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

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 1 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 DB increased to 20 m<sup>2</sup> s 1 , this production would cause a maximum pressure decline 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 production from BRX2Y.

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

#### **Figure 14.**

*Modelling of hydraulic consequences of the pulse of production from well BRX2Y on 23 march 2018. (a) Graphs of -ΔP versus radial distance r within the layer of calcite 'beef' at 600 m depth, which is inferred to connect the Brockham oil reservoir with the Newdigate fault, at different times t after the start of the pulse of production from well BRX2Y on 23 march 2018, time t being measured from the start of production. Calculations using Eq. (15) assume 4 m<sup>3</sup> produced volume, calculated as Q 1.39 <sup>10</sup><sup>4</sup> <sup>m</sup><sup>3</sup> <sup>s</sup> <sup>1</sup> for Δt 8 hours, η 0.9 mPa s, hB 1 m, kB 900 mD, and DB 10 m<sup>2</sup> s 1 . Dashed line indicates ΔP = -51 Pa for r = 8 km after t = 9 days. (b) Graph of Δx versus time following a step pressure reduction within the Newdigate fault. Calculations use Eq. (27) with ΔPO = 51 Pa (cf. (a)), with KD 27 GPa, and DD again 1 m<sup>2</sup> s 1 , for Dinantian limestone. Dashed line indicates Δx = 0.057 μm, which arises after 0.95 hours or 57 minutes. (c) Graph of ΔΦ for the patch of the Newdigate fault that slipped in the 1 April 2018 earthquake versus time for the variations in Δx depicted in (b). Calculations use Eq. (44), with the same parameters as for (b) plus b 21.5 mm, c 0.6, and E 75 GPa for the model asperity on the seismogenic patch of fault. Dashed line indicates ΔΦ = 60 kPa, which arises after 0.95 hours or 57 minutes. (d) Graphs as for (a), except DB is now 20 m<sup>2</sup> s 1 . Dashed line now indicates Δ<sup>P</sup> = -146 Pa for r = 8 km after t = 9 days. (e) Graph as for (b), except DB is now 20 m<sup>2</sup> s 1 . Dashed line now indicates Δx = 0.143 μm, which arises after 0.72 hours or 43 minutes. (f) Graph as for (c), except DB is now 20 m<sup>2</sup> s 1 . Dashed line indicates ΔΦ = 150 kPa, which arises after 0.72 hours or 43 minutes.*

**Figure 15.**

*DB is now 20 m<sup>2</sup> s*

*DB = 10 m<sup>2</sup> s*

**97**

*volume, calculated as Q 1.39 <sup>10</sup><sup>4</sup> m3 <sup>s</sup>*

*1*

*using Eq. (8) assume Q 6 <sup>10</sup><sup>5</sup> <sup>m</sup><sup>3</sup> <sup>s</sup>*

*hP = 2 m, rw = 0.0889 m, and DP =1m<sup>2</sup> s*

*1*

*2.9 MPa, roughly as observed.*

*Modelling of pressure interference of production from well BRX2Y on well HH1. (a) Graphs of pressure variations -Δ<sup>P</sup> in 'beef' adjoining well HH1 caused by the pulse of production from well BRX2Y on 23 March 2018, time t being measured from the start of production. Calculations using Eq. (15) assume 4 m3 produced*

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

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

*Horizontal dashed line indicates Δ<sup>P</sup> = -47 Pa for r = 10 km after t = 35 days. (b) Graphs as for (a), except*

*Graphs illustrating the pressure drawdown caused by 20 years of production from well BRX2Y. Calculations*

*1*

*1*

*dashed line now indicates ΔP = -23.7 kPa for r = 10 km with DB = 10 m<sup>2</sup> s*

*. Horizontal dashed line now indicates Δ<sup>P</sup> = -93 Pa for r = 10 km after t = 35 days. (c)*

*, kB = 900 mD; for other values of DB, kB is adjusted in proportion (cf. Eq. (5)). Horizontal*

