Natural fractures in some US shales and their importance for gas production
Natural fractures in some US shales and their importance for gas production (in Petroleum geology; from mature basins to new frontiers; proceedings of the 7th petroleum geology conference, B. A. Vining (editor) and S. C. Pickering (editor))
Petroleum Geology Conference Series (2010) 7: 1131-1140
- Barnett Shale
- Carboniferous
- clastic rocks
- gas shale
- Illinois Basin
- Mississippian
- mudstone
- natural gas
- naturally fractured reservoirs
- New Albany Shale
- organic compounds
- Paleozoic
- Permian Basin
- petroleum
- production
- reservoir properties
- reservoir rocks
- San Saba County Texas
- sedimentary rocks
- shale
- shale gas
- Smithwick Shale
- source rocks
- Texas
- total organic carbon
- United States
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 (>75 degrees ), 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.