Influence of different shut-in periods after fracturing on productivity of MFHW in Duvernay shale gas formation with high montmorillonite content

Abstract

Drilling multi-stage fracturing horizontal wells (MFHWs) is the most effective way to unlock shale gas resources economically. This paper investigates the influence of different shut-in periods after fracturing, and their impact on the productivity of MFHWs in the Duvernay shale gas formation. This western Canadian formation has a high montmorillonite content which impacts the findings. A Duvernay shale core with high montmorillonite content was used to study gas–water relative permeability. Unsteady state tests were conducted under a variety of soaking times. The experiments demonstrated that the Permeability Jail for relative permeability curves exist in a water saturation range of 60% – 80%. This phenomenon partly explains the rapid gas production decline and a long period of little water production during the formal production in a typical shale gas well. Clay in shale mainly contains illite and montmorillonite. They have surface hydration, which occurs fast enough to cause induced fractures that improves the seepage capacity initially when the core is soaked. As the soaking time prolongs, there is an osmotic hydration expansion of the montmorillonite blocking pore throats and a corresponding reduction of the seepage capacity. The relative permeability increases during the soaking days one to four. After four soaking days, relative permeability begins to slowly decrease over time. Study calculations found that 61.4% – 75.6% of the total fracturing fluid filtrates into the formation. During days one to six of the soaking, the filtration volume increases significantly due to a high initial pressure in the fractures after pump shutdown. After seven days of soaking, the filtration velocity decreases due to the pressure drop and the difficulty of compression caused by more and more fluid in the formation. Based on reservoir simulations of 600 days, the relative permeability value in the near fracture zone and the cumulative gas production are the greatest on day four of the shut-in. The longer the shut-in period, the greater the initial water production rate and cumulative water production reaching a maximum at four shut-in days. In this Duvernay area with high montmorillonite content, the optimal shut-in period after fracturing was determined to be four days.