Introduction to reservoir geophysics
In earth imaging in depth, we reviewed the two-dimensional (2-D) and three-dimensional (3-D), post- and prestack migration strategies for imaging the earth’s interior in depth. In earth modeling in depth, we learned traveltime inversion techniques for estimating a structural model of the earth that is needed to obtain an accurate image in depth. In structural inversion, structural inversion case studies for earth modeling and imaging in depth were presented. By structural inversion, we define the geometry of the reservoir unit, and the overlying and underlying depositional units. Traveltimes, however, are only one of the two components of recorded seismic wavefields; amplitudes are the other component.
In this chapter, we shall turn our attention to inversion of reflection amplitudes to infer petrophysical properties within the depositional unit associated with the reservoir rocks. The petrophysical properties include porosity, permeability, pore pressure, and fluid saturation. Specifically, we shall discuss prestack amplitude inversion to derive the amplitude variation with offset (AVO) attributes (analysis of amplitude variation with offset) and poststack amplitude inversion to estimate an acoustic impedance model of the earth (acoustic impedance estimation). The processes of estimating the acoustic impedance and AVO attributes by way of inversion of amplitudes may be appropriately referred to as stratigraphic inversion. Our goal ultimately is reservoir characterization based on structural and stratigraphic inversion of seismic data with calibration to well data.
We appropriately begin this chapter by investigating the resolution we can achieve from seismic data in defining vertical and lateral variations in the geometry of the reservoir unit. Resolution is the ability to separate two events that are very close together (seismic resolution). There are two aspects of seismic resolution: vertical (or temporal) and lateral (or spatial). Seismic resolution becomes especially important in mapping small structural features, such as subtle sealing faults, and in delineating thin stratigraphic features that may have limited areal extent.
Reservoir characterization involves calibration of the results of analysis of surface seismic data — both from structural and stratigraphic inversion, to well data. One category of well data includes various types of logs recorded in the borehole. Logs that are most relevant to seismic data are sonic, shear, and density. Another category of well data is a vertical seismic profile (VSP) (vertical seismic profiling).
Just as we can seismically characterize a reservoir, we also can seismically monitor its depletion. This is achieved by recording 3-D seismic data over the field that is being developed and produced at appropriate time intervals and detecting changes in the reservoir conditions; specifically, changes in petrophysical properties of the reservoir rocks, such as fluid saturation and pore pressure. Specifically, such changes may be related to changes in the seismic amplitudes from one 3-D survey to the next. Time-lapse 3-D seismic monitoring of reservoirs is referred to as the 4-D seismic method (4-D seismic method). The fourth dimension represents the calendar time over which the reservoir is being monitored.
Some reservoirs can be better identified and monitored by using shear-wave data. For instance, acoustic impedance contrast at the top-reservoir boundary may be too small to detect, whereas shear-wave impedance contrast may be sufficiently large to detect. By recording multicomponent data at the ocean bottom, P-wave and S-wave images can be derived. Commonly, four data components are recorded — the pressure wavefield, and inline, crossline, and vertical components of particle velocity. Thus, the multicomponent seismic data recording and analysis is often referred to as the 4-C seismic method (4-C seismic method).
This chapter ends with a brief discussion on anisotropy. While exploration seismology at large is based on the assumption of an isotropic medium, the earth in reality is anisotropic. This means that elastic properties of the earth vary from one recording direction to another. Seismic anisotropy often is associated with directional variations in velocities. For instance, in a vertically fractured limestone reservoir, velocity in the fracture direction is lower than velocity in the direction perpendicular to the plane of fracturing (azimuthal anisotropy). Another directional variation of velocities involves horizontal layering and fracturing of rocks parallel to the layering. In this case, velocity in the horizontal direction is higher than the vertical direction (transverse isotropy). In seismic anisotropy, we shall review seismic anisotropy in relation to velocity analysis, migration, DMO correction, and AVO analysis.
- Seismic resolution
- Analysis of amplitude variation with offset
- Acoustic impedance estimation
- Vertical seismic profiling
- 4-D seismic method
- 4-C seismic method
- Seismic anisotropy
- Mathematical foundation of elastic wave propagation