Identification of Fluid Flow Zones in the Reservoir and Behind-Casing Through the Second Derivative of the Flowing Temperature

Abstract

The objective of this paper is to identify zones of fluid movements in the reservoir and behind the casing using the second derivative of the flowing temperature (T) log in producing wells. Generally, the combination of static and flowing temperature is required in order to detect flow behind the casing, which implies shuting down the well and deferred production. The temperature logs were acquired with a sensor of resolution of 0.018°F and logging speeds at 30, 60, 90 and 120 ft/min. So, the hysteresis of temperature sensor can be analyzed between runs going up and down. The average temperature of the running down logs were derived with respect to the depth (D), where the first derivative (dTdD) is the dynamic temperature gradient and the second derivative (d2TdD2) is the rate of change of this gradient. In order to identify the flow zones, behavior patterns were settled between production (PLT’s), noise and cementation logs with the second derivative of temperature. For an equilibrium system (non-flow zones) the second derivative values correspond to zero (d2TdD2=0), whilst for a dynamic condition (flow zones) the values are different to zero (d2TdD2≠0). The results indicated that there is a direct relationship between the movement of fluids with the second temperature derivative in a reservoir-wellbore system. Besides, there is a 95% consistency with production logging (PLT´s) results. Coupled with this, the open hole gamma ray (GR) log was compared with the cased hole GR, and a radioactivity increment was observed in the water production intervals which match with the changes of the second derivative of the temperature. Based on those outcomes, it was possible to identify in the reservoir-wellbore system leaks, flow behind the casing, fluid contributions in the perforated intervals and aquifer influx. Finally, the results correlate with increased readings in the total Gamma Ray log, and spectral noise log. This methodology does not require to log the static temperature to identify flow behind the casing, which means that it is not necessary to shut down the well. Consequently, the proposed workflow provides an additional tool for PLT’s interpretation and well integrity since the second derivative of temperature can detect small movements of fluids that the flowmeter can not. Additionally, it can also be used to monitor secondary recovery processes.