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.
This paper covers the development and validation of a hydraulic simulator for subsurface reverse cementing placement in which fluids are placed down drillpipe and diverted into the annulus through a crossover tool above a liner hanger. Returns are taken up the liner inner diameter and are re-diverted through the crossover tool back to surface. Since commercially available cementing simulators are unable to model cement placement through this flow path with a crossover tool, a simulator was developed and validated using downhole pressure data collected during large-scale flow testing and a reverse cementing field trial. Development of this simulator is a major step forward to implementing a subsurface reverse cementing system in deep water. This custom simulator determines the magnitude of equivalent circulating density (ECD) reductions and identifies opportunities in which subsurface reverse cementing is advantageous with regard to pressure. Traditionally, placement through reverse cementing results in reduced bottomhole ECDs compared to conventional cementing. This pressure reduction is not uniform throughout the annulus, and a placement simulator that takes into account wellbore geometry, a crossover tool, fluid properties, and cementing hydraulics is required to assess viability of reverse cementing for specific deepwater wells. Computational fluid dynamics (CFD) modeling was conducted using specific crossover tool geometry and various fluid properties to develop a lumped-pressure loss model mimicking local pressure drops. This lumped model was incorporated into a hydraulics system-level solver to estimate surface and downhole pressures. The hydraulics solver was initially validated by comparing model output with downhole pressure data collected from large-scale flow testing and a field trial in which a liner was cemented using the crossover tool. The resulting subsurface reverse cementing simulator is able to simulate incompressible, multi-fluid placement through a crossover tool. Current capabilities of the simulator include incorporation of a crossover tool to divert flow into the annulus directly above the liner hanger in a deepwater well; estimation of surface pressures, bottomhole pressures, and downhole ECDs at any specified depth; and estimation of u-tubing effect from free fall of fluids. During a large-scale closed-system flow test, model output matched pressure gauge readings to within 11%. Comparisons of field trial surface and downhole pressures correlated with model output for cement placement. This paper will present comparisons of simulator pressure output and collected downhole data used for validation, along with simulator output for an example subsurface reverse cementing job for a deepwater liner.
Deepwater operators continually face technical and environmental challenges to drilling and completing wells safely and efficiently. To address both current and future challenges, the industry has leveraged radio frequency identification (RFID) technology to reduce risk, rig time, and nonproductive time (NPT) and to perform operations that traditional tools cannot perform. RFID technology has been integrated into drilling and completions tools to improve performance and reduce risk for offshore operations, such as drilling underreamed holes, spotting lost circulation materials, setting packers, opening stimulation sleeves, and performing subsurface reverse cementing. These tools use RFID tags released from the rig floor to enable downhole hydraulic power units (HPUs) to operate the tools. This paper describes criteria for selecting RFID-enabled tools rather than traditional tools, integration of RFID tools with operations, and value-added features enabled by RFID. Contingency, safety, and risk assessment factors are discussed, along with case studies validating performance and suitability of selected RFID tools. Three case studies describe how RFID solutions for drilling and completions were selected and applied in high-cost environments to address specific challenges and job objectives. Design and bench testing of RFID tools to enable future subsurface reverse cementing operations are also covered. The first case study describes an RFID lower-completion system that was successfully deployed into a southern North Sea extended-reach well. The system enabled remote control of flapper isolation valves and remote operation of stimulation sleeves to access the reservoir, which aimed to eliminate the need for intervention between treatments and ultimately improved fracture cycle time and reduced risk. In the Gulf of Mexico, an RFID drilling underreamer was used to set a liner shoe precisely at the casing point and eliminate a dedicated hole-opening run that would have been needed with traditional underreamers. The 8 1/2-in. hole section was drilled; but losses prevented the mechanical reamer from opening. Therefore, the 650-ft hole section was drilled to TD using the bit only. To eliminate multiple trips to take pressure samples and underream the hole section to 9-7/8 in., an RFID underreamer was placed below the measurement-while-drilling/logging-while-drilling (MWD/LWD) equipment. After pressure measurements were taken, the underreamer was actuated with RFID tags to enlarge the entire 650-ft openhole section with less than a 13-ft rathole. In the last case study, an RFID circulation sub was deployed above other bottomhole assembly (BHA) components, including an RFID underreamer and a conventional ball drop underreamer. This configuration enabled the operator to ream out the 22-in. cemented show track, underream the openhole section, and efficiently clean the wellbore at total depth. Because of BHA and standpipe pressure limitations, the RFID circulation sub was used in a split-flow application to bypass a percentage of the total flow to allow for a higher downhole flow rate. The sub helped to achieve high flow rates, high annular velocity, and turbulent flow, which contributed to better hole cleaning and improved wellbore integrity. Selecting the best tools and technology for specific applications results in streamlined applications and reduced operational risk. The methodology for selection, design, planning, and implementation of RFID drilling and completions tools identifies when RFID technology can be beneficial to deepwater operations.
