饱和致密砂岩中动态剪切模量硬化的实验研究与理论模拟  

Experimental study and theoretical simulation of dynamic shear modulus hardening in saturated tight sandstone

作　　者：李东庆 司文朋 陈胜红[1] 魏建新[2,3] 狄帮让[2,3] LI DongQing;SI WenPeng;CHEN ShengHong;WEI JianXin;DI BangRang(Sinopec Geophysical Research Institute,Nanjing 211103,China;College of Geophysics,China University of Petroleum,Beijing 102249,China;State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum,Beijing 102249,China)

机构地区：[1]中国石油化工股份有限公司石油物探技术研究院,南京211103 [2]地球物理学院,中国石油大学(北京),北京102249 [3]油气资源与探测国家重点实验室,中国石油大学(北京),北京102249

出　　处：《地球物理学报》2021年第8期2916-2926,共11页Chinese Journal of Geophysics

基　　金：国家科技重大专项(2017ZX05005-004);国家自然科学基金(41474112)联合资助.

摘　　要：Gassmann理论认为岩石的剪切模量在饱和流体前后保持不变,这一认识被广泛应用于高孔高渗常规储层中.然而,致密砂岩等非常规储层通常具有低孔、低渗以及孔隙结构复杂等岩石物理特征,因此Gassmann流体替换理论在此类储层的适用性尚不明确.针对这一问题,本文在1~60 MPa有效压力内分别测量了干燥与饱水致密砂岩样品的超声纵、横波速度,并对饱水前后砂岩的剪切模量变化特征进行了分析.结果表明,致密砂岩的剪切模量在饱水前后可能出现弱化或硬化现象.通过对致密砂岩样品的孔隙纵横比进行定量化分析,发现对于微裂缝主要分布在颗粒间以及颗粒内,并且软孔隙纵横比分布范围宽、软孔隙含量较高的样品更容易表现出剪切硬化的特征.利用一般形式的喷射流模型可以较好的模拟剪切硬化现象,但要考虑微裂缝的闭合情况对输入参数的影响.剪切硬化主要来源于高频弹性波激励下流体的喷射流频散作用.考虑到特定条件下在地震和测井频带也会产生频散现象,因此致密砂岩等非常规储层在进行流体替换时需考虑剪切模量可能会发生变化,盲目使用Gassmann流体替换理论的剪切模量不变假设可能会引起较大的预测误差.According to Gassmann theory,the shear modulus of rock remains unchanged before and after saturation,which is widely used in conventional reservoirs with high porosity and high permeability.However,tight sandstone and other unconventional reservoirs usually have low porosity,low permeability and complex pore structure,therefore the applicability of Gassmann fluid substitution theory in such reservoirs is not clear.In order to solve this problem,ultrasonic P-wave and S-wave velocities of dry and saturated tight sandstone samples were measured under the effective pressure of 1~60 MPa,and the variation characteristics of shear modulus of sandstones before and after water saturation were analyzed.The results show that the shear modulus of tight sandstone may weaken or harden before and after water saturation.Through the quantitative analysis of the pore aspect ratio of tight sandstone samples,it is found that samples of micro fractures mainly distributed between particles and within particles,with wide distribution range of soft pore aspect ratio and high content of soft pore are more likely to show the characteristics of shear hardening.The general squirt flow model can be used to simulate the shear hardening phenomenon,but the influence of micro crack closure on the input parameters should be considered.Shear hardening is mainly due to the dispersion of squirt flow excited by high frequency elastic wave.Considering the frequency dispersion may also occur in seismic and logging frequency bands,it is necessary to realize that the shear modulus may change during fluid replacement in unconventional reservoirs such as tight sandstone.Blind using of the Gassmann theory with assumption that shear modulus does not change in fluid replacement may cause large prediction error.

关 键 词：致密砂岩 剪切模量 Gassmann流体替换 喷射流 微裂缝 

分 类 号：P631[天文地球—地质矿产勘探]