A low frequency, passive seismic experiment over a carbonate reservoir in Abu Dhabi

Abstract

Alow frequency passive seismic experiment was conducted over an onshore carbonate oilfield in Abu Dhabi in an effort to confirm and understand the origins of a low frequency signal that has been observed above several hydrocarbon reservoirs in the area. While the analyses of the data are still on-going the preliminary results of the experiment have confirmed the existence of a narrow band of low frequency (2.5-2.8 Hz) signals over the oil reservoir zone; however, this analysis has found that these signals are also detected over the water saturated zone. More studies will need to be conducted to fully investigate the particle motion of these waves, their apparent velocities, and the azimuth of their wave fronts. Background For the past several years a narrow band of low frequency (1.5 – 4 Hz) signals have been observed and reported over a number of hydrocarbon reservoirs mainly in the Middle East including some oilfields in Abu Dhabi (Dangel et al., 2003; Holzner et al., 2005). The observations suggest that the low frequency signature diminishes towards the rim of the reservoir and is absent above non-reservoir locations. It has been suggested that these signals are caused by non-linear behaviour of the interaction between liquid hydrocarbons, water, and the pore-rock materials in the reservoirs which distort the normal signature of the Earth’s natural ambient vibration spectra. The analyses of such low frequency data have previously been used as a direct hydrocarbon indicator for the optimization of well placement during exploration, appraisal, and production. However, the possible causes of this low frequency energy are not well understood and it has yet to be demonstrated what types of waves are being observed as well as the physical behaviour of the multiphase fluid system in the reservoir. Furthermore, to date most of these observations have only been based on the vertical component of the signal motion. As a result, this experiment aims to better understand the wave systems causing these observations by systematically mapping the temporal and spatial variations of the low frequency waves recorded. The results of this passive seismic experiment will help to determine the source of such low frequency energy and its possible application in the detection and monitoring of hydrocarbons in carbonate reservoirs. The processing and modelling of the data from the experiment are still on-going; hence, the aim of this short paper is to present the preliminary results of the experiment.