Because of unfavorable wetting conditions much residual oil is left when a porous material is flushed by water. Methods suggested to change reservoir wetting to improve oil displacement efficiency are generally expensive. The present laboratory study was undertaken to gain all understanding of the factors which determine reservoir wettability, and to find out if oil displacement efficiency might be improved by a wettability change accomplished at low cost in an oil reservoir. Contact angle measurements were made on mineral surfaces using several sets of reservoir oil and water samples. Results of the contact angle studies suggest that reservoir wettability may be primarily determined by natural surface-active substances present in the reservoir fluids. The effect of changing salinity and pH of the water phase was studied. The results suggest that gross changes in preferential wettability might be accomplished by injection of water containing simple chemicals to alter pH or salinity in the reservoir. Such treatment could be much less expensive than injection of commercial surface-active agents. Waterflood tests have also been made using synthetic cores and oil and water having wetting characteristics similar to those of reservoir fluids. Cores initially oil-wet were flooded in such a way that they were made preferentially water-wet by the advancing flood water. This reversal in preferential wettability achieved greater oil displacement efficiency than when either oil-wet or waterwet conditions were maintained throughout the flood. For the systems studied, the higher the oil viscosity the greater the percentage improvement obtained over conventional waterflood recovery. This suggests that a flooding process making use of wettability-reversal may extend the oil viscosity range over which water flooding is attractive. Because a precise adjustment of reservoir wettability does not seem to be required, and because altering the pH or salinity in some reservoirs may be inexpensive, it appears that a waterflooding process employing wettability- reversal could find successful field application.

A field test has been made in which additional oil recovery was obtained from a previously-waterflooded "oilwet" sandstone reservoir. This recovery improvement was accomplished by adjusting the reservoir wettability through chemical treatment of the flood water.The test was made in the Muddy "J" sand of the West Harrisburg Unit, Neb. The chemical used, sodium hydroxide, was injected as a slug of dilute caustic solution through a waterinjection well. The natural wettability of the reservoir and the chemical requirements for reversing wettability were determined in the laboratory from contact-angle studies using field water and oil samples. Laboratory flood tests with a synthetic system had shown that reversing the wettability of an oil-wet consolidated core would lead to improved oil recovery. The field performance indicates that the mechanism by which increased oil recovery is obtained in the field is the same as that observed in the laboratory.Laboratory studies indicate that higher ultimate recoveries and decreased water-injection requirements result when the adjusting agent is added early in the life of the flood. However, a previously waterflooded area was intentionally chosen for the field test so that unambiguous conclusions could be made about the effects of chemical injection on wettability and the extent of oil-recovery improvement afforded by wettability reversal.

In nettability alteration flooding, a chemical agent is moved IIWOIWIIa reservoir by (he flood water to increase oi[ recovery by decreasing the degree of weuing of the rock by the oil, Sn.bstanrial antoun !s of rhe chemical may be lost dwing nmvement through the reservoir, The exten t of the loss, and therefore the economics of {he process, depends in some case? on factors which are difficr(lt to reproduce in the laboratory. Therefore, a short-dura-Iion. IOMB-COSI field le~t i~lerhod is needed IO pern]ir evahiari(w o~chenl icwl reqnire,nen/r under ocmal field conditions. Thi.r pcrper describes a ,rtilall scale test condltcfed at a singk uell jor mea.wring cflemicaf reql~iretnent.~,(hereby giving a lnore relialde evnll[alion of lh is important factor in the applicab ili(y and econon Iics of the process. In rhe test a SJIIOI1 water slug containing the cilemical agent and a n