| Keywords |
wellbore deformation, multi-finger caliper, caliper logging, shear, buckling, collapse, parting, casing, failure, formation, well integrity |
| Abstract |
Review of the literature suggests that different modes of casing deformations and failures occur in many geothermal operations. In some geothermal fields, such deformations and failures affect a relatively large percentage of the active wells in the asset. In geothermal wells, casing integrity and load capacity can be degraded by both corrosive and erosive wall loss and by mechanical damage modes, such as Trapped Annular Pressure (TAP) collapse, formation movement induced shears and buckles and parted connections. While conventional gyroscopic surveys can be used to reveal global well path changes, they do not provide sufficient resolution to measure highly localized wellbore trajectory or deformation changes, such as localized shear deformations caused by formation movement. Furthermore, while Multi-finger Caliper (MFC) logging tools can be used to identify local changes in casing ovality, the raw MFC data does not provide an indication of the three-dimensional (3D) shape and accessibility. Therefore, casing shears or buckles are often misdiagnosed as ‘collapsed’ intervals using conventional well log data and analysis methods, leading to misinterpretations regarding the underlying mechanism and potential mitigation actions, which may include changes to well design, construction or operating practices. If not properly diagnosed and monitored, such deformations can progress, potentially resulting in the eventual loss of access to lower regions of the well, reduced functionality (e.g. reduced flow capacity), or loss of barrier integrity and early forced decommissioning of the well, impacting the productivity and economics of the asset. To accurately identify and measure casing deformations due to formation-induced shear or thermal-mechanical induced buckling, and to be able to differentiate such deformation modes from other modes (e.g. external pressure-induced collapse), suitable MFC log data interpreted using a three dimensional calculation approach is required. Raw data from MFC logging tools, in particular casing radii values, are routinely used in geothermal wells to assess casing wall loss due to corrosion or erosion, apparent casing breaches due to parted connections, and to investigate potential flow assurance issues (e.g restrictions caused by scale buildup). By combining two-dimensional (2D) radii measurements with the physical tool geometry using a lab-validated transformation algorithm, a 3D profile of the well can be calculated, allowing for accurate identification and quantitative analysis of deformations; in particular, those induced by formation movements at faults or weak bedding planes. As the geothermal sector moves to more challenging and costly wells (e.g. wells completed for hot dry rock, supercritical and Enhanced Geothermal System (EGS) applications), assuring high well reliability and availability is critical to their long-term safe operation, fluid containment and the economics of the asset. This paper describes a summary of the common deformation modes affecting geothermal wells, including deformation statistics from literature; MFC logging technologies and limitations in geothermal wells and applications; the MFC 3D data analysis approach; and presents a case study to illustrate how this workflow can be used to diagnose and quantify deformations in geothermal wells. Supplemental analyses, such as the use of processed MFC log data coupled with wellbore-geomechanical structural analyses methods, potential mitigation options to reduce the magnitude or likelihood of casing deformations and associated consequences, and technology gaps and recommendations are also discussed. |