Natural fractures in some US shales and their importance for gas production

Abstract

Shale gas reservoirs are commonly produced using hydraulic fracture treatments. Microseismic monitoring of hydraulically induced fracture growth shows that hydraulic fractures sometimes propagate away from the present-day maximum horizontal stress direction. One likely cause is that natural opening-mode fractures, which are present in most mudrocks, act as weak planes that reactivate during hydraulic fracturing. Knowledge of the geometry and intensity of the natural fracture system and the likelihood of reactivation is therefore necessary for effective hydraulic fracture treatment design. Changing effective stress and concomitant diagenetic evolution of the host-rock controls fracture initiation and key fracture attributes such as intensity, spatial distribution, openness and strength. Thus, a linked structural-diagenesis approach is needed to predict the fracture types likely to be present, their key attributes and an assessment of whether they will impact hydraulic fracture treatments significantly. Steep (.758), narrow (,0.05 mm), calcite-sealed fractures are described in the Barnett Shale, north-central Texas, the Woodford Formation, west Texas and the New Albany Shale in the Illinois Basin. These fractures are weak because calcite cement grows mostly over non-carbonate grains and there is no crystal bond between cement and wall rock. In bending tests, samples containing natural fractures have half the tensile strength of those without and always break along the fracture plane. By contrast, samples with quartz-sealed fractures do not break along the fracture plane. The subcritical crack index of Barnett Shale is .100, indicating that the fractures are clustered. These fractures, especially where present in clusters, are likely to divert hydraulic fracture strands. Early, sealed, compacted fractures, fractures associated with deformation around concretions and sealed, bedding-parallel fractures also occur in many mudrocks but are unlikely to impact hydraulic fracture treatments significantly because they are not widely developed. There is no evidence of natural open microfractures in the samples studied.

Shale oil occurring between salt intervals in the Dongpu Depression, Bohai Bay Basin, China

“…10A, B). Some of these are filled by calcite, and do not contribute to permeability but they are important planes of weakness that tend to be reactivated by hydraulically induced fractures (Gale and Holder, 2010;Gale et al, 2007;Gasparrini et al, 2014). Some of the open fractures ( Fig.…”

Full-waveform based microseismic source mechanism studies in the Barnett Shale: Linking microseismicity to reservoir geomechanics

“…Possible weak zones in the Barnett Shale include natural fractures and the newly created hydraulic fractures. Two sets of dominant natural fractures were reported to be in the westnorthwest and north-south directions, respectively (Gale et al, 2007;Gale and Holder, 2010). Finally, for reservoirs with a low horizontal differential stress and in normal faulting regimes, such as the Barnett Shale reservoir, rock failure could occur along almost any strike direction.…”

“…From core studies, two major sets of natural fractures were identified. One fracture system had an azimuth of north-south (north-south) and another, west-northwesteast-southeast (west-northwest) (Gale et al, 2007;Gale and Holder, 2010). The natural fractures in the Barnett Shale are completely healed and filled with calcites.…”

“…First, microseismicity is very unlikely to occur in the intact rock because of its large cohesion strength. Therefore, weak zones like natural fractures are critical for hydraulic fracturing in the Barnett Shale (Gale et al, 2007;Gale and Holder, 2010). Second, rock failure could happen on the preferred weak zones in the tip region and the broadside region.…”

Inventory of Shale Formations in the US, Including Geologic, Geochemical, Hydrological, Mechanical, and Thermal Characteristics

“…Formations that have been identified as having at least 20 trillion cubic feet (tcf) of shale gas include the Marcellus Shale (410 tcf), the Antrim Shale (20 tcf), the Haynesville Shale (75 tcf), the Eagle Ford Formation (21 tcf), the Fayetteville Shale (32 tcf), the Barnett and Woodford Shales (97 tcf), and the Mancos Shale (21 tcf). Many of these units are not shales in a strict sense, but may be better described as siliceous mudstones with reduced clay contents; these slightly brittle rocks can be subjected to successful hydrofracture treatment (Gale and Holder, 2010). While many areas within these sedimentary basins are sites of active and prospective oil and gas exploration and The data used for this report represent information that was either digitized using ArcGIS from published isopach and structure maps, or was available as GIS shape files that delineate formation isopachs and structural surfaces relative to a known datum, such as sea level or the ground surface.…”