**4.5 Application of model: Horse Hill**

The first topic to be assessed is the possibility that the resumption in February 2019 of production from the Portland reservoir in well HH1 caused the third 'burst' of Newdigate seismicity. This phase of production is inferred to have begun at 08:00 GMT on 11 February 2019 and caused earthquakes starting with the event (MW 2.5; MO 6.8 1012 N m) at 07:43 GMT on 14 February 2019 (located at TQ 22959 41543, after [1], at a probable depth of 2400 m), event H in **Figure 7**. The time delay in this instance was thus 3 days or 72 hours, for a separation of 3 km (**Figures 1, 7**), the HH1 well bottom being 2.85 km from nearest point where the top Portland sandstone is transected by the Newdigate Fault and 3.0 km from an updip projection of the hypocentre to this cutoff. This production took place at 220 barrels per day or 0.40 l s<sup>1</sup> . The pressure drawdown during this phase has not been reported. However, as detailed in the online supplement, during the flow testing of the Portland reservoir in well HH1 in July–August 2018, the developer reported production at a sustained rate of 190 barrels per day or 0.35 l s<sup>1</sup> (140–160 barrels per day being of oil), with stable bottom hole pressures 1.4 MPa below the initial reservoir pressure of 6.3 MPa; scaling in proportion gives a 1.6 MPa pressure drawdown at the HH1 well bottom in February 2019 (i.e., 1.4 MPa 220/190). Using Eq. (52), with ν = 0.25 and δσ = 10 MPa, gives L = 134 m; using Eq. (49) gives b = 18.9 mm.

earthquakes starting with the event (MW 2.8; MO 1.3 <sup>10</sup><sup>13</sup> N m) at 12:10 BST on 1 April 2018 (located at TQ 21992 41976, after [1], also at a probable depth of 2400 m), event B in **Figure 7**, with hypocentre close to the western or WNW limit of the seismogenic part of the Newdigate fault. The time delay in this instance was 9 days 4 hours or 220 hours. In a straight line, the BRX2Y well bottom is 7.2 km NNW of the point, updip from this hypocentre, on the top Portland cutoff of the Newdigate fault (between circa TQ 18440 48310 and circa TQ 22100 42200). However, as already noted, it is possible that the high throw on the Brockham Fault along this direct path blocks any high-permeability connection, in which case the actual path length between these end points, through localities farther west with low fault offsets (**Figure 8**), and around the western end of the Leigh Fault (**Figure 1**), might be >1 km longer. Using Eq. (52), with ν = 0.25 and δσ = 10 MPa, MO 1.3 <sup>10</sup><sup>13</sup> N m for the 1 April 2018 earthquake gives L = 166 m; using Eq. (49)

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

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

**Figure 14(a)** shows the predicted variation in fluid pressure in the 'beef' layer that is inferred to hydraulically connect the Brockham Portland oil reservoir and the Newdigate fault, calculated using Eq. (15). With the chosen set of parameter values, following 4 m<sup>3</sup> of production the resulting pressure decrease at r = 8 km distance is

appropriate for 7-inch diameter casing), using Eq. (14) the pressure decline at the well bottom at the end of production would be 530 kPa if the production took 4 hours or 1.0 MPa if completed in 2 hours. Within an hour of this idealised 50 Pa step pressure variation affecting the Dinantian limestone, Δx would be 0.06 μm (**Figure 14(b)**). This would be sufficient to unclamp the Newdigate fault

pressure decrease at r = 8 km would peak, circa 9 days after this pulse of production, at 150 Pa (**Figure 14(d)**). Within three quarters of an hour of this idealised 150 Pa step pressure variation affecting the Dinantian limestone, Δx would be 0.14 μm (**Figure 14(e)**). This would be sufficient to unclamp the Newdigate fault by 150 kPa or 2.5% of δσ (**Figure 14(f)**). Evidently, for such a small change in the state of stress to have caused the observed seismicity, the patch of this fault that slipped on 1 April 2018 must have already been within a very small proportion of δσ

The final topic assessed will be pressure interference between the wells. Like the path from the Brockham well to the Newdigate fault, the most permeable connection, through 'beef', between the two wells will exceed the straight line distance; separation r = 10 km is adopted. Again calculated using Eq. (29), with DB = 10 m2 s

the 4 m<sup>3</sup> of production from well BRX2Y would cause a maximum pressure decline in the vicinity of well HH1 of 50 Pa after a 1 month delay (**Figure 15(a)**). With

of 90 Pa after a 15 day delay (**Figure 15(b)**). Variations in HH1 bottom hole pressure of this order, developing and dissipating on timescales of weeks or months in response to attempts at production from well BRX2Y, would be extremely difficult, if not impossible, to recognise given the >1 MPa pressure variations caused by the production from this well. This is consistent with the statement by the OGA [5] that no pressure variation at HH1 was detectable in response to the pulses of

A second test is possible, given that 20 years of production at Brockham (followed by two years of shut-in, during 2016–2018) resulted in a bottom-hole

, this production would cause a maximum pressure decline

by 60 kPa or 1% of δσ (**Figure 14(c)**). With DB adjusted to 20 m<sup>2</sup> <sup>s</sup>

1

, and kP = 200 mD, with rW = 88.9 mm (again

. Assuming hP = 3 m,

1 , the

1

predicted to be 50 Pa after 9 days for DB = 10 m<sup>2</sup> <sup>s</sup>

of the Mohr-Coulomb condition for slip.

