This paper describes the test protocols and procedures of RCX Straddle Packer Microfrac for the estimation of formation Breakdown, fracture propagation and fracture closure pressure at different levels in Vaca Muerta formation. As a result, the horizontal stress field can be estimated, which in turns allows to infer if the fracture scheme and productivity agree well with initial prognosis.

Historically, this tool had been used for the estimation of formation pressure and fluid samples for unconsolidated formations. However, a micro-fracture job, pump capacity, packer hermeticity and a well design test protocol were the key to allow get formation breakdown, fracture propagation, fracture closure through pumping flow back and fracture reopening in various cycles. Such procedure was performed at two depths with success. Selection of the test intervals involved a preliminary geomechanical model for the determination of stress conditions, mechanical properties and wellbore stability analysis to choose proper mud density to reduce the probability of well damage (breakouts mainly), which can compromise test hermeticity. Then, after drilling, well logs including wellbore image and caliper analysis enable the determination of test intervals with minimum damage.

In the test well, Microfrac tests were performed successfully on two intervals out of four selected. Interpretation allowed the estimation of closure pressure related to minimum horizontal stress. Moreover, a rough estimation of maximum horizontal stress, based on breakdown pressure and closure pressure was performed as well. Such results agree well with the initial geomechanical model. Strike slip regime was confirmed in such a way that no alarms were set for future development through horizontal wells. But it is necessary to make a deepper investigation to ensure a favorable anisotrophy stress ratio. Some areas with unfavorable vertical to horizontal stress ratio, have been show low performance productivity. The main advantages of this type of tests are: (i) the availability to acquire information through several depths in a vertical well. (ii) Tests were performed in open hole conditions. (iii) Horizontal in situ stresses can be estimated. (iv) Time efficiency in comparison with classical DFIT tests. (v) Calibration of geomechanical model for future development.

Through the application of this tool, a new methodology was set, which allows for an early diagnose of the horizontal stress field leading to the determination of stress regime. Therefore, if the worst-case scenario appears related to inverse stress regime, development plans can be reorganized, reducing potential economic losses, considering that, as had been demonstrated in Argentina, horizontal wells placed on inverse regime led to EUR lower than breakeven.

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