Improvement of oil recovery and reduction in water-cut in a matured field requires precise time lapse saturation monitoring. Behind casing resistivity, an important member of the comprehensive analysis behind casing services suite, provides the required answer by acquiring deep resistivity information through casing for subsequent formation evaluation. A time lapse saturation figure could be generated immediately after the acquisition which is extremely instrumental to take an immediate decision. The technology is well known in the industry and already proven beneficial in many occasions. In favorable conditions, this is the most effective methods available to date for saturation monitoring as being the deepest through casing measurement in terms of radial investigation. The case study presented in the paper describes a successful water shutoff operation and improved oil recovery from the Bahariya formation in the western desert, Egypt. The well was drilled in early 2006 followed by logging and testing and was put on production immediately after completion. During the production time of little more than one year, the oil rate decreased by 70% with 84% water-cut. This significant fall in oil output with sharp early water break through, a common phenomenon in this mature field required a proper shutoff operation. The present example discusses in detail the reason in choosing resistivity behind casing acquisition and time lapse formation evaluation to monitor present saturation profile. Comparison between original and present water saturation level immediately detects the depleted zones and the degree of depletion across perforated intervals indicates the interval contributes most of the produced water. In the present example the cased-hole resistivity acquisition shows departure (lowering) of resistivity values against some perforated intervals. The zones were isolated after identification of the intervals that showed considerably lower resistivity values. This is considered as an indication of rise in oil-water-contact or lengthening of transition zone. In the present case, the remedial measures for water shut-off operation based on time lapse evaluation have enhanced oil recovery from 540 Barrel to 2250 Barrel oil per day with significant reduction in water cut from 84% to 0.5%. The results were encouraging enough for the operator to reinvestigate the production performance and identify wells where this technique could be meaningfully utilized. The current analysis and workflow offers a fairly good understanding about the reservoir's production scenario, monitoring and future actions for enhanced recovery. Introduction The different techniques of reservoir monitoring and maintenance, enhanced/improved oil recovery, evaluation of bypassed oil, identification of fluid movement etc. depends primarily on the saturation estimation at different stages of the life of a reservoir. Time lapse monitoring of a reservoir is estimated at different wells in the field using incessantly developing and evolving through casing measurement techniques. The ability to detect hydrocarbon behind casing is therefore vital. Two parallel and complementary methodologies have been evolved over the years and numerous patents and articles have been published. Nuclear emission based techniques were the forerunners in this category. Thermal Decay time measurements (TDT) and Reservoir saturation estimation using Carbon-Oxygen ration have been used regularly since its inception. More recently the resistivity measurement through casing opened new vistas in estimating saturations under suitable conditions efficiently and accurately (Béguin et al., 2000, Bellman et al., 2003). The technology proves to be exceedingly effective and beneficial in taking quick decision to perform real-time operations, for example, successful water shutoff operation for the current scenario. Analysis behind casing (ABC) is the presently active methodology that encompasses various techniques used for comprehensive analysis behind casing for complete and elaborate formation evaluation. Cased hole formation resistivity (CHFR) is one of the major components of this category of logs that has been used successfully on many occasions wherever the technology is introduced in the world.
Accurate time-lapse saturation information is the key to making the right decisions on completion strategy, maximizing oil recovery and reducing water cut. This paper presents a case study from the Bahariya Formation, a heterogeneous fluvio-marine channel deposit in the Western Desert, Egypt. All the wells considered in this paper showed significant water production. To identify the main water-producing zones and the bypassed oil, all the wells were logged using a through-casing formation resistivity tool. One well was also surveyed with pulsed neutron capture logs. Based on the log results, depleted zones were identified, and the intervals contributing most to the water production were isolated. Water cut was significantly reduced. In some wells, the saturation analysis revealed that the stacked reservoir zones had variable levels of depletion and that the depletion was not necessarily related to the distance to the original oil-water contact. In these wells, the water shutoff leaves oil behind, and a different completion strategy was recommended. The results from the resistivity and nuclear measurements are discussed in detail with respect to environmental effects. This case study demonstrates that through-casing formation resistivity measurements provide more robust answers compared to neutron measurements in the studied environment. The deeper depth of investigation is extremely valuable, as the wells cannot be logged under dynamic conditions, and fluid reinvasion is always present. Moreover, in view of increasingly high rig rates and limited rig availability, the simple nature of the processing and interpretation of the through-casing formation resistivity log enables fast decisions. These examples from the Western Desert illustrate how analysis behind casing provides critical information to maximize oil production and facilitate water shutoff decisions. Introduction The field studied in this paper is located in the Western Desert of Egypt. It has been producing oil since 1992 from the Bahariya Formation, a heterogeneous fluvio-marine channel deposit. In 2006, the oil production started to decline with a sharp increase in water cut. Four wells were selected by the operator for water shutoff operations. In each of these, the water cut was more than 70%. In each well, through-casing resistivity was acquired. A pulsed neutron capture (PNC) tool was run in one of the wells. On the basis of this data, immediately after the resistivity run, a decision was made about which zones had the highest water saturation and needed to be isolated. This was done by setting a bridge plug. The same rig was used for the logging and the setting of the bridge plug. The water shutoff operations successfully increased the oil production. The purpose of this paper is to demonstrate the potential of the through-casing resistivity measurement compared to its nuclear counterpart, since its deep depth of investigation gives more immunity to reinvasion. The interpretation is fast and hence allows making an almost real-time decision on water shutoff operation so that it can take place immediately after logging. Saturation Measurements Behind Casing Three main types of data are used in saturation monitoring: through-casing resistivity, pulsed neutron capture, and neutron inelastic capture measurements. Aulia et al. (2001) provide a comparison of the applicability of each measurement in different environmental conditions.
The results were encouraging enough for the operator to reinvestigate the production performance and identify wells where this technique could be meaningfully utilized. The current analysis and workflow offers a fairly good understanding about the reservoir's production scenario, monitoring and future actions for enhanced recovery.
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