Radial Differential Temperature (RDT) Logging - A New Tool for Detecting and Treating Flow Behind Casing

Abstract

This paper describes a new cased-hole logging method for detecting vertical flow outside casing caused by faulty cement. The tool measures temperature differences inside the casing wall. By rotating sensors at selected depths, flow channels behind casing are located. Example applications for various flow situations are discussed. Introduction Primary cementing of casing in wells to achieve a Primary cementing of casing in wells to achieve a pressure seal between strata (zone isolation) is a pressure seal between strata (zone isolation) is a critical step in well completions. It is also important to seal casing seats during the drilling process and to achieve a seal at liner tops. Unfortunately, sometimes a seal i inadequate, although numerous steps are taken to place cement effectively.Determining whether the annular space behind pipe is sealed by cement is difficult in some pipe is sealed by cement is difficult in some circumstances. The critical question is whether flow is occurring along the wellbore behind pipe under existing conditions in the wellbore. Flow must be detected through the steel casing, plus as much as 2 or 3 in. (5 or 8 cm) of cement. Several logging methods can detect this flow, including measuring the fluid temperatures inside casing, radiotracer injection, and noise detection. Other logs can determine whether cement is present behind casing - for example, the cement-bond logs and the neutron-acoustic log. The oriented gamma ray (density) log has been used to define channels in cement and to guide the placement of perforations. All these techniques are useful under some conditions, but often more information is needed than is provided by present methods. provided by present methods. This paper describes a new surface-recording logging method, called "Radial Differential Temperature" (RDT) logging, that has been used in more than 18 wells for detecting flow behind casing. Field results of this new method are discussed. RDT logging has been tested under various reservoir conditions. The tool also has been used to orient guns for perforating through casing and into flow channels in the annulus. Squeeze cementing was performed after perforating in several wells. performed after perforating in several wells. Basis for RDT Logging The RDT log measures variations in temperature in the plane of the casing radius on the inside of the casing wall. Normally, two sensors are used, placed 180 deg. apart; one sensor may be used at the wall of the casing and the other sensor in the body of the tool. Temperature sensors contact the casing itself so that thermal properties and fluid movement in the wellbore affect the measurements less.Fig. 1 illustrates the hole geometry and the points at which temperatures are measured by both the RDT log and a conventional temperature log. The radial differential temperature is the difference between Tw2 and Tw2 measured at constant depth as the sensors rotate around the inside of the casing wall. A conventional temperature log measures Tf, the temperature of the fluid inside the wellbore, at different depths as the tool is lowered into the wellbore. JPT P. 676