Summary Water-injection-induced fractures are key factors influencing successful waterflooding projects. Controlling dynamic fracture growth can lead to largely improved water-management strategies and, potentially, to increased oil recovery and reduced operational costs (well-count and water-treatment-facilities reduction), thereby enhancing the project economics. The primary tool that reservoir engineers require to guarantee an optimal waterflood field implementation is an appropriate modeling tool, which is capable of handling the dynamic fracturing process in complex reservoir grids. We have developed a new modeling strategy that combines fluid flow and fracture growth in one reservoir simulation. Dynamic fractures are free to propagate in length and height-direction with respect to poro- and thermoelastic stresses acting on the fracture. A prototype simulator for contained fractures was tested successfully. We have extended the coupled simulator to incorporate noncontained fractures. The new simulator, called FRAC-IT, handles fracture-length and -height growth by evaluating a fracture-propagation criterion on the basis of a Barenblatt (1962) condition. The solution of the 5D problem is computed by use of a tuned Broyden (1965) approach. We demonstrate the capabilities of the coupled simulator by showing its application to a complex reservoir-simulation model. The fracture modeling is used to history match an injectivity test in a five-spot injection pattern using produced water. The coupled-simulation results and the field-data interpretation show a very good match. The outcome of the injection test led to an appropriate waterflood-management strategy adapted to the specific reservoir conditions and, in terms of production, to a net oil-production increase of 50 to 100%. The field example shows how the coupled-simulator technology can be used to achieve optimized waterflood-management strategies and increased oil recovery. Introduction Waterflooding is often applied to increase the recovery of oil in mature reservoirs or to maintain the reservoir pressure above bubblepoint in the case of green fields. Even though often unnoticed, water injection frequently is taking place under induced-fracturing conditions. The rock fracturing has a strong influence on the water injectivity and the areal distribution of the fluids in the reservoir. A qualitative example of the impact of the fracture orientation on the areal sweep is demonstrated in Fig. 1. We show streamlines in two different water-injection-pattern configurations for two fracture orientations (i.e., line-drive and five-spot geometry, and fracture oriented toward the producer and away from the producer. The density of the streamlines indicates that the fracture orientation changes the areal sweep. In order to achieve optimized water-injection management, dynamic fracture propagation needs to be estimated properly before the injection, controlled during operations, and monitored to ensure predictions and reality do not deviate significantly. The tools commonly used to study fracture growth numerically are analytical fracture simulators, which often are based on a single-well model in a simplified reservoir formation. Generally, reservoir heterogeneity is reduced to a number of horizontal layers with homogeneous properties and a laterally infinite extent. Fracture propagation is described using a pseudo-3D description (van den Hoek et al. 1999). For many field developments under waterflooding, fracture propagation is estimated with acceptable error bars using these or similar tools. The major drawbacks areAreal reservoir heterogeneity is not accounted for.Varying poro- and thermoelastic stresses along the fracture are neglected.Injection pressures have large error bars because the reservoir response is not properly captured.Nearby well's influences (e.g., pattern flood) are not captured. In the past, many attempts have been made to address these issues. Common approaches can be grouped into fully implicit simulators (Tran et al. 2002), where both fluid-flow and geomechanical equations are solved simultaneously on the same numerical grid, and coupled simulators (Clifford et al. 1991), where a standard, finite-volume reservoir simulator is coupled to a boundary-element-based fracture-propagation simulator. To our knowledge, both approaches are not standard and currently not used in the industry becauseModels need to be purpose built (i.e., reservoir models from standard reservoir simulator cannot be used).Fracture propagation is oversimplified.Numerical stability is questionable. We have developed an extension to an existing reservoir simulator to circumvent these shortcomings. We use a coupled-simulator approach based on a two-way communication strategy between the fully numerical reservoir simulator and the half-analytical geomechnical-modeling part. The new simulator enables the modeling of fluid flow and dynamic fracture propagation in a combined way. We have applied the tool to field applications for waterflooding projects in which injector/producer shortcuts are a potential risk (pattern floods) and also to environments in which fracture containment and estimating accurate injection pressures are the main concerns. In this paper, we briefly review the coupled-simulator approach and discuss the application to a waterflooding field example.

