Recent technological advances in formation testers enable them to deliver reliable and accurate pressure data by combining downhole automation with real-time control. Conventional methods to estimate drawdown mobility do not provide accurate results in very-low permeability (0.01 to 1.0 mD) formations. In recent years, Computational Fluid Dynamics (CFD) has emerged as a powerful tool to study fluid flow through porous media and provide an alternate way for estimating drawdown mobility and formation permeability.
This paper proposes a new method for estimating drawdown mobility using CFD simulation. This new method uses a diffusion equation to track the fluid flow. The drawdown rate is introduced using the boundary flux term.
After reviewing synthetic cases where the analytical solutions existed, CFD simulations were found to produce more accurate results than those from other approaches. The CFD model is then applied to various formations with a wide range of permeability and anisotropy. The results show that the proposed CFD model can simulate all the testing cases with a broad range of permeability values from very low (< 1.0 mD) to very high (> 2500 mD). In addition, the impact of various drawdown schemes and formation properties were studied using the CFD simulations.
The new CFD method has been tested and validated with synthetic and field data sets. Results clearly show that this method can handle formations with a wide range of permeability and permeability anisotropy. Results also show that the proposed method greatly improves the accuracy in drawdown mobility estimation, especially in low-permeability formations.