Schlumberger

First CO2 EOR Pilot in the Middle East

Well Design for Carbon Dioxide Production

A state-of-the-art suite of well log data was acquired on both wells and interpreted for formation lithology, porosity, saturations, and permeability. Because the injector was drilled and logged with oil-based mud in the borehole, a full porosity partitioning and pore typing on the log data was not performed. However, NMR-derived permeability matches core data very well and calibration was achieved through simple adjustment of a co-efficient in the permeability transform. On the producer, a full porosity and pore typing analysis was performed.

The production tubing was extended to 100 ft (30.48 m) below the deepest perforation, terminating in a mule shoe, to convey a distributed temperature sensor (DTS) array on the outside of the tubing. Hence the flow entered the annulus surrounding the tubing through the perforated casing and flowed up to enter the tubing through a perforated pup joint approximately 50 ft (15.24 m) above the uppermost perforations.

Logging Tools for Flood Front Behavior

Assessment of the borehole environment was the first step toward processing and interpretation of the data. Borehole environment has a significant impact on measurement and greatly influences the interpretation workflow adopted.

When the borehole fluid is supercritical CO2, there is no hydrogen to moderate the neutrons and detector count rates increase dramatically. On the injector, a 50% increase was estimated in near-detector count rates and a 250% increase in far-detector count rates in similar formations. Fortunately, the reservoir saturation tool used an automatic regulation of minitron output based on instantaneous inelastic count rate, which reduced the count rate in the capture windows. Despite regulation, the counts still increase by nearly 100% on the far detector. Such high count rates require very fast detectors for proper measurement. The gadolinium oxyorthosilicate (GSO) crystal used in the reservoir saturation tool has the shortest decay constant, consequently the fastest response.

Results for First CO2 EOR Pilot in Abu Dhabi

The CO2 EOR monitoring exercise for this pilot helped define steps in planning, job execution, and assessing results of the surveys. Corrections of the unusual borehole environment were required because of the limitations of existing characterizations, but were made possible by measurement benchmarks in subject wells. Where corrections were required, suitable benchmarks should be planned in advance to verify the accuracy of those corrections. Corrections should be based on physics of the measurement to the maximum extent possible.

Different modes of pulsed neutron data acquisition (sigma, capture, inelastic CO) complement each other in the individual wells in assessing the borehole environment, providing adequate input data to enable a multiphase reservoir fluid analysis, and yielding independent fluid saturations for effective comparison. The results of the analysis were compared with an openhole evaluation to help create a coherent picture of the reservoir.

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Lessons Learned

  • Ensure the reservoir is at undisturbed saturation in the wellbore vicinity.
  • Develop a monitoring strategy to detect the arrival of flood front.
  • Deduce residual oil saturations after the flood based on observer well.
  • Mitigate effects of parasitic fluids in near-wellbore region(s) following suggested measures.
  • Optimize borehole and casing size, and completion design.
  • Measure flow profile on producer directly with production logging sensors.
  • Improve logging sensor technology to assess fraction of miscible versus free CO2 in formation.

CO2 EOR pilot

Schematic of map of the field showing the area currently under productionBorehole environment in the injector well.Measurement environment in the producer well.
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