Preliminary experimental investigation on long-term fracture conductivity for evaluating the feasibility and efficiency of fracturing operation in offshore hydrate-bearing sediments

Gas hydrate is undoubtedly an essential resource for sufficient energy supply (natural gas) in the near future. Unfortunately, low production efficiency limits its effective development and utilization. Hydraulic fracturing is a common and effective engineering measure to stimulate oil and gas reservoirs, as well as enhance oil recovery. However, its feasibility for clayey silt hydrate-bearing sediments is unclear now. What is more, there is no scientific basis for determining or assuming the value of fracture conductivity used in productivity prediction. In the present work, factors affecting conductivity of fractures in hydrate-bearing sediments were explored first. The experimental results showed that the conductivity of fracture in hydrate-bearing sediments was lower than 50 mu m(2)center dot cm. Besides, it was found that the increase of proppant size, proppant concentration and hydrate saturation can improve the fracture conductivity. However, to achieve a similar effect, the closure pressure needs to be reduced appropriately. Based on these experiment results, the multi-factor influence model of fracture conductivity in hydrate-bearing sediments was developed. The conductivity profile at site SH2 in the northern South China Sea was then obtained. Finally, feasibility and efficiency evaluation of fracturing operation in hydrate reservoir was numerically conducted. The investigation results reveals that fracturing operation can greatly promote hydrate dissociation and gas production in hydrate development process. In addition, the importance of accurately determining fracture conductivity in productivity prediction of hydrate reservoir was also explored. It was found that gas production is only 12.11 x 10(8) m(3) when fracture conductivity is 5.0 mu m(2)center dot cm. However, it was as high as 22.82 x 10(8) m(3) when fracture conductivity is 40 mu m(2)center dot cm, which is 1.88 times that of the former case. The present work provides new perspective for design and optimization of fracturing operation in hydrate-baring sediments.

Automated summary

Gas hydrate is an important resource for future energy supply, but low production efficiency hinders its development. This study explored factors affecting fracture conductivity in hydrate-bearing sediments. It was found that proppant size, concentration, and hydrate saturation can improve fracture conductivity, but closure pressure needs to be appropriately reduced. A multi-factor influence model of fracture conductivity was developed, and the feasibility and efficiency of fracturing operation in hydrate reservoir were numerically evaluated. The results showed that fracturing greatly promoted hydrate dissociation and gas production. Accurately determining fracture conductivity is crucial for productivity prediction in hydrate reservoir.