OVERCOMING THE TECHNICAL CHALLENGES OF SHALE GAS AND TIGHT OIL PRODUCTION

Many people are unaware of the intense and protracted technical struggle researchers and industry went through to develop commercial shale gas and tight oil resources. Because unconventional hydrocarbon production came on quickly and achieved dominance of the U.S. domestic oil and gas supply in less than a decade, it appears to have been a bolt from the blue. In reality, it took more than 30 years of hard work, dedication, persistence, and repeated failures to achieve success. Fluid flow through porous media follows Darcy’s Law. The basic unit of permeability is called a darcy, and it was defined by Henry Darcy himself as a flow of one cubic centimeter per second of a fluid with a viscosity of one centipoise (essentially, water at room temperature) from a cross sectional area of one square centimeter across a flowpath distance of one-centimeter length under a differential pressure of one atmosphere. Most conventional oil and gas reservoirs have permeabilities around a thousandth of a darcy, or a millidarcy, requiring a thousand seconds (about 17 minutes) under the conditions described above to flow one cubic centimeter of water. Gas shale typically has a permeability as low as a nanodarcy, or a billionth of a darcy, and that same cubic centimeter of water would need a billion seconds to emerge from the rock. A billion seconds is nearly 32 years. The combination of horizontal drilling and staged hydraulic fracturing developed by Mitchell Energy in the 1990s to produce these shales modified Darcy’s Law by 1) increasing the cross-sectional area, 2) decreasing the flowpath distance, and 3) increasing the differential pressure. Laterals now reach 3 km in length with as many as 124 frack stages, and initial production rates on some of these shale wells can reach 30 million cubic feet of gas per day. This is stunning after the repeated failures of fracked vertical wells during the shale gas research days of the 1980s.