Deepwater operators continually face technical and environmental challenges to drilling and completing wells safely and efficiently. To address both current and future challenges, the industry has leveraged radio frequency identification (RFID) technology to reduce risk, rig time, and nonproductive time (NPT) and to perform operations that traditional tools cannot perform. RFID technology has been integrated into drilling and completions tools to improve performance and reduce risk for offshore operations, such as drilling underreamed holes, spotting lost circulation materials, setting packers, opening stimulation sleeves, and performing subsurface reverse cementing. These tools use RFID tags released from the rig floor to enable downhole hydraulic power units (HPUs) to operate the tools. This paper describes criteria for selecting RFID-enabled tools rather than traditional tools, integration of RFID tools with operations, and value-added features enabled by RFID. Contingency, safety, and risk assessment factors are discussed, along with case studies validating performance and suitability of selected RFID tools. Three case studies describe how RFID solutions for drilling and completions were selected and applied in high-cost environments to address specific challenges and job objectives. Design and bench testing of RFID tools to enable future subsurface reverse cementing operations are also covered. The first case study describes an RFID lower-completion system that was successfully deployed into a southern North Sea extended-reach well. The system enabled remote control of flapper isolation valves and remote operation of stimulation sleeves to access the reservoir, which aimed to eliminate the need for intervention between treatments and ultimately improved fracture cycle time and reduced risk. In the Gulf of Mexico, an RFID drilling underreamer was used to set a liner shoe precisely at the casing point and eliminate a dedicated hole-opening run that would have been needed with traditional underreamers. The 8 1/2-in. hole section was drilled; but losses prevented the mechanical reamer from opening. Therefore, the 650-ft hole section was drilled to TD using the bit only. To eliminate multiple trips to take pressure samples and underream the hole section to 9-7/8 in., an RFID underreamer was placed below the measurement-while-drilling/logging-while-drilling (MWD/LWD) equipment. After pressure measurements were taken, the underreamer was actuated with RFID tags to enlarge the entire 650-ft openhole section with less than a 13-ft rathole. In the last case study, an RFID circulation sub was deployed above other bottomhole assembly (BHA) components, including an RFID underreamer and a conventional ball drop underreamer. This configuration enabled the operator to ream out the 22-in. cemented show track, underream the openhole section, and efficiently clean the wellbore at total depth. Because of BHA and standpipe pressure limitations, the RFID circulation sub was used in a split-flow application to bypass a percentage of the total flow to allow for a higher downhole flow rate. The sub helped to achieve high flow rates, high annular velocity, and turbulent flow, which contributed to better hole cleaning and improved wellbore integrity. Selecting the best tools and technology for specific applications results in streamlined applications and reduced operational risk. The methodology for selection, design, planning, and implementation of RFID drilling and completions tools identifies when RFID technology can be beneficial to deepwater operations.
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