During the life time of a production well, sometimes it is inevitable that remediation methods have to be adopted for different reasons. Hydraulic fracturing is one of the common ways to boost production by either reopening existing fractures or creating new fractures. Considering the large number of unconventional wells that have experienced sharp production declines and the current tight budget for drilling new wells, refracturing will certainly become an important technology worthy of more investigation. This paper reviews the production and completion data for a number of wells that have been re-stimulated by hydraulic fracturing since 2011. Wells with enough production data were selected to evaluate their production responses by comparing both the cumulative production in a fixed period of time before and after the re-stimulation and by evaluating incremental production increase normalized by amount of proppant used to restimulate the well. Wells involved in the study had been producing from various formations for different periods of time before restimulation. In addition to the analysis above, this study went a step further by attempting to understand the reasons behind inconsistent results after refracturing: high performance of some wells and the failure of the others to meet production expectations. Two re-completion effectiveness indexes are defined in the paper based on production performance before and after restimulation and on re-completion job size. Overall, the study shows mixed results. Although some formations demonstrated much more favorable response than others, it was also possible for wells producing from the same formation to have extremely different responses to the restimulation. Candidates for restimulation should be evaluated carefully, to include their level of depletion, the petrophysical and geomechanical properties of the rock, and previous well completion and stimulation. These data and measurements are required to engineer an appropriate refracturing or recompletion design. The study also shows very promising results from refracturing some long-producing conventional wells, which indicate opportunities to rejuvenate old wells with hydraulic fracturing. This study provides an overview of the lessons learned from examination of a limited data set of hydraulic fracturing restimulations done in the past five years.

No abstract

During the past six years, the technology for shale gas/oil developments in North America has seen many improvements and optimizations as the industry experiences a sharp rise in the contribution of hydrocarbons from these resources. More recently, Europe and Australia have joined the US in expanding recoverable hydrocarbons from these unconventional resources, and initial activities are on the rise in Latin America, China, Saudi Arabia and India. Despite such improvements and optimizations, a closer look at the performance reveals that not all wells are producing commercially. In addition, the hydraulic fracture stages are not all contributing within the producing wells. This scenario potentially suggests that it is important to target the field's sweet spots while dealing with shale resources (like most other hydrocarbon-bearing formations). Hence, resource development based on the current concepts of geometric placement of hydraulic fracture stages (e.g., using fixed well/fracture spacing) may not be appropriate to develop such heterogeneous unconventional resource basins. In the discussion we illustrate certain well-defined criteria that can identify the sweet spot locations within a field/basin for the optimal well placement. We further document the vital formation/zone characteristics that define the locations for hydraulic fracture stages and thus move away from the arbitrary geometric placement. The paper will discuss the well-placement optimization process and identify the required combination of proposed special petrophysical, geochemical, and geomechanical investigations (wireline, Logging While Drilling and cores). The hydraulic fracture stage placement analysis as presented, shoulders on the need to understand the existing natural fracture system. This understanding is achieved through geophysical log measurements and comprehensive analysis of the hydraulic fracture development pattern, as well as interaction of hydraulic fractures at each stage with the natural fractures. A naturally fractured reservoir can be drained more efficiently if a complex fracture network can be created by the hydraulic fracture stimulation. This begins by drilling the well in the direction of minimum principle horizontal stress in the area. The paper concludes by presenting examples demonstrating the practical application of some of the specific aspects of the methodology discussed and with a number of specific conclusions. In summary, the three key points to Proper Placement of Wells and Hydraulic Fracture Stages, in order to maximize the net value of an operator's asset are: Begin With a Complete Understanding of the Reservoir Use a Multidiscipline and Integrated Approach Across Each Phase of the Life Cycle Effectively Use Modern Technology

No abstract

Pulsation of the Sun with a period of PO 160 min discovered about two decades ago, is still waiting explanation. In view of the hypothesis about its cosmological origin, and attempting to find signature of this PO periodicity among other (short-period variable) stars, the pulsation frequencies of 6 Sct stars are subjected to specific analysis. With a confidence level z 3 . 8~ it is found that the frequency vo = PO-' z 104 ~€ 1 2 , within the error limits, appears indeed to be the most "resonant" one for the total sample of 318 pulsating st,ars of 6 Sct type (the most commensurable, or "synchronizing", period for all these stars occurs to be 16? f 4 min). We conjecture that a) the Po oscillation might be connected with periodic fluctuations of gravity field (metrics), and b) the primary excitation mechanism of pulsations of 6 Sct stars, reflected by this "ubiquitous" PO resonance, must be attributed perhaps to superfast rotation of their inner cores (their rates tend to be in near-resonance with tlie "universal" vo frequency). T h e argun1ent.s are given favouring a cosmological nature of the PO oscillation.Key words: variable stars: 6 Scuti stars -pulsation -rotation A A A subject classification: 122 I n t r o d u c t i o nA particular interest to global oscillations of the Sun is connected with the periodicity PO M 160.010 min discovered in the 70-th by Doppler shift measurements of the Fraunliofer spectral lines (Brookes et al. 1976; Severny et al. 1976). This oscillation has been seemingly confirmed by Grec et al. (1980), Scherrer and Wilcox (1983), but was not supported by the observations of Elsworth et al. (1989). Scherrer et al. (1993) recently suggested that an actual cause of the observed variation of the solar 160-min pulsation rriight be rooted well beneath the solar convection zone or even in the (rapidly rotating) inner core of the Sun. The true nature of this solar oscillation which amplitude is dificult to explain on any basis, is yet a mystery. The precise period (in minutes) was determined to be Po = 160.0101 f 0.0001, (1) with the corresponding frequency vo M 104.160 pHz (Kotov 1995). In the present study, being stimulated by previous results (Kotov 1985 and attempting t o get reasonable explanation of the 160min "solar" periodicity, we extend investigation to detailed analysis of oscillation frequencies of the most popular sample of short-period variables -6 Sct stars.