Geomechanical models for induced seismicity in the Netherlands: inferences from simplified analytical, finite element and rupture model approaches

Abstract

In the Netherlands, over 190 gas fields of varying size have been exploited, and 15% of these have shown seismicity. The prime cause for seismicity due to gas depletion is stress changes caused by pressure depletion and by differential compaction. The observed onset of induced seismicity due to gas depletion in the Netherlands occurs after a considerable pressure drop in the gas fields. Geomechanical studies show that both the delay in the onset of induced seismicity and the nonlinear increase in seismic moment observed for the induced seismicity in the Groningen field can be explained by a model of pressure depletion, if the faults causing the induced seismicity are not critically stressed at the onset of depletion. Our model shows concave patterns of log moment with time for individual faults. This suggests that the growth of future seismicity could well be more limited than would be inferred from extrapolation of the observed trend between production or compaction and seismicity. The geomechanical models predict that seismic moment increase should slow down significantly immediately after a production decrease, independently of the decay rate of the compaction model. These findings are in agreement with the observed reduced seismicity rates in the central area of the Groningen field immediately after production decrease on 17 January 2014. The geomechanical model findings therefore support scope for mitigating induced seismicity by adjusting rates of production and associated pressure change. These simplified models cannot serve as comprehensive models for predicting induced seismicity in any particular field. To this end, a more detailed field-specific study, taking into account the full complexity of reservoir geometry, depletion history and mechanical properties, is required.

Insights From Closed‐Form Expressions for Injection‐ and Production‐Induced Stresses in Displaced Faults

“…Qualitatively, the results for regular slip in Figure c are in agreement with those found in earlier numerical studies; see, for example, Figures 7a, 7c, 7e, and 8a in Van Wees et al (), with the exception of one major difference: Van Wees et al () considered production rather than injection resulting in the occurrence of positive peak shear stresses at the internal corners of the reservoir and the fault (located at y =±100 m in our example), whereas for the injection case in Figure c the positive peak stresses occur at the external corners (i.e., at y =±200 m). This has potential consequences as will be discussed in the next section.…”

“…The linear elastic assumptions underlying the analytical solutions in our paper are restrictive, and a full analysis of the development of aseismic and seismic slip along the fault would require nonlinear quasi‐static and dynamic analyses, such as in Wassing et al (), Van Wees et al (), Buijze et al (). However, our linear elastic analytical results do give an indication of some typical features of the development of injection‐ or production‐induced seismic events in displaced faults as will be discussed below.…”

Initiation and arrest of earthquake ruptures due to elongated overstressed regions

“…Stress concentrations play an important role also in some induced seismicity situations. Notably, in producing gas fields, peaked stresses can develop along the top or bottom of the intersection between the reservoir and a fault, due to the effect of differential compaction between reservoir compartments that are offset by the fault (Mulder, 2003;Buijze et al, 2017;Van Wees et al, 2017).…”

Using machine learning for model benchmarking and forecasting of depletion-induced seismicity in the Groningen gas field

“…Forecasting of induced seismicity requires a detailed understanding of both the physical mechanisms governing depletion-induced seismicity, as well as reservoir properties in time and space. On physical mechanisms, research from lab to field experiments and from experimental to complex numerical models has provided insights into the mechanisms behind induced seismicity in general [8][9][10][11][12], in particular depletion-induced seismicity [13][14][15][16][17][18][19] and injection-induced seismicity [20,21]. On reservoir properties, data acquisition efforts [22][23][24] can improve data quantity and quality, but as properties can only be measured directly in wells (which gives very limited spatial coverage) or using seismic reflection data (which needs to be interpreted, and has limits in applicability in regions with induced seismicity), these datasets carry large intrinsic uncertainties.…”