Rare-earth doped crystals have long coherence times and the potential to provide quantum interfaces between microwave and optical photons. Such applications benefit from a high cooperativity between the spin ensemble and a microwave cavity-this motivates an increase in the rare-earth ion concentration which in turn impacts the spin coherence lifetime. We measure spin dynamics of two rare-earth spin species, 145 Nd and Yb, doped into Y 2 SiO 5 , coupled to a planar microwave resonator in the high-cooperativity regime, in the temperature range 1.2 K to 14 mK. We identify relevant decoherence mechanisms, including instantaneous diffusion arising from resonant spins and temperature-dependent spectral diffusion from impurity electron and nuclear spins in the environment. We explore two methods to mitigate the effects of spectral diffusion in the Yb system in the low-temperature limit, first, using magnetic fields of up to 1 T to suppress impurity spin dynamics and, second, using transitions with low effective g factors to reduce sensitivity to such dynamics. Finally, we demonstrate how the "clock transition" present in the 171 Yb system at zero field can be used to increase coherence times up to T 2 = 6(1) ms.

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Openhole gravel packing is one of the most popular completion techniques, due to its high reliability along with the ability to deliver high-productivity wells. Currently, there are two techniques used for gravel placement, one utilizing low-viscosity carrier fluids and low gravel concentration. In this technique the gravel is placed in two waves commonly called Alpha/Beta packing. The second method utilizes a viscous carrier fluid and high concentrations of gravel in conjunction with alternative path screens which mitigate problems caused by unpredicted downhole events. In this paper we present a new approach for gravel packing long high angle openhole intervals without the need for alternative flow path screens but retaining the advantages of high gravel concentration slurries. This is supported by 2 field case histories from a field in India, where two gas wells were drilled with an oil-based drill-in fluid and gravel packed with a viscous water-based fluid. The packing mechanisms and efficiencies in these applications have been verified with downhole gauge analysis as well as mass balance calculations. Both wells are producing sand free with hydrocarbon production that met or exceeded operator expectations, with zero mechanical and extremely low rate dependent skins. Introduction Openhole gravel packing is one of the most popular completion techniques, particularly in deepwater developments with high transmissibility, due to its ability to deliver reliable, high productivity wells.1,2 Current techniques used for gravel packing horizontal wells include Alpha/Beta,3 Alpha/Alpha4 and Alternate Path packing.5 The first two techniques use a low viscosity carrier fluid (typically brine) with a low gravel concentration (typically 1.0 ppa). In both techniques, initial packing takes place in the lower part of the horizontal well until the bottom is packed all the way to the toe (called the Alpha Wave), if circulation can be maintained. This part is dominated by settling of the gravel up to an equilibrium height which is controlled by the circulation rate, with higher rates leading to lower bed heights. In the Alpha/Beta technique, the circulation rate is kept constant, and the packing proceeds from toe-to-heel, covering the upper part of the horizontal well (called the Beta Wave), once the Alpha Wave reaches the toe. For typical Alpha Wave height designs used in these treatments (barely covering the screens), pressure increase during the Alpha Wave is negligible, although the pressure rise during Beta Wave can be substantial. This is because of the narrow annulus between the screen base pipe and the wash pipe, through which the carrier fluid must travel and reach to the entry point into the wash pipe for returns to surface. Such pressure rise can be problematic in cases where the operating window between downhole circulation pressure and the fracturing pressure is narrow. Various hardware and chemistry solutions exist to overcome this problem, including diverter valves that are activated sequentially creating a new entry point upstream into the wash pipe,6 light weight gravel which allows lower pump rates for the same alpha-wave dune height as in conventional gravel7 and drag reducing additives that can be used in the carrier fluid either throughout the treatment or during the Beta Wave.8