Due to geomechanical effects during a quick pressure depletion of a hydraulically fractured reservoir, well operational practices in shale formations have a significant impact over final well economics. Defining operational drawdown as the flowing bottom hole pressure drop over time, this paper aims to determine what drawdown strategy generates the highest economic returns in Vaca Muerta formation, located in the Neuquen Basin, South of Argentina.
Due to the common unavailability of data, absence of a geomechanical simulator or the extensive CPU time required, the typical workflow for conducting this analysis has consisted on including a “proxy” stress calculation into reservoir simulators. The caveat, is the incapability of the “proxy” to fully capture the real stress variations, leading to even more uncertain conclusions and riskier decisions. To better capture geomechanical effects at different operational practices, this paper presents a novel analysis utilizing an in-house geomechanical simulator, fully coupled with a reservoir simulator and considers uncertainties such as reservoir and injected proppant properties. Design of experiment (DOE) was used to check the existence of a single optimal drawdown strategy which holds throughout the expected variations of the considered uncertainties and completion designs.
A proxy function capturing a massive number of DOE results was built to test thousands of new alternatives through Monte Carlo simulation. An optimal operational drawdown was consequently found and further proved to be the same for the whole range of uncertainty. The optimal range found showed an average increase of 8 to 15% in discounted production from the current practice which is a slightly more conservative. These increments are roughly translated to the same increase in NPV, representing a significant economic uplift from the current status. Finally, the obtained optimal drawdown range from 2.8 to 4.8 psi/hr in Vaca Muerta formation presents an 80% confidence interval and is potentially applicable to any other formation if physical properties and completion designs are within the analyzed range.
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