THE SEISMIC REFLECTION RESPONSE TO CHANGES IN PRESSURE

Abstract

Hydrocarbons are often interpreted using seismic attributes, the most common being high anomalous amplitudes or bright spots. Seismic data processing and interpretation is involved in enhancing such anomalies, thereby improving the interpretation for both optimised drilling and volumetric assessment. However, not all bright spots are hydrocarbons since there have been occasions when an economically drillable bright spot unexpectedly produces a dry hole. This paper discusses the first laboratory-based experiments aimed at understanding seismic reflection effects caused by changes in pressure. Water-saturated unconsolidated sand in an unconfining container was pressured from room pressure to over 2 MPa in step manner and then reduced back to room pressure. After each step in application of overburden pressure change, a zero offset 3D seismic survey was performed using ultrasonic transducers. This approach was repeated after the injection of air, to observe the seismic response to gas under changing pressure.

The velocity of sand increased as a function of pressure until a value of approximately 1 MPa was attained, when fracturing became apparent. Upon pressure reduction below 2MPa, fracture healing became apparent; there were also rapid changes in reflection amplitudes due to changes in pressure. It is under high pressure that preferential alignment of parallel grains at specific distances relative to the seismic wavelength may result in ‘seismic tuning’, and apparent bright spots. It was noted that a pressure increase caused anti-clockwise rotation of the AVO fluid line; this may provide an indication of whether a change in seismic character is a result of a pressure or gas reduction. Some apparent bright spots within sand/shale sequences may also result from seismic tuning effects developed at specific seismic wavelengths, rather than from the presence of hydrocarbons; the sense of rotation of the fluid line may also help to differentiate between a pressure change or gas reduction.