Reservoir Description Using Well Logs (includes associated papers 13412 and 13538 )
Authors: Hilchie Douglas W. 
Published: December 2007

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering.
The term "reservoir description" means many things to people. This term, asrelated to well log interpretation, has developed as the petroleum business and technology have advanced. In early times, the term related only to depth and thickness of a reservoir. Today the term includes depth, thickness, porosity, water saturation, and fluid type. Even this is too limiting. Logging data caninfer pore sizes, grain sizes, depositional environments, and lithology.
The need to characterize reservoir rock and pore systems better stems fromthe need to obtain better recovery of oil and gas from the existing reservoirs. A better understanding of the reservoir also will increase the efficiency of interpreting well logs and result in fewer unnecessary tests and completions.
Reservoir description by use of well logs may be divided into three areas: volumetrics, pore description, and matrix description. Each of these arediscussed, although more emphasis is placed on pore description because it moreclosely relates to the interests of petroleum engineers. Where distinction ofthese study areas is not convenient, discussion is handled under Pore Description.

Volumetrics
Determination of the volume of a reservoir is a multiwell problem. In each well, porosity, water saturation, and thickness of the productive or permeable interval must be determined. Porosity and water saturation are considered routine determinations from well logs; usually there is no difficulty inestimating these two parameters.
The thickness of the productive or permeable interval is, however, more difficult to determine because no well log measures permeability directly. Somelogs (like the microlog) and some methods (such as comparison of Ri or Rxo toRt) measure the apparent invasion of mud filtrate into the reservoir and givean indirect estimate of permeability so that the thickness of the productive interval can be estimated. But this estimate is not always reliable, because filtrate invasion is influenced by drilling procedures, mud properties, and rock permeability.
Well logs relate only to reservoir properties around a given well;thus, interpolation between wells is necessary to estimate the volume of an entirereservoir.

Pore Description
Pore description by using well logs is not a precise quantitative procedureat this time. The evaluation of pore-geometry influences on the logs most oftenis overlooked because a backup or check on the conventional interpretation is lacking. This may result from in experience or because the backup technique isnot applicable to the available logs or to the mud and drilling practices used while drilling the well.
In this discussion, conventional interpretation (excluding shaly sandinterpretation) is considered to involve Archie equation(s):
Sw2 = Ro/Rt,................................(1)
where
Sw = fractional water saturation, Rt = resistivity of the reservoir rock, and Ro = resistivity of the reservoir rock at 100% water saturation. Ro = usually is determined by Ro = F.Rw,.................................(2)
where Rw is the resistivity of formation water and F is the formation resistivity factor. F is related to fractional porosity, by
F = a ., ...............................(3)
where a and m are empirical constants that depend on the shape anddistribution of pore geometry.
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