Developing New Advances in 3D Seismic Interpretation Methods for Fractured Tight Gas Reservoirs
Reeves, James J.
GeoSpectrum, Inc., Midland, Texas
Natural fractures are predicted using seismic lineament mapping in the reservoir section. A seismic lineament is defined as a linear feature seen in a time or horizon slice through the seismic volume that has a negligible vertical offset. Seismic attributes investigated may include coherency, amplitude, frequency, phase, and acoustic impedance. Volume based structural curvature attributes may also be computed. It is interpreted that areas having high seismic lineament density with multi-directional lineaments define areas of high fracture density in the reservoir.
Lead areas are screened by seismic attributes, such as seismic amplitude or acoustic impedance, indicating brittle reservoir rock that are more likely to be highly fractured. Seismic attributes are calibrated to clay content measured in existing well control by wireline logs.
Gas sensitive seismic attributes such as the phase gradient (an AVO attribute developed by GeoSpectrum) or frequency dependent seismic amplitude may be used to define a prospective fairway to further screen drill locations having high gas saturation. These attributes may be calibrated to gas saturation determined from existing well control by wireline logs. Reservoir fractures enhance reservoir permeability and volume; they may also penetrate water-saturated zones and be responsible for the reservoir being water wet and ruined.
In a gas field previously plagued with poor drilling results, four new wells were spotted using the methodology and recently drilled. The wells have estimated best-of-12-months production indicators of 2106, 1652, 941, and 227 MCFGPD. A prospect rating system is developed indicating either a “good”, “average”, or “poor” grade.