Cement Placement in Irregular Wellbores

In drilling industry, one of the most important operations performed as part of well construction is primary cementing. The quality of the annular cement barrier affects wellbore integrity. Poor cement quality can result in operational challenges and difficult remedial operations and can prevent cost-efficient abandonment operations of the well. Most importantly, it can lead to devastating environmental consequences with little chance for rectification and possibly irreversible damages to the environment and the society. The purpose of this project is to study the effect of irregular wellbore features on both the bulk annular displacement and removal of mud layers left on the annulus walls. An important motivation for this project is the availability of a new Impulse Radar Caliper tool, or 4D Caliper tool, that can be run behind the bit on drill strings, enabling high-resolution imaging of the open-hole section as it is drilled, and while tripping out. Reviewing the substantial amount of research executed to improve the displacement efficiency, it appears that little emphasis has been placed on fluid displacement in wellbores with irregular geometric features. Since perfectly regular wellbores would never exist, geometric irregularities may be one of the major reasons for why the industry still struggles with achieving effective cementing jobs in both onshore and offshore wells. The project aims to improve the methods, both mathematical and practical, implemented in primary cementing operations in wellbores with irregularities. A combination of mathematical (both analytical and CFD) and experimental work will be conducted. The ultimate goal is to contribute to more effective cementing jobs, thereby reducing the risk of environmental consequences, and ensuring safety and wellbore integrity. The outcome of the project shall propose improvements to current modelling approaches, complementing the models developed for fluid displacement in regular wellbore geometries.

During the cement placement, the drilling fluid/mud that is inside the wellbore prior to cementing will be displaced by a cement slurry, or a sequence of pre-flush, spacer, cement. Incomplete displacement of drilling fluid from the annular space, or excessive fluid inter-mixing and contamination of the cement slurry are among the mechanisms that can be detrimental for zonal isolation. Compromised zonal isolation can in turn result in uncontrolled migration of formation fluids along the wellbore, and seepage into shallower permeable formations or to the surface, manifesting as Surface Casing Vent Flow (SCVF) or Sustained Casing Pressure (SCP). A review of existing works on annular fluid displacements in irregular wellbore geometries indicates that considerable efforts are required before these displacement flows are understood, and before cementing operations in the field can be optimized. Since new downhole radar technology, such as the 4D Caliper tool, now enables very accurate imaging of the wellbore shape, it is the right time to use this data in favor of enhancing our understanding and establishing improved methodologies for realistic annular displacements. Published 4D Caliper runs in wells on the Norwegian Continental Shelf show the presence of washouts, ledges and borehole spiraling, among other features. We intend to study how such geometric irregularities affect mud displacement and primary cementing of casings. With that, the intent is to also develop improved research methodologies, not only the theoretical aspects, but also to establish a more advanced experimental methodology that in addition to relevant displacement flow measurements could be capable of detecting minuscule volumes of residual fluid on the walls, looking into both aspects of bulk displacement and wall cleaning.