Tommy Lakes Field - Landslide Impacting Gas Wells
Client: Enerplus Resources |
Location: Near Ft. St. John, BC |
Type: Slope Stability |
Project Budget: approx. $1.2Million inc. abandonment |
Consulting: $255,000 |
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An active slope failure had encroached on two active gas well leases at a-58-L within the Tommy Lakes Field, in the Peace River Regional District, BC, and posed significant risk in damaging the well infrastructure. The slope failure had progressed within 4 m of the well head a-A58-L and to approximately 10 m of the meter station to the east. ParklandGEO was commissioned to perform a geotechnical investigation and slope stability analysis of the slope and provide slope remedial recommendations for the well lease.
The 108 m high slope had an average gradient of 9° to 10° through the failed zone, and was characterized by a landslide featuring progressive slope failures with distinctive backscraps and benching throughout the slope profile. The upper 40 m of the slope had an average gradient of between 6° and 8° with backscraps of about 18°, while the lower 60 m of the slope had an average gradient of between 10° and 12° with backscraps around 22°. The lower slope profile had two distinct benches that were similar in elevation and gradient as the adjacent slopes which had previously failed. The upper slope had a distinctive backscrap at the top that stretched across the north side of both well leases located at the top of the slope.
The soil conditions consisted of about 48 m of high plastic lacustrine clay overlying high plastic clay till that extended to about 110 m depth, which was at the creek elevation. Both the lacustrine clay and clay till were found to have very low residual friction angles, both of which were similar to the post failure slope angles measured by survey. A slickensided failure plane was also observed at a depth of about 9 m in several boreholes.
The slope failure observed at the site was considered a large, relatively shallow (in relation to the height of the slope) retrogressive failure that started near the toe of the slope and progressed up slope over a number of years. Based on the current condition of the adjacent slope the retrogressive failures appeared to have happened in quick succession. Historically the slope was marginally stable until 2011, at which time the soil profile became saturated due to prolong precipitation events which triggered the slide. The initial slope failures appeared to have occurred on the lower slope in the clay till, likely due to a combination of erosion at the toe of the slope from Trutch Creek and the near surface soil profile becoming saturated. The initial slope failures would have steepened the lower slope causing instability in the upper slope. As the slope started to move, the strength of the clay soils would have dramatically dropped to residual values, thus triggering the slope failures and debris flow.
The extremely weak nature of the native clay soils, particularly the low residual strengths, was a significant contributing reason for the large extent of the failure. Due this extremely weak nature of the clay soils, slope regression will progress further south, eventually enveloping the entire well lease. As such it was recommended that the site infrastructure be removed, the wells cut and capped, and no future development be undertaken near the subject slope. Any directionally drilled wells considered in the future must be set back at least 300 m from the slope.
