Mitigating drilling hazards has been challenging cost-effective well construction for decades. Balancing drilling risks against the optimum well design while preparing for unplanned drilling events combine to be a never-ending set of frustrations. The operator's authorization for expenditures (AFE) can be significantly impacted when he is fighting pore pressure/formation fracture gradient challenges or unexpected shallow flows, unstable formations, overpressure formations or depleted formations are encountered.

Excessive use of lost-circulation pills, and traditional contingency liners drive well costs up and jeopardizes reaching total depth (TD) with an effective completion. A drilling hazard mitigation solution could be as simple as using that contingency liner that was planned into the well, or the use of drill-in casing to fight an unstable wellbore. However, the use of conventional solid expandable drilling liners can drive excessive risks into the well and even cause a costly sidetracking of the well.

Operators in the former Soviet Union (FSU) have been successfully using short expandable openhole clad-thru-clad liners for decades to cost-effectively mitigate expected, and unexpected, well construction challenges. These systems are used only when necessary and cover only the trouble zone, allowing the well construction to continue without the loss of any hole size. Operators have used these solid, expandable systems as "Steel MudcakeTM," minimizing the risk of using solid, expandable liners.

This paper describes applications where this alternative solid expandable liner system has been used over the years and how a wide variety of well construction challenges have been addressed without the need for downsizing a well section.

Short, low-risk, openhole, expandable liners have been used to address drilling hazards during the well construction process, allowing continued, post-remediation drilling of the well with the same size bit. Because there is no loss of hole size through the use of these openhole "clad" liners, subsequent drilling hazards in the same-size hole section can be remediated without downsizing the wellbore.

Installing field-proven, short, solid, expandable, openhole clad liners minimizes operational risks and reduces the cost of these expandable systems. Minimizing the "flat" time on the drilling curve has allowed operators to continue the well construction process quicker, reducing rig time and cutting well construction costs.

Discussion of how risks have been driven out of the expandable drilling hazard mitigation product and case histories will be reviewed for several applications. We will also discuss how this and other conventional and non-conventional hazard mitigation "tools" can be used separately and in conjunction with one another to significantly reduce well construction risks and reduce well costs.

Introduction-Drilling Trouble Zones

Many years of experience have been documented in drilling trouble zones, such as lost-circulation zones, unstable formations, and pressure transistions1, and yet an operator can spend an inordinate amount of time dealing with these zones. On average, drilling these types of trouble zones has added 10 to 20% to well construction costs.

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