5,5′-[Methyl­enebis(sulfanedi­yl)]bis­(1,3,4-thia­diazol-2-amine)

In the crystal structure of the title compound, C5H6N6S4, the mol­ecules are linked by strong N—H⋯N hydrogen bonds into a two-dimensional network and an intra­molecular C—H⋯S inter­action also occurs

Speed Fluctuation Suppression Based on an Adaptive Periodic Disturbance Observer for an Inverter Compressor

Repetitive operations have been extensively used in the inverter compressor refrigeration industry. The approximately periodic disturbance caused by repetitive operations must be compensated to realize stable and high-efficiency operation. In this paper, a periodic disturbance observer (PDOB) is proposed to tackle the speed fluctuation of an inverter compressor in the low-frequency range. Periodic disturbance, consisting of a fundamental wave and corresponding harmonics, can thus be estimated and compensated; in addition, sensitivity and complementary sensitivity can reach a compromise through the use of a certain parameter. Aiming at a different operation environment, an adaptive notch filter based on the Steiglitz–McBride method is employed to estimate the fundamental frequency of periodic disturbance. Finally, the feasibility of our approach is verified by MATLAB simulation, and experiments are implemented to illustrate that speed fluctuation can be more effectively attenuated by the proposed method in comparison with general DOB

Speed Fluctuation Suppression for the Inverter Compressor Based on the Adaptive Revised Repetitive Controller

Repetitive operation is extensively present in the inverter compressor refrigeration field. Research illustrates that an approximate periodic load disturbance deteriorates its self-sensing and efficiency in a low frequency range. According to an internal model theorem, a repetitive controller can realize periodic signal tracking or suppression; however, ideal repetitive control (IRC) can not be directly employed in practical application owing to its excessive sensitivity to period-time uncertainties or aperiodic component, etc. In addition, the ratio between period delay to sampling time can not always maintain an integer which causes the resonant frequencies to deviate from the interested. Even if the ratio is a fixed integer, the auxiliary function for stabilization in a conventional repetitive controller can result in the steady state tracking error. In this paper, a revised repetitive controller with a novel Savitzky–Golay (S–G) filter is proposed, the S–G filter cascaded with a traditional delay element can approximate the delay element with any ratio. Aiming at a different operating condition, a second order generalized integrator (SOGI) based observer is employed to extract disturbance fundamental component, and Steiglitz–McBride based adaptive notch filter (SM-ANF) is adopted for frequency estimation. The proposed scheme varies the S–G filter according to variable fundamental frequency and maintains its resonant frequencies corresponding with the fundamental and harmonic disturbance. In the end, simulations and experiments are implemented and results illustrate that speed fluctuation can be effectively suppressed by the proposed method

Influencing Factors Analysis and Optimization of Hydraulic Fracturing in Multi-Layered and Thin Tight Sandstone Gas Reservoir

With the deepening of exploration and development of tight sandstone gas reservoirs, the remaining recoverable reservoirs gradually become thinner with the vertical stratigraphic structure. The geomechanical properties become complex, and development based on conventional hydraulic fracturing methods often leads to serious problems, such as difficult control of fracture height, penetrating interlayers, too short fracture length, and inadequate proppant filling. In view of the above problems, we conducted a numerical investigation on a hydraulic fracturing scheme in a multi-layered and thin tight sandstone gas reservoir. According to the dataset from wells in a real gas reservoir in China’s Ordos Basin, the relevant geomechanical characteristics of the gas layers, together with the interlayers in the main production interval, were obtained, based on which, a fine numerical model was developed. By using the PL3D fracture propagation algorithm, a 3D hydraulic fracture propagation model was produced, and then using microseismic monitoring and production data matching, a high-precision hydraulic fracture model of the multi-layered and thin tight sandstone gas reservoir was obtained. On this basis, the influence of different geomechanical parameters and fracturing operational parameters on hydraulic fracture propagation was analyzed. Finally, an optimized hydraulic fracturing scheme that fitted the characteristics of the multi-layered and thin tight sandstone gas reservoir was proposed. Using a typical reservoir example, the optimized scheme enabled control of the fracture height in thin layers and the creation of long fractures with better proppant filling, so that the productivity of the fracture was significantly improved

Theoretical Analysis of the Mechanism of Fracture Network Propagation with Stimulated Reservoir Volume (SRV) Fracturing in Tight Oil Reservoirs.

