Abstract

A review of the tidal response in petroleum reservoirs is given. It is caused by periodic changes in overburden stress induced by the ocean tide. The "tidal efficiency factor" is derived by two different approaches and is in line with a recent well test in the Ormen Lange gas field.

For small geomechanical pertubations, like the tidal effect, we show that a simplified coupling of geomechanics and fluid flow is possible. The coupling is easy to implement in a standard reservoir simulator by introducing a porosity varying in phase with the tide. Simulations show very good agreement with the theory.

The observation of the tidal response in petroleum reservoirs is an independent information provider, i.e., it provides information in addition to the (average) pressure and its derivative from a well test. The implementation of the tidal effect in a normal reservoir simulator gives us the possibility to study complex multiphase situations and to evaluate the potential of the tidal response as a reservoir surveillance method. The case studies presented here focus on the possibility to observe water in the near-well area of a gas well.

Introduction

The main objective of this work is to investigate if the tidal pressure response in petroleum reservoirs can be used for reservoir surveillance, in particular to detect saturation changes in the near-well area, e.g., to detect water encroachment towards a gas well. The literature seems sparse in this area. Also, our approach of simplified coupling of geomechanics and fluid flow for small geomechanical effects, and the possibility to implement this in a normal reservoir simulator has to our knowledge not been discussed in the litterature.Several authors have derived a tidal efficiency factor, but a review and comparison study seems to be missing.

Tidal effect

The gravitational pull from the moon and the sun works on both the earth itself, the ocean and the atmosphere. From a point within the earth, e.g., inside a petroleum reservoir, the three effects add up to total tidal dilatation ?e[t] as a sum of the three independent partial effects. These are are the solid earth tide dilatation, ?E, the barometric tidal dilatation ?B and the ocean tide dilatation ?O, and ?e[t] = ?E+ ?B+ ?O, Ref. [1]. The components will have a different magnitude (amplitude), efficiency, frequency and phase.[2]

Tidal response in petroleum reservoirs

During transient well tests, a gauge is placed in the well to continuously record the pressure and the temperature. Modern production wells are often equipped with permanent downhole gauges to monitor well and reservoir behavior.

An important part of a transient well test is the shut-in period when the well is closed in and pressure gradually builds up. If this period is long enough, it is quite common to observe small and periodic pressure variations. These variations occur on a semidiurnal time scale, repeating every half day. In addition, other variations with similar but longer periods, e.g., daily, may also be seen. The sinusoidal variation in reservoir pressure observed in well-test data coincides with the periodic variation in the gravitational pull on the earth by the moon and the sun. In transient well-test analysis the tidal effect appears as unwelcome perturbations troubling the interpretation mainly at the late time periods.

At a reservoir located below the sea floor, the three tidal mechanisms discussed are active at the rock-fluid systems. The ocean tide is, however, by the magnitude of its effects on the reservoir, the dominant source of perturbation.1 The tide gives a certain pressure variation on the sea floor. A much smaller pressure variation is observed in the reservoir. The ratio of the pressure variation at these two locations is known as the tidal efficiency factor. It is further discussed below.

This content is only available via PDF.
You can access this article if you purchase or spend a download.