Effects of Formation Dip on Gas Production from Unconfined Marine Hydrate-Bearing Sediments through Depressurization

Abstract

The effects of geologic conditions and production methods on gas production from hydrate-bearing sediments (HBS) have been widely investigated. The reservoir was usually treated as horizontal distribution, whereas the sloping reservoir was not considered. In fact, most strata have gradients because of the effects of geological structure and diagenesis. In this study, based on currently available geological data from field measurements in Shenhu area of the South China Sea, the effects of formation dip on gas production were investigated through depressurization using a horizontal well. The modeling results indicate that the strategy of horizontal well is an effective production method from the unconfined Class 2 HBS. The predicted cumulative volume of methane produced at the 1000 m horizontal well was 4.51 × 10 7 ST m 3 over 5-year period. The hydrate dissociation behavior of sloping formation is sensitive to changes in the reservoir pressure. As in unconfined marine hydrate reservoir, the sloping formation is not conducive to free methane gas recovery, which results in more dissolved methane produced at the horizontal well. The obvious issue for this challenging target is relatively low exploitation efficiency of methane because of the recovery of very large volumes of water. Consequently, the development of the favorable well completion method to prevent water production is significantly important for realizing large scale hydrate exploitation in the future.

Recent Advances on Natural Gas Hydrate Exploration and Development in the South China Sea

“…Besides the geophysical properties of hydrate reservoirs, the inclination of hydrate formation was another exploitation-affected factor. It is concluded, from the numerical study of Yuan et al regarding the gas production of hydrate-bearing reservoirs in the Shenhu area, SCS, that the increase of dip angle increases the cumulative production of hydrate-released methane, and it might lower the water production by restricting water from the boundary . Chen et al also indicated inclined reservoirs showed higher gas production efficiency and lower water production .…”

“…They had focused on the effect of formation inclination on the gas production and the simulation results indicated that it was a controlling factor in the horizonal well configuration. 135 Sun et al brought up an idea of the multiple vertical wells system based on the actual hydrate reservoir status in the SCS to develop a hydrate reservoir and simulated the dissociation process. 132 The analysis noted that well spacing had a significant influence on the production performance and a multiple well system could bring great benefits for hydrate exploitation under proper operation conditions.…”

“…It is concluded, from the numerical study of Yuan et al regarding the gas production of hydrate-bearing reservoirs in the Shenhu area, SCS, that the increase of dip angle increases the cumulative production of hydrate-released methane, and it might lower the water production by restricting water from the boundary. 135 Chen et al also indicated inclined reservoirs showed higher gas production efficiency and lower water production. 151 In addition, they also discussed the effect of horizontal changes in porosity and hydrate saturation of the hydrate reservoirs.…”

“…Yuan et al simulated depressurization exploitation process of a potential reservoir in the Shenhu area using a horizontal well. They had focused on the effect of formation inclination on the gas production and the simulation results indicated that it was a controlling factor in the horizonal well configuration . Sun et al brought up an idea of the multiple vertical wells system based on the actual hydrate reservoir status in the SCS to develop a hydrate reservoir and simulated the dissociation process .…”

Dynamic Analysis of Exploitation of Different Types of Multilateral Wells of a Hydrate Reservoir in the South China Sea

“…From the results, when the NGH reservoir size was 1495.2 m × 400 m × 800 m, the horizontal well length was 1495.2 m, the bottom hole producing pressure was 2.76 MPa, and the daily production of the horizontal well was 4.2 × 10 5 m 3 /day, which met the requirement of the industrial gas flow rate (3.0 × 10 5 m 3 /day). But in 2018, Yuan 22 pointed out that exploiting hydrate by horizontal wells can bring about high yield water problems. In 2019, Feng et al 23 also pointed out that horizontal wells are not suitable in formations with high initial saturation and permeability of NGH reservoirs.…”