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.
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.
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 project aims to improve the methods, both mathematical and practical, implemented in primary cementing operations in wellbores with irregularities, by conducting a combination of mathematical and experimental works.
With the involvement and support from industry partners, we have acquired access to high-resolution 4D caliper data. A method of caliper data analysis is being developed to automatically detect spiraling (short and long wavelength), washout, keyseat, ellipticity, and irregularities of the borehole upon removal of the data noise.
A mathematical model to study the instability of an elongated (nearly parallel to the wellbore axis) interface between displacing and displaced fluids is being developed. Effect of various non-Newtonian fluid models on linear instability analysis is examined. In the next stages of the project, the model will be enhanced to include for the effect of nonlinearities and turbulence.
Preliminary CFD simulations are performed for typical fluid combinations and spiral wellbore geometries previously reported in literature. Beyond providing insights into computational challenges and requirements, these simulations also guide the decision-making process for defining the simulation matrix and identifying the most challenging cases.
At the initial stage, a medium-scale experimental setup has been constructed, consisting of a transparent annulus with a single irregularity. The setup is designed to allow extension of the annulus length as well as the introduction of various irregularities, either individually or in combination.
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.
