Boosting performance in unconventional shale plays

Vallourec’s pipe mill in Youngstown, Ohio, delivered its first pipe in December 2012.

Unconventional plays have challenged operators to refine drilling and completion techniques everywhere in North America. As an example, production in the Wolfcamp and Cline formations continues to rise. Operators are exploring new technologies to capitalize on the Permian basin's complex multi-layer reservoirs—containing light tight oil, gas and NGLs. Horizontal drilling, a key component of unconventional drilling, has been the answer to many operators’ search for increased efficiency. However, as horizontal drilling develops in the Permian basin, operators face a unique set of challenges while developing the stacked reserves with new drilling methods.

CASING CHALLENGES

Operators in the Permian have sharpened their technique to solve a complex equation. They quickly realized that they needed improved casing performance ratings to increase treatment pressures, and more efficiently fracture the formations. To drill deeper wells, with longer laterals, and to get to bottom, where they couldn’t before—while keeping costs down—they needed to do something different.

The solution came with the chemistry of Vallourec’s P110EC proprietary grade casing. P110EC is compliant with API P110, but it fully meets API Q125 specifications for yield strength. The heat treatment process is done in such a manner, that pipes have a minimum yield strength of 125 ksi, 13.6% higher than the minimum for a standard P110 product. This 13.6% increase applies to all physical attributes of the pipe—burst, collapse, tension and compression. The improved performance ratings have allowed operators to increase pressures and reach these four goals.

Higher injection pressure was the main driver in the need for higher performance materials. Fracturing fluid is pumped through the casing to the total depth of the wellbore through perforations into the formation during fracturing treatment. Formation pressure and total MD define the frac pressure required, which, in turn, drives the OD, wall and grade of casing used. Operators are pushing the performance envelope by drilling deeper with extended laterals. As the laterals are extended, system back-pressure builds, requiring higher burst pressures to efficiently fracture the formation.

A major operator in Texas faced a dilemma with the new fracturing program that it was considering. This operator utilizes a common industry safety factor of 1.1 in its burst load calculations, but it could not achieve that safety factor with standard P110 grade casing. The basis of the new program was to use frac pressures over 10,000 psi that could fracture the formations in a more reliable manner and generate additional production. The standard 14,530-psi-rated burst of 5.5 in. x 23# P110 did not offer enough internal pressure capability to frac these wells after accounting for hydraulic build-up and pop-off pressures of the equipment.

Existing industry options left them with three choices—lower stimulation pressure, increase casing weight per foot, or look at a more expensive casing grade. Working with Vallourec, the operator was able to utilize P110EC in its designs as an alternative to the other three, less-attractive, industry solutions. The increased burst of 16,510 psi (+2,020 psi) allowed the operator to sustain the higher pressures, while maintaining required safety factors. Numerous wells were completed successfully, and the P110EC product continues to deliver the high frac pressure, which is key to
its program.

GOING THE DISTANCE

Easy-to-reach, plentiful oil is becoming a thing of the past. Shallow onshore wells are being replaced by deeper, and longer, horizontal well designs requiring more casing strings. The greater depth and horizontal drilling reach in these wells add significant length and weight to the strings. On top of the physical weight, capped end pressure effects during the frac job intensify tension in the string. Both of these elements combine to create a high-tension stress load on the casing string. In some cases, this load exceeds the usual P110 limitations.

API P110 casing can handle 641,000 lb of tension for the typical size of 5.5 in. x 20#. If an operator were to stimulate the formation with 12,500 psi, the capped end pressure effect would decrease the allowable tension limit over 225,000 lb. With a safety factor of 1.4 for tension, the allowable string length would then drop to 14,860 ft. As an example:

Pipe ID = 4.778 in.

Pipe inside area (A) = 17.930 in.^2

Capped end effect (CEE) = A x Pressure

= 17.930 in.^2 x 12,500 psi = 225,000 lb

Allowable tension (AT) 

= Casing limit – CEE

= 641,000 lb – 225,000 lb = 416,000 lb

Allowable casing length 

= AT/casing weight

= 416,000 lb/20lb/ft

= 20,800 ft

With a safety factor of 1.4 = 20,800 ft/1.4

= 14,860 ft

However, P110EC casing in this same size is capable of 729,000 lb in tension. Running the same calculation allows for a string length of 18,000 ft, while keeping a healthy safety factor of 1.4 in the design. API P110 has been capable in some well programs, but, in today’s environment, operators are seeking wellbore integrity and an increased safety factor. Many large operators around the North American shale plays rely on P110EC, as they drill deeper and run longer casing strings to meet those needs.

TO TORQUE OR SET DOWN

Depending on the shale play, operators have two ways to get to bottom and set casing into the lateral: torque or set down weight. P110EC is capable of 13.6% more compressional load and 10.6% torsion with Vallourec’s DWC/C connection than API P110 in 5.5 in. x 20#. These two performance enhancements give operators additional capability to make it to bottom in otherwise problematic wells.

