Fluid substitution in a shaley sandstone reservoir at seismic scale

Shaley sandstone is heterogeneous at a seismic scale. Gassmann＇s equation is suited for fluid substitution in a homogeneous medium. To study the difference between shaley sandstone effective elastic moduli calculated by mean porosity as a homogeneous medium, and those calculated directly from the sub-volumes of the volume as a heterogeneous medium, computational experiments are conducted on Han＇s shaley sand model, the soft-sand model, the stiff-sand model, and their combination under the assumption that the shaley sandstone volume is made up of separate homogenous sub-volumes with independent porosity and clay content. Fluid substitutions are conducted by Gassmann＇s equation on rock volume and sub-volumes respectively. The computational data show that at seismic scale, there are minor differences between fluid substitution on rock volume and that on sub-volumes using Gassmann＇s equation. But fluid substitution on sub-volumes can take consideration of the effects of low porosity and low permeability sub-volumes, which can get more reasonable data, especially for low porosity reservoirs.

Characteristics of fluid substitution in porous rocks

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.

Reservoir Fluid Substitution Effect in Heterogeneous Seismic Model

Based on the difference of wave impedance between sand layer and surrounding rock, the seismic wave numerical simulation software, Tesseral-2D is used to establish the sandstone formation model containing water, oil and gas respectively, and the three models are treated by post-stack time offset under the conditions of defined channel spacing, wavelet frequency and wave velocity of different rock mass, and the root means square amplitude difference attribute profile under the condition of water-filled oil-filled and gas-filled oil-filled is obtained. From this, it can be obtained that after oil-gas substitution occurs in weak non-mean reservoirs, the root-mean-square amplitude difference from the reservoir to the lower part of the reservoir experiences a mutation process from a positive maximum to a negative maximum, while after oil-water substitution, the root-mean-square amplitude difference from the reservoir to the lower part of the reservoir experiences a mutation process from zero to a positive maximum. For a strong heterogeneous reservoir. Therefore, for Weak inhomogeneous media similar to tight sandstone, the root-mean-square amplitude difference attribute can be used to detect the distribution of fluid in the actual gas drive or water drive oil recovery process.

Fluid substitution in NMR T_(2)distribution and resistivity independent saturation computation using synthetic capillary pressure data

Petrophysicists and reservoir engineers utilise the capillary pressure and saturation-height function for calculating the water saturation of any reservoir,at a given height above the free water level.The results have a big impact on reserve estimation.The majority of capillary pressure studies are carried out in labs with core data.Cores,on the other hand,are usually altered from their original state before being delivered to laboratories.Moreover,the accuracy of discrete sets of core data in describing entire reservoir parameters,is still up for debate.Prediction of the capillary pressure curve in reservoir condition is an important subject that is challenging to perform.The use of nuclear magnetic resonance(NMR)logs for oil and gas exploration has grown in popularity over the last few decades.However,most of the time the utilization of the data is limited for evaluating porosity-permeability,distributions and computation of irreducible water saturation.After the advent of fluid substitution methods,NMR T_(2)distributions may now be used to synthesize core equivalent capillary pressure curves.Using fluid substitution workflow,our study introduces a better approach for obtaining capillary pressure curves from the NMR T_(2)distribution.The available core data has been used to calculate calibration parameters for better saturation height modelling.The workflow introduces a novel approach in resistivity independent saturation computation.In the exploratory wells of our study area,computed water saturation derived from the saturation height function exhibits encouraging agreement with resistivity based water saturation from multi-mineral model.The NMR based saturation height modelling approach has been included in study area for the first time so far.

On the Fluid Dependence of Seismic Anisotropy:Beyond Biot-Gassmann

This work addresses the question of the fluid dependence of the non-dimensional parameters of seismic anisotropy. It extends the classic theory of the fluid-dependence of elasticity, and applies the approximation of weak seismic anisotropy. The analysis shows that reliance upon the classic theory leads to oversimplified conclusions. Extending the classic theory introduces new parameters（which must be experimentally determined） into the conclusions, making their application in the field context highly problematic.