Joint Sets that Enhance Production from Middle and Upper Devonian Gas Shales of the Appalachian Basin
By: Terry Engelder, Gary G. Lash, Redescal S. Uzcátegui (March 23, 2009)
This article explores the marine Middle and Upper Devonian section of the Appalachian Basin, particularly focusing on its black shale units that exhibit two main regional joint sets: J1 and J2. These joints emerged at or near the shale's peak burial depth, forming as natural hydraulic fractures due to abnormal fluid pressures created during the thermal maturation of organic matter. The J1 set, predominantly oriented east-northeast, was the earlier of the two and is often intersected by the subsequently formed J2 set, which has a more northwest alignment. In the foreland side of the basin, J1 joints are denser than J2 ones, yet the J2 joints dominate most of the visible Devonian marine clastic section. Interestingly, the J1 set aligns closely with the current maximum compressive normal stress in the tectonic stress field. This alignment has significant implications for drilling. When extracting resources using vertical wells, the prevailing tectonic stress steers hydraulic fractures east-northeast, enabling them to tap into and drain from J2 joints. In contrast, horizontal drilling, especially in a north-northwest to south-southeast direction, can intersect both joint sets, ensuring more comprehensive extraction. When further hydraulic stimulation is applied, it follows the J1 direction due to the prevailing tectonic stress, subsequently draining J2 joints. This understanding of the shale's joint structure and its interaction with contemporary tectonic stresses provides essential insights for optimizing drilling and hydraulic fracturing strategies in the region.