# Seismic Velocity Changes in the Groningen Reservoir Associated with Remote Drilling – Scientific Reports

As previously mentioned, the transient decrease in the P-wave travel time to the geophone 10 and the increase in the delay time of the PS-waves could be explained by a transient upward movement of the gas-water contact (GWC). Here we examine this interpretation in more detail.

### Height of the GWC from seismic observations

The change in seismic velocities due to the substitution of water for gas in a porous sandstone can be calculated using the Gassmann model of liquid substitution25, 26. The bulk modulus of a fluid-saturated rock is related to the porosity and bulk modulus of the mineral matrix, pore fluid, and dry rock framework. The bulk modulus of the fluid increases in the case of gas-water substitution, and this increases the effective bulk modulus of the rock and hence the P-rate. The shear modulus, on the other hand, does not change since it mainly depends on the solid rock framework. However, due to the small increase in density, a slight decrease in S-velocity is expected.

A quantitative estimate of the level change of the GWC using the Gassmann model would require accurate values ​​of the mass and shear moduli of the matrix, fluids (gas and brine) and rock framework for the local rock. Since these are unknown and rough estimates would have large uncertainties, we have taken a more practical approach. We estimated the average P-velocities above and below the GWC from the sound log data and found P-velocities of 3321 m/s and 3688 m/s, respectively (Supplementary Material, Section 5). Based on these values, a 0.7 ms reduction in the propagation time of the P wave would correspond to an increase in the GWC by 23 m. We further checked whether this increase in GWC could also explain the increase in PS lag time ($$\Delta (t_{PS}-t_P)$$ $$\simeq$$ 1.0ms). Assuming 23m displacement of GWC, S-wave transit time increase of 0.3ms, vertical propagation at S-wave velocity of 2000m/s for the sandstone with gas12, we find a deceleration of the S-wave of only 52 m/s (2.6%). This decrease would be solely the effect of the increase in density on the shear rate caused by the replacement of gas with water. Although the values ​​appear realistic, it should be noted that the uncertainties are large and 23 m should only be interpreted as an indication of the GWC height derived from our measurements.

The other observation is the rapid decrease in noise level and its rapid return to normal levels observed for geophone 10 relative to other geophones (Figure 4). Such rapid changes can easily be achieved by changing the GWC level, although it is not clear how this would reduce the noise level.

### Relationship to well operations at well ZRP-3

If a transient increase in GWC can explain the seismic observations, what caused the increase in GWC arises. Gas production data in the area has been reviewed but shows no correlation with our data. Because the timing of the anomaly appeared to correlate with drilling of hole ZRP-3 4.5 km away, we examined the detailed drill log provided by NAM.

Drilling commenced on May 23 (2015) and the reservoir was reached on July 13 by drilling the Ten Boer claystone (Fig. 3a). Drilling mud well losses occurred on July 18th and the first few hours of July 19th. Deeper drilling occurred in limited periods on single days between July 23 and August 21 when the maximum depth of 3284m was reached. Depth of the GWC was reached on July 31st and the carboniferous shale was drilled on August 11th. The cementing of the borehole took place on 28.-29. August and the well was abandoned on August 30 after the cement had set.

The first conclusion is that there is no correlation between our observations and actual drilling intervals. First, the drilling and coring periods were spread out in time, while our observations show a month-long trend of mostly decreasing travel times (Fig. 3a). Second, drilling noise would affect the propagation times between all geophone pairs, but this is not observed (Supplementary Material, Fig. S3). Thus, drilling noises cannot explain the observations. We also considered the drill hole losses that occurred during drilling in the Ten Boer claystone. However, these drill hole losses began 30 hours after the start of the anomalous observations.

A more likely cause is the pore pressure fluctuations caused by drilling. NAM provided us with data on the static well pressure ($$BHP_s$$), calculated from the borehole depth (H), the drilling mud density ($$\rho _m$$) and the acceleration due to gravity (G): $$BHP_s=\rho _m gh$$. These data are in Fig. 3a. Note that $$BHP_s$$ represents only a portion of the total wellhead pressure (BHP) as dynamic pressure effects are not included. Fast decreases in $$BHP_s$$, such as between July 19 and July 23 related to mud losses, may have been dynamically compensated by well fluid circulation to stabilize BHP. There was a gradual increase from July 23 to August 19 as drilling depths increased from 2919 to 3267 meters $$BHP_s$$ from 36 to 39 MPa as indicated by the dashed blue box in Fig. 3a. The gradual trend of increasing BHP is anti-correlated with the decrease in P-wave travel time from geophone 8 to 10 and correlated with the increase in PS delay time at geophone 10 (Fig. 3a,b).

