分形离散裂缝页岩气藏多级压裂水平井数值试井模型

A numerical well testing interpretation model for multiple fractured horizontal well in fractured shale gas reservoir based on fractal discrete fracture network

关键词:  水平井  数值试井  多级压裂  页岩气  离散裂缝  分形

Key words:  horizontal well  numerical well testing  multiple fractured  shale gas  discrete fracture  fractal

摘要:

基于分形理论,考虑天然裂缝的长度、方向、中心位置、开度等分布规律建立分形离散裂缝网络模型,并在此基础上建立多级压裂水平井试井模型。采用有限差分和嵌入式离散裂缝相结合的方法求解该模型,得到水平井的压力响应及其影响规律。结果表明:裂缝密度、分形维数、裂缝主发育方向和Fisher常数等参数能够较好地控制试井曲线形态;裂缝密度、中心维数和长度维数决定裂缝的数量,裂缝密度和中心维数越大(或长度维数越小),裂缝总数越多,水平井的双重介质特征越显著,即中期拟压力导数下凹越明显;裂缝主发育方向和Fisher常数决定裂缝的走向及其一致性程度,Fisher常数越大,裂缝的方向越一致,渗流时有效渗流面积越小,则晚期拟压力导数曲线越高;Langmuir吸附体积和吸附压力对试井曲线也具有一定的影响,二者越大,中期和晚期拟压力和拟压力导数曲线整体越低。

Abstract:

A discrete fracture network model was established based on the fractal theory, considering the length, direction, center position and width of natured fractures, and then a numerical well testing model for multiple fractured horizontal well in fractured shale gas reservoir was obtained. The model was solved by a finite difference method combined with embedded discrete fractures. The pressure response and its influence characteristics of horizontal wells were studied. The analysis results show that the pressure response curve can be well controlled by parameters such as the fracture density, the fractal dimension, the main propagation direction of fractures and the Fisher constant. The total number of fractures is dependent on the fracture density and the fracture 's center and length dimensions. The larger the fracture density and the fracture center dimension (or the smaller the fracture length dimension), the greater the total number of fractures. As a consequence, the dual-medium characteristics of the horizontal wells can be more visible, or in other words, the pressure derivative curve can depress more obviously during the intermediate period. The analysis also indicates that the orientation and its consistency of the fractures are dependent on the propagation direction of the main fracture and the Fisher constant. The pressure derivative curve can be higher with larger Fisher constant during the late flow period due to smaller effective flow area caused by more consistent fracture orientation. Furthermore, The Langmuir adsorption volume and adsorption pressure also have certain influence on the pressure response. The pressure and its derivative curves can be lower together with larger Langmuir adsorption volume and adsorption pressureduring the intermediate and late flow periods.