The Need for Circumferential Array Production Logging in Vertical Wells

Cased hole production logging tools (PLTs) have been used for almost a century to characterize the downhole flowing system in oil and gas wells. The characterization includes the identification (ID) and quantification of fluid flow (Q) into the wellbore. Because of the notable difference in the fluid electrical properties, capacitance and resistivity are routinely used to distinguish hydrocarbons from water. Fluid density or optical fluid ID sensors can be included to differentiate liquids from gas. Finally, mechanical spinner(s) are included to measure Q, with the ability to quantify the rate of individual fluid phases dependent upon the flow regime and the configuration of the spinners. The sensors in a PLT can be inline or array. Inline PLTs (Inline) are housed in a centralized circular sonde with sensors acquiring fluid data from the center of the casing or tubing; whereas Array PLTs (Array) are either located in a circumferential arrangement with arms extending out close to the inner tubular wall, or arranged axially with sensors, when located on a skid perpendicular to a pad. Established operation practices, discriminate their use based on the deviation angle of the well, and indicate that Inline tools should be used in wells that are vertical or with an inclination of less than 60 degrees, and Array tools in horizontal or highly deviated wells of more than 60 degrees. This practice is due to an assumed segregation of fluids of different densities. The practice of selecting the type of sensor configuration based on inclination violates fundamental physics: inertia, wettability, and Heisenberg’s uncertainty principle (macro), and hence yields uncertain results. The analysis of this ambiguous data has led to numerous misinterpretations of fluid entry zones. Little consideration, if any, has been given to the unavoidable disturbance caused by the sensors of the tool in the wellbore. Hence, a novel array production logging tool, with six arms where multiple fluid ID and spinners are co-located has been modeled to illustrate why the use of circumferential array in conjunction with at least one inline sensor is necessary for any well deviation, including vertical wells. This combination will successfully overcome the uncertainty associated with current practices. Three cases of fluid inflow into vertical wells were modeled at three different rates and with different casing internal diameters. The results indicate that the use of circumferential array logging is not only desirable but mandatory in all wells, regardless of their inclination. The paper also demonstrates the importance of the productivity index of the layer, the producing bed thickness, and the distance between layers to determine operational parameters of the logging runs, since they influence the accuracy and representativeness of the measurements.