致密油藏水驱井间裂缝网络参数反演方法及应用

Parametric inversion method and application of inter-well fracture network for water flooding tight oil reservoirs

致密储层注水开发过程中,天然裂缝会张开和沟通形成井间裂缝网络,导致注入水在井间的低效和无效循环。由于裂缝膨胀、延伸和转向作用,井间裂缝网络特征异于油藏初始天然裂缝,需要从动态数据实现井间裂缝网络的量化。为建立一种裂缝性致密油藏井间裂缝网络反演方法,首先提出用二叉树裂缝对刻画井间裂缝网络,用服从对数正态分布的随机变量描述裂缝开度;然后建立了井间裂缝网络中注入水运移数学模型,求得了理论含水率解析解,分析了井间裂缝网络参数对理论含水率的影响规律,推荐了井间裂缝网络参数的反演次序;最后开展了井间裂缝网络参数反演的实例应用,探讨了反演多解性的来源,分析了致密储层近井和井间裂缝特征。研究结果显示,裂缝开度的对数均值影响采油井见水时间和最大含水率,分别与其呈负相关和正相关关系;裂缝开度的对数标准差影响含水率的上升形状,对数标准差越大则上升形状越缓;裂缝条数影响采油井最大含水率。反演结果表明,三口单井对应第1级裂缝单元开度期望分别为90、108、59μm,致密储层近井地带存在中缝—大缝,范围约为注采井距的两端各1/3段。建立的参数反演方法对监测资料要求低、使用方便快捷,实现了基于动态数据的井间等效裂缝形态、数量和开度量化,可以应用于区块级别的致密储层井间裂缝识别,为堵调等措施提供数据基础。

During the water flooding development of tight reservoirs,natural fractures tend to expand and communicate to form an inter-well fracture network,leading to inefficient and invalid circulation of injected water between wells.Due to the expansion,extension and diversion of fractures,the characteristics of the inter-well fracture network are different from those of the initial natural fractures in reservoirs,and it is necessary to quantify the inter-well fracture network with dynamic data.In order to establish an inversion method of inter-well fracture network in fractured tight oil reservoirs,it is firstly proposed to use bifurcated tree fracture pairs to depict the inter-well fracture network,and adopt random variables obeying log-normal distribution to define the fracture opening.And then,the mathematical model of injected water migration inside the network is applied to obtain the analytical solution of the theoretical water cut,analyze the influence laws of the parameters of the inter-well fracture network on the theoretical water cut,and recommend the inversion sequence of the parameters of inter-well fracture network.Finally,the case examples of inter-well fracture network parametric inversion are implemented to explore the sources of inversion ambiguity,and analyze the characteristics of near-well and inter-well fractures in tight reservoirs.The research results show that the logarithmic mean value of fracture opening affects the water breakthrough time and maximum water cut of oil producers,and there is a negative and positive correlation with them respectively;the logarithmic standard deviation of fracture opening affects the rising shape of water cut,and the larger this deviation,the slower the rising shape is;the number of fractures affects the maximum water cut of the oil producers.The inversion results show that the openings of the first stage fracture units in three single wells are expected to be 90,108 and 59μm,respectively,and there are medium-large fractures in the near-wellbore area of the tight reservoir,covering about 1/3 stage in the injector-producer spacing.The established parametric inversion method has low requirements on monitoring data and is convenient and quick to use.It is able to quantify the shape,quantity and opening of equivalent inter-well fractures based on dynamic data,and can be applied to identify inter-well fractures in tight reservoirs at the block scale and provide a data basis for measures such as blocking and profile control.