Monitoring Reservoir Fluids - Alternatives to 4D Seismic

Abstract

Many giant oil fields in the Middle East produce from prolific carbonate reservoirs. Collectively these reservoirs hold over 50% of world oil reserves. The high rigidity of rock matrix, a small contrast between the elastic properties of pore fluids, low gas-oil-ratio oil and mixed salinity water are responsible for the observed weak 4D seismic effect. An alternative reservoir monitoring technique is therefore considered. Permanent seismic sensors installed in a borehole and on the ground surface detect passive microseisms generated by reservoir pore pressure perturbations. Production and injection operations create these perturbations, induced by shear stress release, along zones of weakness in these rocks. Permanent pressure gauges will also be deployed for monitoring reservoir pressure changes. Microseisms emanating from the reservoir would be recorded simultaneously at multicomponent seismic sensors deployed permanently, at various levels in the borehole and over a surface area, surrounding the borehole. The triggered microseisms at the borehole sensors will be synchronized with microseismograms recorded in the surface sensors. The hypocenters are computed from such dense sensor network. Geophones with frequency response over 10-800Hz frequency would be installed. Reservoir heterogeneities affecting the fluid flow are mapped by the distribution of hypocenter locations of these microseisms.