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Characteristics of fluid substitution in porous rocks

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petroleum Science 2009年第2期6卷 139-145页

作者： Li Shengjie School of Resource and Information Technology China University of Petroleum Beijing 102249 China State key laboratory of petroleum Resource and Prospecting China University of Petroleum Beijing 102249 China key laboratory of cnpc geophysical exploration China University of Petroleum Beijing 102249 China

Analysis of the effect of changes in fluid properties of rocks on the compressional-wave velocity VP and shear-wave velocity Vs is very important for understanding the rock physical properties, especially in oilfield exploration and development. The fluid substitution process was analyzed by using ultrasonic measurement and theoretical calculations. The results showed that the effect of fluid substitution on the rock elastic modulus was mainly controlled by fluid properties, saturation, and confining pressure. For a rock with specific properties and porosity, the result of theoretical prediction for fluid substitution accorded with the experimental result under high confining pressure （higher than 60 MPa for our experimental data）, but failed to describe the trend of experimental result under low confining pressure and VP predicted by Gassmann＇s equation was higher than that measured by experiment. A higher porosity resulted in stronger sensitivity of the bulk modulus of saturated rocks to the change of fluid properties.

关键词： Fluid substitution velocity bulk modulus porosity saturation

A physical model study of effect of fracture aperture on seismic wave

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Science china Earth Sciences 2008年第S2期51卷 233-240页

作者： WEI JianXin & DI BangRang cnpc key Physical exploration laboratory,china university of petroleum,beijing 10224.,china cnpc key Physical exploration laboratory China University of Petroleum Beijing China

Based on Hudson’s theoretical hypothesis of equivalent fracture model,inserting aligned round chips in solid model can simulate fractured media. The effect of fractures on the propagation of P and S waves can be observed by changing the fracture thickness. The base model is made of epoxy resin,and the material of fractures is a kind of low-velocity mixture containing silicon rubber. With constant diameter and number of fractures in each model,one group of models can be formed through changing the thickness of fracture. These models have the same fracture density. By using the ultrasonic pulse transmission method,the experiment records time and waveform of P and S waves in the direction parallel and perpendicular to the fracture orientation. The result shows that,with the same fracture density,changing fracture aperture will affect both velocity and amplitude of P and S waves,and the effect on P-wave amplitude is much greater than that on the velocity. Moreover,the variation in velocity of S wave is more obvious in the slow shear wave (S2),while the variation in amplitude is more obvious in the fast shear wave (S1). These properties of wave propagation are useful for seismic data processing and interpretation.

关键词： fracture aperture Hudson’s fracture theory fractured model P-and S-wave velocity amplitude

Wide/narrow azimuth acquisition footprints and their effects on seismic imaging

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petroleum Science 2008年第4期5卷 308-313页

作者： Di Bangrang Xu Xiucang Wei Jianxin key laboratory of cnpc geophysical exploration China University of Petroleum Beijing 102249 China State key laboratory of petroleum Resource and Prospecting China University of Petroleum Beijing 102249 China key laboratory of Earth Prospecting and Information Technology China University of Petroleum Beijing 102249 China

Acquisition footprint is a new concept to describe the seismic noise in three-dimensional seismic exploration and it is closely related to geometry and observation shuttering. At present, the study on acquisition footprints has become a hot spot. In partnership with the Dagang Oilfield, we used the channel sand body seismic physical model to study the characteristics of wide/narrow azimuth acquisition footprints and analyzed and compared the two types of footprints and their effects on target imaging. In addition, the footprints caused by data processing of the normal moveout offset （NMO） stretching aberration were discussed. These footprints are located only in the shallow or middle layer in the time slice, and possibly affect the imaging of shallow target layers, and have no influence on deep target imaging. Seismic physical modeling has its advantages in the study of acquisition footprints.

关键词： Wide/narrow azimuth geometry acquisition footprint stretching aberration time slice seismic imaging

Study on attribute characterization for reservoir dynamic monitoring by seismic

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Science china Earth Sciences 2008年第S2期51卷 218-225页

作者： CHEN XiaoHong1,3,LI JingYe2,3 & ZHAO Wei4 1 key Lab of geophysical exploration of cnpc,china petroleum university,beijing 10224.,china 2 Basin & Reservoir Research Center,china petroleum university,beijing 10224.,china 3 State key laboratory of petroleum Resource and Prospecting,china university of petroleum,beijing 10224.,china 4 Research Center of CNOOC,beijing 100027,china key Lab of geophysical exploration of cnpc China Petroleum University Beijing China State key laboratory of petroleum Resource and Prospecting China University of Petroleum Beijing China Basin & Reservoir Research Center China Petroleum University Beijing China Research Center of CNOOC Beijing China

