More oil out of limestone

InterOil is drilling  into (Puri and Mendhi) limestone. We know that there are quite a few knowlegeable people who think there is oil there. Here is a technology that could help get it out.

From greencarcongress

  • Researchers at the University of Stavanger in Norway report that injecting a modified seawater fluid—”smart water”—into limestone oil reservoirs for enhanced oil recovery (EOR) could help boost oil extraction from those reservoirs by as much as 60%. Their findings are scheduled for the 10 September issue of the ACS journal Energy & Fuels.
  • In the study, Tor Austad and colleagues note that more than 50% of the world’s oil reserves are trapped in oil reservoirs composed of calcium carbonate, rocks that include chalk and limestone. The average oil recovery from carbonates is generally lower than for sandstone reservoirs. The reason, they note, is that the carbonate rock is neutral to preferentially oil-wet and often highly fractured.
  • The carboxylic material in crude oil adsorbs strongly onto the carbonate surface and makes it partly oil-wet. Injection of water is a cheap enhanced oil recovery (EOR) method, but in this case, the water will mainly follow the fractures. Because of the negative capillary pressure in oil-wet rock, water will not imbibe into the matrix blocks to displace the oil. The water will only displace the oil present in the fractures, and less than 5% recovery is therefore expected in some cases when the oil reservoir is highly undersaturated, e.g., Pres>>Pb and contains a large aquifer for pressure support.  —Strand et al. (2008)
  • Seawater is already being used as an enhanced oil recovery (EOR) fluid for hot, fractured chalk oil reservoirs because it is able to modify the wetting conditions and improve the displacement of oil. The symbiotic interaction between SO42-, Ca2+, and Mg2+, which are all components of seawater, removes some of the carboxylic material from the chalk surface, thereby increasing the capillary forces to promote spontaneous imbibition of water into the matrix blocks.
  • Studies on the use of seawater injection into chalk reservoirs have shown a 40%-60% increase in oil recovery. However, chalk is purely biogenic CaCO3, consisting of fragmentary parts of calcite skeletons produced by plankton algae, and is believed to have a more reactive surface than ordinary limestone.
  • The researchers collected core samples from Middle East oil reservoirs composed of limestone and soaked them in crude oil for several weeks. They then prepared batches of “smart water,”—seawater formulated with sulfate and other substances to improve seawater’s ability to penetrate limestone. In laboratory studies, they showed that irrigating the limestone samples with the “smart water” led to the same fundamental chemical reactions that occur in chalk.
  • In general, the type of interactions between SW [seawater] and limestone were similar to chalk, and the main conclusions drawn were as follows: (i) The chromatographic wettability test based on separation between the tracer, SCN-, and SO42- at the water-wet surface sites is also applicable to limestone; i.e., SO42- is adsorbed onto the surface. (ii) In a NaCl solution at room temperature, Ca2+ and Mg2+ appeared to have similar affinity toward the limestone surface. At higher temperatures, the affinity of Mg2+ was superior to that of Ca2+. (iii) In seawater, the relative interaction between Ca2+ and Mg2+ toward limestone is dictated by the presence of SO42-. Ca2+ appeared to adsorb more strongly than Mg2+ because of the ion-pair formation between Mg2+ and SO42- and the strong adsorption of SO42- onto the rock. (iv) The water wetness of a reservoir limestone core cleaned by toluene and methanol can be improved by flooding the core with seawater at high temperature, 130°C. Seawater can then act both as a cleaning agent and as a wettability modifier. (v) The oil recovery at 120°C by spontaneous imbibition was about 15% higher when the limestone core was imbibed with seawater compared to seawater without SO42- present, confirming that seawater will act as an EOR fluid/”smart water” also in limestone at certain conditions.    —Strand et al. (2008)
  • Upcoming experiments will verify if the efficiency in oil recovery is comparable to the observations in chalk, the scientists note.

This stuff could become useful for InterOil..