An independent operator approached Vallourec, to assist it on a problem that it was having, getting pipe to the end of the lateral. The well program was based on 5.5 in. x 20# P110 casing with semi-premium connections, but torque limitations continued to disrupt the operation. 

After review, the operator agreed that a rotational torque of 20,000 ft/lb would allow it to rotate to bottom. Looking directly at connections, DWC/C offered 19,800 ft/lb in the P110 that was utilized. Rather than increase weight/ft, or move to a premium connection, to improve performance and reduce project costs, P110EC was selected as a more efficient alternative. The boost in yield torque, to 21,900 ft/lb, met the required safety margin. With full fit-for-purpose testing aimed at unconventional wells (fatigue, frac simulation, pressure testing), the DWC/C connection, paired with the higher-strength P110EC, gave the operator the required torque. A new generation of DWC/C-IS PLUS connections offering internal shoulder and thicker coupling OD extends those performances even further when paired with P110EC.

CONSIDERATION FOR PROFITABILITY

Historically, when a higher-performance casing string was required, two options were available—either increase the pipe wall thickness or opt for higher strength grades, such as Q125. Both options would significantly increase the cost of any drilling program. Understanding that the profitability margin of each well in shale plays can be narrow, Vallourec researched how to help operators manage cost.

As an API grade, P110 has a wide yield-strength band that the product can fall into. But understanding and controlling the manufacturing process gives the manufacturer the ability to narrow the product’s yield strength range for a fit-for-purpose material. For P110EC, Vallourec applied that limit to the high side of the yield-strength band. Fully meeting API P110 specifications, P110EC became the standard product for Vallourec when P110 is needed. All Vallourec P110 product is manufactured to the high side of P110 physicals (and the low end of Q125 physicals.) For wells that would otherwise require a thicker wall pipe, or more exotic steel, these savings are realized directly in a product sold by weight. For those that don’t need the higher physicals, standardizing on this product has made inventory management easier, and has offered the customer cost control, combined with better well integrity.

CONTROLLED MANUFACTURING

Starting with raw material, Vallourec’s domestically produced P110EC is designed to meet the company’s customers’ most demanding needs. The company begins the process by melting the steel in its Youngstown, Ohio, facility into billet bars of two sizes. During the smelting process, chemistry is controlled for final steel properties and repeatability of P110EC from one order to another. Billets are rolled into casing in one of two Youngstown facilities from OD’s ranging from 23/8 in. up to 103/4 in. (Larger sizes can be provided.) 

After the pipe has been sized, it moves to one of three heat treatment facilities (Youngstown, Ohio; Houston, Texas; or Muskogee, Okla.) to reach its final physical properties of P110EC. Additional physical testing and collapse testing are performed for every production run, to ensure that it meets the product requirements. Finally, after heat treatment, the pipe is threaded, based on the customer’s well design needs prior to shipment. By controlling all aspects of production—from melting to threading—P110EC is a consistent and repeatable grade that provides operational and economic benefits to operators in shale plays.

APPLICATION SPECIFIC

Fig. 1. Vallourec’s enhanced yield strength range, compared to standard API.

For even more challenging applications, Vallourec offers a large choice of proprietary grades beyond P110EC, Fig. 1. Whether it is mild sour environments, a concern for hydrogen embrittlement, or a need for higher strength, the Vallourec family of proprietary P110 options is available for numerous applications. In the same manner as the enhanced collapse product, the company has taken the non-restrictive P110 that API regulates and developed industry-driven products to meet more specific needs. Each of these proprietary P110 grades—RY, MS, EC, HCE—is fully compliant with P110.

MS—This grade is fully compliant with API P110, but it is enhanced to provide a mild sour service capability. The H₂S resistance comes from controlled yield strength and an enhancement of the chemistry. Vallourec performs a NACE method A test in the mild sour condition, for every heat lot, prior to shipment. This is a cost-effective product for mild sour service environments, which can be found in shale plays around North America.

RY—Fully compliant with API P110, with the enhancement of controlled yield strength. This can be useful in a variety of conditions, where potential high-stress situations need to be managed.

HCE—Collapse and burst properties of OCTG are influenced by a combination of pipe geometry, such as OD, ovality, wall thickness, and eccentricity, as well as material properties, such as yield strength and residual stress. HCE products start with a higher minimum yield strength, which increases base burst and collapse ratings. Controlling the most influencing physical parameters during production allows Vallourec to enhance and guarantee collapse properties over API. All high collapse ratings are confirmed by standardized physical collapse tests.

SUMMARY

Shale reservoir characteristics are quite complex, with each play offering its own unique combination of operational challenges. When pressures are rising, laterals are extending, safety margins are shrinking, and profitability is critical, P110EC is a competitive choice for horizontal drilling and multi-stage hydraulic fracturing that enables the economic production of unconventional oil and gas. With high strength and performance, comparable to a more exotic material, P110EC can extend the operating envelope, mitigate risk, lower the total cost of ownership and increase the life of the well.