*, η 0.9 mPa s, and hB 1 m, for variable kB and DB. For*

*<sup>1</sup> for Δt 8 hours, η 0.9 mPa s, hB 1 m, kB 900 mD, and DB 10 m2 s*

*1*

*, gives a predicted bottom-hole pressure decline at well BRX2Y of*

*. Using Eq. (8), with kP = 20 mD,*

*1 .*

pressure of 3.4 MPa, roughly 3 MPa below hydrostatic, whereas the initial bottomhole pressure in well HH1 was 6.3 MPa, roughly hydrostatic. The pressure drawdown at Brockham evidently had no significant effect on the pressure at HH1. It might thus be inferred that the two wells are not hydraulically connected, in contrast with the arguments in the present study. To explore this issue, Eq. (8) is used to calculate the effect of twenty years of production at the steady rate of <sup>6</sup> <sup>10</sup><sup>5</sup> <sup>m</sup><sup>3</sup> <sup>s</sup> <sup>1</sup> to obtain the 36,900 m<sup>3</sup> produced (**Figure 15(c)**). Taking into

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

#### **Figure 15.**

*Modelling of pressure interference of production from well BRX2Y on well HH1. (a) Graphs of pressure variations -Δ<sup>P</sup> in 'beef' adjoining well HH1 caused by the pulse of production from well BRX2Y on 23 March 2018, time t being measured from the start of production. Calculations using Eq. (15) assume 4 m3 produced volume, calculated as Q 1.39 <sup>10</sup><sup>4</sup> m3 <sup>s</sup> <sup>1</sup> for Δt 8 hours, η 0.9 mPa s, hB 1 m, kB 900 mD, and DB 10 m2 s 1 . Horizontal dashed line indicates Δ<sup>P</sup> = -47 Pa for r = 10 km after t = 35 days. (b) Graphs as for (a), except DB is now 20 m<sup>2</sup> s 1 . Horizontal dashed line now indicates Δ<sup>P</sup> = -93 Pa for r = 10 km after t = 35 days. (c) Graphs illustrating the pressure drawdown caused by 20 years of production from well BRX2Y. Calculations using Eq. (8) assume Q 6 <sup>10</sup><sup>5</sup> <sup>m</sup><sup>3</sup> <sup>s</sup> 1 , η 0.9 mPa s, and hB 1 m, for variable kB and DB. For DB = 10 m<sup>2</sup> s 1 , kB = 900 mD; for other values of DB, kB is adjusted in proportion (cf. Eq. (5)). Horizontal dashed line now indicates ΔP = -23.7 kPa for r = 10 km with DB = 10 m<sup>2</sup> s 1 . Using Eq. (8), with kP = 20 mD, hP = 2 m, rw = 0.0889 m, and DP =1m<sup>2</sup> s 1 , gives a predicted bottom-hole pressure decline at well BRX2Y of 2.9 MPa, roughly as observed.*

account the dependence of DB on permeability, it is evident that for DB 10– 20 m<sup>2</sup> s 1 , as envisaged in the present study, the predicted pressure decline at HH1 was only a few tens of kilopascals, thus not significant in relation to the measured pressure. The large difference in bottom-hole pressure between the two wells in 2018 is thus not evidence against the proposed model.

of well BRX2Y on well HH1. The OGA [6] made no mention of any pressure data for

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

The proposed mechanism for the Newdigate seismicity depends on a pressure

In principle, testing of the proposed mechanism is possible, given the predicted vertical compaction in the Dinantian limestone (Eq. (35)). Such compaction will cause subsidence of the Earth's surface, and so is in principle observable using multiple techniques, including interferometric synthetic-aperture radar (InSAR) and repeated gravity and GPS measurements. A combined dataset of this type has been analysed for a region of southeast England, including the northern Weald Basin, by Aldiss et al. [129]. At the October 2018 workshop attention was also drawn to an InSAR-derived surface deformation map of Britain by GVL [130], spanning October 2015 to October 2017. However, the predicted subsidence, resulting from the two decades of production at Brockham, will be only a small fraction of 1 mm (36,900 m<sup>3</sup> / 200 km<sup>2</sup> <sup>≈</sup> 0.2 mm), even if none of the fluid withdrawal from this limestone were recharged. The Aldiss et al. [129] analysis revealed vertical crustal

shallow groundwater reservoirs. Such rates make it impossible to resolve the much smaller effect expected from compaction of the Dinantian limestone at Newdigate. Much has been made by participants in the OGA [5] workshop regarding the extent to which the Newdigate earthquake 'swarm' might fit the standard criteria identified by Davis and Frohlich [131] for establishing whether instances of seismicity are anthropogenic (e.g., [54]). UKOG [8] have argued that this set of criteria is inapplicable as they relate to seismicity caused by fluid injection, which is not the causal mechanism in this case. However, familiarity with the literature in this field

