Computational Fluid Dynamics (CFD) study to optimize simulated pressure drop across liner hangers: A field trail approach to improve well integrity by optimizing ECD simulations during well cementing.

Abstract

Optimizing cementing parameters during the cement job is a challenging task. Accurate pre-job analysis is essential to elude breakdown of the formation to obtain a successful cement job. If the equivalent circulation density (ECD) exceeds the fracture gradient of the formation, drilling fluids will start to leak into the formation. Mud or cement losses to the formation or inadequate placement of cement could tremendously affect the quality of the well integrity. If cement is insignificantly displaced around the liner, eventually fluids will be able to by-pass the cement leading to a degradation of the primary cement barrier. Remedy cost associated with degraded cement is substantial, therefore accurate analysis in the design phase of the well is critical. In the industry there are several hydraulic simulation software that predict the ECD during cementing. However, there is limited experimental data behind the models and a lack of research around velocity and pressure fields for complex geometries such as cementing with liner hangers. Common industry practice when estimating pressure drop around complex geometries in tight gap scenarios is to use a uniform pipe with a common outer diameter (OD) for the hanger section. The uniform pipe approach can overestimate the pressure drop and the resulting ECD. During placement of cement in pressure sensitive formations there is a small margin between the pore pressure gradient and the fracture pressure gradient. Accurate hydraulic simulation during the planning phase of the well construction is therefore necessary. History shows that the hydraulic simulation software iCem fails to accurately predict the frictional pressure drop across liner hangers with tight annular gap. Simulations indicate formation break down as result of elevated ECD during placement of cement. Simulated ECD tends to be inaccurate and over-estimated by field experience and track records. The immediate response has been to use an effective OD for the liner hanger section. Later these effective ODs seems to be limited to changes in the operational parameters. Therefore, a full-scale flow loop test was initiated at the Norwegian Research Center (NORCE) to examine potential simulation errors in iCem and validate effective ODs to be simulated with iCem for expandable liner hangers. A comparison study between one conventional and two types of expandable liner hanger was also done. Further CFD was investigated as an alternative to flow loop testing to reduce cost associated with optimization of iCem simulations in tight gap scenarios. Results shows that iCem simulations for tight annular gap scenarios tends to be inaccurate for the Standard liner hanger system at low flow rates. Comparison between simulated and experimental data shows that simulation accuracy increases with flow rate for tight gap scenarios. Further iCem shows useful in simulations for the Low ECD liner hanger system. The Low ECD liner hanger shows the least frictional pressure drop and is less susceptible to changes in fluid properties and flow rates. A much better match was found when more detailed geometry input was used. CFD fails to re-produce experimental data and can be disqualified as an accurate alternative to flow loop experiments.