Successful Cap Rock Stress Testing With a Wireline Straddle Packer Tool Configured With an Innovative Controlled Pressure Bleedoff Design

Abstract

Knowledge of the minimum principal stress in the overburden is highly valuable – and even required – for well design, drilling, completion, plug and abandonment, and reservoir injection planning optimization. Access to an effective methodology whereby the stress could in principle be reliably determined at multiple arbitrary depths along the wellbore throughout the overburden is thus highly beneficial to more reliably predict the in-situ stress state. Within the reservoir, such data have been successfully acquired using straddle packer wireline tools, the socalled microfrac test. However, shaly overburden and cap rock formations in particular have very low mobility, therefore the induced fracture bleeds off at an extremely low rate after shut-in. This lack of formation leak-off and fracture closure renders least principal stress analysis difficult without a controlled pressure release option including sufficient volume control. The recent deployment of new technology enables microfrac tests to be successfully applied to tight formations. A new flow-back option has been deployed and utilizes the injection pump in reverse mode as a fluid expander which moves fluid from high to low pressure. Fluid injected into the fracture flows back to the borehole at a controlled rate, enabling high quality tight formation stress testing. A description of the new method and the first field implementation, which turned out to be a success case where data were acquired at multiple depths in a well on the Norwegian Continental Shelf (NCS), is given. The subsequent application of the results in terms of constraining the minimum horizontal stress and pore pressure models for the well is presented, illustrating the value of the tool and the microfrac stress data.