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   The Monobore Open-Hole Liner system is made up of two components, a tieback shoe run on the bottom of the previous string of casing (13-3/8 or 13-5/8 in. conventional string) and the monobore expandable liner itself. The tieback shoe (Fig. 1) is made up of one or more joints of 14-1/2 in. or 15 in. semi flush joint casing that is crossed over to the 13-3/8 in. casing. Its outside diameter (O.D.) is approximately the same O.D. as the collars of the 13-3/8 in. casing. The tieback shoe utilizes a fibre glass inner-string so that its special drift is not exposed to cement during the cementing operation of the 13-3/8” string. After the 13-3/8 in. string is set and the next hole section is drilled, the 11-3/4 in. expandable casing is run and expanded. The top of the expandable liner is expanded into the 14-1/2 in. or 15 in. tieback shoe resulting in an expanded drift of 12-1/4 in., the same as the previous string of 13-3/8 in.

   Since the first conventional expandable liner was run in the GOM in 1999, solid expandable openhole liners have evolved to provide the industry with a large selection of sizes. However, these conventional expandable liners do not meet the demands of pore pressure and fracture gradients of the wells drilled in the region.

   For instance, the collapse of expandable liners is mostly driven by the thickness of the casing. The most effective path to creating an openhole expandable liner with a larger amount of collapse lies in building the liner from a tube manufactured with a thicker wall.

   Compared to a conventional 11-3/4 x 13-3/8 in. solid expandable liner, the 11-3/4 x 13-3/8 in. Monobore openhole liner system has the following features, advantages, and benefits:

Zero hole-size loss (post expansion drift equal to 12-1/4 in.);

Post-expansion collapse over twice that of the conventional expandable openhole liner of the same size (~2,000 psi versus ~900 psi) with tubular equal to 0.582 in.;
An expandable and collapsible cone;

Expandable metal-to-metal connectors qualified to ISO standards;
Seamless tubular;
~30 percent lower expansion pressure required;
System pressure contained using a valve.
The system’s ability to pass through the 13-3/8-in. conventional casing and expand large enough to have a post-expansion ID the same as the ID of the 13-3/8 in. conventional casing lies in two of the system’s attributes.
The first attribute consists of special-sized flush joint casing run on the bottom of the 13-3/8-in conventional casing, forming an oversized tieback shoe. The tieback shoe’s OD is similar to that of the 13-3/8 in. collars with an ID large enough (~13.642 in.) to accommodate the Monobore’s expanded 11-3/4-in. casing and still allow the Monobore liner to have an expanded drift of 12-1/4 in. (the same as the 13-3/8 in conventional casing).

   The second attribute is an expansion cone that is carried into the well in a transverse or collapsed mode and does not require an oversized launcher to house the cone. The cone actually has the ability to create its own launcher downhole when the cone is assembled or “expanded”. The OD of this over-sized shoe is approximately the same as the collars of the previous casing and enables casing to be expanded with a 0.582 in. wall versus 0.375 in. This added casing thickness yields a post-expanded collapse over twice that found with conventional expandable liners, yet exhibits zero hole-size loss after the liner has been expanded.

   Utilising a nine string casing design in many cases limits options, as trouble zones are encountered. This casing design is typical in the Caspian area.
This in many cases allows for completion of the well although potentially seriously reducing productivity and ultimately potential field drainage.
In certain cases this can result in a total loss of the well bore due to the inability to set additional strings as barriers are exhausted.

   The use of expandable technology can provide an additional two or three casing strings this is achieved by utilizing expandables in the upper hole section.

   A single Monobore extension will allow isolation of problematic zones in the upper well bore resulting in the ability to overcome the issues created by the weight of long 16-in. strings which in many cases can exhaust drilling rig capability and result in an operator having to commit to prematurely setting this string of liner prior to the pressure event envisaged.

   The implementation of this technology will allow for the next 13-3/8-in. string with an oversize shoe to be run and still allow for drilling to continue utilising an underreamer giving sufficient hole size to allow for adequate cementation and expansion of the Monobore liner resulting in a 12-1/4-in. drift, an additional 11-3/4-in. flush liner system can be set at a pressure event deeper in the well bore ultimately keeping the planned casing design on track while securing the drilled sections behind pipe.

   In areas such as the Caspian where casing strings are set at short intervals the necessity to have additional barriers is a planned operation, with the utilizing of an oversize shoe this allows for the monobore to remain a planned contingency until required.

   In the event the monobore solution is not utilized the next planned casing can be run and tied back to the previous conventional casing string.
With the addition of this technology it gives the operator the option when required an additional casing string in hand.

   Additional expandable solutions can be utilised as contingency as the well deepens the inclusion of these additional sizes will result in a loss of hole size but in many cases still allow for flush joint casing to be implemented.

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