Page 376 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

34.2 Reservoir Engineering

Laboratory Design for Enhanced Recovery

Prellminary Tests

Water Analysis. A complete water analysis is important to determine the effects
of dissolved ions on the EOR processes (especially the chemical methods) or
to ascertain any potential water problems such as scale or corrosion that may
result when EOR processes are implemented. Water viscosity and density are
also measured.
011 Analysis. Oil viscosity and density are measured as well. A carbon number
distribution of the crude may be obtained, especially if CO, flooding is being
considered.
Core Testing. Routine core analyses, such as porosity, permeability, relative
permeabilities, capillary pressure, and waterflood susceptibility tests are normally
done by service companies that specialize in these types of tests. Specialized
core tests, such as thin sections or scanning electron microscopy, are available
to evaluate the relationship between pore structure and the process being
considered. If required, stimulation or injectivity improvement measures can
be recommended.
Polymer Testlng

The desirability of adding polymers is determined by evaluating all available
data to assess the performance of normal waterflooding. Any problems such
as adverse viscosity ratios or large permeability variations should be identified.
If the results of this study indicate that mobility control of the waterflood is
warranted, the following laboratory tests are undertaken.
Viscosity Testing. Based on the permeability of the reservoir, relative per-
meability data, and the desired level of mobility control, polymers of certain
molecular weights are selected for testing. Various concentrations of the polymers
are dissolved in both the available injection water and in blends of the injection
and formation waters. Polymer solutions may be non-Newtonian at certain shear
rates, that is, the viscosity decreases at high shear rates (shear-thinning or
pseudoplastic). This shear-thinning behavior is reversible and, if observed in the
reservoir, is beneficial in that good injectivity can result from the lower viscosity
observed at high shear rates near the injection well. At the lower shear rates
encountered some distance from the injector, the polymer solution develops a
higher viscosity. In this testing, it is important to consider not only the viscosity
of the injected solution, but, more importantly, the in-situ viscosity that is
achieved in the reservoir. Several things can happen that will reduce viscosity
when the polymer solution is injected into the reservoir. Reduction in viscosity
as a result of irreversible shear degradation is possible at the injection wellbore
if the shear rates or shear stresses are large. Once in the reservoir, dilution with
formation water or ion exchange with reservoir minerals can cause a reduction
in viscosity, and the injected polymer concentration will need to be sufficiently
high to compensate for all viscosity-reducing effects.

Polymer Retention. Retention of polymer in a reservoir can result from adsorp-
tion, entrapment, or, with improper application, physical plugging. Polymer
retention tests are usually performed after a standard waterflood (at residual