Coupled Reservoir Flow Simulation: Fault Leakage Analysis

Abstract

Petroleum project costs may increase tremendously if wells need to be shut in or abandoned. A scenario where this possibility exists occurs in a reservoir hanging a major fault that connects the petroleum field to the seafloor. During water injection operations the fluid pressure within the fault may increase to a value that leads to fault slip and hydrocarbon flow through to the seabed. The hydrocarbon leakage can result in severe environmental damage and must be avoided. So, a proper simulation model that enables the evaluation of the maximum injection pressure allowed to impede oil leakage during water injection projects is a necessity. The focus of this work is an offshore unconsolidated sandstone reservoir field located in Campos Basin, Brazil. This field has used waste water for injection since the beginning of production, and there are strong uncertainties about the limits of pressure injection and reservoir fault properties that are related to the project development. To achieve the objective of examine uncertainties of flow through a major fault in offshore oil reservoir undergoing waterflooding; a methodology is developed that enables the incorporation of key mechanisms (e.g. effective stress law) and parameters (e.g. volumetric strain) to solve a complex numerical field reservoir simulation problem that includes geomechanical process. The proposed methodology uses an external-iterative-coupled reservoir-geomechanical modeling approach to capture the link between fluid flow and in situ stress. As a conclusion this study will reveal the effects of dynamic changes of permeability and porosity on reactivation of faults in a real offshore reservoir (Field "A"), and set up a minimum safe water pressure injection level for the field. The methodology developed in this study is valuable for assessing other oil exploitation project scenarios where limited information and production uncertainties are present. This work is important for reservoir characterization, enhanced oil recovery and production applications such as oil rate leakage through a fault. Introduction Samier et al. (2003) noticed during the depletion or cold-water injection phases that the stress state in and around a reservoir can change dramatically. This process might result in rock movements such as compaction, induced fracturing, and enhancement of natural faults, which continuously modify the reservoir properties, such as permeability and fracture transmissibility. Their work highlighted the importance of incorporating geomechanical effects into reservoir flow simulation studies. Current techniques for coupling fluid flow behavior with geomechanical processes can be summarized as the following: classical approach; iterative loose coupled; one-way coupling; iterative coupling with permeability multipliers correlations (Pseudo-coupling), and fully coupled approach.

Analysis on inflowing of the injecting Water in faulted formation

“…In the development of heterogeneous and low permeability reservoir, [4][5][6] the contradiction between interlayer and the inner layer is prominent, and injection-production process will lead to uneven distribution of pressure which will cause the difference of effective stress throughout reservoir; however, the reservoir's permeability can change when the effective stress varies; therefore, the permeability of reservoir is changing in the whole process of injection-production, and there is a strong non-linear relationship between them. [7][8][9] Consequently, when carrying out numerical analysis in the process of injection-production for this kind of reservoir, the diversification of permeability must be taken into consideration. 10 Here, the main consideration is that permeability of reservoir will change with effective stress.…”