A distributed temperature senor (DTS), that uses a multimode fibre, has been used historically for flow profiling in intelligent wells. In horizontal oil production wells, it is difficult to determine the flow profile due to lack of geothermal gradient. In water injector wells, the DTS can be used to monitor the warm-back temperature changes after the well has been shut-in. If the water injector has been continuously operating over a long period of time, the warm-back may take several days or months, which is not practical.
The intelligent Distributed Acoustic Sensor (DAS) system, that commonly uses single-mode fiber, is a very promising technology for inflow monitoring. In intelligent well completions the acoustic energy generated across inflow control valves (ICVs) and inflow control devices (ICDs) can propagate up through the production tubing. Using an array processing the speed of sound can be measured to monitor the fluid composition. In addition, the flow velocity can be measured by evaluating the Doppler shift between up-going and down-going acoustic waves.
The DAS system was retrofitted to existing optical fibers that were already installed along several wells in Saudi Arabia. The acoustic noise energy generated across the inflow devices and propagating along the wellbore tubing was recorded. The acoustic noise spectrum can be used to monitor the fluid flow through the inflow devices. Using array processing, the speed of sound can be determined over several sections of the tubing to identify the fluid composition and the velocity.
In this paper we report on a horizontal open hole water injector equipped with ICDs. This well was previously instrumented with a multimode fiber for distributed temperature sensor (DTS) measurement. We retrofitted the DAS system to the existing multimode fiber and recorded both amplitude and phase of the sounds waves in real time. The dynamic flow profiles across the ICDs zone were observed as the injection rate was varied from 100% to 31% by changing the surface choke settings. The downhole injection rates were quantified by determining the Doppler shift between up-going and down-going speed of sound propagating along well above the ICDs. A good correlation was observed with a surface flowmeter.
The DAS system can be used with single-modes and multimode fibers and it can be used for dynamic water injection profiling and optimization.