Application of LogFAC* to Coalbed Methane Exploration in Western Canada: A Case History from Ardley Coals near Red Deer, Alberta

Abstract LogFAC fracture detection software was used as the primary exploration tool to delineate a potential coalbed methane reservoir in Tertiary age Ardley coals of the Scollard Formation near Red Deer, Alberta. Drill results show a methane saturated coal with an average of 5.8 millidarcies of permeability over 6.0 meters of pay. Introduction Permeability is considered a major control on coalbed methane producibility and is one of the reservoir characteristics sought by explorationists. An application of a new geophysical log interpretation technology for detection of fractured and permeable coals is outlined in this paper. LogFAC is a software program that re-interprets existing conventional well log data and calculates LogFAC permeability factors (LPF's) for these data [1]. The technique is applicable to developed basins where significant well control exists. In developed basins such as the Western Canada Sedimentary Basin a large database of subsurface information exists in the form of geophysical well logs. In the Western Canada Sedimentary Basin many prospective coalbed methane targets have been traversed and logged in the search for deeper conventional hydrocarbon reserves. LogFAC software uses these conventional well log data to calculate a volume of movable fluid present in the coal's natural fracture, or cleat, system, whereby a larger moveable fluid volume infers greater permeability. The use of existing well log data to locate permeable zones in coal is a cost effective method of delineating exploration targets. LogFAC Theory During drilling operations, drilling mud is used for well control, cooling the bit and cleaning the hole of rock cuttings. Drilling mud is composed of a solid clay faction suspended within a liquid faction. On penetration of a porous and permeable zone, the mud solids separate from the liquids at the well bore and form a mud cake while the liquid faction, or mud filtrate, invades into the permeable formation. LogFAC is based on a well established well log interpretation principle which states that depth of invasion of mud filtrate is inversely proportional to porosity. This statement appears counter-intuitive, as deeper invasion might be expected in zones of greater permeability. However, the opposite occurs as only a certain amount of mud filtrate can be removed from the drilling mud before mud cake formation creates a very low permeability barrier to additional invasion. This amount of mud filtrate must be accommodated within the porous and permeable portions of the formation. In a high porosity formation, invading fluids must move a shorter distance into the formation to be fully accommodated. This creates the counterintuitive inverse relationship between depth of invasion and porosity [2]. Although it is well known that some general, though undefined, relationship exists between depth of invasion and porosity, it is problematic to well log analysts as this relationship does not hold true under all circumstances [3]. The nature and limits of this relationship has remained undefined probably due to a lack of need. Several conventional well logging tools and techniques exist for determining porosity and permeability in clastics and carbonates [3], so there was no pressing reason to investigate this phenomenon further.

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