Abstract
Offshore reservoirs are subjected to pressure loading from the ocean tide. The resulting pressure fluctuation, notably its amplitude and phase, provides valuable information about the formation compressibility and heterogeneity. The purpose of the present study is twofold: first to propose a method for calculating tidal efficiency from harmonic analysis of regional tide stations and detrended bottomhole pressure, and second to compare the compressibility from tidal analysis to that obtained from rock mechanics measurements and material balance. This case study is on a fractured oilfield for which matrix laboratory measurements alone cannot capture the large scale formation compressibility which is driven by the .fracture distribution.
This paper will show how, in the absence of seabed pressure measurement, a synthetic diurnal tide can be simulated by interpolating the harmonic constituents of neighbouring tide stations. A new procedure combining Savitzky-Golay filter and cubic splines gave satisfactory results to filter out the tidal signal residual from the reservoir transient response. The tidal efficiency and pore pressure compressibility, computed through history for several wells, showed a clear correlation with the fracture tendancy. Also fluid effects, caused by higher water saturation in downdip wells or gas breakthrough, produce a clear signature on the tidal efficiency.
