Steven L. Bryant, A. Daniel Hill
The purpose of this project is to consolidate current core-scale understanding of the behavior of strongly coupled flow/reaction systems and to upscale this understanding so as to be applicable to matrix stimulation design. We will determine how to relate core-scale observations to meter-scale behavior using fine-scale and intermediate-scale simulations. We will also incorporate this understanding into a tool for monitoring oil well stimulation treatments in real time.
Matrix acidization of carbonate formations is an extensively used technique for improving recovery from oil and gas wells, with over $200 million/year spent by the industry in the U. S. alone on these treatments. This high level of activity is driven by the fact that successful treatments can increase productivity by several fold, resulting in large increases in economically recoverable reserves. However, the uncertainties about the complex dissolution patterns created during carbonate acidizing make the outcome of any individual treatment unpredictable. Furthermore, treatment design (the selection of acid strength, acid volume, pumping rate, etc.) is based largely on regional experience and rules of thumbs because of the inability to scale up core-scale experimental results and models to the field scale.
The fundamental obstacle to core-based treatment design is that core-scale experiments and models cannot capture meter-scale heterogeneity and flow conditions. These features influence strongly the stimulation process, however, because of the strong coupling between flow field and extent of reaction. Injection of acid recovers and/or enhances the near well-bore permeability via dissolution of matrix material. In carbonates, the ability to improve permeability depends on the density (the number of wormholes created per unit area of wellbore surface) and growth pattern of wormholes formed during acidization. A wormhole is a highly conductive flow channel resulting from preferential dissolution of material along natural heterogeneities in the matrix. The scaleup of wormhole growth characteristics observed in laboratory coreflood experiments to field-scale design models is the focus of the proposed research.
To achieve this objective we will develop and extend advanced large-scale reactive transport simulators to account for fine-scale matrix heterogeneity and the radially unconstrained flow field of the near-wellbore region. Fine-scale simulations of carbonate dissolution will build on a modeling strategy previously shown to capture the cm-scale features of acid flow through natural fractures, adapting the key elements of the model to the matrix flow system. These results will be upscaled, particularly the influence of local velocity on the dissolution regime and the consequent rate of change of permeability, for incorporation into an existing, proven reactive transport simulator and application to meter-scale matrix acid flows. In turn we will upscale the latter intermediate-scale results into a single parameter, the skin factor, that can be used as a real-time indicator of stimulation progress.