Cased Hole Formation Resistivity: Changing the Way We Find Oil and Gas

Abstract

Abstract In today's competitive market it is increasingly important to improve production and increase reserves in the most efficient way possible. One alternative is to add new wells, but this can be an expensive and time-intensive endeavor. Another alternative is adding and optimizing production from the thousands of existing wellbores already in place. To do this, the ability to detect and evaluate bypassed hydrocarbons and track fluid movement in the reservoir is vital. Current methods of through-casing saturation determination using nuclear tools are limited because of their shallow depth of investigation. With the introduction of the CHFR* cased hole formation resistivity technology a new dimension has been added to cased hole evaluation. A deep-reading formation resistivity can now be obtained through steel casing. Although this measurement is analogous to exsisting techniques for saturation determination, it has two distinct advantages:In many cases there is a need to monitor saturation changes in a producing interval. In the past sigma-based interpretation methods were the only option. However, saturation determination in low-porosity reservoirs (<15 pu) using sigma requires a very detailed and accurate lithology description because small errors in the lithology answer have a significant effect on the saturation from sigma. Because many reservoirs in the Permian Basin combine complex lithology with low porosity, answers can be somewhat unreliable. This is often the case in shallow water-flooded carbonates, where the presence of gypsum, anhydrite and salt makes sigma measurements almost impossible to interpret.Cased hole saturation is also needed to detect bypassed pay and occasionally as the primary evaluation method. In these cases, sigma-based measurements fail because of their shallow depths of investigation. Because the zones have not been produced yet, the invading fluid (brine) still occupies the pores around the borehole. This is particularly true in the Permian Basin because no fluid control measures are used during drilling and deep invasion is normal. The CHFR tool reads deep enough to avoid this complication. In fact, it is commonly superior to openhole resistivity since it reads significantly deeper than openhole resistivity tools. When the CHFR tool is used in conjunction with porosity and lithology tools, run either in openhole or through casing, accurate hydrocarbon saturation can be determined. This methodology can be used to find bypassed pay or to determine the depletion level of producing pay intervals. It has also been used extensively in time-lapse surveys to evaluate the sweep efficiency of secondary recovery, ensuring that no producible hydrocarbons are left behind. This paper reviews the application of this technology in several U.S. basins, including the Permian Basin and California. Introduction Ever since Conrad and Marcel Schlumberger first began measuring formation resistivity in 1927, scientists have been working to evolve the measurement to work in all types of conditions. Arguably the most challenging has been adapting the measurement to operate in a cased hole environment. While the measurement principle is quite simple, making it through conductive steel pipe requires highly advanced instrumentation. Potential differences in the nanovolt range must be measured accurately to make the measurement of formation resistivity possible. The CHFR tool provides this technology. Porosity devices such as neutron and sonic tools have been characterized for cased holes for years. However, the ability to calculate water saturation that enables direct comparison to openhole evaluations using standard equations requires the formation resistivity. The CHFR tool can provide that missing piece of the puzzle. Sigma-based interpretation techniques have been tried but the shallow depth of investigation places the measurement in the highly invaded zone. This method of interpretation in cased holes is also hindered by lithology variations that are typical in the Permian Basin.