A Single-Well Tracer Test with In-Situ-Generated C02 as the Oil Tracer – An advent to determine the ROS (Residual Oil Saturation)

Kumar Abhishek Singh Prakhar Sarkar

PDPU

Agenda or Summary LayoutIntroductionA Single-Well Tracer Test With In-Situ-GeneratedCO2 as the Oil TracerGenerator Kinetics of CO2 as the oil Tracer

Limitations and Conclusion

1.

2.

5.

3.

4.

6.

Partition Coefficient Properties andInterpretation of Tracer ResponseGulf Coast Reservoir field Test

IntroductionThe single well chemical tracer (SWCT) test is an in-situ method for measuring fluid saturations in reservoirs.

The SWTT utilizes partitioning chemical tracers that will dissolve in the both the oil and water at known fractions. Esters will hydrolyse 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.

K - partition coefficient (fraction of ester in oil to fraction of ester in water)

β - retardation factor relating the differences in velocity or transportbetween the alcohol and ester

Sor - residual oil saturation to water flood or EOR process

A Single-Well Tracer Test WithIn-Situ-Generated C02 as the Oil Tracer

Hydrolysis of halogen-organic acid salts as a means to generate C02 and to trace ROS.

In the C02 system, the generator chemicals are water-soluble-only when injected.

Generator KineticsGenerator Chemistry- The general hydrolysis reaction and rate controlling step is Cl-RCOO- + (Na+) + H2O HO-RCOO- +Na+ + HCl, ….(A-1)

Where the CI-R group is any halogen atom substituted for hydrogen in an organic radical. The next reaction is instantaneous and essentially converts HCI formed from the first reaction into C02:HCl + HCO3- + (Na+) H2O + CO2 + Cl- + (Na+), ……… (A-2)

The usual halogen is chlorine, and the organic radical usually is attached to a carboxy functional group (organic acid) that is present as a salt to ensure water solubility and oil insolubility.

Partition Coefficient Properties and Determination

Ko is = 3 and is relatively insensitive to salinity and temperature.

The oil-tracer profile is well-defined and adequately separatedfrom the water-tracer profile for sensitive ROS measurements.

There is essentially no differentialchange in Ko with C02 concentration in the range studied.

Frontal analysis was used to obtain the C02 Ko values between methane reconstituted crude and synthetic reservoir brine

The Ko values did not vary significantly when stock-tank oil was substituted for live oil.

Interpretation of Tracer ResponseDuring production, the generated C02 is retarded by the oil and separates from the water-soluble-only tracers that return directly through the water.

Assuming that (1) the hydrolysis rate of the reacting tracer is constant during the process, and (2) the main part of the reaction occurs during the soak period.

The governing equation is

Sor = (Voi/Vow) -1/ (Voi/Vow) -1 + Ko,

When the voi/Vwi ratio is constant throughout tracer production, the required quasi static equilibrium condition for application of chromatographic principles is satisfied and Sor is uniquely determined.

Gulf Coast Reservoir Field TestTwo-chloropropionic acid (2-CPA) was chosen as the appropriate C02 generator for the reservoir temperature and well injectivity.

Well conditions were 142 to 148°F, a 12-ft interval opening, =0.5-bbl/min (720-B/D) injection rate, and =30% porosity.

The well was produced by means of gas lifting.

Steady C02 concentrations between 0.0033 and 0.0041 mol/L (145 to 180 ppm) in the produced brine were measured, pH was 6.3, and HCO) concentration varied between 0.0031 and 0.0033 mol/L over the 2-day period.

The separation of the curves indicates that ROS in the 0.1 to 0.4 saturation fraction range are well-suited for determination by the C02 SWTI method.

Test Results

Other Oil-Tracer-Response FeaturesA valley in the C02 response curve lies directly under water-tracer curve peak and is caused mainly by corrosion interference. The test well had a low bottom hole pressure, and air partially filled the tubing string during hook-up for injection.

Fe+2 in the injected solution is oxidized to Fe+3 by the oxygen. Fe+3 is then reduced by metallic iron (from the tubulars) to produce more Fe+2. This reaction continues until all oxygen is consumed or until the fluid enters the reservoir.

Another factor that reduces the C02 in the water peak is the equilibrium between the increase in HC03 that accompanies the decrease in H+.

Trapped-Gas SaturationSWTT's are generally used in watered-out reservoirs that are pressured above the oil bubble point to keep gas dissolved in the oil and brine.

When this is not the case and trapped residual gas is present, the total trapped residual hydrocarbon saturation is determined because the C02 tracer samples both the trapped-oil and -gas phases.

The partition coefficient for C02 between one crude oil and its equilibrium

gas was found to be close to unity and nearly constant from 100 to 1,000 psia at reservoir temperature.

ConclusionsIntroduction of water-soluble-only injectants in a single-well ROS tracer test allows the injected tracers to travel at the brine velocity previous systems.

In the C02 method, all the generator may be spent during the soak period without harm to the test because the water and oil tracers are independent. This is desirable because C02 cannot be generated production, which simplifies interpretation.

References[1] Richardson, E.A.: "Determining Residual Oil Saturation by Injecting Salts of Carbonic and

Halocarbonic Acids," U.S. Patent No. 4,646,832 (March 1987).

[2] Deans, H.A. and Majoros, S.: ''The Single-Well Chemical Tracer Method for Measuring Residual Oil Saturation," final report, Contract No. DOFJBC/20006-18, U.S. DOE (Oct. 1980) 9-38.

[3] Snyder, L.R.: Principles of Adsorption Chromatography, Marcel Dekker Inc., New York City (1986).

[4] De Monchy, A.R., Gesink, J.C.J., and Van den Bergen. E.A.: ''Tracers for Oil and Gas Field Applications," paper presented at the 1982 Doha Technical Symposium, Qatar, Jan. 11-13.

[5] Gesink, J.C.J., Van den Bergen, E.A., and De Monchy, A.R.: "Use of Gamma Ray-Emitting Tracers and Subsequent Gamma Ray Logging in an Observation Well To Determine the Preferential Flow Zones in a Reservoir,"

JPT (April 1985) 711.

[6] Cooke, C.E. Jr.: "Method of Determining Residual Oil Saturations in Reservoir," U.S. Patent No. 3,590,932 (July 1971).