Any discussion of drill stem requirements for world-class extended reach drilling (ERD) would be incomplete without consideration of advanced material technologies and their potential future use for enabling longer ERD objectives.

Carbon fiber composites
Currently, composite drill pipe (CDP) is approximately three times the cost of conventional steel drill pipe. As the technology improves, this price differential may decrease. CDP offers several potential advantages over conventional steel drill pipe for ERD and other critical drilling applications:
• Lower weight;
• Higher strength-to-weight ratio;
• Superior corrosion resistance;
• Enhanced resistance to fatigue; and
• Non-magnetic properties.

One major disadvantage of CDP relates to hydraulic performance and efficiency. To achieve the necessary structural properties, a composite tube must be made thicker than conventional steel drill pipe (up to twice the wall thickness). This results in a reduced inside diameter (ID) through the pipe with higher pressure losses.

Titanium
Small-diameter titanium drill pipe (TiDP) has been used on a limited scale for ultra-short radius drilling applications. Unfortunately, the cost to manufacture the product is high — approximately seven to 10 times more expensive than conventional steel drill pipe. TiDP offers significant advantages over conventional steel drill pipe for ERD:
• Lower weight — density is 56% that of steel;
• Increased strength to weight ratio of approximately 37% compared to S-135 steel drill pipe;
• Highly resistant to corrosion and erosion; and
• Good fatigue resistance; it doesn’t suffer from corrosion fatigue but can be notch sensitive in fatigue inducing situations.

Titanium is more flexible than steel, which may present certain challenges in ERD. With

Figure 1. TurboTorque double-start thread reduces the number of revolutions to assemble the connection by 50% when all other things are equal. Changes in thread taper and thread pitch further reduce revolutions in total from 13 to four.
roughly twice the deformation for the same induced stress, TiDP may have a delayed response to loading conditions. This can likely be accommodated in drilling operations but there could be a significant learning curve.

Titanium is also known to have significant wear rates. Thus, mid-body wear of TiDP joints is a significant risk that may require development of advanced wear “knots” or other techniques to maximize the usable life of TiDP strings.

Despite these concerns, there is little question that titanium could be used to make a high performance drill string for pushing the ERD envelope. But a serious consideration is cost. TiDP can be an order of magnitude above the cost of steel drill pipe.

Aluminum
Aluminum drill pipe (AlDP) shares some of the desirable features of TiDP at a significantly lower cost. Advantages of AlDP include:
• Lower weight;
• Good corrosion resistance (although it can be susceptible to corrosion in some mud systems);
• Enhanced fatigue resistance; and
• Non-magnetic.

AlDP generally costs about twice that of conventional steel DP although this is highly dependent on the specifications for each product. Some sources indicate that this cost deferential may have decreased recently.

AlDP also has some disadvantages.

It has relatively low yield strength of approximately 69,000 psi (highest yield-strength alloy used for drill pipe).

Consequently, it has a lower strength-to-weight ratio than ultra high-strength steel drill pipe when factoring in the steel tool joints attached to the aluminum tubes. It generally requires a greater wall thickness than steel drill pipe, adversely affecting hydraulic performance. AlDP can suffer from accelerated wear rates to the aluminum tube body. In addition, its yield strength in service can drop off dramatically at temperatures above 250°F (121°C).

High-strength steels
High-strength steels represent a near-, mid- and long-term technology for ERD. Current high-strength grades available on the market today are Z-140 and V-150. These grades provide 4% and 11% improvement in strength-to-weight ratio respectively compared to S-135 drill pipe and carry only a relatively small cost premium over S-135 drill pipe. The industry has been somewhat slow to adopt high-strength grades primarily due to concerns regarding reduced ductility and toughness of the steel. However, manufacturers have significantly improved this technology and can now offer these grades with toughness levels
better than S-135 drill pipe processed to standard API specifications. The substantial improvement in obtaining high toughness in high-strength steels has led drill pipe manufacturers toward development of ultra high-strength steels such as UD-165. The development of a UD-165 grade with 165 ksi yield strength tubes would provide a 22% improvement in strength-to-weight ratio compared to S-135 drill pipe. This would be second only to TiDP by 15% in strength-to-weight ratio. It is likely that the cost of UD-165 would be substantially less than TiDP.

Table 1 provides a summary of advanced material comparisons alongside S-135, Z-140, V-150 and UD-165 steel drill pipe products. When considering strength-to-weight ratio, many have neglected to factor in the steel tool joints attached to the non-steel alternative material tubes. Rather, they have focused on the strength-to-weight ratio improvement of the material density only. This is somewhat misleading and errors on the side of promoting non-steel alternative materials. Table 1, however, factors the presence of steel tool joints and uses a conservative approach (optimistic) by analyzing range three drill pipe products.

Connections
A third-generation, ultra high-torque connection, TurboTorque, has recently been developed by Grant Prideco. It incorporates a double-start thread that reduces the number of revolutions to assemble the connection by 50% if all other things are equal (Figure 1). This new connection is designed to increase the reach on critical ERD wells while improving efficiency and lowering well cost by reducing handling, makeup, running and tripping speeds.
Conservative estimates suggest that the new connection may save a minimum of ~111¼2 hours in planned trip time for a 30,000 ft (9,150 m) well. The new connection provides increased mechanical and hydraulic performance compared to second generation high-torque connections.

BHA connections for ERD
Drill string manufacturers have developed proprietary, double-shouldered bottomhole assembly (BHA) connections aimed at maximizing fatigue performance. These enhanced connections have undergone design verification, laboratory testing and aggressive field trials. Results indicate that one enhanced BHA connection, DC58, provides at least nine times greater fatigue resistance than its API counterpart connection, 65¼8 Regular. In addition, noticeable improvement in running time and material loss upon recut have also been observed.

Changing from a standardized connection to a proprietary connection throughout the BHA requires a significant step change in connection performance. At least nine times improved fatigue life and risk reduction justifies the use of this connection within BHAs in aggressive drilling programs such as ERD wells.

EDITOR’S NOTE: Information in this article is from OTC 18512: “Drillstring Technology Vanguard for World-Class Extended-Reach Drilling,” by M.J. Jellison, R.B. Chandler, M.L. Payne, and J.S. Shepard, presented at the Offshore Technology Conference, Houston, Texas, May 2007, copyrighted by OTC and used with permission.

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