Residual oil evaluation using single well chemical tracer test

The single-well chemical tracer (SWCT) test is an in-situ method for measuring fluid saturations in reservoirs. The most common use is the assessment of residual oil saturation (S_or) prior to improved oil recovery (IOR) operations (post-waterflooding).

Process for single well chemical testing

The SWCT test for S_or uses only one well and involves the injection and back production of water carrying chemical tracers. A typical target interval for SWCT testing is shown in Fig. 1. The candidate well should be completed only to the watered-out zone of interest (zone at S_or). The water used normally is from the formation to be tested, and often is collected during the initial setup for the test.

• 

  Fig. 1 – Candidate SWCT test well.

The injected volume is divided into two parts: the partitioning tracer bank, which carries a small concentration of tracer (usually some type of ester) dissolved in water, and the push volume of water, which pushes the partitioning tracer bank away from the wellbore 10 to 20 ft. A material-balance tracer (normally a water-soluble alcohol) is added to the entire injected volume to differentiate it from the formation water being displaced. This injection step is shown in Fig. 2.

• 

  Fig. 2 – Injection of ester tracer and push volume.

The primary tracer is an alkyl ester. The esters used in SWCT testing usually are more soluble in oil than in water. This solubility preference is expressed quantitatively by the ester’s oil/water partition coefficient, K_e:

....................(1)

where C_eo = the concentration of ester in oil; C_ew = the concentration of ester in water; C_eo and C_ew are values at equilibrium.

For example, if the partition coefficient is four, the ester prefers the oil phase four times more than the water phase. For each tracer to be used in each test, the actual value of K_e must be measured in the laboratory at reservoir conditions. Oil and water samples are collected from the target formation for this purpose.

As the ester tracer enters the pore space containing the residual oil, it partitions between the oil and water phases. The ester maintains a local equilibrium concentration in the oil phase, controlled by the ester’s partition coefficient, even though the water is flowing.

Because the oil is stationary and the water is moving, the ester tracer moves more slowly through the reservoir pore space than does the water with which it was injected. The ester’s velocity thus is a function of the water velocity, the ester partition coefficient, and the S_or. Fig. 2 schematically shows the radial position of the injected ester and water. The material-balance tracer, normally an alcohol, is nearly insoluble in oil, so that it travels at approximately the same velocity as the water, reaching as far into the formation as the injected water.

For simplicity, Fig. 2 illustrates the no-dispersion case however, in all real reservoirs, the tracers disperse significantly, though this effect does not alter the basic mechanism of retardation of ester velocity by the residual oil.

After the ester and push injections are completed, the well is shut in for one to ten days, depending on the reactivity of the ester and the reservoir temperature. This shut-in period allows some of the ester to react with water in the reservoir, which forms a new tracer in situ, the secondary (or "product") tracer.

Reacting an alcohol and an organic acid makes an alkyl ester. At reservoir temperature, however, when dissolved in water, this ester slowly breaks down again into the alcohol and acid: Ester + H₂O = Alcohol + Acid. The shut-in period must be long enough for measurable alcohol to form in situ by this reaction (Fig. 3).

• 

  Fig. 3 – Shut-in (reaction) period.

It is the alcohol formed that makes the S_or measurement possible. The acid formed during the reaction is not observed because it is neutralized by the natural base components of the reservoir. The alcohol, however, is not in the original formation water, and can be detected at very low concentrations in the produced water, thus acting as a unique, secondary tracer.

At the end of the reaction period, the remaining ester and the product alcohol tracers are located together 10 to 20 ft from the wellbore. The tracers then are ready to be back produced to the wellbore and monitored at the surface in the produced water.

Fig. 4 shows the chromatographic separation of the product alcohol and ester tracers. This separation occurs because the product alcohol and water velocities are essentially the same, whereas the ester production velocity is slower because the ester must partition between the oil and water phases during production in the same manner described in the injection step.

• 

  Fig. 4 – Production period.

Throughout the production period, samples of the produced water are collected frequently at the surface. Total produced volume is measured at the time each sample is taken. At a portable laboratory at the wellsite, the samples are analyzed immediately for product alcohol and remaining ester tracer concentrations. A plot of concentration of tracers vs. total volume produced is developed during the production, as the samples are analyzed.

Fig. 5 shows a hypothetical example of typical tracer concentration profiles. All three tracer profiles show the effects of dispersion, which always occurs during flow in porous formations. The unreacted ester, for example, was injected as a square wave and returns as a Gaussian peak.

• 

  Fig. 5 – Hypothetical SWCT test tracer-concentration profiles.

