The purpose of this paper is to present the results of a Gas Research Institute (GRI) evaluation to determine the optimal deepening and completion technique for the Norwood Antrim Shale unit in older Antrim wells in the Michigan Basin, including the potential range of Norwood production responses. There are approximately 500 older Antrim wells not drilled through the Norwood, that could he deepened below their current Lachine unit completion. GRI performed this work because Operators are uncertain of the best deepening/completion procedure, the potential productivity of the Norwood, and the appropriate well spacing for the Norwood completions. In this paper, we show the results of actual field case histories and simulated performance projections to determine the optimal Norwood deepening method and well spacing.
The Michigan Antrim Shale play has been one of the most active in the United States since 1990 due to improvements in well performance caused by different completion, stimulation, and production practices and the IRS Section 29 Tax Credit for Non-Conventional Fuels, The play has been found to be economic even without the tax credit, and several major oil and gas companies have renewed leasing interest including Shell, Chevron, and Amoco. There are currently more than 3,500 wells producing from the Antrim with operators planning to drill as many as 350 wells during 1994.
Fig. 1 is a schematic diagram of a typical stratigraphic section in the Antrim Shale. Operators target the Lachine and Norwood Shales as the primary completion intervals. Both shales are naturally fractured and produce gas and water. As Fig. I shows, the Lachine is the thicker of the two black shales (70 to 100 ft) compared to the Norwood, and contains lower matrix gas porosity (3 to 5%) and total organic content (TOC 5 to 12%). The Norwood is thinner (10 to 30 ft), has a slightly higher matrix gas porosity (3.5 to 5.5%), and considerably higher total organic content (TOC 10 to 16%). Production tests and simulation history matching of well test and production data indicate the Norwood typically has a higher permeability than the Lachine. The two organic shales are separated by 20 to 50 ft of gray Paxton Shale, which apparently acts as a barrier to vertical hydraulic fracture growth between the two black shales. There is a 20 ft thick Traverse formation (limey shale) below the Norwood, and then the thick Traverse Lime formation is encountered, These formations apparently act as a barrier to downward fracture growth when stimulating the Norwood.
Prior to 1990, there were approximately 1,000 wells drilled and completed using "old-style" techniques, including openhole completions, single-stage stimulation treatments, treatments without sand consolidation material, and gas assist as the production method. These lower-cost methods were employed due to the marginal economic returns on an average gas production rate of 50 Mscf/D/well. The typical practices used today, as operators "fine-tune" the completions, include cased hole completions, two-stage stimulations, use of sand consolidation material in fracturing, and the use of beam pumps, progressive cavity (PC) pumps, or modified plunger lifts to more effectively "pump-off' the wells.