The New Albany Shale has great potential for natural gas reserves. Gas-in-place (GIP) measures from 8 bcfg/square mile to 20 or more bcfg/square mile, depending on locations and depths.
Unlike many other shale plays, the New Albany Shale in the Illinois Basin has a continuous 100 foot thick pay zone of shale, capped by a very thick, dense, gray-green shale (Borden Shale). Prior to 1994, over 600 New Albany Shale wells had been produced commercially in the Illinois Basin. In the New Albany Shale, a well commonly produces water along with the gas. It was learned in the early 1900's that a simple open-hole completion in the very top of the shale, would yield commercial gas wells that would last for many years, in spite of producing some water with the gas. Vertical fractures in the shale fed the gas flow at the top of the shale. The potential of these wells was seldom realized, as the production systems for handling the water were limited. Today, we have the ability to deal with the water cost effectively and as a result can keep the water produced off of the shale allowing better rates of gas production. Utilizing the success of horizontal drilling, modern water production systems, and low pressure gas gathering systems, long-term production of natural gas can be achieved.
Current recoverability of the black shale gas in vertical wells is estimated typically at 15% to 20% of GIP from the black shale. On a well-to-well basis, this recoverability varies depending on the natural fracture intensity associated with each well bore. The opportunity to exploit these shale gas reserves is big. Production volumes from the black shale are related mostly to our ability to desorb gas from the shale. Removing the hydrodynamic trap on the shale is the key to producing shale gas. This is accomplished with a large sump drilled downward from the lowest point in the well bore. Water is produced to the surface for disposal in approved SWD (salt water disposal) wells with electric submersible pumps. As the water pressure in the fractures is removed from the shale, the gas begins to release through open natural fractures. The lower the producing pressure of the well bore, the greater its capacity to produce gas.
Simple, low-cost vertical wells are delivering good returns on investment to several operators in the play. Horizontal drilling with only 1,000 feet of lateral wellbore, has demonstrated from a producing horizontal well (Mueller Well) in the Corydon Gas Field, to produce long-term, stable gas flow. Other horizontal test wells drilled recently under joint ventures have also confirmed the excellent production potential of the shale. Commercial production from wells is projected for 40 years or more. Due to the vertical nature of natural fractures/jointing through the shale, horizontal drilling is expected to have the best overall return on investment.
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