论文部分内容阅读
在实验室里对一套富含黏土的储集砂岩频率与地震衰减的关系进行了研究。在有效压力为50MPa,实验频率大约为O.8MHz时,采用脉冲回声技术测定了纵、横波速度(v_P和v_S)和品质因子(Q_P和Q_S)与孔隙流体黏度的关系。实验黏度值从0.3～1000cP,假定为总体流动损耗机制,对水饱和的砂岩来说,其等效频率为2.6MHz～780Hz。我们观测到两种类型的特征:高渗透砂岩(>100mD)表现为可变的Q_P和Q_S值,在数值上与根据Biot理论,对黏度值在0.3～20cP之间的砂岩(等于频率范围为2.6MHz～39kHz)预测的数值相近,而低渗透(<50mD)砂岩表现为几乎不变的Q_P和Q_S值,在实验的黏度值范围内用Biot理论是预测不出来的。Biot理论也不能预测黏度大于20cP的高渗透砂岩里观测到的Q_P和Q_S值,实际观测到的Q_P值一般比Biot理论预测值低很多。高渗透砂岩随孔隙流体黏度变化而呈小的速度差,这与Biot理论一致。而低渗透砂岩的速度随黏度的增加而相对增加较大,这一点根据Biot理论无法解释,而与局部流体损耗机制相一致。结果指出存在两种主要的损耗机制:在仅合少量孔内黏土的高渗透砂岩内的总体流动和低渗透、富含黏土的砂岩在超声频率(在水饱和的岩石里至少低于39kHz)下的局部流动。
The relationship between the frequency of a clay-rich reservoir sandstone and the seismic attenuation was studied in the laboratory. At the effective pressure of 50 MPa and the experimental frequency of about O.8 MHz, the relationship between longitudinal and transverse wave velocities (v_P and v_S) and quality factors (Q_P and Q_S) and the viscosity of pore fluid were measured by pulse echo technique. Experimental viscosity values range from 0.3 to 1000 cP, assuming a total flow loss mechanism. For sand-saturated water, the equivalent frequency is between 2.6 MHz and 780 Hz. We observed two types of characteristics: Highly permeable sandstone (> 100mD) behaved as a variable Q_P and Q_S value numerically proportional to sandstone with a viscosity value between 0.3 and 20cP (equal to the frequency range of 2.6MHz ~ 39kHz), while the low permeability (<50mD) sandstones exhibit almost constant Q_P and Q_S values, which can not be predicted by the Biot theory within the experimental viscosity range. The Biot theory also fails to predict the Q_P and Q_S values observed in high permeability sandstone with viscosity greater than 20 cP. The observed Q_P values are generally much lower than predicted by Biot theory. Highly permeable sandstones show a small velocity difference with changes in pore fluid viscosity, consistent with Biot’s theory. However, the velocity of low-permeability sandstone increases relatively with the increase of viscosity, which can not be explained by Biot theory and is consistent with the local fluid loss mechanism. The results indicate that there are two main loss mechanisms: overall flow and low permeability in highly permeable sandstones that contain only a small amount of intracore clay, clay-rich sandstones at ultrasonographic frequencies (at least below 39 kHz in water-saturated rocks) Local flow.