基于cohesive单元海域天然气水合物储层水力压裂模拟

Numerical simulation of hydrate reservoir hydraulic fracturing based on cohesive units

摘要:

天然气水合物作为一种高效清洁能源，广泛分布于我国南海海域的沉积地层中。我国先后于2017年和2020年成功开展了2次试开采，但由于海域天然气水合物特殊的赋存条件，单井水合物试采仍然面临着开采范围小、高产稳产时间短等问题。为了提高水合物的开采范围，基于cohesive单元进行了水合物储层二维水力压裂数值模型研究，比较了100 m×100 m和20 m×20 m两种模型的裂缝半长和宽度，得出了当注入压力为25 MPa时，压裂裂缝半长均为6 m，最大宽度分别为5.8、5.5 mm，构建尺寸较大的模型得出的实验结果更加准确。并且研究了裂缝宽度随注入时间的变化规律，随着注入压力和注入量的不断增加，初期裂缝宽度急速变大，后续在地应力和注入流体压力的共同作用下裂缝出现“阶梯式”的扩展规律。该研究在页岩气和煤层气等非常规能源储层水力压裂模型分析中得到了成功运用，为海域天然气水合物储层水力压裂提供一定的理论指导。

Abstract:

Natural gas hydrate is an efficient and clean energy， which is widely distributed in the sedimentary strata in the South China Sea. China has successfully carried out two trial productions in 2017 and 2020 respectively. However， due to the special occurrence conditions of marine natural gas hydrate， there are still some problems with single well trial production， such as small production range and short time for high and stable production. In order to improve the exploitation range of hydrate， the two-dimensional hydraulic fracturing numerical model is studied based on cohesive units. Through simulation， the half length and width of cracks of the two models 100m×100m and 20m×20m are compared. It is concluded that at the injection pressure of 30MPa， the half length of the crack is 6m for both models， and the maximum width is 5.8mm and 5.5mm respectively. The more accurate experimental results can be obtained by constructing a larger model. Moreover， the variation law of fracture width with injection time is studied. With the continuous increase of injection pressure and injection volume， the initial fracture width increases rapidly， and then the fracture propagates “step by step” under the action of in-situ stress and injection fluid pressure. The research has been successfully applied in the hydraulic fracturing model analysis of unconventional energy reservoirs such as shale gas， coalbed methane， and provides some technical guidance for marine natural gas hydrate reservoir hydraulic fracturing .