It is well-established that stresses within and outside a petroleum reservoir change as a result of pore pressure depletion. This Paper describes possible consequences of these stress alterations, for reservoir compaction, for compaction drive, for permeability, for reservoir monitoring and for reservoir simulation. The analytical model of Rudnicki [6] is used as a platform for discussing the effects of stress changes within limits of linear poroelasticity. Beyond elasticity, discussion is based on discrete element modeling of a depleting reservoir, permitting a dynamic description of failure processes, and on controlled laboratory experiments with synthetic rocks manufactured under stress.
1. INTRODUCTION
During the last decade, there has been an increasing consciousness within the petroleum industry that stress changes associated with reservoir depletion have an essential impact on field performance. Traditionally, pore pressure reduction has been thought to create essentially uniaxial (vertical) compaction. Through their analysis of stress evolution during depletion of the Ekofisk field in the North Sea, Teufel et al. [1] came to the conclusion that the reservoir did not follow the expected stress path. Later, evidence has been presented that several fields do not behave according to the uniaxial strain hypothesis [2, 3].
The stress path obviously affects geomechanical behavior, i.e. reservoir compaction and associated surface subsidence. It does however also have a profound impact on petroleum recovery, directly through compaction drive, and indirectly through permeability alteration. Obviously, the stress changes may also lead to changes in seismic velocities or seismic attenuation, hence affecting time-lapse (4D) seismic response. Furthermore, issues like borehole stability during drilling, casing collapse and particle production are strongly linked to the stress path of the reservoir and the overburden.
In this Paper, the main ambitions are to demonstrate which parameters that have the strongest impact on the stress path, and to elucidate the effects of stress path both on geomechanical, on fluid flow, and on monitoring aspects of a depleting reservoir.
2. RESERVOIR MECHANICS IN THE POROELASTIC LIMIT
2.1. Reservoir stress path
The reservoir stress path may be defined through the parameters [4]
(available in full paper)
where the subscript j denotes the 3 principal earth stresses and pf denotes the reservoir pore pressure.
The traditional assumptions made by the industry have been (i) that the reservoir compacts uniaxially with zero lateral strain, and (ii) that the full weight of the overburden is carried by the reservoir at all times. From linear poroelastic theory, it then follows that (available in full paper)
These assumptions may be relevant if the lateral extent of the reservoir is much larger than its vertical dimension, if there is negligible contrast between the mechanical properties of the reservoir and the surrounding rocks, and if the reservoir is not tilted with respect to the principal stress directions within the Earth (assumed vertical - horizontal). Notice that an underlying assumption here is that the reservoir is a homogeneous entity or an isolated compartment within a heterogeneous field.
These ad hoc assumptions may however be relaxed.
