High-Collapse, Cost-Effective Solid-Expandable-Monobore Systems Mitigate Subsalt Rubble Zones
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 19008, "Mitigating Subsalt Rubble Zones Using High-Collapse, Cost-Effective Solid Expandable Monobore Systems," by Edwin J. Cruz and Robert V. Baker, SPE, BP plc, and Pat York, SPE, and Lev Ring, SPE, Weatherford, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May.
The reduction of wellbore diameter during well construction has been addressed partially through use of conventional solid-expandable open-hole liners. However, these systems still reduce pass-through diameters and in most cases necessitate significant deviation from the well's basis of design (BOD). For years, end users of solid-expandable liners have needed a simple, cost-effective means of extending casing shoes while maintaining the same wellbore pass-through diameter. The full-length paper describes the application of well-construction techniques using a set of high-collapse, cost-effective monobore systems.

Introduction
Formations entrapped by salt deposits have been the source of significant hydrocarbons, with salt deposits commonly recognized in the Gulf of Mexico (GOM), offshore West Africa, the Middle East, and the North Sea. Hydrocarbons located adjacent to the salt in the GOM have been the growing focus of deepwater exploration over the past decade. Some of these salt deposits are quite thick, with wells being drilled through and around these beds to more than 30,000 ft true vertical depth (TVD). The first drilling challenge of hydrocarbon objectives extending deeper has been just to reach the targets. The second challenge is to complete these wells with sufficient hole size to make these developments economically successful.
To address the challenge of penetrating these salt formations, operators have found that solid-expandable liners have contributed greatly to successful drilling operations. Even with years of successful installations, the low collapse of these thinner-wall expandable liners has caused significant concern. While the use of expandable liners slows the premature slimming of wellbores when contingency liners are necessary, end users have required a liner that afforded zero hole loss after installation of an expandable liner. A solid-expandable liner that has more than twice the collapse of conventional expandable liners plus zero hole loss post-expansion can facilitate successful and cost-effective drilling into, through, and beneath salt formations.

Drilling Challenges
The majority of nonproductive time associated with drilling for hydrocarbon deposits associated with salt formations is actually drilling the sediments into and out of the salt diapir. Examples of these hazardous drilling environments include the following.Shale sheaths.Suprasalt "carapace."Subsalt "gumbo."Subsalt or extrasalt "rubble" or breccia-like zones.Active or relict translation surfaces along the salt face.True caprock.In addition to these formation challenges, higher pore pressures and anomalous rock fabric can be present in the sediments surrounding the salt.