Although the reality of a commercial monobore does not exist yet, most of the essential technology has been proven. This makes a single trip mono-diameter casing string from wellhead to reservoir-toe much more likely.

In the late 1990s, a relatively small group of engineers within Shell E&P, Halliburton and Baker Hughes laid out the plans for a technology that would have made Erle P. Halliburton smile.
By forming technology ventures with Enventure (Halliburton) and the now defunct E2Tech, Shell gave the nascent expandable market the support it needed. Later Shell would go on to sign deals with Weatherford allowing it to enter the expandable market.

In parallel to these deals, some service companies had already developed the expertise to expand slotted tubulars and were realizing commercial downhole applications. However, similar commercial applications for solid tubulars have only become available in the past 2 or 3 years. Now a broad range of operators have expanded solid tubulars to overcome well construction challenges such as preserving wellbore diameter, isolating lost circulation zones below casing shoe and sealing off swelling or poorly consolidated formations.

It is estimated that more than 300 solid expandable applications have been conducted with Enventure appearing to be market leader. In the deepwater arena, the technology offers a real alternative to the seven or eight string casing configurations where "telescoped casing" or "borehole tapering" can severely restrict the production hole diameter in the geological objective. Another feature of the technology is that, through "localized" applications, repairs can be made to damaged or worn casing while patches or entire old casing strings can be replaced without the need for costly cutting and pulling of casing. From an engineering perspective wellbore stability and burst/collapse ratings of casing can be maintained in this way.

Offered as an Openhole Liner (OHL) System, Enventure Global Technology's system provides an effective and cost-saving option for many planned and contingency openhole operations. The system can provide operators with an extra string of casing, which can be the decisive factor in terms of successfully drilling deepwater prospects. Increased casing section length without compromising casing diameter is especially useful in operations where large diameter tophole casing sections are otherwise technically or cost prohibitive. The system enables operators to extend a conventional casing program for an exploratory well to reach promising zones that are deeper than anticipated.

For unexpected lost circulation or shallowwater flow zones in deepwater and sub-salt environments, the OHL System provides affordable contingency solutions.

In sub-salt environments, the system offers a cost-effective solution for reaching total depth with larger production casing. Unexpected trouble zones are a common challenge in sub-salt or deepwater low-fracture-gradient environments. The openhole technology allows the operator to simply drill another hole section to bypass these zones. In older fields requiring redevelopment, the system can help reach deeper reserves and isolate water or gas zones that have penetrated horizontal re-entry wells. The well is drilled to the target reservoir, casing is run, cemented and expanded.

Baker Oil Tools' comprehensive offering of expandable products provide cost-effective solutions to well construction and remediation. Baker's expandable solid tubulars will also be aimed at slimmer monobore wells that will replace conventional telescopic casing. Again, this technology holds much in store for deepwater fields where deep targets below the mudline may not be accessed economically with conventional technology.

But before the economies of scale regarding standardized casing design and supply materialize, there are still "gun-barrel" operational and design challenges. These challenges must be overcome before the monobore can become a commercial reality. These challenges are related to smooth, underreamed gauge hole in all formations, increasing the expansion ratios of underreamers to above 25% of pass through or body size, calipering, cementing type and method, maintaining a consistent internal diameter of casing that has been expanded at connections, reducing the risk of swab/surge dependent on the expansion method. Here, rotary expansion may have some advantages as the application of torque and weight is used to expand the casing as opposed to weight/force applied axially. At any rate, top down expansion is always preferable because if the expansion mechanism fails, then any subsequent fishing can be achieved more easily. In the opposite it is harder to fish a larger diameter component into or within a smaller diameter as would be the case of bottom up expansion.

This column is partly based on a keynote address, "Reaching the potential of the monobore: intelligent tubulars, drill-pipe and underreamers," presented by the author at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, September 2004. Slimhole completions such as coiled tubing fall beyond this scope - our interest lies in 8 1/2-in. or larger diameter conventionally rotary drilled well bores.

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