Stefan Z. Miska

The Buckling Behavior of Pipes and Its Influence on the Axial Force Transfer in Directional Wells

An experimental setup was built at the University of Tulsa to study buckling and post-buckling behavior of pipes constrained in straight horizontal and curved wellbores. Experiments were conducted to investigate the axial force transfer with and without static internal pressure. Different stages of buckling phenomena and their relation to the axial force, the pipe diameter (1/4 and 3/8 in.) and the pipe end-support conditions have also been investigated. Experimental results have shown that the buckling load is a strong function of the pipe diameter and the pipe end-support conditions. Static internal pressure appears to have insignificant influence on the buckling behavior of pipes. A brief review of recently developed mathematical models to predict buckling behavior of pipes in inclined, curved, and horizontal sections of wellbore is also presented. Applications of the current theory are presented by using recently developed computer simulator. Results of the theoretical analysis have confirmed the versatility and effectiveness of computer simulator for better understanding and solving buckling related problems in the field.

An Analysis of Helical Buckling of Tubulars Subjected to Axial and Torsional Loading in Inclined Wellbores

This paper describes new theoretical results for prediction of buckling behavior of tubulars in inclined wellbores. Using conservation of energy and the principle of virtual work improved equations for buckling and post-buckling conditions are derived. The effect of torque on the buckling process is considered. Practical examples are provided showing the influence of torque on the critical buckling force. The equations for critical buckling force reduce to those previously derived when torque is set to zero and weightless strings are considered.

Selecting Drilling Fluid Properties and Flow Rates For Effective Hole Cleaning in High-Angle and Horizontal Wells

This paper presents a method to determine optimum drilling fluid properties and flow rates to minimize cuttings bed height and circulation time in high angle and horizontal wells. The method uses empirical models relating the cuttings bed height and the bed erosion time to drilling fluid properties and flow rates. Bed erosion tests have been conducted using a cuttings transport facility available at the University of Tulsa. Cuttings bed height as a function of time has been investigated by using variable flow rates (200 - 400 gpm) and four different drilling fluid compositions. Experimental results were used together with a non-linear regression analysis program to establish a functional relationship among drilling fluid properties, flow rate, cuttings bed height and the time required to circulate the borehole clean. A numerical example is provided to explain the field application of the method. The sequential calculations involved in determining optimum combination of the Power Law viscosity parameters n and K, and the flow rate to minimize the cuttings bed height and circulation time are also given. Field implementation of the proposed empirical correlations and the new method can aid optimization of circulation practices before tripping, and so reduce the associated risk of non-productive time.

A quantitative investigation of the fracture pump-in/flowback test

Fracture closure pressure is an important parameter for fracture treatment design and evaluation. The pump-in/flowback (PIFB) test is frequently used to estimate its magnitude. The test is attractive because bottomhole pressures during flowback develop a distinct and repeatable signature. This is in contrast to the pump-in/shut-in test where strong indications of fracture closure are rarely seen. Various techniques exist for extracting closure pressure from the flowback pressure response. Unfortunately, these procedures give different estimates for closure pressure and their theoretical bases are not well established. We present results that place the PIFB test on a more solid foundation. A numerical model is used to simulate the PIFB test and glean physical mechanisms contributing to the response. Based on our simulation results, we propose an interpretation procedure which gives better estimates for closure pressure than existing techniques.

Experimental Evaluation of the Lateral Contact Force in Horizontal Wells

In horizontal and extended reach drilling, a large frictional drag may occur. If the pipe buckles laterally or into a helical shape, additional lateral contact force, LCF, is developed between the pipe and the wellbore wall, increasing the drag force. This paper presents the results of an experimental study of the lateral contact force between the drill pipe and the wellbore wall, for helical pipe configuration. Comparison of the experimental results with the current analytical models is also presented. A horizontal well was simulated using a 2-in-dia hole, 86-ft long, and three different sizes of pipe. Two different techniques were used to measure the lateral contact force. Results from both techniques seem to be in good agreement. The comparison with the current analytical models shows that higher values are predicted. The results will find application in directional drilling, horizontal drilling, and coiled tubing operations.

Pressure Profile in Annulus: Solids Play a Significant Role

This paper looks into the effects of solids on the wellbore pressure profile under different conditions. An extensive number of experiments were conducted on a 90-ft-long, 4.5 in. 8 in. full-scale flow loop to simulate field conditions. The flow configurations are analyzed. A solid–liquid two-phase flow configuration map is proposed. Significant difference is found between the pressure profile with solids and without solids in the wellbore. The results of this study show how the pressure profile in the wellbore varies when solids present in the annulus, which may have important applications in drilling operations. [DOI: 10.1115/1.4030845]

A simple model for computer-aided optimization and design of sucker-rod pumping systems

