**3. Seismicity and its correlation with well activities**

As already noted, multiple publications have already documented the 2018–2019 Newdigate 'earthquake swarm', notably those by Baptie and Luckett [54], Verdon et al. [6], and Hicks et al. [1]. Baptie and Luckett [54] presented a preliminary analysis of 14 earthquakes between 1 April and 18 August 2018; their results informed the OGA workshop. The more extensive analysis by Hicks et al. [1] will now be appraised. These latter authors determined hypocentres and other source parameters for 168 earthquakes between 1 April 2018 and 28 June 2019, some with local magnitude ML < 1, their location patterns and timeline being depicted in **Figures 1, 2** and **4** and summarised in **Table 3**, with **Table 4** listing events that post-date their study. The first nine earthquakes up to 10 July 2018 (including one of the largest, with ML 3.02, on 5 July) were located before any local seismograph stations were operational, using only data from regional stations. Hicks et al. [1] explained that due to the limited available data these events were located by assigning each a fixed focal depth. The resulting reported depths vary between 2.33 and 3.08 km (see Table S2 of [1]), it being unclear on what basis different depths were assumed for different events. The next sixteen events, until 11 July, were located conventionally but including data from local stations. For the rest of the events, both 'double difference' relocations (after [55]) and conventional locations were determined, using the velocity structure in **Table 2**. As detailed in **Figures 1** and **2**, most of the earthquakes in a zone 1.5 km long (E-W) by 300 m wide (N-S). The compact width of this zone suggests that many patches of a single strand of the Newdigate fault zone were reactivated.

Focal mechanisms were determined by Hicks et al. [1] for six events, including the largest, of ML 3.18 and moment magnitude MW 3.25, on 27 February 2019, as illustrated in **Figure 1** and listed in **Table 3**. All six events have a nodal plane striking roughly east–west and dipping steeply north. As already noted, this plane is inferred to be the fault plane, indicating predominant right-lateral slip. Available data regarding the state of stress in the Weald Basin are extremely limited; Kingdon *Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.94923*

et al. [56] and Fellgett et al. [57] provided syntheses of in situ stress data across much of Britain. However, these authors wrote little about the Weald Basin, Fellgett et al. [57] noting that many hydrocarbon wells in this area have yielded stress data but these data had not yet been placed in the public domain. The stress dataset available for the Weald Basin thus remains that presented by Evans and Brereton [58]. As is detailed in the supplement, this limited dataset indicates a NW-SE maximum principal stress and a NE–SW minimum principal stress. The Newdigate focal mechanisms (**Figure 1**) are consistent with this stress field orientation, given the standard requirement for the maximum principal stress to lie within dilatational quadrants [59].

## **3.1 Temporal clustering**

As detailed by Hicks et al. [1], the Newdigate seismicity between April 2018 and June 2019 involved four 'bursts' of activity (**Figure 4**). The first began at 11:10 on 1 April (ML 2.66), followed by two events later on the same day, another on 9 April, and a final event on 28 April. The smallest of these events (on 9 April)


*Cataloguing here is complete to 27 August 2020. Data are from https://earthquakes.bgs.ac.uk; these earthquakes have been located using standard BGS procedures, as reported by the International Seismological Centre (http://www.isc.ac .uk). Co-ordinate transformations to the British National Grid, as part of this study, use https://www.bgs.ac.uk/data/ webservices/convertForm.cfm*

*Note: 1. Felt in Newdigate; maximum EMS intensity 2.*

#### **Table 4.**

*Newdigate seismicity since the start of June 2019.*

had ML 1.28. No local seismograph stations were then in operation; Hicks et al. [1] estimated that the completeness threshold for earthquake detection was circa ML 2, so many smaller events were undoubtedly missed.

might have been influenced before this date by surface activities and by activities in the shallow part of the well [2]; if so, this would imply that the bridge plug had

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism…*

*DOI: http://dx.doi.org/10.5772/intechopen.94923*

It is evident from **Figure 4** that production ceased from well BRX2Y in October 2018; production at HH1 switched from the Portland reservoir to the Kimmeridgian reservoirs around the same time. Around this time, seismicity at Newdigate tailed off significantly. The conceptual model in **Figure 5** provides a natural explanation

