Wellbore integrity is a significant problem in the U.S. and worldwide, which has serious adverse environmental and energy security consequences. Wells are constructed with a cement barrier designed to last about 50 years. Indirect measurements and models are commonly used to identify wellbore damage and leakage, often producing subjective and even erroneous results. The research presented herein focuses on new technologies to improve monitoring and detection of wellbore failures (leaks) by developing a multi-step machine learning approach to localize two types of thermal defects within a wellbore model, a prototype mechatronic system for automatically drilling small diameter holes of arbitrary depth to monitor the integrity of oil and gas wells in situ, and benchtop testing and analyses to support the development of an autonomous real-time diagnostic tool to enable sensor emplacement for monitoring wellbore integrity. Each technology was supported by experimental results. This research has provided tools to aid in the detection of wellbore leaks and significantly enhanced our understanding of the interaction between small-hole drilling and wellbore materials.
The main goal of this project was to create a state-of-the-art predictive capability that screens and identifies wellbores that are at the highest risk of catastrophic failure. This capability is critical to a host of subsurface applications, including gas storage, hydrocarbon extraction and storage, geothermal energy development, and waste disposal, which depend on seal integrity to meet U.S. energy demands in a safe and secure manner. In addition to the screening tool, this project also developed several other supporting capabilities to help understand fundamental processes involved in wellbore failure. This included novel experimental methods to characterize permeability and porosity evolution during compressive failure of cement, as well as methods and capabilities for understanding two-phase flow in damaged wellbore systems, and novel fracture-resistant cements made from recycled fibers.
Leakage along wellbores is of concern for a variety of applications, including sub-surface fluid storage facilities, geothermal wells, and CO2 storage wells. We have investigated whether corroded casing is permeable to gas and can serve as a leakage pathway along wellbores. Three specimens were prepared from laboratory steel plates corroded using different mechanisms to reflect different possible field conditions and produce a variety of corrosion rates. Single-phase gas flow measurements were made under a range of gas pressures to investigate flow in both the viscous and visco-inertial flow regimes. Tests were conducted at different confining stresses (range from 3.45 to 13.79 MPa) following both loading and unloading paths. The gas flow test results suggest corroded casing can serve as a significant leakage path along the axis of a wellbore. Transmissivity was found to be sensitive to the variation in confining stress suggesting that the corrosion product is deformable. Gas slip factors and the coefficients of inertial resistance of the corrosion product were comparable to those available in the literature for other porous media. Post-test examination of the corrosion product revealed it to be a heterogeneous, mesoporous material with mostly non-uniform slit type porosity. There was no discernable difference in the composition of corrosion product from specimens corroded by different mechanisms.
In wellbores, cement plays an important role in wellbore integrity. As wells age and are stressed during their life cycle, the cement sheath may deform, altering its permeability and, perhaps compromising its integrity. In this study, we use flow measurements (calculated permeability) to provide real-time insight into damage incurred during triaxial deformation of neat cement. Cracks may be induced during deformation and their linkage may be sensed in the flow measurements. Conversely, cracks and pores may be closed during deformation, arresting fluid flow. We subjected room temperature specimens of neat Portland cement to confining pressures (0.7, 2.1, 13.8 MPa) and measured heliu m flow continuously during triaxial deformation. Axial displacement across a specimen was periodically halted to perhaps assure steady flow rate throughout the sample. We observed the apparent permeability to decrease from 0.8 to 0.7 to 0.2 μD with the imposed confining pressure increase. Each specimen, when subjected to differential stress, exhibited a slight decrease in apparent permeability, implying disconnects of flow paths. For the two lower confining pressures, apparent permeability began to increase just prior to macroscopic failure, suggesting microcrack linkage. For the 2.1 MPa confining pressure test, apparent permeability increased by a factor of three at macrofracture, and for the 0.7 MPa confining pressure test, apparent permeability increased by a factor of thirty at macrofracture. At 13.8 MPa confining pressure, apparent permeability only decreases during triaxial loading, implying that poroelastic compaction restricts flow pathways and connectivity of appropriately oriented cracks for axial flow decreases during deformation. Failure by macrofracture did not occur in this sample. Optical and scanning electron microscopy of deformed specimens indicate that pores and microcracks interact in complex manners, similar microcrack densities are observed in both 0.7 and 13.8 MPa test specimens, and pores represent both microcrack origination and localization sites. Larger pores (entrapped air voids) are sheared, flattened, and sites of crack opening. Micron-scale capillary porosity, determined using SEM image processing, is similar for all specimens. The results from these few experiments indicate that microfracturing of cement during triaxial deformation results in permeabilit y increases at low confining pressures. At the greater pressure, although microfracturing is observed, compaction and lack of microfracture interconnectivity have a greater effect on flow pathways, resulting in a permeability decrease during deformation.