Abstract Design or optimization of a sucker-rod pumping system requires an evaluation of the system performance under different operating conditions. The precise evaluation of the performance is a complex task. A dynamic behavior of the sucker-rod string and complex interactions between components of the system and fluids in motion have to be taken into consideration. In this paper, a simple model of a sucker-rod pumping system is presented, and performance and applications of the model are outlined. A comprehensive, mathematical model of the system accounts for fluid and rod string dynamic behavior. Approximate surface and downhole dynamometer cards, production rates, downhole and surface forces, and energy consumption of sucker-rod pumping system are simulated with high accuracy for a variety of operating conditions. The model is defined by a system of linear algebraic equations derived from the straightforward Bergeron method rather than from numerical differentiation of wave equations. Example results of modeling indicate a good agreement with field data. The model was used to optimize an existing sucker-rod pumping system. Example results indicate significant savings over unoptimized operation. The model may also have an application at the design stage of the sucker-rod system development. The outline of the design procedure with application of the new model is presented.

Drilling and Well Completions

Publisher Summary Derricks and portable masts provide the clearance and structural support necessary for raising and lowering drill pipe, casing, rod strings, etc., during drilling and servicing operations. Standard derricks are bolted together at the well site, and are considered nonportable. Portable derricks that do not require full disassembly for transport are called masts. The derrick or mast must also be designed to withstand wind loads. Wind loads are imposed by the wind acting on the outer and inner surfaces of the open structure. When designing for wind loads, the designer must consider that the drill pipe or other tubulars may be out of the hole and stacked in the structure. For modern derrick and mast designs, American Petroleum Institute (API) Standard 4F is the authoritative source of information, and much of the content of this section of the chapter is extracted directly from this standard. Drilling and well servicing structures that meet the requirements of API Standard 4F are identified by a nameplate securely affixed to the structure in a conspicuous place. The chapter also describes various terms and abbreviations associated with derricks and masts.

Effect of Tool Joints on Contact Force and Axial-Force Transfer in Horizontal Wellbores

An experimental study has been conducted to investigate the effect of tool joints on the buckling/post-buckling behavior of drillpipes constrained in straight horizontal wellbores. Buckling/post-buckling behavior of drillpipes has traditionally been investigated with continuous pipes. To our knowledge, this is the first time the effect of tool joints is included in such a study. The U. of Tulsa Drilling Research Projects experimental buckling facility has been used to carry out the desired experiments. Axial loads at both ends of the pipe and contact forces at the tool joints were measured. Changes in the drillpipe configuration were also investigated visually as the axial load increases. Some of the new findings of this study can be summarized as follows. Sequential occurrence of buckling/post-buckling configuration of jointed pipe is similar to that of continuous pipes, reported previously by various investigators. In other words, in both cases, the pipes buckle first laterally and then helically as the axial compressive load increased. The presence of tool joints does not affect the critical lateral (sinusoidal) buckling load significantly. However, it increases the critical load, causing helical pipe configuration (helical buckling) of approximately 20%. The use of tool joints improved the efficiency of the axial load transfer by approximately 40%. The results of this study will help to improve the design of operational parameters for drilling with jointed pipes as well as with coiled tubing (CT). In particular, improved axial-load transfer performance would allow drillers to use a higher weight on bit and, consequently, faster and possibly less costly drilling.

Stability Analysis of Pipe With Connectors in Horizontal Wells

Except for coiled tubing, most tubular goods used for downhole operations (such as drillpipe and sucker rod) have connectors. Because a connector and the pipe body have different outer radii, the deformation and buckling behavior of a pipe with connectors constrained in a wellbore is much more complicated. However, most buckling models were established by neglecting the existence and effects of connectors. In this paper, buckling equations of a pipe with connectors in horizontal wells were derived with application of elastic-beam theory. The axis of an unbuckled pipe is a 2D curve in the vertical plane and has three configurations—no contact, point contact, and wrap contact. We derived the two critical distances between connectors, Lc1 and Lc2, beyond which a pipe changes its configuration from one to another. The authors proposed an algorithm to determine the critical force (Fcrs) of buckling by numerically solving the buckling equations using the fourth-order Ronge-Kuta method. Both the distance between two adjacent connectors (Lc) and the radius difference between a connector and the pipe body (Drc) have significant impact on the critical force, in addition to net clearance between a pipe and wellbore (r0), bending stiffness (EI), and weight per unit length (w) of pipe. When Lc is small, radial deflection is negligible. Fcrs increases as Drc increases. However, when Lc is close to Lc1, effects of radial displacement become significant, and Fcrs decreases dramatically as Drc increases. Fcrs decreases as Lc increases when Lc Lc1, Fcrs fluctuates as Lc increases. Some curves of Lc1, Lc2, and Fcrs, all in dimensionless forms, were calculated and presented in this paper for practical applications. Our numerical results show that the critical force may reduce by 20 to 60% for commonly used drillpipes and sucker rods with centralizers, which indicates that a pipe string designed without considering the effects of connectors may be risky. The results presented in this paper may provide some practical guidance for optimal design of centralizers for sucker-rod strings, or may avoid some risks because of improper design of drillpipe strings.