Comparing the Accuracy of Fluid Saturation Derived from Different Logging Measurements: Past, Present and Future Methodology

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractObtaining the water and hydrocarbon saturations in any reservoir are one of the main applications of many logging instruments. Resistivity has been used with relative success in many fields and can be considered the main open-hole "saturation" measurement, even though true formation resistivity remains elusive. Pulsed neutron capture is the basic cased-hole "saturation" measurement when formation water salinity is known and high enough. While either one has its own limitations, they often agree well enough that they reinforce the user confidence in their respective accuracy. Thus, they are generally considered adequate for reservoir management decisions.Occasionally, acquiring and comparing a large number of different measurements that respond to fluid distribution in the formation shows that two measurements dose not give the same answer. As a result, Petrophysicists face the task of reconciling and explaining those discrepancies to extract additional and valuable information.In this paper, we review the principles, advantages and limitations of a certain number of measurements acquired in both open and cased hole. We briefly review how data was handled in the past and the way it is handled today. We then review how discrepancies are reconciled today and make suggestions and recommendations as to which measurements are the most appropriate for various types of environments.In the last part of the paper, we are suggesting new ways to acquire, process and interpret log data. Logs free of environmental effects will be obtained through the acquisition of auxiliary measurements that define the Wellbore environment accurately. Models are used to evaluate and correct for the environmental variations that are not resolved by in-built sensors. Tool planners are expanded to become true artificial experts that recommend the right measurements for a given formation and borehole environment. In addition, the uncertainty on the logs, parameters and models will be propagated to the output uncertainty in a meaningful and representative manner.