Brittleness index estimation in a tight shaly sandstone reservoir using well logs

Premise evaluation of the ability of fracture network forming using hydraulic fracturing is crucial for the development of tight shaly sandstone reservoirs. X-ray diffraction, petrographic analysis integrated with petrophysical well logs was used to provide insights to the brittleness of Chang 7 shaly sandstone in the Ordos basin. The mineralogical composition of this unit is quartz, clay, carbonates and feldspar with minor quantities of pyrite. Quartz and carbonate are the critical brittle minerals. The brittleness index defined in terms of mineralogy and geomechanical properties was investigated respectively. The geomechanical brittleness derived from dipole acoustic logs has a wide range from 0.98% to 93.21%, averaging 58.07%. In contrast, the mineralogical brittleness index from X-ray diffraction results is in the range of 28.26-78.85%, averaging 59.31%. The geomechanical brittleness index could be easily calculated using the dipole acoustic and bulk density logs. For wells without dipole acoustic logs, predicting the brittleness index using conventional well log suits is of great importance. The mineralogical brittleness index was then correlated with their well-log signatures. Statistical regression analysis was carried out to find the relations between brittleness index and conventional well logs, and it is found that the ratio of gamma ray to photoelectric absorption cross section index (GR/Pe) shows a good correlation with brittleness index. Therefore, the brittleness index could be estimated using conventional logs (GR/Pe). The geomechanical brittleness index and the test oil results confirm the reliability of the brittleness index estimation model. Favorable layers for hydraulic fracturing are mainly associated with the intervals with high values of brittleness index. This work is valuable for the evaluation of hydraulic fracturing effects in unconventional oil and gas reservoirs in the future.