Evaluation of Water-Oil Displacement Efficiency Using Subsurface Logs
Authors: Rebold J.H. 
Published: December 2007

Abstract
A method is presented for the calculation of in situ values of oil-displacement efficiency by water flushing using conventional subsurface logs. The method has been devised so that a knowledge of the original water saturation vs depth profile can be used but is not essential, provided that a reasonable value for irreducible connate water is available. This feature provides a means of obtaining an approximation of oil-displacement efficiency in wells which were not cored or had original logs of too poor a resolution to yield reliable water saturation values. A reasonable agreement has been obtained between recent field tests and a limited number of laboratory tests performed under reservoir conditions. As secondary products, it is also possible to observe and determine the water-advance rate, the oil desaturation profile with time and the final equilibrium residual oil saturation. All of these are valuable tools in planning and prediction. The steps taken to insure optimum logging results and determination of water saturation and porosity values are discussed in detail, since they form an integral part of the required calculation procedure.

Introduction
As cumulative oil withdrawals from a reservoir subject to edge-water or bottom-water drive increase, it is desirable to:follow and analyze the efficiency with which the water has displaced oil from the pores of the reservoir rocks;determine the actual, in situ, final residual-oil saturation; anddetermine the rate of advance of the oil-water contact.
This information is particularly valuable in determining the methods of operation to gain maximum recovery of original oil in place and in the planning and scheduling of field operations. Knowledge concerning these variables also can be used in substantiating laboratory data and material-balance calculations. In the Ghawar, Abqaiq and Safaniya fields of Saudi Arabia, a planned program is being carried out to complete key delineation wells as water-advance observation wells and, also, to convert suitable wells to water-advance observation wells. In the case of the latter, it has been necessary to mill out liner or casing. To reduce the possibility of end effects near the wellbore, the observation wells are not used for production purposes. The technique employed is simple and consists ofcompleting the well open-hole with a highly resistive fluid in the wellbore such as fuel oil, andinitially running the Induction and Sonic logs through the zone of interest.
Following this, the Induction log is run through the zone of interest at regularly scheduled periods to observe changes in oil saturation with time. By using appropriate calculation procedures, the oil saturation and the relative oil-displacement efficiency can be determined. Results of oil-displacement efficiency calculations using logs have been compared with the oil-displacement efficiency obtained by water flooding reservoir rock under reservoir conditions in the laboratory. The two methods have yielded a very reasonable agreement with each other.

Displacement Efficiency Calculation
For the purpose of this discussion, the oil-displacement efficiency (oil DE) is defined as the efficiency with which water displaces oil in that portion of the reservoir rock volume contacted by the water. It specifically implies that no areal sweep efficiency or vertical conformance efficiency is included in the displacement efficiency. This definition has been used for two reasons:the volume measured by logging is insignificant compared with the entire reservoir volume, anddisplacement efficiencies as measured in the laboratory compared closely with values calculated from logging.
Under certain conditions, it is possible for the volume measured by logging to contain some portion of the areal sweep efficiency and/or vertical conformance efficiency. If this is known to he the case, the efficiency yielded by log calculations will not be the displacement efficiency defined herein. This is a matter of experience and judgment in determining the actual physical significance of the efficiency values yielded by log calculations. When applied in conjunction with subsurface logs, the displacement equations yield instantaneous values.
JPT
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