John J. Zhang

Change of elastic moduli of dry sandstone with effective pressure

Summary The change of elastic moduli of dry sandstone with effective pressure is a result of the subsequent closing of a series of pores. This process is approximated with the KT model and the expression of effective pressure dependence of pore aspect ratio spectra. By inverse modelling, the pore aspect ratio spectra of the water-saturated sandstone samples are obtained from velocity measurements. These pore aspect ratio spectra are then used to calculate the elastic moduli of the corresponding dry sandstone samples. An exponential function is found to best fit the rate of change of the dry elastic moduli with effective pressure.

Evaluating Feasibility of Seismic Fluid Monitoring

Summary Time-lapse seismic monitoring of reservoirs relies on changes in fluid saturation and pressure due to petroleum reservoir exploitation causing observable changes in seismic response. The Gassmann equation can be used to estimate changes in the bulk modulus of the reservoir for given changes in bulk moduli of dry rock and fluids. Gas, oil and water bulk moduli are approximated using published results and commonly available petroleum reservoir data. The lower and upper limits of the fluid mixture bulk modulus are calculated from the saturation-weighted harmonic and arithmetic averages, respectively. The change in the dry bulk and shear moduli due to changes in effective pressure is approximated for sandstones using published data. Estimated changes in the bulk modulus, shear modulus and fluid-saturated density are used to calculate new compressional and shear wave velocities. Percentage changes in velocities and acoustic impedance can be inspected for significance. Finally, synthetic shot gathers are generated which can be compared for changes in seismic attributes such as reflection coefficient, AVO effects or frequency changes.

Reservoir characterization in Leming Lake, Alberta, Canada

The Clearwater formation reservoir in Leming Lake, Alberta, Canada, is identified in well logs by low Gamma ray, low SP potential and high electrical resistivity. The reservoir geometry drawn based on well log picks appears to be regular. The porosity within the reservoir is dominated by 30%-40%, as substantiated by large numbers of core measurements and porosity well log analyses. Strongly cemented limy sands or limestones with zero or near-zero porosity, which are easily distinguished in well logs by spikes of high resistivity, low sonic travel time, low neutron porosity and high density, are sparsely distributed within the reservoir. Their occurrences can be modeled and predicted with universal Kriging method.

Feasibility of seismic monitoring of water drive for clean sandstone reservoirs

The sensitivity of seismic velocities and acoustic impedance to the changes in reservoir conditions is prominent in the case of small dry elastic moduli. Porosity itself has a negative effect on feasibility, but it strongly affects dry elastic moduli. For normally consolidated clean sandstones, reservoirs can be monitored seismically for water drive as deep as 500m. In geopressured zones, 4-D seismic surveys can target 6500 m deep if seismic data quality is fair.