This paper presents the study of the coupled hydro-mechanical behavior of a faulted HP-HT hydrocarbon reservoir during depletion. Because of the large expected depletion, a number of phenomena may be enhanced, all related to the changes during production, of the in-situ stresses, which control the mechanical response of the reservoir. This includes essentially two types of deformation, the first one being the reactivation of in the reservoir existing faults, while the second one is the reservoir compaction. The objective of this paper is principally to present the study of the first phenomenon, and more particularly their hydro-mechanical aspects. The study has been carried out with the help of numerical simulations of the fully coupled hydro-mechanical behavior of 2D reservoir models, using the finite element code LAGAMINE.
The field is a HT/HP field. Sandstones constitute the main reservoir layers that is underlain by sandstones of much lower reservoir quality. The top reservoir is 5500 m deep, reservoir pressure and temperature are close to 110 MPa and 190°C respectively. The cap rock is formed by a thick shales layer. Series of NNE-SSW striking faults divide the reservoir up in four major blocks which appear to be not all at the same pressure.
Because of the expected high level of depletion a number of phenomena may be enhanced, all related to the changes during production, of the in-situ stresses, which control the mechanical response of the reservoir. This includes essentially two types of deformation. The first one is the reservoir compaction which leads to surface subsidence and may affect reservoir permeability-although gas reservoirs are generally less affected than oil reservoirs- as it has already been studied. The second one is the reactivation of existing faults (and fracturing of the formations) within or outside the reservoir. This phenomenon may induce changes in fault hydraulic transmissivity (including loss of sealing capacity), triggered seismicity, loss of cap rock integrity, well failure and casing collapse.
The simulation of the mechanical behavior of the reservoir is done using a Finite Element Model. The reliability of the predictive output will largely depend on the quality of the input data. From all the seismic data, early borehole drilling and logging data, a relatively reliable geological model has been constructed. Mechanical parameters were provided by means of laboratory testing on core material or were estimated from internal or published databases. Generally, for similar studies, the current in-situ state of stress is estimated from hypotheses on the local tectonic state; but for this specific and complex HP/HT field, these simple assumptions may be far from realistic.
In order to circumvent this problem it has been decided to consider a preliminary phase which simulates the last major tectonic event that has led to the current structuration, state of stress and pressure conditions. The subsequent modeling of the reservoir (the second phase of the study) will use as input data the results of this first simulation.
Although computationally more intensive than the classical approach, this strategy has the advantage that the results of this preliminary exercise provide realistic information on the current reservoir conditions and particularly the in-situ state of stress. Besides, the initial geological conditions are simplified and hypotheses on the mechanical behavior of the various lithologies can be finely tuned in order to reproduce closely as possible the current situation.