On The Impact Of Depletion On Reservoir Seal Integrity: Geomechanical Model Application
Authors: Schutjens P. , Ita J. , van den Bogert P. , Hermsen F. , Bakker P. , Watts L. , Webers J. , van den Heuvel Erik 
Published: March 2015

Abstract
For a safe and efficient reservoir development, it is important to understand if, how, when and where faulting and fracturing will occur as a function of reservoir production or stimulation operations, or in case of a dumpflood or an internal blow-out situation. This paper describes 1) the making of a numerical geomechanical model to achieve this understanding, 2) the uncertainty in geomechanical model results, and 3) how the model results were applied in operational decisions for production and on reservoir fluid containment. The case study presented here is one of deep-gas production from stacked thin (few meters) sandstone reservoirs vertically separated by shale layers and laterally cut by steeply-dipping sealing normal faults, with pore pressure differences of several MPa across the faults in many sand-shale and sand-sand juxtapositions. We calculated the effective normal stress (sn) and maximum shear stress (tmax) along the faults and in the country rock as a function of pore pressure changes documented in the field development plan. The sn - tmax data were compared with fault slip and fracture-opening criteria based on Mohr-Coulomb frictional slip and tensile fracturing laws using fault cohesion, fault-friction-angle, and tensile strength as input. The geomechanical model results indicate that the current operational criterion of a maximum pore pressure difference of 7 MPa across the faults can be increased to 10 MPa without creating shear failure or tensile fracturing. This would lead to greater operational flexibility, cost reduction (less wells), and accelerated yet safe production.

Introduction
Reservoir rocks deform when their pore pressure is changed due to hydrocarbon production or because of fluid injection. A pore pressure reduction (depletion) increases the average effective stress on the rock, and leads to compaction (densification) and porosity reduction. A pore pressure increase, on the other hand, decreases the average effective stress on the rock, and leads to a bulk-volume increase (dilation) and to a porosity increase. Because the compacting or dilating reservoir remains connected to the rock around it, there will be deformation, displacements and total stress changes in these rocks too (Teufel et al. 1991, Zoback 2007). These total stress changes in the non-reservoir rock do not "stop" at the boundary of the reservoir, but are transmitted to the reservoir as well, interacting with the total stress changes within the reservoir proper. Therefore, pore pressure changes in the reservoir are typically accompanied by changes in the total stress in and around the reservoir. Together, they make up the operation-induced change in effective stress in and around the reservoir, also known as the stress path (Addis et al. 1996, Hettema et al. 2000, Nelson et al. 2006, Sayers and Schutjens 2007, Davison et al. 2013). Particularly strong total stress changes (due to depletion or injection) are expected in fault-bounded reservoir compartments, because of the (often) strong fault-position-controlled lateral variation in operation-induced pore pressure change. Hence, rather than the common "pancake"-model where the overburden acts like a dead weight (see Figure 1a), in pore pressure compartments.