Measuring geomechanical properties of the caprock and shale gas reservoirs for enhanced oil recovery (EOR) or storage reservoirs has become an integral part of asset evaluation services. The steam-assisted gravity drainage (SAGD) process, as well as other EOR and storage applications (gas, nuclear waste, CO2), requires caprock integrity testing. Measuring in-situ stresses is an important input to caprock integrity solutions not only for the technical management of projects but also for environmental reasons.
A Wireline Formation Testing (WFT) tool is one of the commonly used techniques used for direct measurement of the minimum in-situ stress at different depths. The process is commonly called a mini-frac or micro-frac and is typically performed in an openhole wellbore. A suitable tool string includes a straddle packer arrangement, downhole pump, gamma ray sonde for depth correlation, motorized valves, and pressure gauges. To perform a stress test, an interval of the wellbore is isolated by inflating the straddle packer arrangement. The interval is then pressurized by pumping fluid until a tensile fracture is initiated. In an open hole, the fracture initiates and propagates normal to the minimum stress. Multiple injection and fall-off cycles are performed to ensure fracture growth beyond the hoop stress regime. The data is analyzed to determine fracture initiation pressure, fracture propagation, and closure pressure.
This paper describes the process of minimum in-situ stress measurement using a WFT. Lessons learnt over the years and best practices are highlighted along with their importance for proper job planning. Case studies of WFT testing including SAGD caprock stress testing from Canadian fields are presented and discussed.