Canterra Caroline 12-12-32-6 W5M: The Drilling Of A Critical Sour Gas Well In The Post — Lodgepole Era
Authors: Landsiedel A. 
Published: March 2010

Abstract
Canterra Caroline 12-12-32-6 W5M was drilled with the objective of penetrating a Leduc reef known to contain high concentrations of H2S. Seven wells have been drilled into this reef with none of them currently being on production. Following the Energy Resources Conservation Board's (ERCB) Lodgepole inquiry and a public hearing into the proposed well, Canterra was issued a well license containing numerous conditions. This article describes the drilling of the Caroline well under these constraints.

History
Projected increasing demands for sulphur throughout the world combined with steadily decreasing sulphur reserves in Alberta prompted Canterra Energy Ltd.'s search for additional sulphur supplies. Canterra's Exploration Department identified a Leduc reefal structure known as the Bentz Carbonate Complex as having high concentrations of hydrogen Sulphide. This complex has been drilled into on seven occasions by other operators. The H2S content of the Bentz complex was found to be approximately 90%, based on DST and production test samples. None of these wells is currently on production.
Following land negotiations with partners and mineral rights owners, Canterra elected to drill the exploration well Canterra Caroline 12-12-32-6 W5M, as shown in Figure 1. The well's objective would be to penetrate the reefal edge of the Bentz Complex, above the known gas/water contact and in the 'sweetest' porosity section of the reef. Canterra began drilling preparations, including Right of Entry application, lease construction and Well License application, in February 1982. Canterra disclosed to the ERCB the nature of the reservoir fluids expected. The ERCB was in turn compelled to make public Canterra's proposal. Local public reaction was immediate and very disfavorable. The ERCB decided to conduct a public hearing into the proposal; the hearing being originally scheduled for the autumn of 1982. Unfortunately, the occurrence of the Amoco Brazeau River 13-12-48-12 W5M blowout and the resulting ERCB Lodgepole Inquiry delayed the Caroline hearing until August 1984.
The public hearing into Canterra's proposed well lasted eight days, resulting in numerous demands by the interveners and commitments by Canterra. After due consideration the ERCB handed down its ruling in the form of a well license with the final "conditions " attached. This well license is shown in Appendix A to give the reader an appreciation of these constraints. The seventh condition required that Canterra prepare and submit for approval a detailed drilling program. The drilling program, in its final approved form exceeded 100 typewritten pages and contained twenty-four separate appendices.
Caroline 12–12 was spotted July 31,1985 and drilled to total depth November 21, 1985, All provisions and constraints of the well license were essentially and successfully met. Unfortunately, the well failed to meet its geological objective of encountering a Leduc reef development.

Drilling Summary

Drilling Curve
The projected and actual drilling curves are shown in Figure 2. Differences between the two were caused in large part by the directional drilling requirements not originally expected, by slow penetration rates encountered while directionally drilling with weighted invert drilling fluid, and to circulation losses encountered with the weighted invert.