Innovation meets long-established.

A part of conventional tubing/casing stress analysis was the “shock-load” calculation, suggesting the importance of dynamic analysis. As shown in surge pressure calculations, dynamic effects produce unexpected and significant results. Motivated by these ideas, a fully dynamic tubing and casing stress analysis was developed. An interesting side effect of this formulation was a more direct and accurate friction analysis. Friction is known to have two values, a static and a dynamic value. But what meaning does a dynamic friction coefficient have for a static analysis? One major drawback to dynamic analysis was the large number of calculations needed. Fortunately, modern laptop computers are sufficiently fast to make this no longer a problem. 

R. F. Mitchell, Nola R. Zwarich, Hai L. Hunt, A. R. McSpadden, R. Trevisan, M. A. Goodman, “Dynamic Stress Analysis of Critical and Cyclic Loads for Production Casing in Horizontal Shale Wells,” SPE-194062-MS, IADC/SPE Drilling Conference and Exhibition, The Hague, Netherlands, 5-7 March 2019.

N. R. Zwarich, A. R. McSpadden, M. A. Goodman, R. Trevisan, R. F. Mitchell, “Application of a New Dynamic Tubular Stress Model with Friction”, IADC/SPE-151235-MS, IADC/SPE Drilling Conference and Exhibition held in Fort Worth, Texas, 6–8 March 2018.

The first comprehensive analysis of the post-buckling equilibrium of tubulars posed as a boundary value problem

The original analysis of this problem was done by Arthur Lubinski, with help from Henry Woods. The formulation was in terms of an energy formulation, that predicted the equilibrium configuration of an elastic helix. This original paper was titled “Helical Buckling of Tubing Sealed in Packers”, but this terminology is counter to the usual meaning of buckling, such as the Euler buckling of columns. While this paper presented a plausible solution, the solution did not match with the actual boundary conditions of the problem, in particular, the packer itself. Resolving these issues required much fundamental research. Eventually, general solutions to the equilibrium of a buckled pipe were found, which allowed the correct solution to the overall problem. This background knowledge then made the solution of other related problems possible. 

Mitchell, R. F., "Buckling Behavior of Well Tubing: The Packer Effect", Society of Petroleum Engineers Journal; October 1982.

Mitchell, R. F., "Simple Frictional Analysis of Helical Buckling of Tubing," SPE Drilling Engineering; December 1986.

Mitchell, R. F., "New Concepts for Helical Buckling," SPE 15470 presented at the 61st Annual Technical Conference and Exhibition, New Orleans, Louisiana; October 1986.

Mitchell, R. F., "Numerical Analysis of Helical Buckling", SPE 14981 presented at the 1986 Deep Drilling and Production Symposium of the SPE, Amarillo, Texas; April 1986.

Mitchell, R. F., "The Twist and Shear of Helically Buckled Pipe," SPE Drilling & Completion, March 2004.

Mitchell, R.F., “The Pitch of Helically Buckled Pipe,” SPE 92212 presented at the SPE/IADC Drilling Conference, Amsterdam, February 2005.

Comprehensive prediction of dynamic wellbore surge pressures

The original method used to estimate surge/swab pressures was steady-state flow, where the wellbore pressures were determined by the fluid density and the frictional flow characteristics. This ignored the dynamic effects of fluid inertia and compressibility. The first attempt to quantify dynamic effects was written by Arthur Lubinski. This analysis, based on “water hammer” type calculations, was not comprehensive, due to ignoring the elastic coupling of the fluid inside the pipe to fluid outside the pipe. Developing a comprehensive model required a complete reformulation of the dynamic equations to include all dynamic effects. Conventional wisdom held that surge pressures were increases and swab pressures were decreases. Dynamic analysis showed that fluid inertia could produce surge pressure decreases and swab pressure increases. For situations where wellbore pressures are near fracture or pore pressures, ignoring dynamic effects could result in serious problems. 

Mitchell, R. F., "Dynamic Surge/Swab Pressure Predictions," SPE Drilling Engineering; September 1988.

Mitchell, R. F., "Surge Pressures: Are Steady State Models Adequate?" SPE 18021 presented at the 63rd Annual Technical Conference and Exhibition, Dallas, Texas; October 1988.

Mitchell, R.F., “Surge Pressures in Low Clearance Liners,” SPE 87181 presented at the SPE/IADC Drilling Conference, Dallas, February 2004.

