In Shale plays, EURs relay almost exclusively on "primary" production with practically no account for Enhanced Oil Recovery (EOR) practices. However, many recent works show the feasibility of using different techniques to improve the well's EUR by means of some kind of assistance. Lean gas, CO2 and water injection, mostly in a huff-n-puff manner, seem to lead the industry's most accepted EOR methodologies in ultra-tight (shale) formations. Conceptual, theoretical and experimental work has been made in the water huff-n-puff arena including lab and field work and documented in previous work published by the authors. The methodology consists on a novel operative sequence to enhance oil production in multi-stage fractured Tight/Shale Oil scenarios involving countercurrent water imbibition processes and consists of a cyclic scheme of (i) high pressure water injection, (ii) soaking and a (iii) production period. These cycles could be repeated until capillary effects fade out. These phenomena, which should occur in areas where a substantial portion of the shale matrix contains water-wet effective porosity, could explain some anomalous behaviors such as the low water recoveries observed in many cases, rapid salinization of the flow-back water and the rapid appearance of hydrocarbons at early flow-back times, when no substantial % of the injected water is yet recovered.

This work shares the partial results of the second field test performed in the Vaca Muerta resource play, implemented on a vertical, three-stage well in the black oil window, similar to the first field experience from previous work. The test design was modified (i.e. lower water volumes and alternation of injection and shut-in periods) based on the insights gained in previous tests, to minimize the test complexity, costs and to extract additional information from the transients. The results of this new field test showed that it was possible to inject significant water volumes without reaching original fracturing pressure and without damaging the fracture network productivity. Compared to previous tests, the surface and downhole operations were simpler and inexpensive. Partial results were encouraging, as the pre-test oil rates were recovered and even slightly improved with less than 40% of the injected water volumes recovered. Despite this, due to the shut in periods included and operative issues, in this particular trial, oil rates were not as instantaneous and as important yet, as to compensate for the cumulative oil losses during these shut-ins.

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