Abstract

The drilling of deep foothills wells in western Alberta presents numerous challenges. Intermediate casing in these wells is typically set at 3200 to 3400m TVD, but may be set as deep as 3600m TVD without regulatory exceptions. Regulatory requirements dictate the need for cement at least 100m above the top of the highest potential productive interval. It is also required that no annular gas is present at surface. To eliminate surface casing vent flows, it has been determined that nuisance gas stringers that are above the regulatory top of cement requirement must be isolated. This has resulted in a need to bring top of cement back inside of the previous casing.

Historically, operators opted to utilize a stage tool in this casing string to achieve a top of cement sufficient to meet regulatory requirements. While this approach has been successful at meeting regulatory requirements, remedial cementing has often been required to seal surface casing vent flows. A limitation of this approach is that the stage tool is exposed to periods of contact with the drillstring while the subsequent hole section is being drilled, resulting in wear and potential leaks.

Development of a new high performance, low-density cement system has provided the ability to address all of the above issues. Utilization of this cement system allows wells to be cemented in a single stage with the top of cement lifted into the previous casing. This paper describes the new cement system and its application in solving the problems described above. Case histories including evaluation are provided.

Introduction
Background

Successful cementation of wells in the Central Alberta Foothills region presents a difficult challenge with regards to gas migration and/or surface casing vent flows (SCVF). Specific fields where difficulties have been encountered include: Bighorn, Brown Creek, Cordel, Deanne, Lovett River, Mountain Park and Stolberg. Surface casing is normally set at approximately 610m (2,000 ft) to cover all groundwater in this region. The wells are cased and cemented with an intermediate casing at a measured depth range of 3200 to 3600m (10,500 to 11,812 ft). Then another ±1000 meter (3,280 ft) horizontal section is drilled and completed openhole.

Numerous formations can be identified as potential gas sources in these wells (Fig. 1). The Belly River sands, while not considered producible in this area, are a source of nuisance gas that may result in a SCVF. The Cardium sand, which may repeat as many as three times due to faulting, is another potential gas source. The Wapiabi, Blackstone and Blairmore shales may all contain coal seams and high-pressure inter-bedded sand stringers that can be sources of nuisance gas. The Cadomin sand and Nordegg Lime may also contain gas although they may not always be sufficiently porous in this area to be of concern.

Alberta Energy Utilities Board (AEUB) regulations1 require that the uppermost Cardium sand must be covered since it is the shallowest producing interval in the area. The AEUB also requires that the well cannot have a SCVF.

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