Norman R. Morrow

Influence of brine composition and fines migration on crude oil/brine/rock interactions and oil recovery

Increase in recovery of crude oil with decrease in salinity has been observed for numerous laboratory waterfloods. Repeated waterfloods on a single reservoir sandstone core indicated that potentially mobile fine particles play a key role in the sensitivity of oil recovery to salinity. This conclusion was tested using Berea sandstone after fines had been stabilized by firing at 800°C and metal oxides removed by acidizing. Recovery of crude oil from this fired and acidized sandstone was essentially independent of salinity. The presence of crude oil was also a necessary condition for sensitivity to salinity. When refined oil was used rather than crude oil, with all other conditions held the same, salinity had no effect on recovery. There was also no effect for cores that were initially 100% saturated with crude oil so that the fines were initially immersed in the oil phase. Adsorption from crude oil, the presence of potentially mobile fines, and initial water saturation are all necessary conditions for increase in oil recovery with decrease in salinity. Observed recovery behavior is ascribed to partial stripping of mixed-wet fines from pore walls during the course of waterflooding.

Effect of Wettability on Waterflood Recovery for Crude-Oil/Brine/Rock Systems

This paper presents a definitive account of the effect of wettability on oil recovery from Berea sandstone based on the results of more than 50 slow-rate laboratory waterfloods. Closely reproducible wettability conditions and waterflood recoveries were obtained with wettability, depending on the crude oil, brine composition, aging temperature, and initial water saturation. Maximum oil recovery by waterflooding was obtained at very weakly water-wet conditions from shortly after breakthrough up to discontinuation of the test at 20 PV of water injected. In most of the tests, coproduction of oil and water continued long after breakthrough.

Magnitude and Detailed Structure of Residual Oil Saturation

Experimental results are presented that demonstrate the effect on residual oil, under water-wet conditions, of particle size, particle-size distribution, macroscopic and microscopic heterogeneities, microscopic dimensions such as ratio of pore-body to pore-throat size, and pore-to-pore coordination number. Experiments were performed in random packs of equal spheres, heterogeneous packs of spheres with microscopic and macroscopic heterogeneities, two-dimensional (2D) capillary networks having various pore geometries, and Berea sandstone. Detailed information on residual oil structure is presented, including blob-size distributions of residual oil. Major conclusions are (1) residual saturations are independent of absolute pore size, per se, in systems of similar pore geometry; (2) well-mixed two-component aggregates of spheres gave virtually the same residual saturations as random packings of equal spheres; (3) clusters of large pores accessible through small pores will retain oil; (4) high aspect ratios tend to cause entrapment of oil as a large number of relatively small blobs, each held in single pores; and (5) the role of pore-to-pore coordination number is generally secondary; hence, correlations that have been proposed between residual oil and coordination number are unreliable.

Recovery of oil by spontaneous imbibition

Spontaneous imbibition is of particular importance to oil recovery from fractured reservoirs. There has been a surge in the growth of technical literature over the past 5 years. This review is centered on developments in the scaling of laboratory imbibition data. Results for variation in interfacial tension, wetting and non-wetting phase viscosity, sample size, shape and boundary conditions, and initial wetting phase saturation have been correlated for a variety of strongly water-wet rocks as plots of normalized oil recovery vs. dimensionless time. Correlations have been tested for weakly water-wet conditions induced by adsorption from crude oil. In situ fluid saturation measurements have been used to distinguish between modes of imbibition that range from frontal to global displacement. Research on surfactant-enhanced imbibition has advanced from laboratory to field tests.

Salinity, Temperature, Oil Composition, and Oil Recovery by Waterflooding

The effect of aging and displacement temperatures and brine and oil composition on wettability and the recovery of crude oil by spontaneous imbibition and waterflooding has been investigated. This study is based on displacement tests in Berea sandstone with three crude oils and three reservoir brines (RB`s). Salinity was varied by changing the concentration of total dissolved solids (TDS`s) of the synthetic brine in proportion. Salinity of the connate and invading brines can have a major influence on wettability and oil recovery at reservoir temperature. Oil recovery increased over that for the RB with dilution of both the initial (connate) and invading brine or dilution of either. Aging and displacement temperatures were varied independently. For all crude oils, water wetness and oil recovery increased with increase in displacement temperature. Removal of light components from the crude oil resulted in increased water wetness. Addition of alkanes to the crude oil reduced the water wetness, and increased oil recovery. Relationships between waterflood recovery and rate and extent of oil recovery by spontaneous imbibition are summarized.

