Induced Fracturing in Reservoir Simulations: Application of a New Coupled Simulator to a Waterflooding Field Example

Abstract

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.

Coupled discrete element modeling of fluid injection into dense granular media

“…[3] In engineering practices, subsurface fluid injection has been widely employed for applications such as grouting for ground improvement to reduce the liquefaction potential of cohesionless soils, to raise the ground elevation, or to compensate the volume loss due to ground surface settlement [Mitchell and Katti, 1981;Au et al, 2003;Woodward, 2005;Germanovich and Murdoch, 2010]; construction of permeable reactive barriers for soil remediation [Hocking, 1996]; injection of carbon dioxide for geological storage [Bachu, 2000;Hovorka et al, 2004;Lucier et al, 2006] or for enhanced oil or coalbed methane recovery [Orr Jr. and Taber, 1984;Blunt et al, 1993;White et al, 2005]; subsurface disposal of liquid or slurrified solid waste such as drill cuttings [Moschovidis et al, 1998;Schmidt et al, 1999;Keck, 2002;Clark et al, 2005;Guo et al, 2007;Tsang et al, 2008]; and hydraulic fracturing and waterflooding for hydrocarbon recovery [Ayoub et al, 1992;Morales and Marcinew, 1993;Economides and Nolte, 2000;Hustedt et al, 2008;Khodaverdian et al, 2010]. Although the engineering objectives vary in this list of applications, they share a common operation procedure in that clean fluid and/or slurry is injected into the subsurface via a circular wellbore over a certain interval.…”

Three-dimensional finite element simulation and parametric study for horizontal well hydraulic fracture

“…Varieties of numerical models have been developed for hydraulic fracture simulations (Fischer et al, 1994;Siebrits et al, 2001;Smith et al, 2001;Miskimins and Barree, 2003;Hustedt et al, 2006). Hydraulic fracture height containment for thin-bedded reservoirs could be analyzed by the software GOHFER (Miskimins and Barree, 2003).…”

“…However, assumption and simplifications are made in current models. Only partial fluid-solid coupling is considered, even the fluid flow is completely ignored (Hustedt et al, 2006;Xue et al, 2006). Finite difference method is adopted in some models (Miskimins and Barree, 2003;Adachi et al, 2006).…”

Estimates of Fracture Lengths and Injection Profile in an Injection Well by History Matching Bottom Hole Pressures

“…al. (2001) and Hustedt et al (2008) have presented such coupled reservoir simulators. These coupled reservoir simulators are intended to simulate the entire reservoir, are not designed to capture the short term and long term pressure transients associated with fracture opening and closing and are not easy to use for history matching the performance of single injection wells.…”

“…Estimates of the induced fracture lengths and their impact on the reservoir sweep have been studied in the past (Gadde et. al., 2001, Hustedt et. al., 2008.…”