Geomechanical Modeling of Fault Reactivation Associated to Depletion in an Offshore Poorly Consolidated Turbidite Reservoir
Authors: Lima Claudio , Lavorante Luca , Carvalho Leandro , Oliveira Fabio , Santos Marcelo de , Moraes Anderson , Nogueira Melissa , Moriss Mathieu 
Published: June 2014

Abstract
The hydraulic behavior of faults during production is a key issue in reservoir management. The correct assessment of dynamic fault hydraulic behavior can optimize the number of wells, the injection design and the well design used in the production network. In an offshore sandstone turbidite reservoir, water breakthrough was detected and attributed to fault reactivation induced by depletion. Assuming that the fault leakage can be attributed to fault reactivation, we describe a geomechanical modeling that was able to support the hypothesis that fault reactivation should be expected in response to depletion in the studied reservoir. The modeling main inputs are the fault and reservoir geometries as well as the complete geomechanical model, built by integrating data from well logs, seismics, pressure measurements, image logs, leakoffs, minifracs, drilling events, measured stress paths and mechanical properties obtained in laboratory or derived by published and in house empirical equations. The program simulates in real time the reactivation of any number of faults in a 3D visualization environment in response to the estimated stress tensor and fault zone mechanical properties for different time steps of the flow model. Any number of scenarios can be simulated in a batch-processing mode, in order to assess the uncertainty associated to the value of several input parameters (e.g. cohesion, friction angle, magnitude and orientation of the maximum horizontal stress, etc.) and to evaluate different production strategies. A key parameter to the successful replication of fault reactivation in the reservoir was the stress path value (A), i.e, the ratio between the variation of the total horizontal minimum stress with respect to the variation of pore pressure during production (A=deltaShmin/deltaP). Leak-off tests performed within the reservoir provide A=0.8, which is a consistent one for an ensemble made by a cemented fault zone in contact with a pristine poorly consolidated sandstone reservoir. Given that such an ensemble is a widespread one, our results might be useful to address dynamic fault hydraulic behavior in poorly consolidated reservoirs in general.

Introduction
Accessing the hydraulic behavior of faults during production is a key issue in the reservoir management. Faults can produce unexpected compartmentalization and/or faults can be reactivated during injection or depletion, in both cases, dramatically changing the expected distribution of pressures, stresses and water breakthrough in a petroleum field. Conversely, the correct assessment of the dynamic fault hydraulic behavior during production can optimize the number of wells used in the production network, the injection design and the well design as well.
Here we present a case of an offshore deep water sandstone turbidite reservoir, where unexpected water breakthrough was detected and attributed to fault reactivation induced by depletion. The reservoir is made by thick channels, with an average porosity of 25% and permeability around 1000 mD. Several faults intersect the oil/water contact within the reservoir.
Assuming that the leakage of a fault can be attributed to fault reactivation, in this paper we describe an analytical geomechanical modeling that was able to support the hypothesis that fault reactivation should indeed be expected in response to the observed depletion.
First, we will briefly present some key aspects of the methodology. Second, we will describe how the geomechanical model was built and applied to the faults using in-house software solutions. Finally, we will compare and discuss results of fault stability obtained from our analytical geomechanical model under original pressures and depleted pressures.