Developed the industry standard wellbore thermal analysis software

One of the first wellbore thermal simulators was developed by Enertech Engineering and Research Company for the Department of Energy to use for geothermal wells. This became the basis for the WellTemp thermal simulator. As principle developer of this code, we went through several complete rewrites as more functionality was needed, together with comprehensive fluid flow and thermodynamic simulation of the single and multi-phase flow of typical drilling and production fluids. 

Mitchell, R. F., "Simulation of Air and Mist Drilling for Geothermal Wells", Journal of Petroleum Technology; November 1983.

Galate, J. W. and Mitchell, R. F., "Behavior of Oil Muds During Drilling Operations", SPE 13158 presented at the 59th Annual Conference and Exhibition, Houston, Texas; September 1984.

Mitchell, R. F., Schuh, F. J., and Serocki, S. T., "Steam Flash Conditions While Drilling into an Ocean Hydrothermal Reservoir", presented at the 11th Annual Energy‑Sources Technology Conference and Exhibition, New Orleans, Louisiana; January 1988.

Goodman, M. A., Mitchell, R. F., Wedelich, H., Galate, J. W., and Presson, D. M., "Improved Circulating Temperature Correlations for Cementing," SPE 18029 presented at the 63rd Annual Technical Conference and Exhibition, Dallas, Texas; October 1988.

Wedelich, H. F. and R. F. Mitchell, "Prediction of Downhole Temperatures is Key for Optimal Wellbore Design," 1989 SPE Production Operations Symposium, Oklahoma City, OK. March 13‑14, 1989.

Mitchell, Robert F. and Ronald Sweatman, “Wellbore Stability and Integrity Contributors Revealed by Thermal Modeling and Fluid Analysis”, paper presented at 2013 Offshore Technology Conference, Houston, May 2013.

Developed the industry standard casing and tubing stress analysis software

The buckling development referred to earlier was fundamental to formulating casing and tubing stresses, necessary for design. The treatment of friction was also fundamental in treating tubing/casing design. The resulting program was WellCat, where the “CaT” referred to “casing and tubing”. Following this development came a full review of the concepts behind torque and drag modeling. This review produced innovations in wellbore trajectory calculation and new torque drag modelling concepts.

Robert F Mitchell, “Fluid Momentum Balance Defines the Effective Force,” SPE 119954 prepared for presentation at the SPE/IADC Drilling Conference and Exhibition, Amsterdam, The Netherlands, 17–19 March 2009.

Mitchell, R.F. and Robello Samuels, “How Good is the Torque-Drag Model?,” SPE Drilling & Completion, March 2009.

Mitchell, R.F., Bjorset, A., and Grindhaug, G., “Drillstring Analysis with a Discrete Torque/Drag Model”. SPE Drilling & Completion. SPE-163477-PA, 2014.

The first correct analysis of permafrost thaw subsidence and associated surface casing design

Original thinking on thaw subsidence was that it was similar to other forms of subsidence. The thawed permafrost was assumed to become “heavier” and to move downward. Instead, the problem of thawed permafrost about a producing well is completely different. Thawing the permafrost results in a decrease in the pore pressure of the formation surrounding the wellbore, due to the reduced volume as the ice melts to water. The thawed permafrost is a cylinder about the well, rather than the horizontal plane for traditional subsidence calculations. This different geometry causes a squeeze, like a toothpaste tube, that causes vertical motion in the cylinder, due to the Poisson effect, resulting in alternating compression and expansion of the formation, due to rock of different properties pushing against each other. At the base of the permafrost, reduced pressure produces an upward motion, the opposite expectation for subsidence.

Mitchell, R. F., "A Mechanical Model for Permafrost Thaw‑Subsidence", presented at Petroleum Mechanical Engineering Conference, Mexico City, Mexico; September 1976.

Mitchell, R. F., "Loading Mechanisms in Thawed Permafrost Around Arctic Wells", formal discussion presented at 1977 ASME Energy Technology Conference and Exhibition, Houston, Texas, September 1977.

Mitchell, R. F., and Goodman, M. A., "Permafrost Thaw‑Subsidence Casing Design", Journal of Petroleum Technology, November 1977.

Mitchell, R. F., "Loading Mechanisms in Thawed Permafrost Around Arctic Wells", Transactions of the ASME, Journal of Pressure Vessel Technology, Vol. 100, No. 3, pp 320‑321, August 1978.