A completion fluid should not be damaging to the formation, should be solid free, and its weight can vary over a wide range. There is an organic fluid1 that is reusable after recuperating it with a brine and has low toxicity and corrosiveness. Given all these properties, we decided to evaluate it as a completion fluid.

This reusable organic fluid is prepared by dilution of a concentrated very viscous organic base with a low viscosity and toxicity oil to reach concentrations in the range of 9 to 16 ppg.

The permeability tests were conducted in a Hassler type cell using the organic fluid (15.7 ppg) within different sequences of fluids, as is shown in Tables 1 and 2. All the tests were performed at 320°F with cores and oil from Naricual Formation (Venezuela) and soaking times of 3 hrs for mud filtrate or calcium brine and 16 hrs for the organic fluid.

The results in Table 1 show that the organic fluid by itself does not damage the formation but instead is capable of cleaning it, allowing a better mobility of the oil. If the calcium brine (11 ppg) will follow the organic fluid into the. formation, a damage of near 30% will appear; this damage can be reduced to almost zero by adding 1% of a surfactant to the brine.

A severe impairment to the oil mobility appeared when mud filtrate had leaked into the formation, which in all three cases shown in Table 2, is enlarged by. the calcium brine. The use of the organic fluid as completion fluid partially removed the damage made by the water-based mud filtrates, but it did not clean or added damage in the oil-based mud case.

Table 2 shows a further reduction in the mobility of oil in all three cases if calcium brine goes into the formation after the organic fluid and the mud filtrate; these reductions are not as severe as when no organic fluid was used. Table 2 also shows that the addition of 1% surfactant to the brine lowers the additional damage in all the cases; the effect is more pronounced for the water-based muds.

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