Dynamic Induced Fractures in Waterflooding and EOR

Abstract

Abstract It is well established within the Industry that water injection mostly takes place under induced fracturing conditions. Particularly in low-mobility reservoirs or when injecting contaminated water (e.g. PWRI), large fractures may be induced during the field life. This paper presents a new modeling strategy that combines fluid-flow and fracture-growth (fully coupled) within the framework of an existing 'standard' reservoir simulator. We demonstrate the coupled simulator by applications to a model five-spot pattern flood model, and to a number of actual field cases (waterfloods, produced water disposal) worldwide. In these field cases, validity checks were carried out comparing our results with available surveillance data. These applications address various aspects that often play an important role in waterfloods, such as short-cut of injector and producer, vertical fracture containment, and reservoir sweep. We also demonstrate that induced fracture dimensions can be very sensitive to typical reservoir engineering parameters, such as fluid mobility, mobility ratio, 3D saturation distribution (in particular, shockfront position), positions of wells (producers, injectors), and geological details (e.g. flow baffles, faults). The results presented in this paper are expected to also apply to (part of) EOR operations (e.g. polymer flooding). 1. Introduction Water injection will generally result in rapid injectivity decline unless it takes place under induced fracturing conditions (e.g. 1,2). Important risks associated with waterflooding under induced fracturing conditions are related to potential unfavorable areal and vertical sweep. These risks can be managed if one has a proper understanding of dynamic induced fracture behaviour as a function of parameters such as injection rate, voidage replacement, reservoir fluid mobility and reservoir / injection fluid mobility ratio3. In order to enable building and using such an understanding as part of field development planning and of reservoir management, we developed an 'add-on' fracture simulator to our existing in-house reservoir simulator4. In the past, several attempts were made to address the coupled problem of reservoir simulation and induced fracture growth. Common approaches can be grouped into fully implicit simulators (Tran et al.5) where both fluid flow equations and geomechanical equations are solved at the same time on the same numerical grid, and coupled simulators (Clifford et al.6) where a standard, finite-volume reservoir simulator is coupled to a boundary-element based fracture propagation simulator. Both approaches are not standard and currently not used in the industry mainly because reservoir models need to be purpose-built, and numerical stability is questionable. Our approach, as briefly described in 4, uses a 'standard' reservoir simulator, thereby enabling reservoir engineers to model induced fracturing around injectors using their 'standard' reservoir models (sector, full-field). Moreover, our specific methodology of coupling induced fractures to the reservoir via special connections 4 helped to eliminate most of the numerical instabilities that are generally encountered in the coupled (reservoir flow)-(fracture growth) problem. The current paper presents an important application of coupled reservoir flow and induced fracture growth. The focus is on demonstrating how dynamic fracture growth around injectors is largely driven by reservoir engineering parameters. It is shown that the degree of induced fracture growth / shrinkage in waterfloods depends strongly on oil-water mobility ratio and can vary strongly with time because of changing reservoir saturation distribution (e.g. shockfront position). For example, induced fracture growth in an injector can be strongly accelerated at the moment of water breakthrough in nearby producers. Once water has broken through, the induced fracture shrinks again. These results imply that an optimized waterflood strategy will generally require variable injection rates over the field life in order to prevent jeopardizing sweep by excessive induced fracture growth.