(e.g., [132]) indicates that these criteria are widely used irrespective of the

geomechanical cause of any particular anthropogenic earthquake. Verdon et al. [6]

, caused by processes such as extraction from or recharge of

drop within the Dinantian limestone alongside the seismogenic strand of the Newdigate fault zone, as a result of depressurization of the water within this fault, caused by oil production from neighbouring wells (**Figure 5**). For the production from BRX2Y to have caused seismicity by this mechanism, the seismogenic fault must already have been extremely close (maybe within 60 kPa; see above) to the Mohr-Coulomb failure condition. It can be inferred that the same mechanism, operating during the previous production from this well, contributed to creating this state of stress by progressively depressurizing the Dinantian limestone. To test this possibility, one may use Eq. (36), noting the 2.7 MPa depressurization of the reservoir over 20 years and estimating from the previous analyses (e.g., **Figure 15 (d)**) that the resulting value of ΔP (and, thus, δP) within this limestone would be 10 kPa. With BE = 27 GPa, <sup>α</sup> = 0.46, H = 70 m, and <sup>Δ</sup>V = 36,900 m<sup>3</sup> to balance the production, depressurization of a 6000 km<sup>2</sup> area would be indicated; if roughly equidimensional, this would have a radius of 40 km. However, in reality, it is to be expected that such depressurization would be largely cancelled by recharge of water into the Dinantian limestone from other directions, which is not incorporated into the model. For example, if after this cumulative production, xM were 6 km and ΔPO were 1 kPa, then substituting Eq. (21) into Eq. (41), and taking b 0.1 m (obtained for L = 1.5 km, the length of the seismogenic part of the Newdigate Fault, using Eq. (49)), ΔΦ would be 6 MPa. Notwithstanding the approximations made in the model, it is thus indeed plausible that the cumulative production at Brockham brought this fault to the condition for shear failure, assuming that it was already

well BRX2Y that might be used to test this deduction.

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

critically stressed before this production began.

motions at 1 mm a<sup>1</sup>

**99**

**5. Discussion**

The production test from well HH1 that started on 11 February lasted until late June 2019, some 140 days or 20 weeks, albeit with some intermittency (see supplement). Using Eq. (8), the resulting pressure decline is depicted in **Figure 16(a)** for DB = 10 m<sup>2</sup> s <sup>1</sup> and in **Figure 16(b)** for DB = 20 m<sup>2</sup> s 1 . At r = 10 km, in the vicinity of well BRX2Y, the maximum pressure decline is 40 kPa (**Figure 16(a)**) or 60 kPa (**Figure 16(b)**). The production from well HH1 thus influenced bottom hole pressure in well BRX2Y by three orders of magnitude more than for the effect

#### **Figure 16.**

*Modelling of pressure interference of production from well HH1 on well BRX2Y. (a) Graphs of pressure variations -Δ<sup>P</sup> in 'beef' adjoining well BRX2Y caused by the phase of production from well HH1 starting in February 2019, time t being measured from the start of production. Calculations using Eq. (8) assume Q <sup>4</sup> <sup>10</sup><sup>3</sup> <sup>m</sup><sup>3</sup> <sup>s</sup> 1 , η 0.9 mPa s, hB 1 m, kB 900 mD, and DB 10 m<sup>2</sup> s 1 . Horizontal dashed line indicates ΔP = -37.8 kPa for r = 10 km after t = 140 days. (b) Graphs as for (a), except DB is now 20 m<sup>2</sup> s 1 . Horizontal dashed line now indicates ΔP = -56.6 kPa for r = 10 km after t = 140 days.*

of well BRX2Y on well HH1. The OGA [6] made no mention of any pressure data for well BRX2Y that might be used to test this deduction.
