The Auca Mahuida Volcano and Las Manadas field produces oil and gas from the Mulichinco, Lower Centenario and Rayoso formations. However this high quality reservoir has been severely damaged due to volcanic activity. This volcanism occurred after the main hydrocarbon migration and trapping, although there are hydraulically isolated bodies related to the igneous intrusions, confirmed from pressure testing and the distribution of the fluid along the stratigraphy column. The attitude and distribution of these intrusions in the reservoir is not generally known in part due to the lack of the surface seismic over this geologically complicated area. The big challenge is to be able to accurately identify and rank the intrusive igneous features such as dykes, sills, laccolith and other geological features. A new borehole image was introduced in Argentina for oil based mud systems. It has very good coverage with 80% in 8 inch wells. It is composed of 6 pads mounted in 6 independent articulated arms, each pad has 10 sensors resulting 60 micro-resistivity measurements. This tool works on different frequencies for different formation resistivity range. Depending on the known resistivity two frequencies can be simultaneously selected to acquire two images in one single pass. Recommended logging speed is 5.5 m/min. As a result of the operations more than 2000 meters have been logged with high quality borehole imaging. In spite of hostile weather conditions and because it is a natural protected area, there were no incidents registered during these jobs. At this point of the project dykes seem to follow pre-existing fractures to intrude the formations so the recognition of the fracture attitude is very important to prevent those intrusions at depths where the oil and gas is trapped. This information was considered as a high value to improve the existing geological model, providing knowledge about the complex net of intrusive bodies by the accurate recognition of the type of intrusion and its attitude close to the borehole.
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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