TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractMitigating drilling hazards-balancing drilling risks against the optimum well design while preparing for unplanned drilling events-has been a challenge to cost-effective well construction for decades. The pore pressure/formation fracture gradient balancing challenges mixed with the unexpected encounters with shallow flows, unstable formations, overpressure formations and depleted formations makes AFE (authorization for expenditures) goals dim.Excessive use of loss circulation pills and traditional contingency liners drive well costs up and jeopardize reaching total depth (TD) with an effective completion. A drilling hazard remediation solution could be as simple as using that planned contingency liner or using drill-in casing to fight sloughing formations. 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 Gulf of Mexico, the North Sea and in Asia Pacific have been successfully using a set of proven well construction tools with a more "Fit for Purpose" application to mitigate drilling problems that have resulted in excessive nonproductive time (NPT) during drilling operations. These systems are used only when necessary to mitigate the well challenge, allowing the well construction to continue while minimizing their NPT fighting these well problems.This paper describes some of these "fit for problem" well construction tools and their applications in recent case histories.

The objective of this work is to investigate the influence of cuttings size in Casing Drilling to plug pores for fluid loss control, and thereby reducing formation damage. Historically, the emphasis of drilling operations has been to drill wells cheaply and quickly without much consideration for the resulting impact on well productivity. Formation damage occurs frequently and rapidly during drilling operations with potentially severe consequences. The Plastering Effect of Casing Drilling reduces solids and filtrate invasion, resulting in less skin damage and improved productivity. It also reduces formation damage due to cement filtrate by creating a gauged wellbore and proper casing/wellbore standoff. The Casing Drilling process grinds drill cuttings as they travel up the annulus and creates a larger particle size distribution (PSD) profile than conventional drilling operations. These finer cuttings are subsequently smeared into the wellbore face by the mechanical contact of the large diameter casing with the borehole wall. The result is a very high quality, tight, thin, almost impermeable mud cake that isolates the formation from the wellbore. Evidence shows that cement binds very well to this type of mud cake. The PSD analysis determined that the smaller size and wide range of Casing Drilling cuttings make it possible for these particles to readily adhere to the wellbore; this helps seal the pore spaces of the formation and prevent further solids and filtrate invasion. Pore throats can most effectively be plugged when the cuttings are in the proper micron size range as any possible gap between the mud and cuttings PSD can be covered by adding minimal amounts of properly sized lost circulation materials. Casing Drilling has proven to be a unique approach in mitigating formation damage due to the drilling process. One case study confirms that reservoir sections drilled with casing show enhanced productivity as much as twice the wells drilled conventionally. The Plastering Effect, as an inherent benefit of Casing Drilling, keeps the producing formation as intact as possible and reduces formation damage.

The gas fields of the western Piceance basin in northwestern Colorado present significant challenges to drilling and casing operations. This area has complex geology, with dipping formation beds that lead to "crooked hole" drilling. Fractured formations cause problems, including lost circulation while drilling, and failure to return cement to surface during primary cement jobs of the 9 5/8-in. casing. Sometimes casing cannot be run to total drilled depth. After review of the problems, an operator in the region concluded that a different approach was warranted and selected drilling with casing (DwC) as an alternative to investigate. DwC, combined with stage cementing of the surface casing, was expected to yield a significantly more effective surface-hole drilling and casing operation, reducing nonproductive time (NPT) and the associated cost. This paper reviews the problems encountered in conventional surface-hole drilling and casing running operations in the Piceance basin. It also reviews the operator's decisions for a DwC approach. A case history of a DwC-with-stage-tool cementing operation and its successful conclusion is presented. Background The operator has been drilling in the gas fields of the Piceance basin since 2003. Figure 1 shows a general map of the area. The operator has experienced difficulty drilling and casing the surface hole, which is typically targeted to approximately 3,100 ft measured depth (MD). Problems are caused by dipping formation beds, rock stresses, and lost-circulation intervals. Conventional drilling practices use mud motors and low weight on bit (WOB) to drill a 12 1/4-in. surface hole because high WOB with conventional drilling assemblies often results in severe inclination increases, sometimes in excess of 7°. Table 1 shows a typical conventional bottom hole assembly (BHA) previously used to drill surface hole in this area. Table 2 shows maximum hole inclinations measured in the surface holes drilled with both conventional BHAs and DwC. The surface-hole geology predominantly consists of sandstone, siltstone, and limestone stringers with interbedded shales. Figure 2 shows a typical mud log section of the surface interval. The naturally fractured formations often lead to lost-circulation problems. With insufficient volumes of drilling fluid being circulated, sloughing shales and subsequent pack-off and sticking situations result. After drilling, the hole conditions encountered make running the 9 5/8-in. surface casing to the planned depth problematic, even when the casing is washed down. Many times, as seen in the conventionally drilled holes, the operator has not been able to get the 9 5/8-in. casing to its planned setting depth. On two of the conventionally drilled wells, the casing was set 300 ft and 427 ft short of total depth (TD). Unplanned hole conditioning trips have often resulted in NPT and, in the worst cases, fishing operations have been required because increased open-hole exposure time has led to stuck pipe. NPT was as much as 15 days in the worst case. The hole conditions also often have led to poor-quality primary cement jobs and problems in satisfying the requirements of the local regulatory authority, the Bureau of Land Management (BLM). Failure to circulate cement to surface resulted in the requirement to run a cement bond log (CBL) to evaluate the integrity of the cement job which, in turn, often led to the need for remedial cementing and wireline operations to reach an acceptable quality of cementation. This remedial work often has resulted in 3 to 4 days of NPT. Table 3 shows NPT as a result of drilling problems, casing running problems, and cementing issues before and during implementation of DwC technology. Evaluation of DwC With knowledge of surface-hole difficulties, the operator began to investigate alternative solutions to fulfill the following objectives:

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