Stimulated reservoir volume (SRV) fracturing in tight oil reservoirs often induces complex fracture-network growth, which has a fundamentally different formation mechanism from traditional planar bi-winged fracturing. To reveal the mechanism of fracture network propagation, this paper employs a modified displacement discontinuity method (DDM), mechanical mechanism analysis and initiation and propagation criteria for the theoretical model of fracture network propagation and its derivation. A reasonable solution of the theoretical model for a tight oil reservoir is obtained and verified by a numerical discrete method. Through theoretical calculation and computer programming, the variation rules of formation stress fields, hydraulic fracture propagation patterns (FPP) and branch fracture propagation angles and pressures are analyzed. The results show that during the process of fracture propagation, the initial orientation of the principal stress deflects, and the stress fields at the fracture tips change dramatically in the region surrounding the fracture. Whether the ideal fracture network can be produced depends on the geological conditions and on the engineering treatments. This study has both theoretical significance and practical application value by contributing to a better understanding of fracture network propagation mechanisms in unconventional oil/gas reservoirs and to the improvement of the science and design efficiency of reservoir fracturing

Geological Characteristics and Development Techniques for Carbonate Gas Reservoir with Weathering Crust Formation in Ordos Basin, China

The carbonate gas reservoir is one of the most important gas formation types; it comprises a large proportion of the global gas reserves and the annual gas production rate. However, a carbonate reservoir with weathering crust formation is rare, and it is of significant interest to illustrate the geological characteristics of this kind of formation and present the emerging problems and solution measures that have arisen during its exploitation. Therefore, in this research, a typical carbonate gas reservoir with weathering crust formation that is located in Ordos Basin, China, was comprehensively studied. In terms of formation geology, for this reservoir, the distribution area is broad and there are multiple gas-bearing layers with low abundance and strong heterogeneity, which have led to large differences in gas well production performance. Some areas in this reservoir are rich in water, which seriously affects gas well production. Regarding production dynamics, the main production areas in this gas reservoir have been stable on a scale of 5.5 billion cubic meters for more than a decade, and the peripheral area has been continually evaluated to improve production capacity. Nevertheless, after decades of exploration and development, the main areas of this reservoir are faced with several problems, including an unclear groove distribution, an unbalanced exploitation degree, low formation pressure, and increases in intermittent gas wells. To deal with these problems and maintain the stability of gas reservoir production, a series of technologies have been presented. In addition, several strategies have been proposed to solve issues that have emerged during the exploration and exploitation of peripheral reservoir areas, such as low-quality formation, unclear ancient land and complex formation-water distribution. These development measures employed in the carbonate gas reservoir with weathering crust formation in the Ordos Basin will surely provide some guidance for the efficient exploitation of similar reservoirs in other basins all over the world

Absolute open flow (AOF) potential evaluation for watered-out gas wells in water-drive gas reservoir

This paper presents a gas-water two-phaseLaminar-Inertial-Turbulent (LIT) flow equation for watered-out gas wells, which can be solved through the incorporation of daily production, gas-water relative permeability and PVT data, and then Q (AOF) and formation factor (Kh) are obtained. Field case study in Long Wang Miao (LWM) water-drive gas reservoir shows that for watered-out gas wells, the single-phase LIT equation can result in overestimation of Q (AOF) by 30-80%. Water invasion performance can be relieved after reducing the production rate of watered-out gas wells, and water invasion behavior before breakthrough can be diagnosed with the decline trend of Kh values