Drilling trouble zones has haunted drilling operations experts through the years. Conventional methods for mitigating formation problems encountered during the drilling phase of a well, such as unstable formations, overpressured formations, and loss zones, are often costly and ineffective. Age-old methods of simply setting a casing and drilling ahead with a different mud weight and a smaller hole size are becoming increasingly unacceptable as the industry strives to reach deeper reserves.

The use of cement, chemical "cocktails," and loss-circulation materials can significantly increase well costs. Using conventional solid expandable drilling liners which are tied back to the previous casing can also be cost prohibitive and lead to non-standard equipment requirements for the following section of the well.

This paper explains how a non-cemented, solid expandable open-hole clad can be installed remotely from a previous casing to isolate problem formations so that the target setting depth can be reached with conventional casing, without sacrificing hole size.

Installation of a solid expandable open-hole clad will result in mitigation of problem sections and allow drilling to continue with the same hole size. The problem formation is straddled with a short length of expandable casing or clad with a resultant drift ID equal to the diameter of the original hole or drill bit. With this full bore ID, further clads can be run through, and deployed lower in, the same hole section.

In addition to reducing operational risks associated with expanding long lengths of conventional tied-back solid expandable open-hole liners, the open-hole clad or "Steel Mudcake" significantly reduces well costs and results in higher and earlier production associated with the increased hole size in the reservoir section.

Open-hole patch systems have been used extensively with excellent results in carbonate formations, in remote locations over the past 20 years. This type of system is expected to have a significant effect on the global drilling industry because it will allow downsizing of conventional casing designs by eliminating conventional and expandable contingency casing strings without sacrificing completion and production performance. This advantage allows the operator to drive down well costs by minimizing the well's "flat" time on the drilling curve and using conventional long, tied-back expandable drilling liners.

Introduction---Mitigating Casing Trouble Zones
Application of Expandable Solutions for Mitigation of Casing Trouble Zones

Studies have shown1 that 7% to 11% of intermediate casing strings are damaged in all wells drilled. Some of these damaged casings have allowed contamination of upper fresh water formations. Casing and cement sheath integrity can be damaged in the process of production due to corrosion and aging of wells and the average well life before some kind of casing failure is ∼10–12 years. The remediations of these casing trouble zones have approached 15% of the cost of drilling a replacement well, with the total rig time exceeding the rig time it would take to redrill the well.

Traditionally, casing repair techniques such as cement squeezes and simply running an intermediate string of casing have been used. However, cement squeezes can be costly when multiple squeeze jobs result in little or no isolation of the trouble zone. The use of a conventional intermediate string of casing can significantly reduce the working wellbore diameter (Figure 1). This wellbore diameter loss can come back to haunt the operator when the well needs to be deepened later in its life, or a larger, more efficient completion would yield more production.

Product adoptions of the cased-hole expandable products have been slow in the past. As with any product the value that it offers needs to surpass its cost and therefore the application for these products must be strictly analyzed to weight their benefit against their cost. Figure 2 illustrates how multiple squeeze jobs can be a non-cost effective means of solving poor zonal isolation compared to the use of a cased-hole solid expadandable liner.

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