A new method for sleeve fracturing directly measures the strain of the borehole wall by leveraging high-definition fiber optic sensing (HD-FOS) in a helically wound optical fiber integrated in an inflatable packer. This paper presents results of a field demonstration where the sensing assembly detected the initiation and growth of fractures as pressure was incrementally increased. The distributed nature of the measurement allowed for detecting a pre-existing horizontal fracture between formations, detecting new bi-wing vertical fractures in each of the two formations, and the determination of different breakdown pressures and stress orientations for each formation. After inducing the primary fracture, the sleeve pressure continued to increase until a secondary fracture occurred. Correlation of pressure measurements and the fracture opening and closing events detected by the fiber optics allows for calculation of the far-field maximum and minimum horizontal in-situ stress in the rock formation based on cohesive zone finite element modeling and an accompanying inversion algorithm.
The in-situ stress in the subsurface is of great interest to the oil & gas, geothermal, and geotechnical industries, as rock stress is a key driver in many underground phenomena. The new sensing technique presented in this paper will provide drastically increased amounts of data that can be leveraged to increase safety, production efficiency, and optimization of future subsurface engineered systems.
The existing methods for in-situ stress measurement have been summarized and categorized by Ljunggren et al., 2003, Sano et al., 2005, Zang and Stephansson, 2010, and Schmitt et al., 2012, among others. The methodology, advantages, and disadvantages of the primary methods are summarized below.
Borehole relief or stress relief methods attach strain gauges to the rock at the bottom of a borehole and measure the strain in the rock caused by stress relief from subsequent over-coring. These methods tend to work best in fracture free zones, which limit their usefulness for deep borings where borehole walls may be seriously damaged or fractured (Sano et al., 2005). The rock is typically assumed to be isotropic and linear elastic with a modulus of elasticity that must be measured separately.