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:
