Direct current resistivity with steel-cased wells

Abstract

The work in this paper is motivated by the increasing use of electrical and electromagnetic methods in geoscience problems where steel-cased wells are present. Applications of interest include monitoring carbon capture and storage and hydraulic fracturing operations, as well as detecting flaws or breaks in degrading steel-casings -such wells pose serious environmental hazards. The general principles of electrical methods with steel-cased wells are understood, and several authors have demonstrated that the presence of steel-cased wells can be beneficial for detecting signal due to targets at depth. However, the success of a DC resistivity survey lies in the details. Secondary signals might only be a few percent of the primary signal. In designing a survey, the geometry of the source and receivers, and whether the source is at the top of the casing, inside of it, or beneath the casing will impact measured responses. Also the physical properties and geometry of the background geology, target, and casing will have a large impact on the measured data. Because of the small values of the diagnostic signals, it is important to understand the detailed physics of the problem and also to be able to carry out accurate simulations. This latter task is computationally challenging because of the extreme geometry of the wells, which extend kilometers in depth but have millimeter variations in the ra-arXiv:1810.12446v2 [physics.geo-ph] 8 Jul 2019 2 Lindsey J. Heagy, Douglas W. Oldenburg dial direction, and the extreme variation in the electrical conductivity (typically 5-7 orders of magnitude between the casing and the background geology).In this paper, we adopt a cylindrical discretization for numerical simulations to investigate three important aspects of DC resistivity in settings with steel-cased wells.(1) We examine the feasibility of using a surface-based DC resistivity survey for diagnosing impairments along a well in a casing integrity experiment. This parameter study demonstrates the impact of the background conductivity, the conductivity of the casing, the depth of the flaw, and the proportion of the casing circumference that is compromised, on amplitude of the secondary electric fields measured at the surface.(2) Next, we consider elements of survey design for exciting a conductive or resistive target at depth. We show that conductive targets generate stronger secondary responses than resistive targets, and that having an electrical connection between the target and well can significantly increase the measured secondary responses.(3) Finally, we examine common strategies for approximating the fine-scale structure of a steel cased well with a coarse-scale representation to reduce computational load. We show that for DC resistivity experiments, the product of the conductivity and the cross-sectional area of the casing is the important quantity for controlling the distribution of currents and charges along its length.To promote insight into the physics, we present results by plotting the currents, charges, and electric fi...

Identification of Breaches in a Regional Confining Unit Using Electrical Resistivity Methods in Southwestern Tennessee, USA

“…It is unclear why the ERV domain shifts boundaries for spacings at SF; however, the sensitivity appears to be greatest at depths < 10 m using the dipole-dipole array, similar to the results of [3]. Steel casing near the survey lines at SF may introduce errors in the depth calculation of the stratigraphic units [55], but this is likely only a local effect. The influence of the steel casing on the electrical resistivity values around the well needs to be further investigated, as well as the potential influence of other infrastructure at SF (e.g., [7]).…”

“…Q-125 contains a metal protective casing and is ~5 m from the survey lines. The metal casing can act as a conductor, allowing electrical current to bypass the subsurface and travel directly through the casing [55]. This phenomenon can introduce biases or distortions in the survey data near the well, leading to overestimating subsurface resistivity.…”

A Feasibility Study of Land CSEM Surveys With a Near-Casing Source for Hydraulic Fracturing Monitoring

“…Although the impact of steel casings on EM signals is not a new issue in geophysics, conducting efficient 3D numerical simulations that account for steel casings remains a challenging problem. Among various studies, some researchers discretize steel casings at a millimeterscale grid resolution and employ numerical algorithms to achieve detailed simulations of arbitrarily complex 3D models at the cost of computational efficiency [12], [13], [14], [15]. However, the implementation of this method in complex steel-casing systems poses significant challenges.…”

Geophysical inversions to delineate rocks with CO$_2$ sequestration potential through carbon mineralization

“…Geophysical data, including seismic, InSAR, and electromagnetics can be employed to monitor wellbore integrity, injection, and leakage; there are multiple field examples worldwide (e.g. Vasco et al (2010); Ajo-Franklin et al (2013); Heagy and Oldenburg (2019); Wilt et al (2020)).…”