Wireline Formation Stress Testing – Successful Implementation of a New Controlled Rate Bleed Off Design for Caprock and Reservoir Stress Testing on the Norwegian Continental Shelf

Abstract

While drilling a well, extended leak off tests (XLOT) can be conducted after drilling a short openhole section below the casing/liner shoe to obtain the minimum principal stress. Alternatively, the data may be obtained during plugging and abandonment (P&A) of the well. The data is used for well design, drilling, completion, abandonment, and injection optimization. The XLOT is the industry standard to obtain minimum stress data. Although it is possible to conduct more than one XLOT per section in a well cemented and perforated casing/liner, the XLOT is typically conducted just below a casing/liner shoe so only one minimum stress data point is acquired per well section. For minimum principal stress profiling over a well section, there is now an option to use wireline (WL) formation testing tools such as the Reservoir Description Tool (RDT) configured with a Straddle Packer Section (SPS). For WL stress testing, the packers are set at the desired depth and inflated to isolate approximately 1 m of formation. Filtered mud is then pumped into the packed-off interval to increase the interval pressure until the rock breaks down. Pumping is continued to propagate the fracture beyond the near wellbore stress region. The pumps are then shut down to monitor the pressure falloff which is analyzed to estimate the minimum principal stress as given by the fracture closure pressure (FCP). The volume of fluid injected in an XLOT is typically two orders of magnitude higher than the injected volume in a WL stress test, and consequently the induced fracture extends about 10-15 m from the wellbore as compared to 1-3 m for a WL stress test. This microfrac technique is applied to rocks with some permeability for fluids to naturally leak off into the formation. For very low permeability rocks like shales where there is little to no leakoff, a recent technology development provides a controlled flowback option to depressurize the packed-off interval and enable the fracture to close for minimum principal stress determination. This involves a modification of the existing injection pump design and moves the fluid from the high pressure inside the packed-off interval to the hydrostatic pressure at a controlled rate, enabling full volume control during the flowback phase. The controlled flowback pump was successfully used for the first time in a Norwegian Continental Shelf (NCS) well in 2021 and has since then been used in several wells on the NCS and other parts of the world. One of the NCS wells provided a unique opportunity to compare the results from WL microfrac with XLOT to verify its performance relative to the industry standard. There was good agreement in the estimated minimum stress from both measurements. This paper provides details on the design of the new flowback pump, operational details, and field results when applied in four wells at the NCS, summarizing the experience with the new WL tool.