Minimum-Cost Casing Design for Vertical and Directional Wells (includes associated papers 17100 and 17107 )
Authors: Wojtanowicz Andrew K. , Maidia Eric E. 
Published: December 2007

The potential of casing cost reduction is investigated by use of an optimization technique. A simple recurrent computer procedure for the minimum-cost casing in vertical wells is derived. Simulation analysis reveals 24% casing cost reduction, as well as several special applications. A new model of the minimum-cost casing design in directional wells is developed. Directional data are used for casing load calculations, then the minimum-cost casing string is select. An analysis of the effect of directional-well geometry and the borehole dragon the minimum casing cost is performed. This paper and the optimization computer programs provide the casing designer with the complete cost-related information necessary to make the best decision.

Introduction

The tubular program of most oil wells represents thegreatest single item of expense in well cost. It can be asmuch as 18% of the completed well cost. Therefore, evena small reduction in casing cost can save a considerableamount of money. This objective has traditionally beenachieved by initially minimizing the number of strings andthe length of each string and then by designing acombination casing string. In vertical wells, an optimum combination casing stringhas been a challenge for casing designers. Its principleis based on consideration of several combinations of thegrade/weight/length sections of casing string. Eachcombination satisfies some predetermined external loadcondition. Eventually a combination casing string is selectedthat allows the minimum total cost. Because of the verylarge number of combinations, there is currently nomethod for the minimum-cost casing calculations. Instead, several stepwise procedures have been developed for casinggrade/weight selection without explicit cost expressions. In these procedures, the general observation usedis that the casing price increases with increasing casinggrade, weight, burst strength, and joint strength. Therefore, the lowest-grade and -weight casing with thelowest possible values of mechanical strength should givethe lowest cost. Unfortunately, this "minimum grade/minimum weight" approach cannot always yield the minimumcost simply because casing grade, weight, and cost cannotbe simultaneously minimized. Moreover, usually onlya small selection of casings can be considered because ofthe computational problems. Another approach to theproblem is presented by the set of graphs known as the problem is presented by the set of graphs known as the Casing Quick Design Charts. The charts are based onsome computer calculations of the minimum cost. Thougheasy to use, the charts have limited applications becauseof the simplifying assumptions regarding external loads, borehole conditions, and prices. Hence, they can berecommended only as a general guidance in casing design. A few theoretical investigations of the minimum-costcasing problem are based on the mathematical theoryof optimization and provide interesting concepts with verylittle practical use. In directional wells, designers' effort has been focusedon the mechanical aspects of casing design. The mostpopular is the method of the vertical projected depth. In popular is the method of the vertical projected depth. In this method, the directional well is assumed to be vertical, and the effect of wall support on casing weight is ignored. The unfavorable effect of the wall friction on thetensional load is sometimes considered as a constant valueof tension drag. This approach is oversimplified. Itdisregards a geometric profile of the directional well. Besides, the cost factor is not included in directional-wellcasing design. In summary, a systematic analysis, is needed of apotential cost reduction through casing optimization in potential cost reduction through casing optimization in vertical and directional wells. Also, there is a lack ofa simple computational procedure for minimum-cost casing. This research addresses several basic problems related tothe impact of casing cost on casing design.

1. How is the absolute minimum cost of casingcalculated, given external loads, design factors, and casing supply?

2. What is the quantitative effect of certain decisionsmade by the casing designer (value of the designfactors or number of sections) on the cost of casing?

3. How significant, given specific loads, is the conflictbetween minimum weight/minimum grade and minimumprice of casing?

4. How do the external casing loads in directional wellsaffect casing cost?

5. What is the correlation between the directional-wellprofile and its minimum casing cost?

6. What is the effect of the borehole friction on casingdesign in directional wells?

Optimization Model of Casing Cost

The development of the model has been based on boththe casing design theory and the theory of optimization.

JPT

P. 1269