Fracture patterns and their origin in the Upper Devonian Antrim Shale gas reservoir of the Michigan basin: A review

Robert T. Ryder

Open-File Report 96-23

PATTERNS OF GAS PRODUCTION AND CHEMISTRY OF ASSOCIATED RESERVOIR WATER

The influence of fractures on gas production is demonstrated by a plot of fracture frequency, measured from fracture-detection logs in three wells in Otsego County, against gas production from these wells (Decker, 1992). The plot shows a direct relationship between fracture frequency and gas production. The three wells used in the plot were also used in fracture studies by Decker and others (1992) and Caramanica (1993). Gas production from the three wells, measured over 24 hours, ranged from 243 to 500 thousand cubic feet of gas per day (MCFGPD). Accompanying water production from the wells ranged from 25 to 85 barrels of water per day (BWPD). All the wells are vertical, received similar stimulation treatment, and were tested for their gas yield at similar times in their production history.

Average gas production for Otsego County wells in March 1993 was contoured by Richards, Walter, and others (1994). A strong northeast-southwest trend in the 100- and 200-MCFGPD contours imply that "sweet spots" in the producing trend are controlled by the northeast-southwest fracture set. Moreover, contoured variations in water chemistry (e.g. HCO3- ion) across the gas-producing trend show the same northeast-southwest trend as the gas production (Walter and others, 1993; Richards, Walter, and others, 1994).

Gas production superimposed on the 28-township regional structure map of Decker and others (1992)(fig. 8) provides an insight to the influence of large- and small-scale structures on gas production and fracture frequency. Because most gas production in the Antrim Shale is reported by lease or project rather than by well, it is expressed on the map as average gas flow rates per quarter township. The highest gas production per quarter township in the region, 100 to 225 MCFGPD, is in south-central to southeastern Otsego County (fig. 8) where structures are characterized by small-scale flexures and terraces (Decker and others, 1992). Somewhat surprisingly, the production is rather low (12 to 86 MCFGPD) along the prominent southeast-plunging anticlinal nose in western Otsego County shown on 1:1,000,000 scale maps by Fisher (1980). The one exception is an 8 sq mi area in western Otsego County and adjoining Antrim County where gas production ranges from 113 to 123 MCFGPD (fig. 8).

A comparison of gas production and structural configuration in the 4-township "sweet spot" of southern Otsego County also shows that the best gas production, and probably the highest fracture frequency, is not necessarily associated with the most obvious anticlines and anticlinal noses(fig. 9). Production assigned to each of the 200+ wells in the data set was prorated from net production data for a given lease. Although good production rates in the 4-township area are associated with the easily recognized south- to southwest-plunging anticlinal nose interrupted by small-scale flexures and terraces, better production is generally located on the west flank of the structure (fig. 10).

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