Abstract

In the area of this study, the Quintuco-Vaca Muerta complex is an ultra-low permeability, highly over pressured, laminated calcareous and shaly rock that extends over 5000 km2 with an average gross thickness of 700 m. It overlies some 30 meters of what is known as the kitchen of Vaca Muerta, a world-class, high TOC source rock, the main source rock for the Neuquen basin in Argentina. Whenever found naturally fractured, it can produce high quality oil at attractive initial choked rates as high as 1500 bopd.

Historically, throughout many drilling campaigns, it has been very difficult to anticipate where it will have enough good reservoir properties such as to grant a commercial completion. Many times in the past decades, as soon as the wells showed important kicks during the drilling phase in Quintuco-Vaca Muerta, the wells were completed without any further evidences other than high initial productivity indexes. This led more often than not, to wrong decisions and the completions turned out to be sub-economical.

Drilling operations through this geological sequence present severe well control related problems such as difficulties anticipating pore pressures, predicting fluid influx levels and mainly the fact that an increase in mud weight in most of the wells, instead of leading to well control, exacerbates formation fluid kicks and influx pressure.

Combining geological knowledge, production history, downhole pressure while drilling (PWD), surface underbalanced drilling (UBD), mud logging data and drilling experience, a specific underbalanced drilling methodology was developed assuming one main hypothesis which is that in some areas of the field, the formation is naturally fractured and it is potentially critically stressed. This drilling methodology defines how the optimum mud weight is identified, how to deal with formation fluid influx and the well killing procedure; additionally a "dynamic soft well shut-in method" was defined.

An integrated effort of drilling and reservoir engineering allowed the operator to design and execute a meaningful short reservoir limit test that facilitated the evaluation in real time of the size of the accumulation and the reaching of an informed decision on whether or not to complete the well from this interval.

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