The 4D seismic response observed at Valhall is closely related to production-induced rock deformations within and outside the reservoir. The seismic signal is very sensitive to the changes in shear stress associated with the deformation. The associated seismic travel-time response is almost an order of magnitude higher than the travel-time response caused by the deformation effect.
Implication of Reservoir Compaction
The net impact of reservoir compaction is improved recovery. In most cases the value of the added production is much greater than the possible costly challenges associated with compaction. The impact of subsidence is well known from both the Ekofisk and the Valhall fields, where increasing water depth has called for modification and replacement of existing platforms.
Subsidence often results in reduced well life due to casing deformation, and local stress changes can introduce wellbore instability during drilling. Consequently, understanding the impact of the compaction on the surrounding rocks all the way to the surface becomes an important task for safe and optimal resource development.
The Compaction Process
Some compaction occurs in all reservoirs where the pore pressure is reduced. Compressibility, thickness and the amount of pressure reduction are the primary factors controlling the degree of compaction. The larger these parameters are, the more compaction can be expected, and this relationship sometimes becomes complex and non-linear. When the reservoir compaction is significant the rocks are normally subject to plastic deformation. The onset of this behaviour is called yield, and if the rocks experience yield, the compressibility of the rock can increase by a factor of 10. The microscopic process in the rock is related to grain structure breakdown, often referred to as pore collapse. If the pressure reduction is arrested by, for instance, water injection, we could stop compaction. However, some rocks, like the North Sea chalks, are sensitive to seawater, and the pores collapse as a result of exposure to sea-water, as experienced at Ekofisk in the 90’s.
The relative movement of the pressure and water fronts is controlled by the microscopic displacement of oil by water. Normally the pressure front travels faster than that of water. The seawater weakening of chalk is also dependent on the reservoir temperature, so Ekofisk at 130 degrees Celsius will be weakened to a greater extent than Valhall at 93 degrees Celsius. Loading and unloading are the terms used to describe impact on the decrease and increase in reservoir pressure and stress state changes on the individual grains.
A fit for purpose rock constitutive model is needed to capture the implications of these complex processes. The constitutive model describes the deformation under all the load conditions and possible stress paths that the rock could be exposed to. Ability to handle pore collapse and load rate dependency are key features of these models. For chalk the water weakening and temperature effects also need to be incorporated. In some reservoirs the temperature change itself can cause significant deformations as seen when steam flooding is used. The rock mass around and away from the reservoir deforms less and a linear elastic transversal isotropic model is often sufficient for these situations, as found to be the case in subsidence prediction above mines.
First order approximations to the impact of a compacting reservoir on its surroundings may be found using simplistic geometrical models and analytical methods (see box on Geertsma). For complex reservoirs and structures like Valhall we need numerical models to predict rock deformations (see box on Coupled Modelling).
Using a calibrated rock physics model the stress changes and deformations can be translated into velocity changes in various layers in and around the reservoir, and forward seismic modelling is used to predict the effect on the seismic waves. Travel-time changes and polarization of the waves are two examples of such attributes.
Linking 4D seismic observations and rock mechanics
The 4D seismic data are employed to validate the rock mechanical modelling. At the same time insights from rock mechanical effects are used to define work-flow for applying 4D seismic in building and matching the reservoir models, to support well planning and interventions. This functionality is available as an integrated modelling tool (mech2seis) which has been developed by BP and Sintef.
The frequent 4D seismic data from the Valhall LoFS array have increased the level of detail that can be observed in the changes taking place 2500m below seabed. Time-lapse time shifts as small as 0.25 ms are important, since such changes correspond to a reservoir compaction of 5 cm.
A fundamental mechanism in compacting reservoirs is stress arching, which is a load re-distribution process where some of the load above the compacting area in the reservoir is reduced and re-distributed to less condensed materials to the side of the compacting area. The shale in the cap-rock above the compacting body, in the arch, is unloaded and the velocity is reduced. At the side of the compacting area the load is increasing and the velocity increases.
At the seafloor an irregular reservoir compaction, caused by faulting for instance, has been smoothed out by 2.5 km of overburden and results in a smooth subsidence bowl. As a result, large lateral movements associated with the subsidence are predicted at the seafloor.
The appreciation of the sensitivity of seismic methods to deformations in the subsurface is still in its infancy. Monitoring of integrity around CO2 sequestrations is an application that has increased the interest in this methodology significantly. We expect that in the future the analysis will be based on more rigorous analysis of the data.
The technology for making use of seismic passive recordings to qualify wellbore stimulations is progressing rapidly. Work still remains in verifying the results; however, the observations fit well with expectations from a geomechanical understanding of stress changes around a producing reservoir.
A better understanding of field processes can be achieved by integrating geomechanics and 4D seismic. The field examples from Valhall demonstrate applications that reduce development cost and increase recovery.