This paper describes a simple method to identify, prioritize, and evaluate restimulation candidates in the Antrim Shale of the Michigan Basin. This work is being performed as part of an ongoing field-based Gas Research Institute (GRI) project investigating the Antrim Shale. There are between 500 and 1,000 Antrim Shale wells which could be candidates for restimulation due to previous screenouts and/or flowback problems, when sand consolidation material was not used. However, all of these wells might not benefit from restimulation, due to either poor reservoir quality or because the wells are already effectively stimulated. Based on historical results, we estimate the increase in reserves from restimulation could be between 50 and 400 MMscf per well, which could add 50 to 200 Bscf in future reserves from the 500-1,000 candidate wells.
We have developed a novel injection test unit and procedure to help operators identify the best restimulation candidates in a cost-effective manner and in a reasonable amount of time. The test is designed to determine if there is an effective hydraulic fracture connected to the wellbore. To our knowledge, a test of this nature has not been used before for specifically identifying restimulation candidates. To date, we have run 50 tests, and identified potential restimulation candidates from about 1/3 of the tests.
The Antrim Shale play in the Michigan Basin has been very active in recent years with over 500 well completions in 1993 alone. However, the Antrim Shale boom is relatively new. As recently as 1988, the majority of the activity was concentrated in Otsego Co., Michigan. But, since 1988, producing Antrim wells increased from about 500 to over 3500 wells with completions in six counties (see Fig 1). Antrim gas production also increased dramatically, rising from about 10 Bsc/year in 1988 to over 90 Bsc/year in 1993. The Gas Research Institute (GRI), recognizing the potential of the Antrim for adding to long term gas reserves, is currently involved in a detailed research project designed to advance technology in the Antrim. Objectives of this field-based project include increasing the effectiveness of fracture stimulations, developing methods for screening restimulation candidates, identifying optimal recompletion candidates and procedures, and improving the understanding of Antrim reservoir. This paper focuses on methods for screening restimulation candidates.
The Antrim Shale is a black, organic rich shale which is currently being produced from depths between 500 to 2,300 fL It consists of two primary completion intervals, the Norwood and Lachine, separated by the gray Paxton shale, as illustrated in Fig 2. The Norwood is commonly about 20 ft thick and the Lachine is 80 ft thick. Generally, both intervals are fracture stimulated separately with nitrogen foam and sand.