Microfracture Stress Tests, Anelastic Strain Recovery, and Differential Strain Analysis Assist in Bakken Shale Horizontal Drilling Program

Abstract

ABSTRACT The economical completion of the Bakken Shale often dictates the drilling of a horizontal wellbore. A horizontal wellbore placed through a naturally fractured shale formation introduces drilling problems of wellbore collapse, the opening of existing natural fractures, or creation of fractures. Knowledge of magnitude and direction of the in-situ stresses is important in determining the optimum direction to drill a horizontal wellbore. The stress direction is important because a horizontal wellbore has its maximum stability when it is in the direction of minimum principal in-sttu stress. Combining the in-sttu stress magnitude data from microfracture stress tests, the in-sttu stress direction from anelastic strain recovery and differential strain analysis, and mechanical properties from laboratory tests, one can determine the minimum and maximum allowable drilling mud weights and optimize wellbore direction if necessary.1 The microfracture stress test was used to determine stress magnitude. Successfully performing the microfracture tests has meant overcoming problems inherent in performing these tests at high temperatures and high injection pressures associated with deeper wells in addition to concerns with the naturally fractured shale's stability. Some special equipment was required for the tests. Special surface injection equipment, developed in recent years was used to perform the tests. Standard downhole drillstem testing tools were deemed to be the most appropriate and were used in a special configuration to isolate the zones of interest and to overcome some of the problems associated with temperature and pressure (increasing depth), the natural fractures, and problems with borehole sloughing. Problems peculiar to naturally fractured shale affected the anelastic strain recovery (ASR) and differential strain analysis (DSA) testing. This paper presents a systematic approach for determining in-sttu stress field direction and magnitude and applying these measurements to determining the minimum and maximum limits of mud weight and wellbore direction for optimum drilling design in the Bakken Shale.