Core-shell structures have been proposed to improve the electrical properties of negative-temperature coefficient (NTC) thermistor ceramics. In this work, Al 2 O 3 -modified Co 1.5 Mn 1.2 Ni 0.3 O 4 NTC thermistor ceramics with adjustable electrical properties were prepared through citrate-chelation followed by conventional sintering. Co 1.5 Mn 1.2 Ni 0.3 O 4 powder was coated with a thin Al 2 O 3 shell layer to form a coreshell structure. Resistivity (ρ) increased rapidly with increasing thickness of the Al 2 O 3 layer, and the thermal constant (B) varied moderately between 3706 and 3846 K. In particular, Co 1.5 Mn 1.2 Ni 0.3 O 4 @Al 2 O 3 ceramic with 0.08 wt% Al 2 O 3 showed the increase of ρ double, and the change in its B was less than 140 K. The Co 1.5 Mn 1.2 Ni 0.3 O 4 @Al 2 O 3 NTC ceramics showed high stability, and their grain size was relatively uniform due to the protection offered by the shell. The aging coefficient of the ceramic was less than 0.2% after aging for 500 hours at 125°C. Taken together, the results indicate that as-prepared Co 1.5 Mn 1.2 Ni 0.3 O 4 @Al 2 O 3 NTC ceramics with a core-shell structure may be promising candidates for application as widetemperature NTC thermistor ceramics. K E Y W O R D S ceramic matrix composites, core-shell structures, electrical properties, NTC thermistors 4394 | LIU et aL.

Hydraulic fracturing has been widely used to develop coal seam gas (CSG) resources worldwide and improve gas production from CSG wells. An optimal stimulation in CSG, in particular with lower permeability coals lacking a well-developed cleat system, creates a fracture with sufficient length and orientation along the maximum horizontal stress (σHmax) direction such that the fracture intersects the existing hydraulically-conductive natural fracture network in an a way that maximizes the stimulation effect. In the Walloon Coal Measures (WCM) in the Surat Basin in eastern Australia, induced fractures have been observed to grow in a way which does not optimally intersect the natural fracture network or conform to the σHmax azimuth. This resultant stimulation of the natural fractures, as opposed to the construction of a conductive path crosscutting the natural fracture system and linking it to the wellbore, has proven to reduce the effect of stimulation in vertical wells. This paper will present a two-well program in the WCM where the wells have been directionally drilled along the σHmax direction with different deviation angles from vertical. Both wells have been stimulated with a combination of treated water with a low proppant concentration, followed by crosslinked-gelled water with a higher proppant concentration. There are two microseismic observation wells and twenty one tiltmeters to map the fracture propagation and determine the fracture geometry. The diagnostic results from microseismic wells and surface tiltmeters for each hydraulic fracturing stage will be outlined in this paper. These results, compared with similar diagnostic results from nearby offset vertical wells, are encouraging and indicate consistent fracture growth along the maximum horizontal direction for all stages. These results indicate that hydraulic fracture geometry can be improved by directional drilling.

Exploring the wax removal process by numerical simulation is beneficial for guiding field operations. In this paper, enthalpy-porosity and volume of fluid (VOF) methods were adopted to simulate the melting process of wax in the crude oil gathering pipeline. The melting patterns and liquid fraction of the wax were used to validate the mathematical model. The results show that the wax melts quickly before the liquid fraction reaches 80%, while the remaining 20% melts very slowly. Since the water with higher density sinks to the lower part of the pipeline, the wax in the lower part of the pipeline melts first, while the wax in the upper part of the pipeline melts slowly. The water temperature and flow rate disproportionately affect the melting process. Increasing the water temperature and flow rate can accelerate the melting process, but the effects on shortening the melting time of wax gradually decrease. Increasing the flow rate, the heat transfer rate and the melting rate are increasing progressively, the change of flow rate also affects the outlet temperature of the pipeline.

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