Summary of SWCT test features

The important SWCT test features are summarized below:

• The S_or measurement is made in situ in the waterflooded layers of the target formation. The tracers can go only where the injected water goes.
• Compared to coring or logging method results, the S_or results are from a relatively large reservoir volume.
• The S_or measurement is carried out on an existing well, and usually in an existing completion, which can be perforated or openhole.
• Because the S_or measured actually is the volume fraction of oil in the pore space, the measurement is independent of porosity.

Calculating saturation quantitatively

Before discussing the design and interpretation of SWCT tests, we need to establish the quantitative relationship between tracer velocity, the tracer distribution coefficient, and S_or. Fig. 6 schematically shows a local population of ester tracer molecules in a control volume (V_c) of a pore. The tracer is assumed to be locally in equilibrium, even though the water phase is moving and the oil phase is fixed (residual oil conditions).

• 

  Fig. 6 – Ester distribution between oil and water in a pore.

The number of ester molecules in the water (n_ew) is given by

....................(2)

and the number of ester molecules in the oil (n_eo) is given by

....................(3)

where C_ew and C_eo = the concentration of ester (molecules/unit volume) in water and oil, respectively; and S_w = saturation of water, in fraction of PV.

We determine the retardation factor for ester (β_e) by dividing these two equations:

....................(4)

The larger β_e is, the more the ester tracer is retarded by the residual oil. Substituting in Eqs. 2 and 3 and canceling V_c yields:

....................(5)

Because C_eo/C_ew = K_e (see Eq. 1), the equilibrium distribution coefficient of ester between oil and water, and S_w = 1 – S_or, this becomes

....................(6)

The typical ester molecule spends a fraction of its time (f_t) in water and the rest of its time (1 – f_t) in oil. Elementary probability theory requires that

....................(7)

The probable behavior of each ester molecule is the same as the behavior of a large population of identical molecules. From Eq. 6, then:

....................(8)

Solving for f_t:

....................(9)

The typical ester molecule will travel at the time-weighted average velocity:

....................(10)

where v_e, v_w, and v_o = the time-weighted velocities of the tracer molecule, water, and oil, respectively. Because v_o = 0 if oil is at residual saturation, the last two equations combine to give

....................(11)

Eq. 11 is the fundamental equation for tracer chromatography in a porous medium. Solving Eq. 11 for β_e:

....................(12)

If we can develop a way to measure v_e and v_w using an in-situ test, then β_e can be evaluated. We then can measure K_e in the laboratory (at reservoir conditions) and substitute it into Eq. 6 to solve for S_or:

....................(13)

Both the well-to-well flow of two tracers and the SWCT test are methods that have been used to measure v_e and v_w in the reservoir. Well-to-well was the first suggested of these and in it, tracers A and B are injected (dissolved in water) into Well 1, and water is produced from nearby Well 2 until the tracers arrive. Tracer A is a partitioning tracer, such as an ester, that has a partition coefficient ranging from 2 to 10. Tracer B is assumed to have an equilibrium distribution coefficient (K_B) of zero (insoluble in oil), so that its time-weighted velocity (v_B) = v_w. Tracer B arrives first, after time t_B, and tracer A later, after time t_A. Because the two tracers travel the same distance,

....................(14)

where v_A = the time-weighted velocity of tracer A. β_A then is determined from Eq. 12. Once the equilibrium distribution coefficient of tracer A (K_A) is measured, S_or follows from Eq. 13.

Often, the well-to-well method is impractical for determining S_or because well spacing is too large. At the velocities that can be achieved between such wells, t_A is months, or even years. Also, where different permeability layers exist, the different travel time in each layer causes excessive dispersion of tracers traveling from Well 1 to Well 2.

Nomenclature

Ceo	=	concentration of ester in oil, mol/vol
Cew	=	concentration of ester in water, mol/vol
Ke	=	oil/water partition coefficient for ester
neo	=	number of ester molecules in oil
new	=	number of ester molecules in water
Sor	=	residual oil saturation, fraction of PV
Sw	=	saturation of water, fraction of PV
tA	=	arrival time of tracer A, day
tB	=	arrival time of tracer B, day
vA	=	average velocity of tracer chemical A, ft/D
ve	=	time-weighted velocity of ester
vo	=	time-weighted velocity of oil
vw	=	time-weighted velocity of water
Vc	=	control volume of a pore, bbl
βe	=	retardation factor for ester

Noteworthy papers in OnePetro

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read

External links

Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro

See also

Single well chemical tracer test

Designing single well chemical tracer test for residual oil

Well to well tracer tests

PEH:The_Single-Well_Chemical_Tracer_Test_-_A_Method_For_Measuring_Reservoir_Fluid_Saturations_In_Situ