The seismicity then re-intensified as the third 'burst', recognised by Hicks et al. [1], starting on 14 February 2019, which followed the resumption on 11 February 2019 of production from well HH1, now at rates of up to 220 bopd, from the Portland reservoir. As is detailed in the online supplement, production from this reservoir continued until late June 2019, after which it switched back to the Kimmeridgian reservoir, then during December 2019 to the newly-completed horizontal lateral, off well Horse Hill-2 (designated HH2Z), in the Portland reservoir. Seismicity at Newdigate remained significant during this phase of production from the Portland reservoir at HH1. However, production was not continuous; Hicks et al. [1] reported shutdowns during 9–12 April and 4–10 May, the latter corresponding to the start of the fourth 'burst' of seismicity as recognised by these authors. Seismicity subsequently tailed off following the end of production at HH1 from the Portland reservoir in late June 2019 and the switch to production from the Kimmeridgian reservoir in early July (**Figure 4** and **Table 4**). Furthermore, seismicity did not resume during the initial flow testing of well HH2Z in December 2019, even though the production rates from the Portland reservoir were much greater, up to 1087 barrels of fluid per day, than they had been from well HH1 (see

Overall, the correlation between phases of production from the Portland reservoir, from well HH1 or well BRX2Y or both, and 'bursts' of seismicity has been compelling (**Figure 4**). Hicks et al. [1] did not recognise this pattern, apparently because they did not differentiate between the Portland and Kimmeridgian reservoirs as sources of production from well HH1, as is now done (based on details in the supplement). There are particularly clear patterns for the first and third 'bursts': the first began 9 days after the resumption of production from well BRX2Y in March 2018; the third began 3 days after the resumption of production from well HH1 in February 2019. However, the patterns are less clear for the other two 'bursts' of seismicity, nor has the flow testing of well HH2Z, starting in December

The conceptual geomechanical model already summarised (**Figure 5**), which might account for seismicity beneath Newdigate, caused by pressure decreases in the Portland reservoir resulting from production (or other activities) from the HH1 or BRX2Y wells, will now be developed quantitatively. The basis of this model (**Figure 5**) is as follows. The Upper Portland Sandstone reservoir adjoining these

seismogenic Newdigate fault zone via a permeable fabric formed in calcite 'beef' in the stratigraphically adjacent Lower Portland Sandstone. The seismogenic fault is assumed highly permeable and to provide a downward hydraulic connection to the rocks beneath the Jurassic succession. These rocks are assumed to include the dolomitic conglomerate and Dinantian limestone, as in the HH1 well (**Table 1**),

wells is assumed to make a subhorizontal hydraulic connection with the

2019, been associated with any significant seismicity.

**4. Conceptual geomechanical model**

failed.

for such a variation.

supplement).

**79**

The second 'burst' (**Figure 4**) began at 12:28 on 27 June (ML 2.52), and included four other events above ML 2.0 (on 29 June and 5 July, and two on 18 July) including the second largest event overall (ML 3.02), at 10:53 on 5 July. The installation of local seismograph stations in mid June and early July lowered the completeness threshold for earthquake detection to below ML 0 [1], resulting in many small events being thereafter detected and enabling use of the aforementioned relative location procedure. After these initial relatively large events this 'burst' of earthquakes began to tail off, in terms of both magnitude and frequency of occurrence. The last event with ML > 0 occurred at 03:21 on 18 August (ML 0.30), with infrequent smaller events persisting into early 2019.

The third 'burst' (**Figure 4**) began on 14 February 2019 with a relatively large event at 07:43 (ML 2.47), followed by two other events of ML ≥ 2, at 17:03 on 19 February (ML 1.98) and at 03:42 on 27 February (ML 3.18), this being the largest event of the overall sequence. After these initial relatively large events this 'burst' also tailed off, although two events with ML > 0 occurred during April 2019 (on 11 and 22 April; ML 0.73 and 0.56).

The fourth 'burst' (**Figure 4**) began with a relatively large event (ML 2.35) at 00:19 on 4 May 2019. As for the preceding 'bursts', this seismicity thereafter began to tail off, although events with ML 0 persisted until the end of June 2019. Locations by BGS confirm the tailing-off trend through July and August 2019 (**Table 4**), with a ML 1.1 event on 2 September, three smaller events later that month, one during October, and none more before the end of 2019 (**Table 3**).

