Brittleness investigation of producing units in Three Forks and bakken formations, williston basin

• Presented a method to predict the brittleness index without geomechanical test.
• Built a correlation between elastic properties and mineralogy.
• Verified the correlation by the measured properties of the middle Bakken member.
• Predicted the brittleness of the upper Three Forks by the correlating approach.
• The results indicate the upper Three Forks is conducive to fracking.

Brittleness is a key factor in identifying intervals and areas for fracking in tight oil and gas reservoirs. The brittleness index of reservoir rocks can be calculated by elastic properties, but laboratory measurement of elastic properties is usually expensive and time consuming. In this paper, a method is proposed to predict the brittleness from known mineralogy and applied to assess the upper Three Forks formation in the Williston basin. In this method, a correlation between the elastic properties of the formation rock and mineralogy was first established based on rule of mixture. This correlation was verified by the measured elastic properties of the middle Bakken member. Then the correlating approach was used to predict the elastic property-based brittleness index of the upper Three Forks formation. The results indicate that the upper Three Forks formation should be conducive to reservoir stimulation as its predicted brittleness index ranges from 53 to 67 in the direction parallel to the bedding plane and 58 to 70 in the direction perpendicular to the bedding plane. Its brittleness is comparable to that of the middle Bakken member, which has been already successfully stimulated. The correlating method presented in this paper is also applicable to other major tight reservoirs.