A Comparative Hydraulic Analysis of Conventional- and Reverse-Circulation Primary Cementing in Offshore Wells

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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.