Field Test Results of a Slim Through Casing Resistivity Array Instrument Prototype
Authors: Fanini O.N. , Hunziker J. , Maurer H.M. , Tchakarov B.J. 
Published: December 2007

Abstract
Two field test examples of formation resistivity logs acquired through metal casing are presented. There is good agreement between the acquired logs and the open hole resistivity logs measured prior to the completion of the wells. The casing collar effects are successfully compensated by the measurement scheme. Error propagation analysis shows that through casing resistivity measurements can have better accuracy of water saturation estimation than provided by pulsed neutron tools for low porosity formations. A comprehensive review of the history of the technology is presented.

Introduction
If additional producible reserves can be located or if recovery from already identified producing zones can be increased, the infrastructure currently in place in a developed field could be further utilized once production life is extended. These advantages can be accomplished by using the new Through Casing Resistivity Tool® (TCRT®). Under certain formation conditions, reservoir monitoring with nuclear measurements can be problematic, as discussed in previous publications.1 The magnitude of this problem prompted the development of the Through Casing Resistivity (TCRSM) technology as a complementary measurement to existing production logging tools. The TCR technology has the inherent ability to improve recovery efficiency by properly identifying depleted, flooded, and unswept zones. Utilizing formation resistivity measured through casing with established resistivity interpretation methods and with the proper formation evaluation models can improve monitoring of water, hydrocarbon, and gas movement. When used in combination with standard production logs, the TCR instrument yields more accurate and reliable reservoir management and assists in maximizing hydrocarbon production.
The industry's first slim through casing resistivity array tool is able to provide accurate and reliable resistivity logs through metallic casing. The TCR prototype instrument has undergone an extensive field-testing program in producing, injecting, and monitoring wells. The TCR has logged intervals that included perforated zones in vertical and deviated wells. These field tests have demonstrated the capabilities, safety and value of this technology. Based on its consistency with open hole resistivity logs in the published field test results, the TCR measurement has gradually gained industry acceptance.2,3
The Houston Technology Center of Baker Hughes has developed and field-tested this ground breaking slim through casing resistivity prototype under the sponsorship of Gas Technology Institute (previously Gas Research Institute) and an industry consortium formed by major oil companies.

TCR Technology Applications
The TCR can offer advantages over nuclear measurements such as providing a deeper measurement with a wider dynamic range over various salinity and porosity conditions.
Measurement of electrical resistivity is useful because it allows distinguishing high-resistance oil- or gas-bearing zones from low-resistance, water-filled formations. Formation resistivity is fundamentally a measure of the type and amount of fluid (i.e., water or hydrocarbon) present in the rock's pore space. In a formation with a given porosity and water salinity, the measured resistivity will be higher when oil or gas occupies an increasing part of the available pore space.
The main application areas for TCR are reservoir monitoring and locating bypassed hydrocarbons.

Reservoir Monitoring.
During the life cycle of a hydrocarbon producing reservoir, resistivity data can help to track fluid and gas movements within the reservoir:Water migrates upward as oil and/or gas depletion occurs during production in water-drive reservoirs. Using resistivity logs, the upward migration of the connate water can be monitored.