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Technical Paper: Monitoring Acid Stimulation Treatments in Naturally Fractured Reservoirs with Slickline Distributed Temperature Sensing

Society: SPE
Paper Number: 173640
Presentation Date: 2015
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Abstract

Production from naturally fractured reservoirs can be greatly enhanced by stimulation with acid. When pumping the treatment from the surface (a bullhead treatment), the acid tends to enter the reservoir at the most permeable interval (conductive natural fractures). Without any other injectivity control, the acid will not likely divert from this path. Mechanical diversion techniques using swab cups or inflatable packers on jointed pipe or coiled tubing (CT) can ensure injection into the various intervals.

With these methods, surface pressure can be monitored to assess fluid placement effectiveness for each zone treated, but uncertainty of the friction pressure in the pipe while pumping can result in an incorrect interpretation of fluid entry. These mechanical isolation techniques are typically more costly and time consuming than bullhead jobs. Diversion techniques for bullhead stimulation treatments using chemical or foam diverters are more efficient, but lack the confirmation that all the zones received acid because of uncertainty of interpreting downhole behavior from surface pressure. A new technique using fiber-optic distributed temperature sensing (DTS) measurements offers a solution when bullheading by providing an indication of where the acid has been injected into the fractured reservoir.

A system has been developed consisting of a fiber-optic element encased in an acid-resistant slickline (commonly referred to as SL-DTS) that can be deployed in wells to monitor acid treatments in real time. The entire length of the fiber-optic strand functions as a temperature sensor with a vertical resolution of approximately 3.5 ft. The SL-DTS is gravity deployed or can be pumped into highly deviated or horizontal wells prior to the stimulation treatment. Once in place, the fiber is interrogated with pulsed laser energy to record temperature profiles versus time over the entire fiber length. These time-based data allow easy observation of thermal events due to fluid injections as well as any exothermic reaction of the acid system with the formation. Surface-temperature fluid pumped into perforations provides a cooling effect along the flow path. Interaction of the acid system with carbonate-containing minerals in the formation generates heat. Temperature profiles observed during and after pumping can be used to detect and quantify the distribution of the fluid system into the various intervals.

This method was used to successfully delineate acid placement in several deviated wells during bullhead stimulations. The data illustrate zones successfully treated as well as zones that may be targeted for remedial treatment. SL-DTS offers a unique opportunity to optimize stimulation placement in fractured reservoirs or any reservoir with variable injectivity.

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