1. Drill-String and Casing Design for Horizontal and Extended Reach Wells - Part I

SPE/Petroleum Society of CIM/CHOA 79001 Drill-String and Casing Design for Horizontal and Extended Reach Wells – Part I J. C. Cunha, SPE, Petrobras Copyright 2002, SPE/PS-CIM/CHOA International Thermal Operations and Heavy Oil Symposium and International Horizontal Well Technology Conference This paper was prepared for presentation at the 2002 SPE International Thermal Operations and Heavy Oil Symposium and International Horizontal Well Technology Conference held in Calgary, Alberta, Canada, 4–7 November 2002. This paper was selected for presentation by the ITOHOS/ICHWT Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers, the Petroleum Society of CIM, or CHOA and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, the Petroleum Society of CIM, or CHOA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers, Petroleum Society of CIM, or CHOA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract This paper will address some important factors that should be considered when designing drill-strings for horizontal and extended reach wells (ERW). A second paper will look into the issues of casing design for the same type of wells and present some practical field cases and examples of drill-string and casing design for ERW. Buckling of the string and its influence on reach capability, fatigue and directional control will be emphasized. Introduction Drill-string design is of utmost importance for operations in highly deviated, horizontal and extended reach wells. It is a well known fact that drill-string failure represents one of the major causes for fishing operations which may lead to millions of dollars in losses for the Industry1, 2. This problem will be intensified when the string is submitted to the more rigorous conditions present in highly deviated wellbores. Besides that, use of an inappropriate string will have influence in the operation performance since it may impede the use of the optimized mechanical and hydraulic parameters. In extended reach wells, hydraulics plays a major role since long high-inclined sections are very difficult to clean and there is a tendency to accumulation of cuttings in the low side of the wellbore. High flow rates may be necessary to provide an efficient cuttings transport mechanism, which may result in pump pressures higher than the ones the rig pumps can handle. Among the factors that should be considered when designing drill-strings, it may be mentioned: • •

maximum expected loads; accumulated fatigue;

• • •

buckling; hydraulics; equipment availability.

There are so many variables involved in drill-string design that it is difficult to obtain a completely optimized string. However, careful consideration of the above mentioned factors will allow the operator to obtain a design that will successfully carry on the job in a cost-effective way. The Buckling Factor Drill-string buckling prediction will be very important while drilling extended reach wells. The behavior of the string in a long, high inclined slant or in a horizontal section of the well will sometimes be determinant in terms of maximum reach and steering capability. When drilling an ERW, the trajectory of the well may need adjustments according to the lithology being encountered. A body of shale, for example, may intercalate a sandstone oil reservoir. Since the shale should be avoided in order to prevent low productivity and completion problems, the well must be deviated in this point. However, in a long reach well, to deviate from this shale can be a difficult task due to the high friction forces generated by the contact between the wellbore and the helically buckled string. A helically buckled string will cause the friction force along the pipe to increase and, therefore, less force will be transferred to the bit making difficult further advances. Nowadays, with the use of rotary steering tool systems, this problem can be minimized, however, there a number of wells that are still drilled using the regular steering tools. Critical Buckling Force As stated in Ref. 3, buckling occurs when the effective compressive load exceeds some critical value. There are a number of articles3,4,5,6,7,8,9 dealing with models for prediction of the critical buckling force. Those models simulated buckling for different wellbore configuration such as vertical, inclined, curved and horizontal. Also, some of those models presented results that were apparently conflictants. An interpretation10 of those results suggests that they were derived from different situations or, as better explained in Ref. 3, different loading stages. Table I and II summarize the conclusions from Ref. 10 and Ref. 3, respectively, in terms of the axial force applied to the pipe and the shape it will assume.

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This will be a good guide in terms of buckling prediction for inclined/horizontal wells. Table I – Axial Load x Pipe Configuration10 Load F