Surveillance of CO₂ floods is critical in establishing maximum oil recovery.  Injection and production profiles are used to determine the inflow and outflow of fluids into the various layers of the reservoir.  Radioactive tracer logs are often used for this purpose.  A slug of radioactive tracer (typically Iodine 131) is added to the injection fluid and as the slug moves down the well, several gamma ray logs are recorded at specific time intervals.  The reduction in tracer material as it moves down the wellbore indicates how much of the injected fluid goes into each zone.  The position of the tracer slug is seen as a large gamma ray spike whose size is proportional to the flow rate.  A reduction in the size of the peak indicates a loss of fluid into the formation.  Fluid velocity can also be calculated from the time interval and the distance the peak has moved using time-slug analysis.  The radioactive tracers can be used to detect channels and leaks within the casing and cement jobs.  Correction of these wellbore integrity issues is critical in maximizing vertical sweep.

Tracer logs can also be used to determine inflow from the producing zones.  The tool is operated in reverse mode to that of an injection profile.  Capacitance sensors are installed to determine the percentage of oil and water flow from each zone.  The dielectric constant of water (80) is so high relative to that of oil (1.9 to 2.3) that the capacitance is a direct indicator of the amount of water.

In addition to inflow and outflow from perforations in the wellbore, the breakthrough of CO₂ provides an indication of the sweep.  Dimensionless curves can be generated for the various patterns and poorly performing patterns can be identified.  This deviation from normal flow behavior provides an indication that the injector and producers should be investigated for potential mechanical integrity issues.  The adjustment of injection and production flowrates can also be used to maximize areal sweep based upon the formation thickness and pattern size.

For relatively mature CO₂ floods, 3-D seismic has been used to determine areal sweep.  Operators often do not consider the fact that CO₂ can move downdip toward plugged and abandoned wells.  Should this occur and these wells have mechanical issues, extreme losses of CO₂ can occur.  Reservoir pressure in all parts of the reservoir should therefore be monitored closely.

A&A Consulting Team of high-level and seasoned CO₂ EOR consultants has the expertise and extensive CO₂ EOR project experience to help our clients on all aspects of their CO₂ EOR reservoir surveillance projects in order to optimize production rates and maximize reserves, and to maximize project profitability.