Porosity Calibration of Neutron Logs, SACROC Unit (includes associated papers 15799 and 15846 )
Authors: Swulius T.M. 
Published: December 2007

A major field study has been initiated at the Scurry AreaCanyon Reef Operators Committee (SACROC) unit to provide a detailedgeologic description of the reservoir. To supply porosity information to the digital database, a new technique has been developed thattransforms neutron deflection logs into accurate neutron porosity logs. The technique uses computer programs to analyze statistically the neutronlog to be rescaled as well as nearby modern porosity logs. An equationis derived that transforms neutron log deflection into porosity. Oncethe porosity response of a particular tool is determined in areas ofgood porosity control, that response can be applied successfully inareas of little or no control.

Introduction

The SACROC unit reservoir description project (RDP)is a major field study and has as its overall goal thecreation of a digital geologic database of wide scope andrelevance to the operation's current and foreseen needs. The purpose of the RDP is to provide support to theengineering staff at this major CO2 and waterflood siteand to assist the geologists in infill wellsite selection. To accomplish these goals, an immense amount of data mustbe incorporated into the system.

This paper describes a major part of the system: thegeophysical well log processing. All gamma ray andneutron logs have been digitized, but only the neutronlogs are undergoing transformation. They are being rescaledin terms of calibrated limestone porosity to provide adense three-dimensional control on the porositydistribution.

The SACROC unit covers about 50,000 acres [200 x 10(6)m2], the major part of the giant Kelly-Snyder field(Fig. 1). The reservoir is in the Canyon limestone, a late Pennsylvanian stratigraphic reef or carbonatebuildup(Fig. 2). Although composed for the most partof clean limestone, it also contains shale, shaleylimestone, dolomite, and chert.

Having produced over 10(9) bbl [160 x 10(6) m3] of oil, the unit has been converted to tertiary recovery methods(CO2 injection). If reliable porosity logs could beobtained for each of the 1,600 + wells in the unit, fluidmovement would be much more predictable, and floodefficiency could be improved.

Throughout the unit is a total of 400+ modern neutronporosity logs(mostly sidewall neutron porosity [SNP]). The remaining 1,200 + wells have been logged with theold-style neutron deflection logs. Core control is limitedto 124 wells, 95 of which have a cored interval greaterthan 30 ft [10 m].

Preliminary tests showed that the old neutron logs couldbe transformed to accurate limestone porosity if sufficientmodern porosity-log control existed in nearby offset wells. Because virtually all of the porosity control exists in thenorthern one-third of the unit, accurate porosity transforms could be developed in that area, but the remainingtwo-thirds of the wells would have to be treateddifferently.

We decided to develop carefully the most accuratetransforms possible for well logs in the area of good controland then to apply the appropriate transforms, accordingto the particular neutron tool used, throughout the restof the field.

A survey of the log files indicated that 11 differentlogging companies have used at least 70 different neutronlogging tools at SACROC since the field's discovery in1948. Each tool was given an alphanumeric tool code sothat as transform equations were developed and storedalong with other well data, they then could easily berelated to the particular service company and toolconfiguration used. This would eventually permit anygroup of transforms to be called up and compared to others. With nearly 1,000 logs transformed to date, this automaticcross-referencing system has been an obvious advantage. Also, as logs are processed in those areas that lack control, the tool code permits the proper, previouslygenerated transform to be selected and used automatically.

Digitization

As each well is digitized, additional information to be usedin various phases of the project is recorded with a keypunch. Fig. 3 is a printout of these data for a typical well.

After a log is digitized, a playback plot of the log asdigitized is overlaid on the original log for quality-controlpurposes. After the playback plot passes quality control, a second plot is requested with all of the backups shiftedto their correct position and any neutron-zero baseline shifts removed. The result is a continuous trace of neutrondeflection in inches from neutron zero. At this point, the log is ready for transform development.

Porosity Transform Development

The relationship between neutron log deflection andporosity was presented by Brown and Bowers.

JPT

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