Water Injection and Water Flooding Under Fracturing Conditions

Abstract

Abstract With 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. Introduction Water injection is a well-established practice for reservoir pressure maintenance and secondary recovery, and for disposal of production water. Nowadays it is recognized industry-wide that water injection almost always leads to rapid well injectivity decline unless it is taking place under fracturing conditions. However, fracturing during injection is often unnoticed. In waterflood applications, the latter can result in sweep patterns being entirely different than perceived by the Operator. Therefore, it is essential to study fractured injection scenarios prior to waterflood development and collect data to limit the uncertainties. In order to properly predict the sweep of waterfloods under fracturing conditions, the rock (fracture) mechanics aspects have to be properly coupled to the reservoir engineering aspects. Due to the complexity of such modeling, validation through cautious field monitoring is required. Particularly, techniques for monitoring the growth of induced fractures are being deployed. Reservoir Simulation of Induced Fracturing With the objective to better manage an increasing number of water injection projects, Shell is currently working on a major effort to couple a recently developed in-house fracture simulator1,2,3 to its widely used in-house reservoir simulator. Improved Modelling of Induced Fracturing The fracture simulator consists of a single well multi-layered model including poro and thermo-elastic backstresses and injection fluid properties in its fracture propagation calculations. The effects of varying rock properties are included as stress modifiers in the present modeling program. Although application of single well models is valuable in a number of simple projects, in most cases the complexity of the field development plan (multiple production and injection wells) and the complexity of the reservoir (e.g. layering of geomechanical facies and pre-existing faults and fractures) need to be accounted for in an integrated model. The developed coupling between this single well fracture simulator and the reservoir simulator can often handle this complexity. The strength of the proposed coupling mainly relies on the two following facts :the reservoir simulator can only estimate a realistic pressure and temperature field if induced fractures are included in the modelusage of a realistic pressure and temperature field (that is translated into a stress field) is essential input to the numerical model used to estimate fracture dimensions. Therefore both softwares benefit from the output of the other one and the overall simulation is improved.