The Impact of Compression Forces on Casing-String Designs and Connectors

Abstract

Summary The effect of downhole axial compression forces on casing string designs and connectors is a major concern to engineers designing high-temperature, high-pressure, extended reach, and horizontal wells. Widespread concern regarding the significance of compression loads on casing string design is relatively recent. For decades engineers were either not aware that high-compression loads may occur or were not concerned about the effect of compression on the casing. This paper discusses the sources of axial compression forces in straight and directional wells. Methods to analyze casing subjected to compression and bending loading are presented including the effect of compression and bending on the von Mises equivalent stress state of the pipe. Connector selection for casing applications where high axial compression loads are anticipated is critical. Compression ratings for API casing connectors have never been addressed in API standards. The paper offers methods to evaluate API connectors for applications with significant compression loading. Compression ratings for premium casing connectors are generally available, but the justification for these ratings varies substantially from one manufacturer to another and the basis for the compression ratings may not be well documented. The paper discusses compression rating methods for various premium connector configurations including threaded and coupled, flush and near flush. Desirable and undesirable premium connector features that relate to compression capacity are reviewed. Results from compression testing of premium connectors are also presented. The paper benefits engineers involved in the design of casing strings for applications with high compression and bending loading. Background Conventional casing string design analysis has traditionally been based on calculating minimum design factors for internal pressure (burst), collapse, and axial tension. Axial tension design factors are often calculated based on the string weight in air. If the effect of buoyancy is accounted for, the adjusted weight per foot of the pipe in the drilling fluid is generally used to determine string weight. Axial compression forces acting on the casing are generally not considered. The trend in more high-temperature, high-pressure (HTHP), extended reach and horizontal wells combined with the availability of sophisticated casing design models has increased awareness that casing can experience severe axial compression loads and that these compression forces have a significant impact on the overall safety of the string design and the well. High temperatures, high pressures, and borehole friction can result in substantial axial compression forces acting on the casing. Downhole Causes of Axial Compression Understanding the impact of compression forces on casing and casing connectors starts with recognizing the sources of compression forces in the well. Several downhole factors cause axial compression loading of casing strings: buoyancy, Poisson's effect or (reverse) ballooning, thermal expansion, borehole friction or drag in directional wells, and slack-off. Buoyancy. Buoyancy from the drilling fluid results in axial compression forces at the bottom of the casing string and at crossovers in the string (Fig. 1). In conventional casing design, buoyancy is accounted for by using an adjusted weight per foot for the casing in fluid to determine the tension force at the top of the string. This approach yields the correct tension force at the top of a string suspended in fluid, but has several inaccuracies and shortcomings if applied to detailed evaluation of casing string designs. The correct method to account for buoyancy is the pressure/area method.1 The actual fluid pressure at the bottom of the string is multiplied by the cross-sectional area of the pipe to arrive at the buoyancy force. This is a compressive force acting on the bottom of the string. The axial load on the casing is the hanging weight of the pipe (in air) minus the buoyancy force acting on the string. The equation for buoyancy force for open casing is as follows: F B = − P o A p b . ( 1 ) For casing with a plugged end (float valve holding): F B = p i A i − p o A o . ( 2 ) Throughout this paper tension forces and stresses are positive and compressive forces and stresses are negative. Fig. 2 shows plots of axial force vs. depth for a 12,000 ft, 9 5/8-in. 53.5-lbf/ft (0.545-in. wall) casing string suspended in 16-lbm/gal fluid based on the adjusted weight method and the pressure/area method. Note, the adjusted weight method yields the correct value at the top of the string, but this is the only place where the force profile is correct and no indication that axial compression is effecting the lower portion of the casing is shown. The compression force due to buoyancy at the bottom of the string is equal to 155,000 lbf. Once the casing string is cemented the buoyancy force is "locked in" below the cement top and effects all subsequent load conditions. Poisson's Effect of (Reverse) Ballooning. After the casing string is cemented changes in internal or external pressure effect the axial force profile in the casing. Pipe that is free to move (not fixed) shortens when internal pressure increases or external pressure decreases. This is often called ballooning or Poisson's effect. Conversely, decreases in internal pressure or increases in external pressure cause pipe that is free to move to get longer (reverse ballooning). Since casing is generally cemented and cannot move, changes in pressure cause a change in axial force (Fig. 3). Net increases in external pressure and net decreases in internal pressure generate axial compression forces. The force due to ballooning or Poisson's effect is calculated with the following formula: F P = 0.6 [ Δ p i A i − Δ p o A o ] . ( 3 ) Partial or complete evacuation load conditions can generate high axial compression forces in the casing. For example, a lost circulation condition inside of 9 5/8-in. 53.5-lbf/ft casing that results in a net reduction in average internal pressure of 7,000 psi generates an axial compression load of 240,000 lbf in the casing.

Casing structural integrity and failure modes in a range of well types - A review

“…As such, they developed a semi-analytical model that can be applicable to study various wellhead loading situations that can potentially cause motion in the upward direction. In contrast, Awe et al (2015) and Jellison and Brock (1998) identified connection failure, local buckling and shear failure as the main types of failures on casing strings. Even though both API and ISO have issued recommended practice for connection evaluation procedure (API RP 5C5 and ISO/PAS 12835) left much to be desired for hydraulically fractured wells (Hamilton 2019).…”