Water-injection induced fractures are key factors influencing successful waterflooding projects. Controlling dynamic fracture growth can lead to largely improved water management strategies and potentially to increased oil recovery and reduced operational costs (well count and water treatment facilities reduction), thereby enhancing the project economics. The primary tool that reservoir engineers require to guarantee an optimal waterflood field implementation is an appropriate modeling tool that is capable of handling the dynamic fracturing process in complex reservoir grids. We have developed a new modeling strategy that combines fluid-flow and fracture-growth in one reservoir simulation. Dynamic fractures are free to propagate in length- and height-direction with respect to poro- and thermo-elastic stresses acting on the fracture. A prototype simulator for contained fractures was successfully tested. We have extended the coupled simulator to incorporate non-contained fractures. The new simulator handles fracture length and height growth by evaluating a fracture propagation criterion that is based on a Barenblatt condition. The solution of the five-dimensional problem is computed using a tuned Broyden approach. We demonstrate the capabilities of the coupled simulator by showing its application to a complex reservoir simulation model. The fracture modeling is used to history match an injectivity test in a five-spot injection pattern using produced water. The coupled simulation results and the field data interpretation show a very good match. The outcome of the injection test led to an appropriate waterflood management strategy adapted to the specific reservoir conditions and, in terms of production, in a net oil production increase of 50–100%. The field example shows how the coupled simulator technology can be used to achieve optimized waterflood management strategies and increased oil recovery. Introduction Waterflooding is often applied to increase the recovery of oil in mature reservoirs or to maintain the reservoir pressure above bubbelpoint in the case of green fields. Even though often unnoticed, water injection is frequently taking place under induced fracturing conditions. The rock fracturing has a strong influence on the water injectivity and the areal distribution of the fluids in the reservoir. A qualitative example of the impact of the fracture orientation on the areal sweep is demonstrated in Figure 1. We show streamlines in two different water-injection pattern configurations for two fracture orientations i.e., line-drive and five-spot geometry, and fracture orientated towards the producer and away from the producer. The density of the streamlines indicates that the fracture orientation changes the areal sweep. In order to achieve optimized water injection management, dynamic fracture propagation needs to be properly estimated prior to the injection, controlled during operations, and monitored to ensure predictions and reality do not deviate significantly. The common tool used to study fracture growth numerically are analytical fracture simulators that often are based on a single well model in a simplified reservoir formation. Generally, reservoir heterogeneity is reduced to a number of horizontal layers with homogeneous properties and laterally infinite extent. Fracture propagation is described using a pseudo-three dimensional description (van den Hoek et al.1). For many field developments under waterflooding fracture propagation is estimated with acceptable error bars using these or similar tools. The major drawbacks are that:Areal reservoir heterogeneity is not accounted forVarying poro- and thermo-elastic stresses along the fracture are neglectedInjection pressures have large error bars since the reservoir response is not properly capturedNearby wells influences in e.g. patternfloods are not captured.

Summary It is well established within the industry that water injection mostly takes place under induced fracturing conditions. Particularly in low-mobility reservoirs, 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 repeated five-spot pattern flood models, addressing various aspects that often play an important role in waterfloods: shortcut of injector and producer, fracture containment to the reservoir layer, and areal and vertical reservoir sweep. We also demonstrate how induced fracture dimensions (length, height) can be very sensitive to typical reservoir engineering parameters, such as fluid mobility, mobility ratio, 3D saturation distribution (in particular, shockfront position), 3D temperature distribution, positions of wells (producers, injectors), and geological details (e.g., layering and faulting). In particular, it is shown that lower overall (time-dependent) reservoir transmissibility will result in larger induced fractures. Finally, it is demonstrated how induced fractures can be taken into account to determine an optimum life-cycle injection rate strategy. The results presented in this paper are expected to also apply to (part of) enhanced-oil-recovery operations (e.g., polymer flooding).

It is well established within the Industry that water injection mostly takes place under induced fracturing conditions. Particularly in low-mobility reservoirs, 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 five-spot pattern flood models, addressing various aspects that often play an important role in waterfloods: shortcut of injector and producer, fracture containment, 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). The results presented in this paper are expected to also apply to (part of) EOR operations (e.g. polymer flooding).

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWith the realization that water injection is generally taking place under fracturing conditions, tools capable of better modelling fractured injection and its impact are being developed. Models integrating rock (fracture) mechanics and traditional reservoir simulation are now applied to water injection projects with a number of applications in the Middle East. Fracture dimensions are a key input to those models. Monitoring techniques to track the evolution of induced fractures with time are also being deployed. Amongst those techniques microseismic and specific fall-off test procedures are used.