1

DB increased to 20 m<sup>2</sup> s

production from BRX2Y.

**95**

1

**4.7 Application of model: interference between the wells**

gives b = 21.5 mm.

η = 0.9 mPa s, DP = 1.3 m2 s

**Figure 13(a)** shows the predicted pressure variation in the 'beef' layer that is inferred to hydraulically connect the Horse Hill Portland reservoir and the Newdigate fault. With the chosen parameter values, the resulting pressure decrease at 3 km distance is calculated using Eq. (8) as 6.5 kPa after 2.5 days and 8.7 kPa after 3 days. Taking hP = 11 m, η = 0.9 mPa s, D = 1.3 m<sup>2</sup> s 1 , and kP = 35 mD (somewhat higher than the 20 mD reported by Xodus [26], with rW = 88.9 mm (appropriate for 7-inch diameter casing), using Eq. (14) the pressure decline at the well bottom, 2–4 weeks after the start of production, would be 1.6 MPa, as expected. If η were adjusted to 11 mPa s, to reflect oil rather than water, kP 430 mD would be required to match this pressure variation.

**Figure 13(b)** shows the variation in fluid pressure inside the Dinantian limestone alongside the seismogenic part of the Newdigate fault, as a result of the imposed pressure variation in the 'beef'. **Figure 13(c)** shows the corresponding variation in Δx, the poroelastic separation of the opposing faces of this fault, and **Figure 13(d)** shows the corresponding variation in ΔΦ. The upper limit to ΔΦ is taken as c δσ, or 6 MPa, the coseismic stress drop, δσ = 10 MPa, also being the increase in shear stress during a complete earthquake cycle; this value of ΔΦ corresponds (using Eq. (44)) to Δx = 0.005 mm. These conditions are taken as indicating the condition for an earthquake to occur on the specified patch of fault. It is evident that with the specified combination of parameter values, these changes will occur very rapidly, in just 15 minutes after the idealised step onset of the pressure variation in the fault. In reality, **Figure 13(a)** indicates a gradual onset of this pressure variation, rather than an abrupt step, so the calculated changes Δx and ΔΦ will occur gradually, leading to an earthquake at the calculated time, roughly three days after the start of production from the well. This modelling demonstrates that, with the parameter values adopted, production from the Portland reservoir by well HH1 has been readily able to cause seismicity on the Newdigate fault.

#### **4.6 Application of model: Brockham**

The second topic assessed will be the possibility that the 4 m<sup>3</sup> of production from well BRX2Y on 23 March 2018, inferred to have started at 08:00 BST, caused

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

earthquakes starting with the event (MW 2.8; MO 1.3 <sup>10</sup><sup>13</sup> N m) at 12:10 BST on 1 April 2018 (located at TQ 21992 41976, after [1], also at a probable depth of 2400 m), event B in **Figure 7**, with hypocentre close to the western or WNW limit of the seismogenic part of the Newdigate fault. The time delay in this instance was 9 days 4 hours or 220 hours. In a straight line, the BRX2Y well bottom is 7.2 km NNW of the point, updip from this hypocentre, on the top Portland cutoff of the Newdigate fault (between circa TQ 18440 48310 and circa TQ 22100 42200). However, as already noted, it is possible that the high throw on the Brockham Fault along this direct path blocks any high-permeability connection, in which case the actual path length between these end points, through localities farther west with low fault offsets (**Figure 8**), and around the western end of the Leigh Fault (**Figure 1**), might be >1 km longer. Using Eq. (52), with ν = 0.25 and δσ = 10 MPa, MO 1.3 <sup>10</sup><sup>13</sup> N m for the 1 April 2018 earthquake gives L = 166 m; using Eq. (49) gives b = 21.5 mm.

**Figure 14(a)** shows the predicted variation in fluid pressure in the 'beef' layer that is inferred to hydraulically connect the Brockham Portland oil reservoir and the Newdigate fault, calculated using Eq. (15). With the chosen set of parameter values, following 4 m<sup>3</sup> of production the resulting pressure decrease at r = 8 km distance is predicted to be 50 Pa after 9 days for DB = 10 m<sup>2</sup> <sup>s</sup> 1 . Assuming hP = 3 m, η = 0.9 mPa s, DP = 1.3 m2 s 1 , and kP = 200 mD, with rW = 88.9 mm (again appropriate for 7-inch diameter casing), using Eq. (14) the pressure decline at the well bottom at the end of production would be 530 kPa if the production took 4 hours or 1.0 MPa if completed in 2 hours. Within an hour of this idealised 50 Pa step pressure variation affecting the Dinantian limestone, Δx would be 0.06 μm (**Figure 14(b)**). This would be sufficient to unclamp the Newdigate fault by 60 kPa or 1% of δσ (**Figure 14(c)**). With DB adjusted to 20 m<sup>2</sup> <sup>s</sup> 1 , the pressure decrease at r = 8 km would peak, circa 9 days after this pulse of production, at 150 Pa (**Figure 14(d)**). Within three quarters of an hour of this idealised 150 Pa step pressure variation affecting the Dinantian limestone, Δx would be 0.14 μm (**Figure 14(e)**). This would be sufficient to unclamp the Newdigate fault by 150 kPa or 2.5% of δσ (**Figure 14(f)**). Evidently, for such a small change in the state of stress to have caused the observed seismicity, the patch of this fault that slipped on 1 April 2018 must have already been within a very small proportion of δσ of the Mohr-Coulomb condition for slip.
