The Use Of Subsidence Data To Monitor Reservoir Behaviour
Authors: Marchina P.J.M. 
Published: December 2007

Abstract
In this paper, the use of surface subsidence data for monitoring reservoir behaviour is discussed. Under certain assumptions, depletion induced surface subsidence data can be inverted to obtain the compaction of the depleting reservoir, including lateral aquifers. The resulting compaction distribution can then be used to derive a pressure depletion pattern covering the zone of interest. This technique is therefore particularly interesting for monitoring the possible depletion of zones where no pressure measurements are available, such as lateral aquifers or undrilled reservoir blocks in the case of compartmentalised reservoirs, and can provide additional data for the modelling of reservoir behaviour.
The details of the inversion technique are presented. The applicability and limitations of the method are highlighted on the basis of synthetic examples as well as real field data.

Introduction
Land subsidence is a natural consequence of the production of underground fluids, water or hydrocarbons, caused by the compaction of the rocks that are being depleted. This phenomenon has received a great deal of attention, since it can have a wide range of negative consequences, such as well failures, damage to surface infrastructures, or impact on the environment. In the context of exploitation of oil and gas fields, it is essential for an operator to be able to predict the amount of reservoir compaction and future subsidence from the production of oil and gas from the reservoir(s) as the consequences of subsidence could alter the economic value of a field development.
The data required in a subsidence study are the geometry of the reservoir (depth and lateral extent), the mechanical characteristics of the overburden, and the amount of reservoir compaction. The geometry of the reservoir can be determined from seismic surveys and well data, and the mechanical properties of the overburden can be easily estimated provided the elasticity assumption is valid. Reservoir compaction depends on the pressure variations induced by hydrocarbon production, the thickness of the reservoir and the rock compressibility. Pressures in the reservoir are obtained from simulations calibrated with measurements in wells and thickness is obtained from petrophysical data. The compressibility of the reservoir rock, which is a crucial parameter in a subsidence study, can be measured in the laboratory on a representative piece of core. However, there seems to be a large discrepancy between the values measured in the laboratory and the value that can be derived from field observation. The direct measurement of reservoir compaction using radio-active markers is another method to determine reservoir rock compressibility, but it remains an expensive exercise.
However, depletion, and therefore compaction, is usually not limited to the hydrocarbon bearing zone only, and aquifers may also undergo compaction which, in some cases, can have a significant impact on surface subsidence. Compaction in an aquifer is usually more difficult to determine than in the reservoir itself as no well data are usually available. Data such as depth and thickness may be extrapolated beyond the boundaries of the reservoir based on seismic data and geological models. Aquifer pressures can be calculated from reservoir simulations, which requires a number of assumptions on the behaviour of these aquifers. Finally the rock compressibility can only be assumed on the basis of geological and petrophysical information. As a result, the contribution of aquifer depletion to the amount and extent of subsidence may be wrongly assessed which can have technical as well as economic consequences on the development project.
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