Refining advanced technologies for the successful completion of wells is paramount in high-risk, high-cost environments. Challenges are associated with complex well architectures and with the successful completion of wells deploying conventional or floated casing or liner strings. A key part of the design process is to be able predict both maximum downhole surge pressures and dynamic and static friction effects. The analysis also needs to take into account the impact of using centralizing devices. Understanding these effects during both casing running and reciprocation is important so that appropriate operational decisions can be made. Specific challenges are associated with running liners through casing strings or hole sections with limited annular clearance.
Centralizers are primarily installed on a casing string to provide adequate stand-off for primary cementing operations; however they are also sometimes used to aid in the deployment of casing strings. To achieve adequate stand-off, calculations are performed to determine the number of centralizers, their placement, and spacing frequency. Drag modeling is performed to ensure that the casing string will reach its target depth. This requires predictions of both static and dynamic drag effects that take into account the impact of centralizers and surge pressures on drag. In cases where non-rigid centralizers are used, conventional torque and drag models underestimate frictional drag effects. Similarly it is considered that steady state surge models under predict maximum surge pressures.
To address these concerns both improved models and comprehensive analysis is required. This paper describes the analytical models that are used to incorporate additional forces associated with centralizers. Both conventional and floated casing running scenarios, especially in close tolerance extended reach wells, are considered.
Introduction
The challenge of running casing to its desired target depth using floated casing requires a more comprehensive in-depth analysis than is currently available. Such analysis is required when non-rigid centralizers, that exert additional frictional drag through running forces, are used. Previous methods of analysis based on adjusting friction factors to compensate for this additional drag or simply adding running forces to drag is considered inadequate. Such techniques may be acceptable for elementary analysis, however such basic approaches are considered inadequate for modeling long casing strings in extended reach wells. In such cases the results may significantly underestimate frictional drag effects, resulting in casing not reaching its target depth.
Increasingly more difficult wells are being drilled with a narrow margin between pore and fracture pressure. Both swab and surge pressures must be maintained within narrow limits during tripping, casing running and cementing operations. Operating outside this window for even short durations has historically led to the onset of costly wellbore problems. Monitoring downhole pressures in real-time with downhole drilling data measurement tools is, in principle, a reliable method and is common practice in critical wells; however it is not currently possible to run such devices with casing strings.
Therefore, if a reliable and validated model was available that could be used with real time data; it would help in the accurate evaluation of transient wellbore pressures. A validated model would also offer a viable tool not only to plan a well but also to more accurately define operating limits for casing running operations. This would be of particular value when using floated casing techniques where collapse pressures are often a significant concern.
