背景为各向异性的含裂缝岩石频散和衰减计算方法研究

Calculation method of dispersion and attenuation in the fractured rock with anisotropic background

裂缝广泛分布于各类储层岩石中,并且会显著提高储层的渗流能力.因此,裂缝的评价和表征对于提高油气产能具有重要意义.由于裂缝与背景介质之间的波致流会显著影响地震波的频散和衰减特性,所以地震勘探是评价裂缝性储层的有效手段.裂缝地震定量表征的前提是要基于含裂缝岩石中波致流对频散和衰减的影响建立含裂缝岩石物理特性与地震性质的关系.然而,目前相关的理论研究大部分基于各向同性背景这一假设,难以有效应用于常见的各向异性储层.本文针对背景为各向异性的含裂缝岩石提出了频散和衰减的计算方法.该方法首先将含裂缝岩石中的各向异性背景介质等效为层状背景介质;然后,通过分析不同频率下层状含裂缝岩石中的流体压力分布,理论计算了两个特定的中间频率并求解得到两个中间频率下的弹性参数;进一步,以计算得到的两个特定中间频率以及高低频极限下的弹性参数为基础,应用数值方法求解得到弛豫函数中的未知参数,最终实现了背景为各向异性含裂缝岩石中频散和衰减的理论模拟.通过将理论预测结果与实验测量和数值模拟结果进行对比,验证了该方法在背景为各向异性含不同分布裂缝岩石中的有效性.本文提出的方法考虑了常见的各向异性背景对含裂缝岩石频散和衰减的影响,因而在裂缝性储层的地震勘探中具有广泛的应用前景.

Fractures are widely distributed in virtually all kinds of rocks,and their characterization is of great meaning for improving the hydrocarbon production due to the fact that the presence of fractures can greatly improve the permeability of reservoirs.Seismic exploration is a useful tool to evaluate fractures because the wave induced fluid flow between the fractures and background pores will dramatically affect the seismic dispersion and attenuation.The quantitative characterization of fractures needs to establish the relations between the physical properties and seismic properties for fractured rocks according to the effects of fluid flow between the fractures and the background pores on the dispersion and attenuation.However,most relevant studies are based on the ideal case of isotropic background and are therefore inapplicable to the anisotropic formations that are commonly existing in nature.We presented a theoretical method to obtain the seismic dispersion and attenuation in a fractured rock with anisotropic background.In this method,we first assumed the anisotropic background of the fractured rock to be a layered background.Then,we developed theoretical methods to calculate the elastic coefficients in two specific middle frequencies that are determined by the fluid pressure distributions in the layered rock with fractures.After obtaining these elastic coefficients as well as those in the high-and low-frequency limits,the dispersion and attenuation were finally simulated by numerically determining the parameters in the relaxation function.We further compared the theoretical predictions with the experimental and numerical results,and the excellent agreements indicated the applicability of the proposed method in simulating the dispersion and attenuation of the fractured rocks with anisotropic background permeated by fractures of various orientations.Because of the universal presence of anisotropic background in fractured formations,the proposed method that considerer the effects of anisotropic background has a broad application range in the seismic exploration of fractured reservoirs.