Overall, this pattern of seismicity, consisting of 'bursts' of events, each involving activity tailing off after a peak, with the largest event increasing during successive 'bursts', bears a striking resemblance to other earthquake swarms that are inferred to be caused by fluid pressure changes in a fault (e.g., [60]). However, the Newdigate earthquake population is insufficient to permit statistical testing of the patterns expected for this mechanism.

#### **3.2 Correlation of seismicity with well activities**

**Figure 4(c)** indicates how these four 'bursts' of earthquakes correlate with activities affecting the Portland reservoir in the HH1 or BRX2Y wells. Production from well BRX2Y resumed in late March 2018: from Hicks et al. [1] 4.0 m3 (<sup>25</sup> barrels) of oil were produced on 23 March followed by 1.1, 0.9 and 1.0 m<sup>3</sup> (7, 6 and 6 barrels) on 25–27 March. Reservoir pressure during this and subsequent production has not been reported, but from standard theory (e.g., [61, 62]) one expects it to have again decreased. This start of production occurred nine days before the first Newdigate earthquake on 1 April 2018. Furthermore, as is detailed in **Figure 4** and in the online supplement, other brief 'pulses' of production occurred from well BRX2Y in June, respectively 20, 19, 16 and 6 days before the start of the second 'burst' of seismicity on 27 June.

Although the activities that were planned in the HH1 well in 2018 have been disclosed [63], most of the actual activities that took place, and any associated pressure variations in the well, have not been, other than in the very general terms reported by Hicks et al. [1]. An attempt is made in the supplement to piece together the sequence of events, based on fragments of information available. It is thus evident that before 4 July 2018, the Portland reservoir was reported as isolated from the surface by a removable bridge plug. Claims have been made that the reservoir

*Seismicity at Newdigate, Surrey, during 2018–2019: A Candidate Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.94923*

might have been influenced before this date by surface activities and by activities in the shallow part of the well [2]; if so, this would imply that the bridge plug had failed.

It is evident from **Figure 4** that production ceased from well BRX2Y in October 2018; production at HH1 switched from the Portland reservoir to the Kimmeridgian reservoirs around the same time. Around this time, seismicity at Newdigate tailed off significantly. The conceptual model in **Figure 5** provides a natural explanation for such a variation.

The seismicity then re-intensified as the third 'burst', recognised by Hicks et al. [1], starting on 14 February 2019, which followed the resumption on 11 February 2019 of production from well HH1, now at rates of up to 220 bopd, from the Portland reservoir. As is detailed in the online supplement, production from this reservoir continued until late June 2019, after which it switched back to the Kimmeridgian reservoir, then during December 2019 to the newly-completed horizontal lateral, off well Horse Hill-2 (designated HH2Z), in the Portland reservoir. Seismicity at Newdigate remained significant during this phase of production from the Portland reservoir at HH1. However, production was not continuous; Hicks et al. [1] reported shutdowns during 9–12 April and 4–10 May, the latter corresponding to the start of the fourth 'burst' of seismicity as recognised by these authors. Seismicity subsequently tailed off following the end of production at HH1 from the Portland reservoir in late June 2019 and the switch to production from the Kimmeridgian reservoir in early July (**Figure 4** and **Table 4**). Furthermore, seismicity did not resume during the initial flow testing of well HH2Z in December 2019, even though the production rates from the Portland reservoir were much greater, up to 1087 barrels of fluid per day, than they had been from well HH1 (see supplement).

Overall, the correlation between phases of production from the Portland reservoir, from well HH1 or well BRX2Y or both, and 'bursts' of seismicity has been compelling (**Figure 4**). Hicks et al. [1] did not recognise this pattern, apparently because they did not differentiate between the Portland and Kimmeridgian reservoirs as sources of production from well HH1, as is now done (based on details in the supplement). There are particularly clear patterns for the first and third 'bursts': the first began 9 days after the resumption of production from well BRX2Y in March 2018; the third began 3 days after the resumption of production from well HH1 in February 2019. However, the patterns are less clear for the other two 'bursts' of seismicity, nor has the flow testing of well HH2Z, starting in December 2019, been associated with any significant seismicity.
