High Resolution Seismic As A Reservoir Analysis Technique

Abstract

Abstract The seismic reflection method is well established in the exploration end of the petroleum industry but little used in reservoir analysis by petroleum engineers. As the major structures have been discovered in many areas, seismic profiling has been turned more and more toward smaller structures and toward exploration for stratigraphic traps which require a far greater precision and fineness of detail to locate. This redirection has precision and fineness of detail to locate. This redirection has resulted in improved seismic equipment design, field recording techniques and seismic data processing procedures. This means that the petroleum engineer, for the first time, may be able to utilize this technique to assist in delineation of the fine subsurface stratigraphy within individual reservoirs. The technique used for high resolution reservoir profiling is a greatly scaled-down version of that employed in most exploration operations. Every 5 to 20 meters (16 1/2 to 65 feet), a detector produces a new seismic "pseudo" log trace which will subsequently produces a new seismic "pseudo" log trace which will subsequently be correlated with borehole logs and other seismic log traces. Structural information derivable from the log trace displays includes location of very small faults, sand channels and shale-outs. Shape variations in the log traces can provide the petroleum engineer with valuable reservoir information regarding localized depositional environments, pore fluid characteristics, and even porosity and permeability variations across a given reservoir bed, porosity and permeability variations across a given reservoir bed, once a local reference has been established. Acoustical amplitude analysis of injected, produced or in situ reservoir gases should be a valuable method of monitoring a variety of enhanced oil recovery techniques. Carbon dioxide or other gas injection projects are ideal for seismic pseudo log monitoring. Combustion products from fire floods or injected steam should also be easily seen with high resolution seismic techniques. High frequency surface to surface seismic profiling offers great potential as a reservoir analysis tool. This potential will be developed by the petroleum engineer working with seismic log displays and field well logs rather than exploration geophysicists using smoothed structural displays. Introduction Porosity, fluid saturation, rock compressibility, permeability and capillary pressure are all terms familiar to the permeability and capillary pressure are all terms familiar to the reservoir engineer but seldom used by the geophysicist. On the other hand root mean square velocity, predominant reflection frequency, normal moveout, wavelet amplitude, deconvolution and other terms in the geophysicists vocabulary are almost never involved in reservoir analysis. Until recently, this language problem created no particular difficulty since the geophysicists were almost exclusively involved in exploration activities where the targets were big structures and the geological "details", involving potential reservoir rock type, matrix porosity and permeability as well as pore fluid identification, were considered inconsequential as they routinely fell beyond the resolving power of the seismic reflection method. This has now changed. As the larger structures have been discovered, exploration activities are being redirected to smaller features and especially to stratigraphic traps which require much greater geophysical precision to locate. Demand for increased fineness of detail is resulting in improved geophysical equipment, field recording techniques and computer processing procedures. These improvements allow one for the first time to look at the individual producing formations of immediate interest to the petroleum engineer. petroleum engineer. The change started a few years ago when seismic "Bright Spots" were heralded as a direct hydrocarbon indicator. The "Bright Spot" is caused by the increased seismic reflection amplitude caused by the relatively lower density and velocity of gas filled reservoirs compared to those filled with water. As many operators discovered somewhat later, coal also has low density and velocity and produced bright spots which were easily mistaken for gas. Also depending on the reservoir material, the "Bright Spot" may in fact be a "Dim Spot" or may have no measurable amplitude effect at all.