Estimating Fracture Geometry in the Naturally Fractured Antrim Shale

Abstract

Abstract The Antrim Shale of the Michigan Basin has been an active gas play with over 3,500 wells drilled over the last 5 years. There is substantial evidence that the Antrim must be fracture stimulated to be economical and that two-stage treatments provide the best results. However, due to the shallow depths (500-2300 ft) and naturally fractured nature of the Antrim, fracture geometry is complex, and determination of optimal fracture treatments is not straight forward. Because historical field comparisons did not provide insight on the optimal fracture treatments, the Gas Research Institute (GRI) instituted a field-based project for the specific purpose of evaluating the geometry of hydraulic fractures in the Antrim. Open- and cased-hole tests were performed on two separate Antrim wells - a shallow producer (600 +/- ft) and a deep producer (1550 +\- ft). Open-hole testing and data collection consisted of in-situ stress and mechanical property testing with Halliburton’s THE™ Tool (9 tests) and a detailed suite of geophysical logs including dipole sonic logs and natural fracture detection logs. Cased-hole testing consisted of pre-and post-fracture injection/falloff tests, minifracture treatments, multiple isotope tracer and tracer logs, and treating pressure and production data analysis. Analysis of open- and cased-hole data from the shallow and deep wells suggests that subvertical fractures are being created and are probably following existing natural fracture planes. The shallow depths, low in-situ stresses, and extremely fractured nature of the Antrim probably results in the preferential opening of existing fractures instead of the creation of new fracture planes. As a result, the creation of multiple fractures and severe near wellbore tortuosity is likely. Therefore, the natural fractures are responsible for increased leakoff and will greatly impact created fracture geometry. The results also suggest that creating long propped hydraulic fractures in the Antrim is not likely due to the creation of multiple fractures.