孔隙含气压力对不同孔隙结构砂岩声学属性的影响

Influence of Pore Gas Pressure on Acoustic Properties of Sandstones with Different Pore Structure

为研究孔隙含气压力对不同孔隙结构砂岩声学属性的影响,以大港油田3块不同孔隙结构砂岩为研究对象,利用自研的实验装置在多个含水饱和度点测量了砂岩纵波波形、幅度和速度随孔隙含气压力的变化。定义了1个声波幅度衰减变量I来表征首波幅度的衰减。结果表明:随着孔隙含气压力的增加,声波幅度下降,波形后移,中高孔渗砂岩在低含水饱和度下的声波波形甚至发生一定程度的畸变。变量I与含气压力呈线性负相关,且孔隙结构越好,含水饱和度越低,线性负相关关系越好,随着孔隙结构变差,含气压力降低,声波幅度的衰减减弱。砂岩的纵波速度随孔隙含气压力的增加而降低,在含气压力小于5 MPa时,纵波速度随含气压力的增加缓慢下降,含气压力大于5 MPa后,纵波速度随含气压力的增加而快速下降。岩心孔隙含气压力增大后,与声波幅度衰减相比,纵波速度变化不明显,纵波幅度的衰减对含气压力更敏感。砂岩孔隙含气压力增大所造成的声波衰减主要由黏滞性吸收衰减所引起,含气压力的增加对岩石体积模量和体积密度影响较小,纵波速度的变化主要是由有效应力减小造成岩石孔裂隙张开和颗粒接触刚度减小所引起。实验结果可为砂岩地层的含气性评价和孔隙含气压力预测提供借鉴。

In order to study the influence of pore gas pressure on the acoustic properties of sandstones with different pore structure,three different pore structure sandstones of Dagang Oilfield were researched.The variations of P-wave shape,amplitude and velocity of sandstones with gas pressure were measured under several water saturations by self-developed experimental equipment.A acoustic amplitude attenuation variable I was defined to characterize the attenuation of the first wave amplitude.The results show that with the increase of pore gas pressure,the amplitude of wave decreases,and the waveform shifts backward,acoustic waveform of medium-high porosity sandstone with low saturation even distorted.The variable I was linearly negatively correlated with the gas pressure,the better the pore structure and the lower the water saturation,the better the linear negative correlation.As the pore structure deteriorates and the gas pressure decreases,the attenuation of the acoustic amplitude decreases.The P-wave velocity of sandstone decreases with the increase of pore gas pressure.When the gas pressure is less than 5 MPa,the P-wave velocity decreases slowly.After the gas pressure is greater than 5 MPa,the P-wave velocity decreases rapidly.After the pore gas pressure increases,the P-wave velocity does not change significantly compared with the wave amplitude attenuation,and the attenuation of the P-wave amplitude is more sensitive to the gas pressure.The acoustic attenuation caused by the increase of pore gas pressure in sandstone is mainly caused by the viscous absorption attenuation.The increase of gas pressure has little effect on the bulk modulus and bulk density of rock.The change of P-wave velocity is mainly caused by the opening of pore fractures and the decrease of particle contact stiffness caused by the reduction of effective stress.The results can provide reference for the gas evaluation and pore gas pressure prediction of sand formations.