Real-Time Fiber-Optic Distributed Temperature Sensing: Oilfield Applications
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 103069, "Real-Time Fiber-Optic Distributed Temperature Sensing (DTS) - New Applications in the Oil Field," by D.K. Nath, D.B. Finley, SPE, and J.D. Kaura, SPE, Halliburton Energy Services, and B. Krismartopo, SPE, and W. Yudhiarto, SPE, PT Chevron Pacific Indonesia, prepared for the 2006 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, 24–27 September.
Reservoir-performance monitoring can provide real-time information for more-timely operating decisions. Often, the data can be collected without performing an intervention. One reservoir-monitoring technique is fiber-optic distributed temperature sensing (DTS). DTS can provide a temperature profile of the entire wellbore in real time, which, in turn, can provide an enhanced understanding of the downhole flow (or injection) profile.

Introduction
DTS can measure temperature distribution along a fiber-optic line. The optical fiber comprises concentric layers of materials—the core and the cladding. The core is the light-carrying element. The surrounding cladding provides the lower refractive index that enables total internal reflection of light through the core. The back-scattered light from the fiber-optic line provides information about the temperature from where the reflection originated.
Installations. The most common installation uses a retrievable fiber rod or stem deployed from the surface into preinstalled capillary tubing clamped onto the production tubing. When production stabilizes, the fiber (in the form of a flexible rod or stem) is inserted into the capillary tubing and the surface equipment is connected to the fiber. Data are recorded continuously from the start to obtain the baseline profile and profiles during the cooling-down period (when cold water is pumped through the tubing/casing annulus) and throughout the production period. This method facilitates retrieval of the fiber-optic sensing device (fiber stem) upon completion of the survey so that it can be used in another survey. This procedure provides temperature data at a relatively low cost and within a short time frame.
A second method uses a 0.156-in.-outside-diameter steel tube with preinstalled multimode fiber material. The physical dimension of the fiber is a 50-µm core surrounded by a 125-µm cladding or jacket. The preinstalled-fiber tube can perform a retrievable survey in production wells with open-ended tubing and pressure equipment. As with wireline, the fiber tube is spooled back onto a cable drum and is moved from one location to another as required. The retrievable fiber tube also is used for surveys in observation wells.
A third method is fiber pumping. Fiber-optic material is pumped into preinstalled capillary tubing. A check valve installed below the capillary tubing allows pumping the fiber through the capillary tubing as a semipermanent or permanent (cemented in place with casing) system, either single-ended or double-ended. In this type of installation, the fiber is left in place after it is pumped. This method is used for wells where other methods cannot be applied.