Location of induced earthquakes in the Netherlands gas fields
Dirk Kraaijpoel, Bernard Dost, Reinoud Sleeman, Femke Goutbeek
The catalogue of earthquakes induced by gas production in the north of the Netherlands contains 688 events to date (Feb 2011). The emerging seismicity patterns suggest delineation of specific faults at reservoir level. To improve location quality we are in the process of incorporating a 3D velocity model from the gas industry.
The north of the Netherlands contains a number of large on-shore gas fields that are in production since 1960. The first earthquake in the area was recorded in 1986. The KNMI monitors the area with a network of seismic sensors in shallow (200m) boreholes as well as accelerometers (Figure 1).
Figure 1.Seismicity in The Netherlands. Symbols indicate natural (red circles) and induced seismicity (yellow circles), KNMI stations (blue symbols). Gas fields are shown in green. Red and blue lines refer to Figures 2 and 4 respectively.
Whereas the surface of most of the Netherlands is flat, the subsurface is highly complex. Near the gas fields the stratification is distorted severely mainly due to salt tectonics (Figure 2). The reservoirs are cut into compartments by vertical faults systems. The earthquakes are associated with differential compaction due to gas extraction and reactivation of the existing faults.
Figure 2. Geological cross section through the Netherlands along the red line in Figure 1. The black box indicates the Groningen gas field. Gas is trapped in Rotliegend sandstone reservoirs beneath the Zechstein salt. Tops of fault planes are shown at base Zechstein. Vertical extent downwards and precise dip are not well known.
(Figure from “Geological atlas of the subsurface of the Netherlands”, TNO, 2004)
From 1986 to February 2011 we have recorded 688 induced events with magnitudes ranging from -0.8 to 3.5. Routine location is performed using a 1D velocity model. Depth is fixed to reservoir level of 3.0. Horizontal uncertainties are in the order of 0.5 km. Figure 3 shows a subset of the catalogue with the expression of the faults at base Zechstein. The seismicity pattern shows some lineation, especially for the larger earthquakes. It is expected that more accurate location with a 3D velocity model will allow us to more precisely identify active fault systems.
Figure 3. Subset of the current induced earthquake catalogue in the Groningen gas field with the faults at base Zechstein superimposed.
The complexity of the geology on and above reservoir level strongly demands the use of a 3D velocity model to improve location accuracy. We use the velocity model VELMOD1.0, constructed by TNO based on seismic and borehole data from the gas industry (Figure 4).
Figure 4. P-velocity models. In red currently used 1D model; in blue lateral median of 3D model including ranges.
Figure 5. South-North(left-right) cross section for 3D model along blue line in Figure 1.
To improve the accuracy of induced earthquake locations in the gas fields and to identify active fault planes we have decided to incorporate 3D velocity variations. We are currently evaluating forward and inverse algorithms to perform this task.