Introduction Some of the North Sea's largest and most important oilfields are in chalk reservoirs. In these fields it is important to measure the compaction and compressibility of the reservoir because the compaction can result in platform subsidence. Also a major drive mechanism in these fields is compaction drive and so an accurate estimation of reserves cannot be made without first measuring compressibility. Estimating compaction and reserves is difficult because compressibility changes during the field life. The installation of accurate permanent downhole pressure gauges on offshore chalk fields makes it possible to use a new method to monitor compressibility: the measurement of reservoir pressure changes caused by the tide. The tidal monitoring technique is an in-situ method which can greatly increase information about compressibility. It can be used both to estimate compressibility and to measure the variation in compressibility with time. This paper concentrates on the application of the tidal monitoring technique to chalk reservoirs in the North Sea. However, the method is applicable in any tidal offshore area and can be applied whenever it is necessary to monitor in-situ rock compressibility. One such application would be if platform subsidence was expected. platform subsidence was expected. Tidal Effect Tidal pressure variations have been observed on a large number of North Sea oilfields. Several North Sea oilfields have such high permeability that production tests achieve only a small drawdown. When the build-up analysis is made on such tests it is often necessary to first remove the formation pressure variations made by the tidal effect before proceeding pressure variations made by the tidal effect before proceeding with the test interpretation. The great majority of the tests however are not affected significantly by the tidal effect. This is because the pressure variations caused by the tide are small in comparison to the pressure changes measured in a test. Typically the tidal formation pressure variations for North Sea sandstone reservoirs are in the range 0.1 - 0.5 psi [.7 - 3.5 kPa]. On the Piper field, for example, the tidal reservoir pressure variation was 0.2 psi peak-to-peak [1.4kPa]. pressure variation was 0.2 psi peak-to-peak [1.4kPa]. All reservoirs are affected to some extent by the tidal effect. The moon and sun exert a tidal pull not just on the sea but also on the air and the earth. The air tide can he measured as barometric changes and so is often referred to as the 'barometric" tide. The earth tide results in only a tiny deformation of the earth's surface; the maximum deformation of the earth's surface is only 36 cm [14 in]. Nevertheless the attempt by the solid earth's crust to deform under gravitational changes results in measurable changes in reservoir formation pressures. There are many published examples of observations pressures. There are many published examples of observations made in wells of the air tide and the earth tide. Most of these observations have been made in water wells or geothermal wells mostly in the continental United States. As a result the sea tide is usually absent from these observations. There are very few examples published of the tidal effect in offshore oilfields. In offshore oilfields it is the effect of the sea tide on reservoir pressure that is the easiest tide to measure. For this reason the rest of this paper concentrates on the effect of the sea tide on reservoir formation pressures.
During a three year waterflood pilot program at the Valhall field, injection at a pressure higher than the Formation Parting Pressure (FFP) was employed to improve injectivity. Due t o concerns about premature water breakthrough and reduced sweep efficiency, an engineering study was performed simulating the dynamic growth of the induced fracture. The work provided a better understanding of the dominant physical processes occurring during the Valhall waterflood pilot and gave a tool for predicting future performance.
During a three year waterflood pilot program at the Valhall field, injection at a pressure higher than the Formation Parting Pressure (FFP) was employed to improve injectivity. Due t o concerns about premature water breakthrough and reduced sweep efficiency, an engineering study was performed simulating the dynamic growth of the induced fracture. The work provided a better understanding of the dominant physical processes occurring during the Valhall waterflood pilot and gave a tool for predicting future performance.
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