Filtering Of Tide Effects In Formation Evaluation Data

In this work, a method is proposed for filtering the data recorded in formation tests and subtract the noisy parcel due to the tidal effects. The tidal effects originate on the variation of the gravitational potential applied to the reservoir as the moon and the sun change their position in space. This phenomenon, named tidal effect, can be observed when measuring the pore pressure of any accumulations of fluids in the pores of rocks in the underground, including petroleum reservoirs. Tidal effects were observed in petroleum reservoir pressure tests by the mid-seventies and a correlation between the amplitude of the tidal effect and the reservoir parameters were noted. From Biot poroelastic theory, methods were developed, using the tidal effect, to evaluate reservoir parameters such as compressibility, porosity, permeability. The selective extraction of the tidal effect is an important part of these methods. Fourier transform appears as great resource for this purpose, since the tidal effect is a sinusoidal signal with well-known component periods. The selective extraction of signals using the Fourier transform can also be important to assist in the determination of reservoir parameters, since the presence of the tidal effects may prevent or hinder the interpretation of the formation test from the diagnostic log-log plot. This paper shows the Fourier transform application to extract noise and tidal effect observed in formation evaluation data. The Fourier transform techniques were successfully used in three set of data recorded in formation tests performed in offshore wells. Copyright 2012, Society of Petroleum Engineers.

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