Study on characterizing reservoir parameters dynamic variations by time-lapse seismic attributes is the theoretical basis for effectively distinguishing reservoir parameters variations and conducting time-lapse seismic interpretation,and it is also a key step for time-lapse seismic application in real oil fields. Based on the rock physical model of unconsolidated sandstone,the different effects of oil saturation and effective pressure variations on seismic P-wave and S-wave velocities are calculated and analyzed. Using numerical simulation on decoupled wave equations,the responses of seismic amplitude with different offsets to reservoir oil saturation variations are analyzed,pre-stack time-lapse seismic attributes differences for oil saturation and effective pressure variations of P-P wave and P-S converted wave are calculated,and time-lapse seismic AVO (Amplitude Versus Offset) response rules of P-P wave and P-S converted wave to effective pressure and oil saturation variations are compared. The theoretical modeling study shows that it is feasible to distinguish different reservoir parameters dynamic variations by pre-stack time-lapse seismic information,including pre-stack time-lapse seismic attributes and AVO information,which has great potential in improving time-lapse seismic interpreta-tion precision. It also shows that the time-lapse seismic response mechanism study on objective oil fields is especially important in establishing effective time-lapse seismic data process and interpreta-tion scheme.

关键词： time-lapse seismic dynamic monitoring rock physical model pre-stack attribute Amplitude Versus Offset (AVO)

Potential risks of spectrum whitening deconvolution——compared with well-driven deconvolution

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petroleum Science 2009年第2期6卷 146-152页

作者： Li Guofa Zhou Hui Zhao Chao State key laboratory of petroleum Resource and Prospecting China University of Petroleum Beijing 102249 China key laboratory of cnpc geophysical exploration China University of Petroleum Beijing 102249 China

Deconvolution is widely used to increase the resolution of seismic data. To compare the resolution ability of conventional spectrum whitening deconvolution to thin layers with that of welldriven deconvolution, a complex sedimentary geological model was designed, and then the simulated seismic data were processed respectively by each of the two methods. The amplitude spectrum of seismic data was almost white after spectrum whitening, but the wavelet resolution was low. The amplitude spectrum after well-driven deconvolution deviated from white spectrum, but the wavelet resolution was high. Further analysis showed that if an actual reflectivity series could not well satisfy the hypothesis of white spectrum, spectrum whitening deconvolution had a potential risk of wavelet distortion, which might lead to a pitfall in high resolution seismic data interpretation. On the other hand, the wavelet after well- driven deconvolution had higher resolution both in the time and frequency domains. It is favorable for high resolution seismic interpretation and reservoir prediction.

关键词： Well-driven high resolution spectrum whitening deconvolution seismic wavelet

Anisotropic characteristics of electrical responses of fractured reservoir with multiple sets of fractures

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petroleum Science 2009年第2期6卷 127-138页

作者： Shen Jinsong Su Benyu Guo Naichuan Department of Geophysics School of Resource and Information Technology China University of Petroleum Beijing 102249 China key laboratory of cnpc geophysical exploration China University of Petroleum Beijing 102249 China State key laboratory of petroleum Resource and Prospecting China University of Petroleum Beijing 102249 China

In fractured reservoirs, the fractures not but also form the main flow channels which connect productivity of reservoirs. However, because of the only provide the storage space for hydrocarbons, the pores of the matrix, so fractures dominate the heterogeneity and randomness of the distribution of fractures, exploration and evaluation of fractured reservoirs is still one of the most difficult problems in the oil industry. In recent years, seismic anisotropy has been applied to the assessment of fractured formations, whereas electrical anisotropy which is more intense in fractured formations than seismic anisotropy has not been studied or used so extensively. In this study, fractured reservoir models which considered multiple sets of fractures with smooth and partly closed, rough surfaces were established based on the fractures and pore network, and the vertical and horizontal electrical resistivities were derived as a function of the matrix and fracture porosities according to Ohm＇s law. By using the anisotropic resistivity equations, variations of the electrical anisotropy of three types of fractured models under the conditions of free pressure and confining pressure were analyzed through the variations of the exerted pressure, matrix porosity, fracture aperture and formation water resistivity. The differences of the vertical and horizontal resistivities and the anisotropy between the connected and non-connected fractures were also analyzed. It is known from the simulated results that an increase of the confining pressure causes a decrease of electrical anisotropy because of the elasticity of the closed fractures and the decrease of the fracture aperture. For a fixed fracture porosity, the higher the matrix porosity, the weaker the electrical anisotropy in the rock formation.

