APOTRIA, T. G., Shell Development Company, Houston, TX; and S. J. NARUK, Shell Western E&P Inc., Houston, TX

ABSTRACT: Stress History and Fracturing of the Bakken Shale in the Williston Basin

An elastic stress history algorithm models fracturing of the Bakken shale in the Williston basin. The model computes the vertical and horizontal stresses as a function of time. Data includes burial and thermal history, elastic rock properties measured from core, and fluid pressures form the Shell Connell 24-27 well. In the productive "fairway," the interval of interest includes the lower Lodgepole limestone, upper Bakken shale, middle Bakken dolomite, lower Bakken shale, and the Three forks dolomite. Horizontal drilling in this overpressured interval has focused primarily on the upper shale. In the Connell well, the Bakken shale was buried to 11,516 feet, and uplifted during the Tertiary to 10,499 feet. Assuming a fluid pressure/overburden ratio of 0.75 imposed near the onset of Laram de-age rapid burial, burial and uplift alone does not reduce the modeled horizontal effective stress sufficiently to generate extension fractures in any lithology within the interval. Additional extensional strains are required, which may preferentially fracture the nonshale lithologies due to relatively high elastic moduli. Extensional strains could occur if the Bakken interval is located along the outer arc of subtle folds in the fairway area. The tendency for fracture is strongly lithology-dependent, correlates positively with measured density and Young's modulus, and shows significant variability within each rock unit, based on rock property measurements on the Connell core. Horizontal drilling within the Lodgepole limestone or middle Bakken dolomite rather than the upper or lower sh le should optimize the intersection of vertical fractures in the fairway area.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.