Abstract

Earth, barometric and ocean tide effects hove been observed for many years, principally in hydrogeolgical work. During testing of wells in the Timor sea we have observed ocean tide effects in the reservoir with an amplitude of - 0.07 bar (1 psi) about 70 times larger than earth or barometric tide effects.

This paper has two aims: to provide an overview of analytical interpretation of tidal affects from a petroleum engineering viewpoint and second, based petroleum engineering viewpoint and second, based upon our simulations and observations, to propose that in addition to previous methods of interpretation the phase shift of ocean tide effects can be used to gain an insight into the presence of major reservoir heterogeneities, particularly fluid contacts.

There is a need for more field data and further investigations, however we believe we have demonstrated the existence of a useful and previously unused aspect of pressure measurement in offshore wells.

Introduction

The effects of a periodic tidal stress - having their origin in the gravitational attraction between the sun, moon and the earth - on the fluid accumulations in porous strata in the earth have been observed for more than one hundred -years-). The majority of the observations were made in mines and open water wells in which even the smallest periodic fluctuation of water level was easily detectable and recordable. It was only in about the last decade, after the advent of high sensitivity pressure gauges (crystal and strain gauges). that a similar observation could be made in petroleum reservoirs. In these circumstances it is understandable that the main theoretical analysis of this phenomenon and all attempts at its practical interpretation and application ere accomplished in the field of hydrogeology. Yet we can expect that in future the tidal effect will be observed and recorded more frequently during the testing of petroleum wells and that it will eventually be accepted as a new piece of information which will be worthwhile interpreting.

The tidal effect on the fluid pressure in buried porous strata is a complex one. The cyclic porous strata is a complex one. The cyclic fluctuation of pressure as it is measured in aquifers and reservoirs is generally a consequence of a tidal dilatation of the porous system (AA). However there are three different mechanisms by way of which the prime cyclic fluctuation of the strength of gravity prime cyclic fluctuation of the strength of gravity field produces this final effect. The total dilatation (AA) can be considered as a sum of three independent partial effects:

  • the solid earth tide dilatation

  • the barometric tidal dilatation

  • the ocean tide dilatation

so that ........................... (1)

All the components naturally arise from the same lunar and solar tide generating forces however each of them will have a different magnitude (amplitude), efficiency, frequency and phase 5. The complexity of this phenomenon is further aggravated by the complex nature of the tide generating potential itself which consists of several tidal components with varying frequency and amplitude. 1. .)

Consequently the final product of these forces is a complex periodic pressure fluctuation with many harmonic constituents. For each particular tidal constituent of angular frequency (w) the variation of the induced dilatation (AA) with time (t) can be described by the equation

(2)

in which 0 A' 0 El (D B and (Do are phase differences between the tide generating potential under consideration and the aquifer or reservoir dilatation the earth tide dilatation A El the barometric dilatation 1/2. the ocean tide dilatation A. The main results of past studies are summarised in Sections 2 and 3 of this paper. The majority of these studies had several assumptions in common:

  • they were based on data from onshore wells and therefore concentrated on earth and barometric tide effects;

  • they assumed a closed, homogeneous reservoir with no induced flow;

  • the earth, barometric and ocean tide dilatations were assumed to occur in phase with their respective tidal generating forces.

There is, in principle, no obstacle to the application of these results in petroleum reservoir engineering once sufficiently accurate data is available. However, the petroleum engineer now has available a new piece of information for offshore wells: observation and accurate measurement of the ocean tide effect, an effect which we can expect will be measurable in many offshore reservoirs.

The importance of the ocean tide effect is that:

  • it has a much greater amplitude (we have measured a value of 0.07 bar, I poi) than the earth or barometric tide effects (a typical value being 0.001 bar, 0.015 psi). Thus, when interpreting ocean tide effects, all others can be ignored.

  • As can be seen in Section two of this paper, interpretation of ocean (and barometric) tide effects is based upon a cyclic change to the head of water (or air) which alters the overburden and thus the reservoir pressure. Accurate measurements of surface ocean tide are simple to make and can be compared in phase to (simultaneous) measurements of the reservoir pressure.

Thus. not only can we gain a measure of formation compressibility and porosity (as was possible previously) but we may also be able to investigate the previously) but we may also be able to investigate the variation of compressibility away from the wellbore through interpretation of the shift of phase between surface and bottomhole measurements.

The main objective of this paper is to present our current understanding of these effects and the results arising from them. This is done in Sections 4 and 5 with the aid of mathematical modelling and the results from observed ocean tide effects in the Timor Sea.

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