Summary Reverse-circulation primary cementing (RCPC), a technique in which cement is pumped down the annulus, has historically been used for specialized cases as an alternative to conventional-circulation primary cementing (CCPC), in which cement is pumped down the casing and circulates up the annulus. As the potential application of this placement technique has extended to deep water, traditional conventional hydraulic analysis is insufficient because of the complex flow path required by deepwater RCPC. The focus of this study is to provide a hydraulic analysis of this flow path, to determine causes of apparent equivalent-circulating-density (ECD) reductions, and to provide operators and well engineers with simple tools to estimate the changes in ECDs throughout the casing annulus. Investigations of the specific hydraulic considerations of RCPC have been explored and evaluated since its first applications. This analysis builds upon previously published case studies and evaluations of hydraulics for traditional RCPC in which fluids are directly injected into the annulus from surface. By use of a graphical analysis, the hydraulics of deepwater RCPC, which requires an unconventional-flow path to divert flow from the work string into the annulus below the seafloor, is evaluated and compared with conventional placement. The results of this study can be used for an initial determination of whether RCPC will produce the desired results for a specific wellbore geometry. By developing expressions for the pressure in the casing annulus for both conventional and reverse circulation, an analytic equation for the critical depth can be derived, assuming a constant pressure drop per unit length in the casing annulus. This study also evaluates the cause of pressure differences between conventional and reverse placement and the relationship of frictional-pressure drops, hydrostatic effects, and the elimination of applied lift pressure. If the ECD is reduced at the bottom of the hole and increased at the previous casing shoe, then there is a point between those two where the pressures in conventional and reverse circulation are equal. A critical depth analysis has previously been performed for traditional RCPC applications. For deepwater applications that take into account the unconventional-flow path, analysis in this study shows that well geometry and location of a weak zone in the formation affect which placement method results in the lowest ECDs in a targeted area. For deepwater RCPC to be effective, the weakest part of the formation should be below the determined critical depth of the well.

A two-year government funded project is being conducted to evaluate the viability and applicability of Reverse-Circulation Primary Cementing (RCPC) in deepwater wells. This project focuses on the identification of technical issues that must be addressed before routine RCPC operations can occur in deepwater wells, and a recommended path forward to full evaluation of the viability of this placement technique in deepwater. Analysis includes numerical models and simulations, mechanical placement controls, cementing materials, and operational challenges.While RCPC has been used on land and on a few shallow water offshore wells, it has not yet been fully evaluated for use in a challenging deepwater environment. The application of RCPC to deepwater wells is expected to reduce bottom-hole circulating pressures and prevent lost circulation during cementing, as well as increase safety, environmental sustainability, zonal isolation, and improve cement seals.Standard commercially available software packages are unable to directly model the flow path through the complex configuration of a deepwater reverse-circulation cementing process. A multi-physics finiteelement software package has been used to develop a model to predict temperatures and pressures during the reverse-circulation cementing process. Evaluation of mechanical placement controls has found that a major challenge will be the development of a switchable crossover between a conventional and reverse flow path, and the modification of float equipment. Also, with the application of RCPC it is anticipated that the design methodology of cementing fluids may be affected by changing the placement method.One major challenge in deepwater cementing is the narrow formation fracture gradient, so the application of RCPC has clear beneficial potential. By lowering the Equivalent Circulation Densities (ECDs) during the job, the risk of fracturing the formation and lost-circulation is decreased. Less fluid lost to the formation during placement can potentially lead to higher tops of cement (TOC) and improved cement bonding and zonal isolation.