Implication of Understanding Diagenetic Evolution to Completing a Deep, High-Pressure Devonian Limestone Reef Well in Alberta

Jack Wendte, Howard King, Hairuo Qing

In 1990 Canadian Hunter encountered overpressured gas in a Leduc limestone reef at 14,000 feet of burial in west-central Alberta. The pressure gradient of .91 pounds/foot was more than twice the normal gradient. A primary concern on completing this well was whether a potentially uncompacted reservoir would collapse around the well bore due to gas withdrawal during the initial stage of production. This concern was addressed by petrographic and analytical compressibility analysis.

Porosity in this reef is largely confined to intercrystalline, micropores in the reefal allochems and to micro-fractures cutting the allochems. Larger primary voids within the reefal allochems had already collapsed. The overcompacted fabric of these rocks implied that the reef had been buried to deep depths and had nearly reached its point of maximum competency prior to overpressuring. Thus, the reservoir would not undergo any further significant rock collapse and resulting destruction of permeability during gas withdrawal on production.

Analytical compressibility analysis, undertaken to simulate the effect of gas withdrawal, verified this conclusion. Thus, production from this well could be sustained at economical rates for a normal duration upon completion and pipeline tie-in.

The well initially produced at 8 MMcf/d and has sustained an average production rate of over 4 MMcf/d for the past year. It has an accumulative production of 3 Bcf to date. These results support the conclusions based on petrography and compressibility analysis.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995