In a Gas Research Institute sponsored study, enhanced formation image analysis was performed on twelve wells in the Antrim Shale, Michigan Basin. The analysis revealed that the abundance of open and partially open fractures as well as their degree of interconnection are controlling factors in gas production. Borehole maps show the interconnection with reference to the borehole.
A fracture factor (Zf) was derived and plotted against gas flow rates for nine wells. Six wells treated with a single- stage stimulation show a linear relationship between (Zf) and Q. Two were stimulated with a two-stage treatment and produced more gas than single-stage wells with comparable values of Zf. The methods of fracture analysis derived for the Antrim Shale should have immediate practical application to other unconventional and tight reservoir rocks.
In many tight and unconventional types of reservoirs, natural fractures have been recognized as one of the key geological features that controls production rates. This study was performed to determine the relationship between gas production rates and the natural fracture network for the Antrim Shale in the Michigan Basin. Log and production data were provided on 12 wells in which either formation microresistivity (FMSTM) or acoustic reflectance (CASTTM, BHTVTM) formation imaging data were available as shown in Table 1. The formation imaging data were analyzed within each shale unit to determine fracture abundance and spacing, orientation (strike, dip), type of fracture (open, partially open, closed), and interconnectivity of the fractures as they are projected away from the wellbore. These fracture attributes were then combined and related to gas production rates.
As mentioned above, twelve wells have formation image data that were made available for analysis (Table 1). Eight wells are located in Otsego County, three are in Ogemaw County, and one is in Sanilac County (Figure 1). The Antrim Shale is a Devonian-Age fractured gas reservoir within which rock types vary from gray shales to organic rich black shales (Figure 2). From top to bottom, the Antrim Shale consists of the following units: Upper Antrim (black) Ellsworth (grey) Lachine (black) Paxton (gray) Norwood (black).
Recent work by Apotria, Kaiser, and Cain has shown that the black shales are more susceptible to fracturing than are the gray shales. Studies by Caramanica in the same three counties in Michigan show that where natural fractures are highly developed and numerous, the Lachine Shale tends to contain more fractures than do any other black or gray shale units (Tables 2, 3). These fractures are especially well developed in the Lachine Shale in Otsego County.
In the following, a brief description is presented of the types of imaging data used in the study and the data processing steps used. Next is presented the method of mapping the fractures in the wellbore to determine abundance, spacing, and interconnection.