One of the major uncertainties in the analysis of unconventional well productivity is the estimation of the hydraulic fracture height generated during stimulation operations. This study was carried out in a well located in the Neuquen Basin, Argentina with main focus on the development of unconventional shale oil. The strategy consisted in the application of two combined techniques based on different and independent physical principles for the estimation of the hydraulic fracture height. The initial technique consisted in the pumping of proppant which contains elements with great neutron absorption. Thereby, the presence of traced proppant is identified through the differences in the neutron absorption capacity before and after the stimulation at approximately 6 inches from the wellbore. The second technique is based on the characterization of the anisotropy on the shear wave obtained by the dipolar sonic curve in two perpendicular directions. This acquisition should be carried out before and after the stimulation and it is sensitive in the area close to the wellbore. (Between 5 and 40 in) Results from both techniques showed a reasonable good consistency in the results, thus allowing the validation of both methodologies. The results also allowed defining intervals that act as barriers of the hydraulic fracture vertical growth. This permit us the optimization of the fracture design in other wells, thus minimizing the vertical overlapping of the fractures and maximizing the connectivity of the stimulated interval in the well. In addition, traced proppant was confirmed close to the wellbore in several intervals that together represent approximately 60% of the pay which represents a 170 m. To improve completion efficiency, this information can be used to place hydraulic fracture stages and define clusters geometry. Finally, it could be determined that a set of factors allowed the control of the vertical growth of the hydraulic fracture with proppant in the proximity of the wellbore. The main control on hydraulic fracture height was the magnitude of the minimum horizontal stress. The presence of discontinuities in the rocks such as calcite veins volcanoclastic intervals and limestone beds may also play a role. These new data provide confidence on the current geomechanical model helping to optimize the upcoming stimulation operations in the area.

In the Vaca Muerta shale of the Neuquén basin, Argentina, the most prolific intervals tend to be the most difficult to hydraulically fracture because of the abnormally high fracture gradients present in some parts of the basin. Thus, it becomes very important to have a good understanding of the anisotropic geomechanical properties of this heterogeneous formation prior to developing the completion strategy. A calibrated, anisotropic 1D mechanical earth model (1D MEM) was developed and used to optimize the completion strategy for a vertical well in the Vaca Muerta shale. The output from the 1D MEM, including the principal stresses, anisotropic elastic properties, pore pressure, and rock strength, were used to define the reservoir intervals with the best characteristics for initiation, propagation, and maintenance of a conductive complex fracture network. Next, the reservoir intervals with the highest hydrocarbon generation tendency were determined from petrophysical and image logs acquired in the well. This formed the basis for selecting the optimum number of stages and perforation strategy for the well. Sensitivity analysis revealed the impact of the hydraulic fracture properties on the production performance. The analysis showed that higher fracture conductivity greatly improves the well performance in the deeper Vaca Muerta intervals, whereas larger fracture surface area is more beneficial across the shallower intervals. Thus, a unique completion strategy was developed for each interval to optimize the well performance. Three hydraulic fracture stages were planned initially, but because of casing limitation, only the first stage was executed. A time-lapse acoustic measurement acquired from the well corroborated the propped fracture height predicted during the completion design phase. The study showed that proper characterization of the anisotropic geomechanical behavior of the Vaca Muerta formation improves the development of a completion strategy, which ultimately optimizes economic performance of the well.

Low-permeability formations must be hydraulically fractured to produce at commercial rates. A good understanding of the formation stress conditions is critical for completion design, but requires in-situ measurement as calibration to support the geomechanical evaluation. This calibration is commonly done using diagnostic formation injection test (DFIT) methodology by creating a hydraulic fracture and then waiting for it to close through leakoff to the formation. However, in a low-permeability low-leakoff environment, application of this approach might become limited because of the time to fracture closure and the non-uniqueness of the interpretation. This paper demonstrates the applicability of the fracture flowback method to define closure pressure. Although proposed in the 1980s, this method has been underused by the industry. One of the objectives of this paper is, therefore, to advocate through field examples its simplicity of development, interpretation, and repeatability, particularly in low-leakoff reservoirs such organic shale formations. The procedure is composed of a sequence of various cycles of pump-in flowback through a fixed choke, pressure rebound, and fracture reopening. The proposed methodology offers several minimum stress measurements for repeatability and quality check purposes, reduces interpretation non-uniqueness, and can be completed within 1 hour, making it compatible with the hydraulic fracturing operations. Test design considerations, such as well geometry, pump rate, fluid volume, choke size, or perforation requirements, are reviewed to maximize the chance of success. Interpretation of the different possible patterns that can be observed is discussed and illustrated with practical examples from the Vaca Muerta shale. Comparison is made between the pump-in flowback and calibration decline approaches performed over the same interval. Repeatability is evaluated, and discrepancies between cycles are investigated. A direct application of the method is the calibration of a stress profile when applied to a vertical well. However, additional observations related to the fracture closure mechanism or residual fracture conductivity can be drawn by detailing the flowed-back volumes or rate of rebound pressure. These observations can be related to the lithology when the procedure is implemented in different intervals of the same formation.

Deliberate inhalation of volatile substances is a common and harmful phenomenon among young persons around the world. Clinical and cytogenetic studies were performed in 35 children (33 boys and 2 girls) chronically habituated to inhalation of volatile agents, with the aim to define whether this practice is associated with chromosome abnormalities and if so to what extent. A significant increase in the rate of chromosome abnormalities and in the frequency of sister chromatid exchanges (SCE) was found in sniffers over the values in controls (p less than 0.001). Clinical, socioeconomic, and cytogenetic findings are discussed.

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