Evaluation Model for Shale Brittleness Index Based on Mineral Content and Fracture Toughness

Some uncertainties exist in shale brittleness index evaluation by using rock mechanics parameters.While it cannot present actual formation brittleness by using mineral content.A new evaluation model for shale brittleness was established based on conventional calculation method using mineral content and integrating fracture toughness as weighing coefficient for each mineral.In this paper,the new model,rock mechanical model and mineral content model were compared.It was found that,for an interval with abnormal rock mechanics properties,the new model provided a result aproximately 10% higher than the rock mechanical model and consistent with the stimulation results.Moreover,a negative linear relationship existes between fracture toughness and brittleness index-higher fracture toughness,lower brittleness index,and vice versa.Analysis showed that the new model avoided the defects of single mineral content model and was irrelevant to gas and organic materials in shale formation.Therefore,the new model is efficient for shale formation evaluation.