Investigation into the Physical Process of Sand Production in Unconsolidated Sands by Laboratory Experiments using X-ray CT for Methane Hydrate Development

Methane gas hydrate exists between sand particles of sandy sediment. Since gas hydrates cement grain contacts, the sediment acts in nature as a solid-like material. However, after starting operation of gas production, gas hydrates dissociate and the sediment turns into unconsolidated sands. It is a key issue how to block out such sands flowing into a production well for maintaining the efficiency of gas production. Sand screen is a typical solution to realize sand control. As a part of the MH21 research program in Japan, we carried out laboratory tests to examine the effect of sand screen on sand production and observe visually the change in structure inside sand formation during sand production. To do this, we developed a cylindrical test vessel for tests using cylindrical specimens under 3D axi-symmetric confining stresses. The test vessel was designed to carry out the tests with real time in situ X-ray CT observations of specimen. The vessel was made from a Carbon Fiber Reinforced Plastic, CFRP. This material has both properties of a very-good transparency of X-rays and a high strength. The specimen of 200 mm in diameter and 150 mm in axial length was made of sands mixed with a little kaolin. Mean particle size of sand was 125 mm. In the tests, water flew uniformly into the specimen from one end, and it flew out from the other end through a circular hole of 40 mm in diameter, where the hole was covered with two layers of stainless steel wire screen. We used three kinds of screen with different opening sizes of 390, 460 and 630 mm respectively. The results show that sand production rate changed drastically with screen opening, even though the difference in opening size was so small to be equivalent to the grain size. On the other hand, it was clearly detected from the CT images that with progress of sand production, multiple flow channels in a shape of elongate semi-ellipse in axial cross-section were induced and they grew in axial direction of specimen. The flow channels play likely a role to collect water and drain it directly out of the specimen. As a result, the velocity of water flow through the rock matrix may be reduced nearby the drain side of the specimen. The reduction in water velocity should contribute to suppress the sand production.