人工隔板在海域层系天然气水合物藏降压开采中的适应性分析

Adaptability of artificial barrier to depressurization production of marine stratified gas hydrate reservoir

海域天然气水合物(以下简称水合物)藏降压开采期间储层内形成的“压力漏斗”会导致邻近层大量孔隙水入侵井筒,引发井筒水淹,阻碍生产。为此,基于南海神狐海域水合物藏地质特征,同时考虑了水合物、自由水及自由气三相共存体系,模拟了人工隔板敷设时海域层系水合物藏连续降压开采5年的生产过程,研究了井口产气/产水特性、压力/温度场与水合物/自由水饱和度空间分布特性,分析了隔板半径对水合物藏降压生产特性的影响规律。研究结果表明:①人工隔板敷设情境的井口产水速率及累积产水量均低于无隔板情境,尤其上、下隔板同时敷设30 m时,其5年井口累积产水量减少约20.88%;②上、下隔板的敷设在井口累积增气或减水方面均存在一定的可叠加性,即人工隔板对水合物藏降压生产存在积极效用;③上、下隔板同时敷设可有效阻碍敷设区域附近沉积层内压力垂向传播,同时整个生产期间沉积层内压力径向传播更为均整,隔板末端附近区域水合物分解显著快于无隔板情境,且隔板的敷设可缩减井筒附近自由水有效汇入面积;④随着上、下隔板敷设半径从0增至60 m,5年井口累积产水量显著降低,井口综合含水率从13.45%降至7.8%,水合物分解率从3.84%提升至6.32%。结论认为,人工隔板的敷设虽然有助于海域层系水合物藏降压生产,但仅靠人工隔板联合降压开采技术,仍不足以实现商业化开发。

Due to the existence of"pressure funnels"formed in reservoirs during depressurization production of marine natural gas hydrate(hereinafter referred to as hydrate)reservoir,a large amount of pore water flows into the wellbore from adjacent layers,resulting in wellbore flooding and consequently hindering the production of hydrate reservoir.Therefore,based on the geological characteristics of hydrate reservoirs in Shenhu sea area of the South China Sea,this paper simulates the 5-year continuous depressurization production process of the marine stratified hydrate reservoir with an artificial barrier by considering the three-phase coexistence system of hydrate,free water and free gas simultaneously.In addition,the wellhead gas/water production characteristics,the pressure/temperature field and the spatial distribution characteristics of hydrate/free water saturation are investigated,and the influence laws of the artificial barrier's radius on the depressurization production performance of hydrate reservoir are analyzed.And the following research results are obtained.First,the wellhead water production rate and cumulative water production in the case of artificial barriers are lower than those without artificial barriers,and especially when the upper and lower barriers(30 m)are laid at the same time,the 5-year cumulative water production at wellhead can be reduced by about 20.88%.Second,the laying of upper and lower barriers can be superimposed in the wellhead cumulative gas increase or water decrease,which means that artificial barriers have a positive effect on the depressurization production of hydrate reservoirs.Third,when the upper and lower barriers are laid simultaneously,the vertical pressure propagation in the sedimentary layer near the laying area can be hindered effectively,and the radial pressure propagation in the sedimentary layer is more uniform in the whole production period.The hydrate dissociation near the end of the barrier is significantly faster than that in the case without barrier,and the layering of the barrier can reduce the effective inflow area of free water near the wellbore.Fourth,as the laying radius of the upper and lower barriers increases from 0 m to 60 m,the 5-year cumulative water production at the wellhead decreases significantly,the composite water cut at the wellhead decreases from 13.45%to 7.8%,and the hydrate dissociation rate increases from 3.84%to 6.32%.In conclusion,the laying of artificial barrier is conducive to the depressurization production of marine stratified hydrate reservoir,but it is still not enough to achieve commercial development only by using artificial barrier combined with depressurization production technologies.