D.C. McKeon, ED. Scott, G.L. Patton
Schlumberger
ABSTRACT
One of the problems frequently encountered in oilfield production and injection operations is the vertical movement of water in channels within the casing-borehole annulus. A new method of using oxygen activation, the Water Flow Log (WFL) service, has been developed for detecting water movement inside and outside casing. This oxygen activation service uses the new impulse activation technique for detecting and quantifying water flow. The new technique consists of a short neutron activation period followed by a longer data acquisition period, flow is detected when the measured count-rate profile does not match the expected profile for a static environment.
The flow velocity is determined by measuring the time that the activated water passes a detector. Theoretical models and laboratory experiments have demonstrated that the simple v=d/t relationship is unsatisfactory at low and high velocities. Algorithms to correct the measured velocity have been developed from the theoretical models and laboratory experiments. The volumetric flow rate can be estimated from the total flowing signal, but the relationship depends on the source-to-detector spacing, the flow velocity, and the distance to flow. Algorithms to predict the volumetric flow rate are based on theoretical models and laboratory experiments.
Logging procedures have been established to assist with planning and performing WFL measurements. An important application for this measurement is testing for fluid migration in the wellbore as part of the Mechanical Integrity Testing process for Class I and Class II disposal wells. The new oxygen activation measurement has been used also in numerous production wells to identify water flow behind casing. Additional applications include the identification of open fractures in horizontal wells and the quantification of water flow in the tubing-casing annulus of injection and production wells having high flow rates.