Tracer tests

In order to verify the homogeneity of a sample core, tracer tests were performed.

Tracer tests may also be performed to calculate the pore volume (PV) of fluid in a core, either from 100% saturation or from a residual saturation. A tracer is an injected substance that is both measurable (e.g. through UV-visible detection) and conserved (i.e. not retained, destroyed or created by the core). If tracer concentration at the outlet of the core being tested is recorded against time, an S-shaped tracer curve will form. This S-shape is caused by dispersion (mixing and diffusing of the two fluids);

if there were no dispersion there would be an instantaneous change to tracer concentration when breakthrough occurred at 1 PV. The symmetry of the curve is an indicator of homogeneity; a perfectly homogeneous core will have a perfect S-shape.

Several primary assumptions are made in tracer testing (Green and Willhite 1998):

• Fluid B is displacing Fluid A, and the two fluids are miscible

• There is no viscous fingering of one fluid into another, only dispersion

• Flow is single phase

• Fluids are incompressible

• Fluids are of equal density

• Flow is in only one direction

• Fluid velocity is constant

• Flow is through a porous medium of constant porosity and cross-sectional area

These are all reasonable assumptions for the experiments performed in this research with oil and water tracers displacing pure oil and water, respectively in core samples.

Theoretically, for a homogeneous core PV is indicated at the time when effluent tracer concentration reaches 50% of injected concentration, that is, at a normalized tracer concentration of 0.5. Tracer data is typically normalized to scale the tracer concentration values from zero to one.

Normalized concentration is calculated as:

C_B = normalized tracer concentration C_B^* = measured tracer concentration

CBi = injected or maximum tracer concentration C_B0 = initial concentration of fluid B in the system Core PV can then be calculated as:

^PV=

( (

)

)

t50 = time normalized concentration equals 0.5 t_o = time tracer begun

q = tracer flow rate dv = dead volume

Note that any dead volume (tubing etc. filled by tracer solution before or after the core) must be subtracted from the calculated pore volume. However, cores are rarely perfectly homogeneous and so a tracer “tail” typically forms due to core in-homogeneity and/or pore space that is not readily accessible to the main flow paths in the core (Green and Willhite 1998). From the effluent concentration curve the core PV may be calculated by means of an equal area method (Figure 3-13). The time that areas on both sides of the normalized concentration curve versus time becomes constant, is used to obtain the core PV as:

^PV=

( (

)

)

t_eq = time equal area reached t_o = time tracer begun q = tracer flow rate d_v = dead volume

Again, note that any dead volume in the setup must also be subtracted from the

in Figure 3-13 below. For a perfectly homogeneous core the pore volume calculated by equations 5 and 6 would be identical; the equal area point would lie exactly at a normalized concentration of 0.5

Figure 3-13 Equal area technique in calculating the PV of a core from tracer test

For water-based tracer tests, potassium nitrate can be used as a tracer based on the assumption that there is no retention of potassium nitrate in the cores. Laboratory grade KNO₃ and NaCl can be dissolved in deionized water to form a 0.1 M/L KNO₃, 1% NaCl tracer solution. Two pore volumes of 0.1 M KNO3 solution in 1% NaCl were injected into a core saturated with 1% NaCl brine solution and displaced by the same pore volumes of 1% NaCl brine solution. The concentration of KNO3 in the effluent can be detected by an in-line UV spectrophotometer at the wavelength of 302

nm, where the NaCl absorbance is zero, and then it can be plotted as a function of pore volumes injected.

Before performing a tracer test the UV-visible detector was turned on and set to read at a desired wavelength of 302 nm for aqueous tracer and 323 nm for oil tracer. The flow cell of the UV-visible was cleaned before each use by injecting 20 mL of acetone followed by 20 mL of RO-water with a syringe. The UV-visible detector was zeroed after injecting pure RO water or Soltrol 130 as appropriate before beginning a tracer test. The flow diagrams for water and oil based tracer test are shown in Figure 3-14 and Figure 3-15 .

Constant flow rates were used when performing tracer tests. When tracer testing at a residual saturation, care was taken that the tracer test pressure drop did not exceed that of the previous flood to establish residual saturation. This is necessary in order to keep the capillary number lower or equal to the previous flood so that no additional residual fluid would be displaced during the tracer test. Before starting flow it was checked that the fluid A and fluid B (tracer) transfer cylinders were both full and connected to the same system pressure. In this setup it was possible to switch the flow between tracer and non-tracer fluid by changing the direction of only one valve.

After flow had begun, the flow was allowed to reach steady state with absorbance in the UV-visible detector reading a steady initial value CB0. The valve was switched to tracer injection and concentration values were recorded. The switchover time was recorded in LabView by setting the recorded flow rate to 0.0 ml/min at the exact time

the valve was turned. The flow rate was then changed back again several seconds later. Once the tracer tail was complete, flow was returned to fluid A and the backside of the tracer was recorded to displace the tracer solution from the core. The switchover time was recorded in Lab View exactly as was done before. Once a tracer test was complete the data was exported to an Excel file; graphs were generated showing pore volumes of fluid injected versus normalized tracer values. Areas on each side of the tracer curve were calculated using trapezoidal rule, and the areas were summed until the areas were as closely equal as possible. Pore volume was then calculated by using Equation 3.6. Dead volumes were calculated for each cabinet setup both by separate tracer test calculation and by measurement of fluid injected with a syringe through the tubing lines making up the dead volume.

Figure 3-14 Schematic of flow set up during water (0.1 M KNO₃) tracer test

Figure 3-15 Schematic of flow set up during oil (20 ppm trans-stilbene) tracer test (drawn by Stephen Johnson, used by permission)