Using Multiple Time Lapse 3D Seismic Surveys for Fluid Characterization in Consolidated Sandstone Reservoirs

Abstract

Abstract Analysis of wireline sonic and density measurements of a study well in a h a y consolidated sandstone reservoir shows changes of only 2-8 microseconds/ft increase in transit time and .lg/cc increase in density as water saturation (Sw) changes from 10% to 100% as oil is replaced by brine. The zero offset acoustic impedance between the reservoir and non-reservoir material (encasing shales) shows a 3%-5% increase in impedance contrast over the same changes in water saturation (Sw). Forward seismic modeling of the acoustic logs that change as a function of water saturation from 25% to 100% shows detectable changes in thereservoirs' seismic signature both prestack and poststack. These changes range from 10%-35% increases in amplitude and reflection strength as water saturation increases creating a "dun spot" condition. Seismic reprocessing of a 1982 and a 1991 2D seismic line through the study well confirms themodel predictions of stacked seismic amplitudes of the reservoir. Subsets of a 1982 and a 1995 3D survey covering the study well were normalized to each other using poststack processing techniques. The 3D "region grown"' reflection strength difference map, extracted along the horizon identified by the seismic forward modeling and confirmed bythe 2D seismic test, shows changes in reflection strength that, at least on a gross scale, relates to the production data in the study area. Introduction The use of multiple time lapsed 3D seismic sweys for the analysis and characterization of fluids in reservoirs of aproducing field has emerged as one of our most important technical developments in the oil and gas industry for thls decade. Recent published case histories show that time lapsed 3D seismic surveys can track the movement and change of fluids in porous unconsolidated sandstone reservoirs. This successhas been achieved primarily because of the large effect that fluid types such as gas, oil, and water have on the acoustic impedance of this type of reservoir relative to its encasing material (usually shale). Can time lapse 3D seismic data be used in consolidated sandstone's for fluid movement and characterization wherethe effects of fluid types on the acoustic impedance of the reservoir is small? This paper will show that one can identify and quantlty the seismic response as a function of fluid type and saturation of a consolidated sandstone by using seismic forward modeling techniques with well log data measured at a study well. Coupled with newly developed seismic processing techniques that can enhance and detect even subtle changes in acoustic impedance, the paper will then show how the time lapsed 2D data sets compare to the modeled seismic response of the log data at the study well. Upon confirmation of the time lapsed 2D seismic data to the model seismic data, the paper will then show how the study progressed to a time lapsed volumetric evaluation of the reservoir using subsets of 1982 and 1995 3D surveys over the study well.