Chemical Flooding Technologies

Single Well
Tracer Test

Single Well Tracer Test

The SWTT measures residual oil saturations in the reservoir and has several advantages over logging and core analysis.

The Single Well Tracer Test (SWTT) was first developed at Exxon by Harry Deans in 1968 and patented in 1971. The SWTT does not alter reservoir wettability and measures Sor over a much larger volume than near wellbore techniques such as core analysis or logging. In addition, SWTT are non-destructive and can be run in either sandstone or carbonate reservoirs over widely varying formation characteristics.

The SWTT can also be used to evaluate the effectiveness of EOR processes to mobilize residual oil (Sor) or “trapped oil”. First, SWTT is used to determine Sor to waterflood. Then the EOR process is carried out at the test well. Typically, the EOR process is chemical EOR or miscible CO2. Lastly, the SWTT is carried out a second time to determine Sor to the EOR process. The results of the test will give direct indications of the effectiveness of the EOR process to mobilize residual oil.

The SWTT utilizes partitioning chemical tracers that will dissolve in the both the oil and water at known fractions. The partitioning chemical tracer, typically esters, will hydrolyze only in the water phase forming an alcohol. The ester and alcohol will travel at different velocities in the reservoir as the ester is continuously partitioning into and out of the oil during injection and pullback. The difference in travel time is directly related the residual oil saturation.

The SWTT procedure consists of the following steps:

Pre-Test

Inject a ~1-pore volume pre-flush of brine to ensure oil saturations are at waterflood residual over the test volume (typically a 10-15 foot radius from the wellbore)

STEP 1

Inject ester tracer into zone of interest.

STEP 2

Push the ester slug away from wellbore and into the formation with brine.

 

 

 

 

 

 

 

1 – J. F. Tomich, R. L. Dalton, and H. A. Deans, JPT, February 1973.
2 - US Patent 3,623,842

STEP 3

Shut-in well to allow the ester in the water phase to hydrolize into an alcohol.
 

STEP 4

Produce the well back and continuously sample and measure concentrations of produced chemical tracers, both the ester and produced alcohol.

1 – J. F. Tomich, R. L. Dalton, and H. A. Deans, JPT, February 1973.
2 - US Patent 3,623,842

STEP 5

Determine Sor from measured separation of ester and produced alcohol and simulate results.

  • K - partition coefficient (fraction of ester in oil to fraction of ester in water)
  • β - retardation factor relating the differences in velocity or transport between the alcohol and ester
  • Sor - residual oil saturation to waterflood or EOR process

 

1 – J. F. Tomich, R. L. Dalton, and H. A. Deans, JPT, February 1973.
2 - US Patent 3,623,842