[Editor's note: A version of this story appears in the May 2020 edition of E&P. Subscribe to the magazine here.]
HP/HT wellbore designs are addressing deeper targets using more complex well paths. Solid expandable tubular (SET) systems have evolved to enable these wells to reach these deeper objectives with the planned wellbore diameter. Advancements in expandables have increased the mechanical ratings and maximum temperature of specific SET system sizes.
High-performance SET systems significantly increase pressure ratings in three sizes for a range of applications:
- 7⅝ inches (for 9⅜-inch, 9⅝-inch, 9⅞-inch and 10⅛inch base casings);
- 8⅝ inches (for 10¾-inch base casing); and
- 11¾ inches (for 13⅜-inch, 13⅝-inch and 14-inch base casings).
Compared to standard SET systems, the collapse rating doubled for the 7⅝-inch and 8⅝-inch systems and tripled for the 11¾-inch system. These systems increase the options available for designing HP/HT wellbores.
In the 21 years that SET systems have been available, more than 2,200 systems have been installed globally. More than 35,000 ft of specialized SET pipe have been installed in HP land, shelf and deepwater wellbores. Each application added significant value to the wellbore construction process by resolving unique problems, both in drilling and repairing the well. Bottomhole temperatures in these extremely deep wellbores can exceed the 400 F rating of standard SET systems. HT and HP SET systems were developed and successfully deployed that increase SET ratings by 100 F to 450 F to address these extremes. In the challenging operational and economic conditions presented by HP/HT wellbore construction, the use of these HP/HT SET systems allow an operator to add additional casing strings that may be critical in reaching the planned depth with the optimal wellbore diameter.
In drilling these wells, there is often a very narrow window between pore pressure and fracture gradient, which requires close management of equivalent circulating density (ECD). Since fluid properties are typically dictated by pore pressure and formation sensitivity, one variable to work with is the annular flow area. SET is useful in managing ECD when drilling the next hole section. Since the flow area within a wellbore increases by the square with each increment of diameter increase in the annulus, the additional diameter provided by SET versus conventional casing can have a profound effect on ECD. The rule of thumb is that each time conventional casing is run, the diameter is decreased by about 30%. With the use of SET, about half of that loss might be regained. Engineered into the optimal section of a wellbore, SET can help manage ECD through narrow pore pressure/ fracture gradient areas, which can potentially save millions of dollars on a deepwater HP/HT wellbore.
High-performance solid expandables
The need for an HP expandable was initially recognized for a North Sea operator drilling HP/HT wells in the Central Graben area. The operation required an additional casing string to isolate HP shales prior to drilling a lower-pressure interval immediately below. The drilling was being executed from a wellhead platform that had been set some years earlier; therefore, adding an additional string to isolate the shales or changing the upper and/ or intermediate casing strings were not options (Figure 1). In collaboration with the operator regarding the use of a solid expandable liner to cover the HP shales, it was calculated that the pressure when drilling the lower-pressure interval would create a collapse scenario with a negative differential of 2,000 psi to 3,000 psi on the expandable liner. This differential could exceed the rating of a standard solid expandable liner. Thus, the need for a high- performance expandable liner was determined.
The 7⅝-inch HP SET shipped to the location was run, cemented and expanded in a 17.5-ppg oil-based mud environment to cover the known HP shale section at a depth below 15,000 ft. This first 7⅝-inch HP SET system had a pre-expanded length of 1,665 ft. After the liner was installed, the shoe was drilled out and mud weight was reduced. These operations exerted more than a 2,000-psi collapse differential pressure across the post-expanded liner. Drilling continued to the next casing point where the planned 6¾-inch conventional casing was run and cemented. The wellbore was then successfully drilled to total depth (TD) and completed with the planned tubular diameters.
After the first 7⅝-inch installation, additional HP SET systems have been run in the North Sea, Caspian Sea, Middle East region and the Gulf of Mexico (GoM).
The first GoM HP SET system used in a drilling application was run in a subsalt exploration well. The objective of an exploratory wellbore is to reach planned depth with an adequate diameter to properly evaluate the potential reservoir. As an exploratory wellbore progresses, there can be numerous anomalies involving pore pressures, fracture gradients and wellbore stability. In this wellbore, the operator experienced significant wellbore instability at the base of the salt. Due to insurmountable challenges, including loss of an entire bottomhole assembly that required a sidetrack, the 9⅞-inch conventional casing was 3,400 ft higher than originally planned (Figure 2).
The operator needed a system that would preserve wellbore diameter and enable the planned depth to be reached. A 7⅝-inch by 9⅞-inch HP SET was ideal as it provided a post-expanded drift of 7.524 inches. This diameter enabled the use of a rotary steerable assembly to drill an additional 5,000 ft of directional hole below the HP SET liner, allowing the planned TD objective to be reached and evaluated. The liner had a pre-expanded length of 1,936 ft and provided for a subsequent 7-inch liner option.
The development of HP/HT SET systems with increased mechanical ratings and maximum temperatures extends expandable capabilities to address the growing extremes of HP/HT wellbore construction. Throughout HP/HT wellbore construction, SET systems can be utilized in equivalent circulating density management by increasing the annular flow area while drilling the next hole section. Attaining section TD and getting casing to the planned section TD is critical to reaching overall TD in an HP/HT wellbore.
Three HP SET sizes (11¾ inches, 8⅝ inches and 7⅝ inches) provide a range of options in dealing with unpredicted pore pressure changes.
HP SET liners also have demonstrated their value in remedial operations for restoring the pressure integrity of wellbores with severe casing wear and to cover unwanted perforations to restore production.
References available. Contact Brian Walzel at email@example.com for more information.
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