Stuart Haszeldine and his team at the University of Edinburgh proposed a theory that tremors are induced by rapid pressure changes in the wellbore after periods of well shut-in. Several events appear to support this hypothesis:
- On 27th June 2018 UKOG reported that it commenced “production flow test operations.” A 2.6ML tremor was recorded on the same day.
- On 18th February 2019 UKOG reported that it re-established production from the Portland reservoir after a 6-months shut-in period. Four days earlier, on 14th February 2019, the earthquakes resumed after several months of no activity.
- On 11th April 2019 UKOG reported pressure testing of the HH-1 well, which the announcement said was shut in for a 60-hr pressure build up. An earthquake (0.7ML) was recorded on the same day, and another one (-0.3ML) 4 days later.
- On 17th 2019 May UKOG reported that it resumed production from the Portland following a week-long pressure build-up test. A tremor of 0.6ML magnitude was recorded on the same day.
- UKOG reported that test production was switched from the Portland to the deeper Kimmeridge oil pool, from which production started on 6th July 2019. Nine small tremors were registered on that day.
- UKOG reported that “final site works to accommodate the arrival of the rig” started Friday 30 August and were completed on 11th September 2019, and that “test production from the Horse Hill-1 (“HH-1″) Kimmeridge oil pool was temporarily shut-in”. Several tremors were registered during this period: on 2nd, 3rd and 6th September (1.1ML, 0.2ML and 1.2ML respectively).