Natural and Drilling induced Fractures in the Grosmont Formation, Alberta: Implications for the State of Stress

Abstract

The Devonian Grosmont formation is a huge heavy oil carbonate reservoir located in northeastern Alberta that has an estimated 64.5 x 〖10〗^9 m^3 initial oil in place. This thesis will focus on issues related to the in situ state of stress and the natural fracture statistics within the Grosmont formation as determined from the numerous image logs obtained at the site. Natural fractures were interpreted from an image log data set of 22 wells and analyzed for preferred orientation using statistical tests and density contouring. Of the wells, 6 tested positive for a preferred fracture dip orientation striking north-south. Equal area lower hemisphere statistical contouring results also showed that fracture orientations preferred a north-south strike. Drilling induced tensile fracturing were observed in log data of 20 of the wells and the maximum and minimum in-situ horizontal stress orientations were determined using these to be 50°NE and 140° SE respectively with circular standard deviation 9°. Vertical stresses were calculated by integrating bulk density logs to the Grosmont C and D units and values ranged from 6 - 9MPa. Since data for evaluating in-situ horizontal stress magnitudes were limited, a procedure was devised using concentrations of azimuthal hoop stress at the borehole wall to bound horizontal stress magnitudes. The procedure took advantage of the lack of borehole breakouts caused by shear failure and the existence of tensile failure in the borehole in conjunction with laboratory measurements of carbonates in literature. Horizontal stress magnitudes in the area were constrained to 11.6 MPa-15.0 MPa for maximum horizontal stress and 5.4 MPa-6.6 MPa for minimum horizontal stress. The stress regime was ascertained to be strike slip