Influence of horizontal well section length on the depressurization development effect of natural gas hydrate reservoirs

Abstract

The second production test of natural gas hydrate (referred to as hydrate) reservoir in the Shenhu sea area of the northern South China Sea shows that horizontal wells have greater potential than vertical wells in the depressurization production of hydrate reservoirs, but the length of horizontal section to achieve remarkable development effect has not been determined. To this end, a set of production simulation experiment device was independently designed. After the influences of horizontal well section length on gas and water production behaviors and temperature and pressure change laws during the depressurization development of hydrate reservoirs were studied by means of physical experiment, an equal-scale numerical simulation model was established by numerical simulation method. And based on history matching, the change laws of hydrate saturation and gas saturation were analyzed. And the following conclusions were reached. First, for the hydrate reservoirs where water and hydrate coexist, a great amount of high-pressure movable water is produced in the early stage of depressurization development while the formation pressure drops quickly and hydrates are mostly dissociated. In the later stage of depressurization development, there is no stable heat supply. Therefore, the gas production rate presents a trend of fast rise to the peak in the early stage and then concussive decline. Second, horizontal well can effectively increase the water and gas drainage areas, so the longer the horizontal well section is, the higher the peak gas production rate and cumulative gas production are and the shorter time it takes to reach the peak gas production rate, but the faster the gas production decline is in the case of no heat supply. Third, an obvious low hydrate saturation area is formed near the horizontal well, so a long horizontal section is helpful to enlarge the hydrate dissociation area, but in the later stage of depressurization development, there are still a lot of undissociated hydrates in hydrate reservoirs, so the development mode shall be modified to further promote the dissociation of hydrates. Fourth, due to the heat transfer of cap rocks and the gas/water gravity difference, a secondary gas cap is formed easily in the process of depressurization development, so if the horizontal well is drilled near the upper part of a hydrate reservoir, the overlay of dissociated gas can be alleviated, and consequently the production and development effect of dissociated gas are improved.