Though hydrocarbons are widely viewed as being thermally unstable in
nature, cracking to
gas in reservoirs, there has been very little work to address the
chemical feasibility of the concept.
The present effort provides a test of this view by developing from
laboratory experiments criteria
for recognizing thermally altered hydrocarbons and then applying the
criteria to two condensates
that are believed to have been exposed to temperatures of 175−200
°C for 30 million years. The
observation that the two samples exhibit none of the characteristics
expected for thermal alteration
products suggests that hydrocarbons are far more stable in nature than
is generally recognized.
The stability of hydrocarbons in nature means that an alternative
mechanism must be devised
to explain the presence of gas in deep reservoirs. A new mechanism
is proposed which is expected
to yield the appropriate isotopic and molecular distribution of
products, operates in the source
rock, uses established chemical pathways, and does not require any
exotic catalysts.
Chemical EOR can be a very efficient enhanced oil recovery method when properly designed, but specialized laboratory tests must be done to tailor it to the specific reservoir fluids and mineralogy. The final step in the laboratory evaluation process is to test the chemicals in reservoir cores under reservoir conditions. Unfortunately, available reservoir cores are typically not in their native state. In particular, they are usually in a highly oxidized state (high E h ) compared to the reservoir formation. The oxidation state of the minerals affects the surfactants and polymers used for chemical EOR. In particular, at high E h some of the iron-containing minerals may have surficial ferric ions that were originally ferrous under the low E h conditions in the reservoir. Ferric ions can cause polymer degradation, poor polymer transport, and increased surfactant adsorption to the matrix, among other problems contributing to lower oil recovery. In general, to obtain valid test results, such cores must be restored to reservoir conditions to ensure valid results from core floods. We have investigated methods suitable for removing these ferric ions and restoring the core to reservoir condition. The new restoration method described in this paper dramatically improved polymer transport, reduced polymer and surfactant retention, and improved the oil recovery performance by the chemicals in several core floods. It should also be considered for other types of core floods to avoid getting laboratory results that are not representative of the behavior in the actual reservoir, which is in a reduced oxidation state compared to old cores.
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