decreased as the volume percent of brine in the mixture decreased. However, the IFT increased with further dilution of reservoir brine with DIW. A decreasing trend was observed in the behavior of water-advancing contact angle with brine dilution. This indicated that the initial oil-wet nature of the system was changed to intermediate-wettability simply by diluting the reservoir brine. However, a strange behavior of spreading of crude oil drop against brine on the dolomite surface (with large water-receding contact angles) was observed at certain brine dilutions. This spreading of crude oil appears to be related to interfacial tension in a manner similar to Zisman's observations in solid-liquid-vapor systems. The use of surfactants to enhance oil recovery through reduction in IFT is well known in the industry. However, this study examines the capability of certain surfactants to alter wettability in addition to reducing IFT. For the Yates reservoir rock-fluids system, an Ethoxy Alcohol surfactant altered the strongly oil-wet nature (advancing angle of 158°) to water-wet (advancing angle of 39°) at a concentration of 3500 ppm. While the DDDC technique yielded significant changes in wettability due to surfactant addition, the Wilhelmy Plate Technique remained insensitive throughout the range of surfactant concentrations. The practical significance of this study is that it identifies two simple modes through surfactant addition and brine dilution to alter wettability to minimize capillary trapping of oil. Introduction The primary and secondary oil recovery processes currently being practiced have been successful in recovering only about a third of the original oil in place leaving behind nearly two-thirds as residual oil. This points out the need to study and implement new and innovative methods to recover the remaining oil. This in turn requires an understanding of the interactions that take place between crude oil, brine and the rock surface, which are collectively represented by the term wettability. Reservoir wettability is affected by several factors including roughness and mineralogy of the rock surfaces and the compositions of brine and crude oil. The effect of rock mineralogy and surface roughness have been reported previously (SPE # 75211) and this paper presents the effects of fluid characteristics brine dilution and surfactant concentration) on spreading and wettability as characterized by receding and advancing contact angles, respectively. Effect of Brine Composition on Wettability Several previous studies have been reported in the literatures that describe the effect of brine composition on formation damage and waterflooding. Mungan(1) investigated the role of pH and salinity changes on core damage. He concluded that the primary cause of permeability reduction was blocking of the pore passages by dispersed particles. A change in the salt concentration (salinity) or pH of the reservoir fluid can liberate these clay particles and cause the clay particle to swell and clog the pores thus resulting in lower permeability.

No abstract