Optimization of Design Using Computational Fluid Dynamics Technique for New P&A Down Hole Milling Technology - Case Studies of Rig Run Comparisons Validating Design Changes

Abstract

Hydrocarbon migration after plug and abandonment (P&A) operations negates well integrity and threatens the environment. A section milling procedure for P&A that achieves rock to rock bond eliminates the risk of releasing fluids and gases to surface. This section milling procedure usually involves milling a window in the inner casing and then creating another window in the adjacent outer casing. The tool developed for this process is commonly known as dual string section milling (DSSM) tool. Simulations using advanced finite element analysis (FEA) and computational fluid dynamics (CFD) prior to prototype manufacturing is an extremely useful guide for predicting performance, optimizing design and avoiding costly oversights. This results in higher quality of design of pioneering or innovative products while reducing development costs and time to deliver tools to field. The scope of this paper is to discuss design optimization using CFD that resulted in efficient flow distribution leading to elimination of wash out issues during field runs, while highlighting case studies summary based on design changes made to the new dual string section milling tool. Initial testing of dual string section milling tool design was done on test drilling rig that revealed some erosion prone areas in the tool; this design was modified using CFD to eliminate these erosion prone areas. However, during field runs in salt dome reservoir applications, the erosion issues reappeared in critical flow areas. Further CFD analysis was conducted to eliminate erosion in critical areas to formulate the final product design. The goal was to develop an optimized design delivering extended milling times when the section window requirements were 100 feet or more without any washout or erosion issues while using high pump flow rates. This paper discusses the optimized dual string section milling technology using advanced computational techniques. The development of this technology is an enormous time saving for operators versus using conventional methods- in one offshore field case it resulted in more than 20 days savings in milling time.