Capillary behavior of a perfectly wetting liquid in irregular triangular tubes

Abstract Triangles provide a useful example of simple pore shapes: they have angular corners, which can retain liquid, and irregular triangles give a wide range of shapes. The exact meniscus curvature of perfectly wetting liquids draining from pores of general triangular cross section is derived. The appropriate normalized shape factor for capillary action in triangular pores is the ratio of the area of cross section, A, to the square of the perimeter length, P. The drainage penetration curvature is calculated for all possible shapes of triangular tubes. The amount of wetting phase that drains at the penetration curvature decreases as aspect ratio increases. The remaining liquid is retained in the corners of the triangular pore. Thus, after drainage, there is dual occupancy of the pore and continuity of both wetting and nonwetting phases. The decrease in liquid retained in the corners with increase in curvature of are menisci subsequent to penetration is also calculated. The relation between saturation and the square of the curvature is shown to be hyperbolic. Imbibition occurs by the progressive filling of corners. At low saturations imbibition is the exact reverse of drainage. Corner filling continues even when the meniscus curvature falls below the drainage penetration curvature, thus giving hysteresis. When the menisci in the corners meet, the liquid spontaneously redistributes. A portion of the tube length refills and the meniscus curvature jumps to the drainage penetration curvature. Spontaneous redistribution is an example of “snap-off” and may give rise to the entrapment of nonwetting phase. Both the curvature and saturation at which spontaneous filling occurs are derived as a function of shape factor. The triangular pore has much greater versatility than the commonly used cylindrical pore, and models, in a simple way, several basic features of the capillary behavior of highly complex porous materials.

Correlation of Capillary Number Relationships for Sandstone

Capillary number relationships are presented for displacement of both residual and initially continuous oil from water-wet consolidated sandstones having permeabilities that varied over about two orders of magnitude. It was found that the critical displacement ratio, (..delta..P/Lsigma) /SUB cr/ , for the onset of mobilization could be correlated with sample permeability. Relationships between normalized reduced residual oil saturation and capillary number (taken as k /SUB w/ ..delta..P/Lsigma) also were correlated satisfactorily. For sandstones, capillary numbers for displacement of continuous oil were lower than values for mobilization of discontinuous oil for down to 50% of normal waterflood residual. Thereafter, capillary number relationships for the two types of displacement were indistinguishable. Conditions for complete recovery of residual oil correspond to values of (k /SUB w/ ..delta..P/Lsigma) of about 1.5 X 10/sup -3/ as compared with about 2 X 10/sup -5/ for onset of mobilization.

Generalized scaling of spontaneous imbibition data for strongly water-wet systems

Mass transfer between fractures and matrix blocks is critical to oil recovery by waterflooding in fractured reservoirs. A scaling equation has been used for rate of oil recovery by spontaneous imbibition and presented their results as oil recovery vs. dimensionless time. Many conditions apply to this scaling equation, including identical core sample shapes and fluid viscosity ratios. Recent investigation by experiment of these two factors has resulted in a more generalized scaling equation for strongly water-wet systems with a general definition of characteristic length and a viscosity ratio term included in the definition of dimensionless time. In this paper, published data on oil recovery by imbibition have been analyzed and correlated through application of the new definition. These data sets were for different porous media, core dimensions, boundary conditions, and oil and water viscosities. All of the systems were strongly water-wet. The generalized correlation was fitted closely by an empirical mass transfer function with the new definition of dimensionless time as the only parameter.

Experimental Verification of a Modified Scaling Group for Spontaneous Imbibition

Spontaneous imbibition is of critical importance to oil recovery from fractured reservoirs. A widely used approach to prediction of oil recovery involves scale-up of laboratory results to reservoir conditions. Scaling involves the effects of sample size, shape, boundary conditions, viscosity and viscosity ratios, interfacial tension, pore structure, wettability, capillary pressure and relative permeability. This work addresses the problem of scaling the combined effects of sample shape, boundary conditions, and viscosity ratios with only minor variations in other parameters. Imbibition measurements are presented for cylindrical Berea Sandstone cores of different lengths. For some experiments, core surfaces were partially sealed with epoxy to give different boundary conditions. Cores were initially saturated with refined mineral oils of different viscosities. A synthetic reservoir brine was used as the wetting phase. A characteristic length was defined as the square root of the ratio of volume to the summation of the ratios of area of core surface open to imbibition to the corresponding distance from the surface to the no-flow boundary. The characteristic length, in combination with a term that compensates for the effect of viscosity ratio, gave close correlation of all data.

Interrelationship of Wettability, Initial Water Saturation, Aging Time, and Oil Recovery by Spontaneous Imbibition and Waterflooding

od on Summary Previous studies of crude oil/brine/rock ~COBR! and related systems showed that wettability and its effect on oil recovery dep on numerous complex interactions. In the present work, the w tability of COBR systems prepared using Prudhoe Bay crude a synthetic formation brine, and Berea Sandstone was varie systematic change in initial water saturation and length of ag time at reservoir temperature ~88°C!. All displacement tests were run at ambient temperature. Various degrees of water wet were achieved and quantified by a modified Amott wettabi index to water, the relative pseudowork of imbibition, and a new defined apparent advancing dynamic contact angle. Pairs of spontaneous imbibition ~oil recovery by spontaneou imbibition of water! and waterflood~oil recovery vs. pore volumes of water injected ! curves were measured for each of t induced wetting states. Several trends were observed. Imbib rate, and hence, water wetness, decreased with increase in time and with decrease in initial water saturation. Breakthrou recoveries and final oil recovery by waterflooding increased w decrease in water wetness. Correlations between water we and oil recovery by waterflooding and spontaneous imbibition presented.