Measuring fracture closure pressures serves as calibration points to constrain the geomechanics model necessary for conducting cap rock integrity and seal assessment analysis for subsurface injection projects. Formation breakdown, permeability, and in-situ stress contrast are required to constrain the CO2 injection rate. Frequently, the measured formation breakdown pressure is higher than the elastic breakdown pressure predicted through geomechanical characterization. Additionally, operational constraints and tool capabilities impede successful formation breakdown in high-strength reservoirs and cap rocks.
The main challenges are either increasing the differential pressure capability of straddle packer tools or reducing the formation breakdown pressure required to initiate a fracture. This paper consolidates multiple innovations in downhole formation testing from both tool design and testing methodology to acquire accurate and reliable subsurface information by assuring the success of downhole in-situ stress testing operations. In addition to open-hole assessments, the paper introduces cased-hole fracture closure measurements opening opportunities for stress characterization in existing wells.
Straddle packers are equipped with high-strength elements to achieve higher differential pressures and redesigned to reduce the setting time to improve operational safety and efficiency. In instances where the breakdown pressures in permeable formations are elevated by a strengthening effect from a thick mud cake, a near-wellbore fatigue pump-out methodology is applied to remove the mud cake before pressurization and thereby, reduce formation breakdown pressure significantly. A single packer (Sleeve Frac stress test) is employed when the straddle packers cannot achieve formation breakdown. The sleevefrac test can achieve higher wellbore pressurization that produces borehole weakening and fracture reopening with the dual packer. In challenging borehole environments where straddle packer operations involve deployment risks, a cased-hole stress measurement illustrates the successful measurement of fracture closure through perforations in the casing. Implementation of a novel high-rate pump allows accurate MiniSRT (Step Rate Injection Tests) to investigate frac initiation and interval injectivity. Integration of analysis of offset wells, experience from prior stress testing attempts, meticulous planning of equipment, and flawless execution through detailed workflows are centric to assured success in these operations.
This paper opens a new chapter in geomechanical characterization of mature fields through cased-hole stress measurements in existing wells. In mature fields, geomechanical inputs are critical to evaluate sand production, EOR techniques, hydraulic fracturing, etc. Predictive geomechanical models are uncertain without in-situ calibration measurements. The presented case studies highlight the value and operational impact of employing these new microfrac testing techniques to ensure formation breakdown for successful stress measurements in challenging formations, and especially in CO2 injection projects.