关键词： Fractured reservoir partly closed fracture electrical anisotropy fracture roughness

Enhancing the resolution of seismic data based on the generalized S-transform

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petroleum Science 2009年第2期6卷 153-157页

作者： Tian Jianhua Song Wei Yang Feizhou State key laboratory of petroleum Resource and Prospecting China University of Petroleum Beijing 102249 China key laboratory of cnpc geophysical Prospecting China University of Petroleum Beijing 102249 China

In this paper, we analyze the seismic signal in the time-frequency domain using the generalized S-transform combined with spectrum modeling. Without assuming that the reflection coefficients are random white noise as in the conventional resolution-enhanced techniques, the wavelet which changes with time and frequency was simulated and eliminated. After using the inverse S-transform for the processed instantaneous spectrum, the signal in the time domain was obtained again with a more balanced spectrum and broader frequency band. The quality of seismic data was improved without additional noise.

关键词： Time-frequency domain generalized S-transform spectrum modeling instantaneous spectrum balanced spectrum

2.5-D modeling of cross-hole electromagnetic measurement by nite element method

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petroleum Science 2008年第2期5卷 126-134页

作者： Shen Jinsong Sun Wenbo Department of Geophysics School of Resources and Information Technology China University of Petroleum Beijing 102249 China cnpc key Lab of geophysical exploration China University of Petroleum Beijing 102249 China

A finite element method is developed for simulating frequency domain electromagnetic responses due to a dipole source in the 2-D conductive structures. Computing costs are considerably minimized by reducing the full three-dimensional problem to a series of two-dimensional problems. This is accomplished by transforming the problem into y-wave number （Ky） domain using Fourier transform and the y-axis is parallel to the structural strike. In the Ky domain, two coupled partial differential equations for magnetic field Hy and electric field Ey are derived. For a specific value of Ky, the coupled equations are solved by the finite element method with isoparametric elements in the x-z plane. Application of the inverse Fourier transform to the Ky, domain provides the electric and magnetic fields in real space. The equations derived can be applied to general complex two-dimensional structures containing either electric or magnetic dipole source in any direction. In the modeling of the electromagnetic measurement, we adopted a pseudo-delta function to distribute the dipole source current and circumvent the problem of singularity at the source point. Moreover, the suggested method used isoparametric finite elements to accommodate the complex subsurface formation. For the large scale linear system derived from the discretization of the Maxwell＇s equations, several iterative solvers were used and compared to select the optimal one. A quantitative test of accuracy was presented which compared the finite element results with analytic solutions for a dipole source in homogeneous space for different ranges and different wave numbers Ky. to validate the addressed the effects of the distribution range τ of the homogeneous medium. code and check its effectiveness. In addition, we pseudo-delta function on the numerical results in

关键词： 2.5-D electromagnetic modeling frequency domain isoparametric finite element pseudo-delta function

The attenuation anisotropy of mudstones and shales in subsurface formations

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Science china Earth Sciences 2005年第11期48卷 1882-1890页

作者： SHI Ge1 & DENG Jixin1,2 1. The key Labratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking university, beijing 100871, china 2. geophysical Prospecting key Lab (cnpc), university of petroleum, beijing 10224., china The key Labratory of Orogenic Belts and Crustal Evolution School of Earth and Space Sciences Peking University Beijing China geophysical Prospecting key Lab (cnpc) University of Petroleum Beijing China

We have measured the relationships in the laboratorial condition between the at-tenuation in three samples of mudstones and shales as well as two samples of layered sand-stones and the alteration in direction, confining pressure and pore fluid. The relative contributions of different attenuation mechanisms are also discussed. Studies show that the attenuation in dry and saturated samples is strongly related to pressure and direction. The attenuation coefficient of P-wave propagating parallel to bedding (PH ) is less than that of P-wave propagating vertical to bedding (PV ), while the attenuation coefficient of S-wave both propagating and vibrating parallel to bedding (SH ) and that of S-wave propagating vertical to bedding while vibrating parallel to bedding (SV1) are less than that of S-wave propagating parallel to bedding while vibrating verti-cal to bedding (SV ). Under oil saturated conditions, the Biot flow attenuation acts as the domi-nant mechanism for waves propagating parallel to bedding, while the frame anelastic attenuation and squirt flow attenuation seem to play important roles in condition of propagation vertical to bedding.

关键词： mudstone, shale, attenuation, anisotropy, attenuation mechanism