Improved Techniques of Deepwater SBM Displacements: A Case History

Abstract

Abstract Wellbore cleanups are an integral part of the effort to reduce formation damage during completion operations. One operation that can significantly impact the cost of the well completion is the displacement from mud to completion brine. SBM (synthetic-based mud) cleanups pose unique challenges due to the emulsifying tendency of the additives to the synthetic base fluid. In deepwater, there are at least two displacements that compound an already difficult task, i.e., the riser and the casing. This paper demonstrates how a combination of software design tools, laboratory optimization and characterization, and post job evaluation led to improved displacement practices for the deepwater wells evaluated for this paper. Introduction In early 2001, the authors began an investigation of how to improve well cleanup and displacement techniques in conjunction with a deepwater project in the U.S. Gulf of Mexico. This investigation was commenced at the request of the operator following a failed attempt at well cleaning on this same project. Two goals were identified early on. First, a clear understanding of the impact of friction pressures related to the pumping of chemical spacers had to be achieved. This was important for reasons relating to cement-shoe integrity. This was achieved through computer wellbore modeling. Second, the high daily rig cost mandated that the well displacement have minimal impact on the critical path. This meant that the pipe must be cleaned on the first attempt while pumping at very high rates. This was accomplished through spacer compatibility and performance testing in the laboratory, taken through computer simulations, and then implemented in the field. In the failed attempt to clean the riser, it was felt that the chemical washes and related pumping rates had been least engineered in terms of appropriate pill size, chemical concentration, wall contact time, inter-chemical compatibility, and desired flow regimes. Using advanced computer wellbore simulation software, optimal pumping characteristics were modeled for each pill in specific pipe geometries. Once the modeling was complete, laboratory work began to develop technical limit criteria for each individual pill. In this fashion, the viscosity transfer and cleaning efficiencies were determined for the lead and secondary pills using optimal chemical concentrations. During the actual implementation of these displacements, numerous samples were caught for post-job analysis. This paper will show how, using these methods, very predictable results were achieved in these displacements. Rig time was also saved by the ability to pump these displacements at very high rates. As a result the engineers were better able to predict the timing of other completion operations and minimized the impact of the displacement on the critical path.