New insights into microcosmic characteristics of sand production via hydrate decomposition behaviours in the sediments captured by Cryo-SEM

The non-diagenetic reservoir was a weak point in sand production and sand control studies of conventional petroleum industry due to their poor economy. However, nature gas hydrate (NGH) mainly exists in non-diagenetic strata, and sand production often happens during mining, which restricts the safe and sustainable production of NGH. In order to study microcosmic characteristics of sand production, the hydrate decomposition behaviours in the sediments were captured by the Cryo-SEM method. The micromorphology of different particle sizes sand samples containing NGH (the sand median diameter d₅₀ is 150, 87, 55, 38, 24 and 13 𝜇m) and the microcosmic process of NGH decomposition was observed. Then the microcosmic characteristics of sand production during the decomposition process were analyzed. (1) The gas hydrate decomposition increases pore space and reduces reservoir strength: The expansion action of decomposition water & gas, softening action of decomposition water and compression action by overlying stress (crustal stress) promoted sand production, deformation and subsidence of NGH reservoirs in the mining process. (2) Decomposition of NGH has a greater impact on sediments with smaller particle size. (3) The particle size of NGH may be larger than the particle size of mud in the reservoirs, and acting as "gravel" plays a certain role in sand control. Therefore, the particle size of NGH cannot be ignored in the design of sand control. (4) It has been revealed and verified that sputtering (splashing) is a unique process for sand production caused by NGH decomposition. In other words, the rapid expansion of the volume by decomposed gas and water from NGH leads to eruption and sputtering of hydrate particles, providing the driving force for sand migration, which is not the way of sand production in conventional oil and gas.