Feasibility Study of WAG Injection in Naturally Fractured Reservoirs

摘要

The fundamental aspects of Water Alternating Gas (WAG)injection are still not well understood. There are a fewapplications in fractured media and these show potentialpotential1. Thisstudy looks at the sensitivity of production to reservoir andfluid properties on a pattern scale using tools derived fromexperimental design. Also a look has been taken into the drivemechanisms in the fractured media that play an important roleduring WAG injection.. Earlier studies claimed that WAG injection in fracturedreservoirs is not the best improved oil recovery (IOR)methodmethod2. However when the conditions are fully understoodand the injection is modeled correctly, it is optimal in somefractured reservoirs. The proper modeling of fracturedreservoirs remains difficult.. The standard dual porosityformulation in commercial simulators is based on a continuousmatrix grid overlaid by a continuous fracture grid. The transferof fluid from matrix to fracture is described by a transferfunction. The heart of this transfer function is the shape factor,sigma. As can be seen in a fine-scale grid, the shape factor isdifferent for different reservoir properties and injection types.The upscaling of this sigma from the fine-scale grid to thepattern scale model is important for correct modeling. Thisstudy performs several fine scale single porosity simulations toproperly upscale the flow model to a dual system.The process of determining sensitivities in an organizedmanner on properly upscaled models will be shown here witha limited number of parameters. Fractured reservoirs can bedivided into different categoriescategories3. Based on a fine grid modelof one type of fractured media, a dual porosity model withvarying parameters is set up. This yields recovery outputsacross a range of reservoir properties that are represented bytwo dimensionless numbers. These numbers represent thecapillary over viscous and gravity over viscous forces. Bylooking into the mechanisms that are behind the recovery in.different balances of forces, an insight into when and whyWAG is working is given.