Abstract

A complex carbonate oil reservoir off the coast of Qatar has been under production since 1997. The field development is based on horizontal oil producers, water injectors and the use of Electrical Submersible Pumps (ESP) as the artificial lift method. A full field 4D seismic data acquisition was undertaken in 2014 to support the future development phases. A Distributed Acoustic Sensing (DAS) acquisition was included in the 4D seismic program to capture a Vertical Seismic Profile and observe the fluid flow dynamics inside the well bore together with the reservoir inflow. The dynamics of gas fraction was imaged through a tuned operating sequence of the well. A digital movie was captured for each location in the wellbore from toe to pump intake.

The first of its kind sonic data capture involved a DAS system using fiber optics. Thanks to a permanent fiber optic into the well, the simultaneous captures of DAS system reduce the incremental cost for the DAS information.

Gas locking in the ESP is the major cause of down time and reduced ESP run life. The selected well is in an area with limited water drive. The high cost of replacing failed ESPs affects the economic limit of the well and thus reduces the recoverable oil.

The applied operational sequence successfully observed intermittent ingestion of gassy fluids into the ESP intake. This provided key information to diagnose the root cause of gas-lock.

The incident free operation has clearly imaged (filmed) the fluid dynamics of gas and liquids interacting with the ES pump intake.

How detailed the challenges of preparing and conducting this operation were overcome is described. The operational field constraints cover safety, weather constraints, unmanned well platforms, the effects of streamer shooting and simultaneous production operations. On-shore details are described in relation to unusual job planning requirements including logistics and statutory requirements.

The results show DAS images that identify the specific gas volumetric fraction of fluids all along the well and entering the pump intake. The acquisition program fully captured the development and movement of large gas bubbles towards the ESP and which subsequently develop into a gas-lock event. The sensing includes images from the drain toe, to ESP intake and onto the well head. The acquired data shows the root causes of operational difficulties particularly during an ESP restart. One can see the actual gas handling performance of the ESP system as the specific gas volume fraction at ESP intake evolves over time.

The full sequence of this DAS advanced application has fully captured a gas lock event. This brings key information to improve ESP performance in a gassy environment from the completion design and operating points of view.

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