Gas saturation prediction and effect of low frequencies on acoustic impedance images at Foinaven Field

• Published in: Geophysical Prospecting Vol. 54; no. 1; pp. 75 - 87
• Main Authors: Wagner, Sean R., Pennington, Wayne D., MacBeth, Colin
• Format: Journal Article
• Language: English
• Published: PO Box 1354, 9600 Garsington Road , Oxford OX4 2XG , UK Blackwell Science Ltd 01.01.2006; Blackwell
• Subjects: Applied geophysics; Earth sciences; Earth, ocean, space; Exact sciences and technology; Internal geophysics; projects; mixing; surveys; reservoirs; low frequency; frequency; saturation; acoustical impedance; solution; management; hydrophones; interpretation; prediction; high resolution
• ISSN: 0016-8025; 1365-2478
• DOI: 10.1111/j.1365-2478.2006.00517.x

ABSTRACT Low frequencies are necessary in seismic data for proper acoustic impedance imaging and for petrophysical interpretation. Without lower frequencies, images can be distorted leading to incorrect reservoir interpretation and petrophysical predictions. As part of the Foinaven Active Reservoir Management (FARM) project, a Towed Streamer survey and an Ocean Bottom Hydrophone (OBH) survey were shot in both 1995 and 1998. The OBH surveys contain lower frequencies than the streamer surveys, providing a unique opportunity to study the effects that low frequencies have on both the acoustic impedance image along with petrophysical time‐lapse predictions. Artefacts that could easily have been interpreted as high‐resolution features in the streamer data impedance volumes can be distinguished by comparison with the impedance volumes created from the OBH surveys containing lower frequencies. In order to obtain results from the impedance volumes, impedance must be related to saturation. The mixing of exsolved gas, oil and water phases involves using the Reuss (uniform) or Voigt (patchy approximation) mixing laws. The Voigt average is easily misused by assuming that the end‐points correspond to 0% and 100% gas saturation. This implies that the patches are either 0% gas saturation or 100% gas saturation, which is never the case. Here, the distribution of gas as it comes out of solution is assumed to be uniform until the gas saturation reaches a sufficiently high value (critical gas saturation) to allow gas to flow. Therefore, at low gas saturations the distribution is uniform, but at saturations above critical, it is patchy, with patches that range from critical gas saturation to the highest gas saturation possible (1 minus residual oil and irreducible water saturation).