Detailed study of stress-related borehole features and the stress regime near the Wilmington Blind-Thrust Fault in the Los Angeles area

Justin O. Kain, & Patricia Persaud

Published August 15, 2020, SCEC Contribution #10682, 2020 SCEC Annual Meeting Poster #126

To obtain stress constraints near the inferred ~20-km-long Wilmington blind-thrust fault in the southwestern Los Angeles basin, we analyze digital 4- and 6-arm caliper data and resistivity image logs from 36 wells (16 with image logs). The wells are distributed in a 12 x 3 km2 area and cover up to ~3140 m in depth. They have surface locations within ~5 km of the Wilmington blind-thrust fault and are as close as 500 m to the inferred fault trace. Our image logs complement the caliper data by providing high resolution images of features in the borehole wall such as borehole breakouts and open fractures, but also show layer dips, smaller fractures, drilling induced tensile fractures (DITFs), and lithological changes.

We focus on DITFs and borehole breakouts because they can be used to determine the SHmax orientation and stress regime. We identify 71 DITF zones over a 2600-3100 m depth range (~170 m total length) that occur in the image logs from four wells, and 450 borehole breakout zones over a 1000-3100 m depth range (~670 m total length) that occur in 21 wells based on the caliper logs. No breakouts or DITFs were observed from 2000 m to 2500 m depth. To identify the DITFs in the image logs, lithological changes, dipping strata and natural fractures were first interpreted.
The Wilmington field is one of the few study areas where transverse DITFs are observed. Transverse DITFs are expected to form roughly perpendicular to the vertical borehole axis, and have been previously used to restrict the possible stress regimes to near the boundary between reverse and strike slip stress regimes. We observe transverse DITFs in a near-vertical section of Well 34 at ~1700 m depth and ~1.5 km from the Wilmington blind-thrust fault. Inclined DITFs more commonly occur and are at an angle to the borehole axis in deviated wells. We also observe several instances of DITFs and borehole breakouts occurring in the same zones, which implies a changing mudweight that over and under pressured the wellbore. There are areas where breakouts change orientation as they approach faults in the resistivity images, which indicates a changing SHmax orientation in close proximity to faults. This occurs in Well 27 within 1-2 m of an imaged fault. Using both the borehole breakouts and DITFs, we will present an update on our misfit analyses that determines the best fit stress regime and SHmax orientation, and allows us to examine any fine-scale variability in the stress field.

Key Words
Drilling Induced Tensile Fractures Stress Borehole Breakouts Wilmington Southern California

Citation
Kain, J. O., & Persaud, P. (2020, 08). Detailed study of stress-related borehole features and the stress regime near the Wilmington Blind-Thrust Fault in the Los Angeles area. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)