Assuming that our anomalous observations at SDM-1 are related to borehole pressure (BHP) at ZRP-3, it is likely that they are related by changes in pore pressure. An increase in GWC of $$\sim$$ 23 m would correspond to an increase in pore pressure in the water-bearing part of the sandstone $$\sim$$ 0.23MPa ($$\Delta P = \rho _w g \Delta h$$). By relating the start and finish times of drilling in the reservoir to the start and finish times of our anomalous observations, we calculated the time it took for the pressure front to propagate from ZRP-3 to SDM-1. Drilling in the Ten Boer mudstone occurred between 7:45 am and 5:00 pm on July 13, while anomalous seismic observations began on July 17 at $$\sim$$ 00:00 (Fig. 4a). This results in a time delay of 3 days and 7-16 hours. A similar calculation can be performed for the end of the abnormal period. The hardening of the cement at the ZRP-3 took place on August 30 (00:00 – 07:30). After the cement had hardened, the well was sealed and there was no longer any interference from drilling operations. Combining this with the end of the anomaly at SDM-1 on September 2 at 19:00 (Fig. 4b) results in a time lag of 3 days and 11.5-19 h.

### pore pressure diffusion

From our seismic observations and their correlation with the bottom hole pressure at ZRP-3, it is concluded that variations in pore pressure may have caused changes in the level of the GWC in SDM-1. Next, it should be verified that the pore pressure diffusion process can explain the time lag between the reservoir drilling at ZRP-3 and the GWC response at SDM-1 4.5 km away.

For isotropic and spherical diffusion, the hydraulic diffusivity (D) in connection with the pore pressure diffusion in a liquid-carrying porous medium can be derived from the time (she) the pressure front needs to reach a certain distance (right)27

\begin{aligned} r = \sqrt{4 \pi D t}. \end{aligned}

(1)

The pore pressure diffusivity is estimated from the propagation time of the pressure front, given the time lags of 3 days and 7-16 h (onset) and 3 days and 11.5-19 h (end). The longest (3 days and 19 h) and shortest (3 days and 7 h) times result in diffusivities of 4.9 m$$^2$$/s and 5.7m$$^2$$/s or

An independent estimate of the hydraulic diffusivity (D) can be calculated from material properties, including average porosity (0.1528) and permeability (120 mD29) with more details under “Method”. We find a pore pressure diffusivity of 3.9 m$$^2$$/s, which is similar, if slightly smaller, to the previously estimated diffusion range of 4.9–5.7 m$$^2$$/s. This diffusion range requires permeabilities of 151–176 mD, slightly higher than our assumed value of 120 mD, but within the wide range of 1–1000 mD measured for the Groningen gas reservoir30. From this it is concluded that pore pressure diffusion in the water-bearing part of the reservoir can explain the time lag between the overpressure caused by drilling at ZRP-3 and the change in GWC at SDM-1.

The Groningen reservoir is highly faulted, and faults can act either as barriers or as efficient channels for pore pressure, depending on direction: permeability is generally high within the fault zone parallel to the fault and low across the fault31. The NAM fault map for the top of the reservoir (Fig. 5a) shows a fault offset about 150 m midway between SDM-1 and ZRP-3, separating two compartments of the reservoir with the wells on either side (Fig 5b ) . This disturbance probably impedes the direct pore pressure diffusion through the gas-carrying parts between the two compartments. On the other hand, the $$\sim$$ 20 m level changes of the GWC at a distance of 4.5 km from the drilling site and the high diffusivity ($$\sim$$ 5 m$$^2$$/s) indicate a high permeability line between the two locations. The NAM fault map shows no connecting fault, although there are speculative two ENE-WSW trending fault segments that may be connected at the bottom of the reservoir (Fig. 5a).

It is important to note that SDM-1 is an open-top and open-bottom well that will be perforated at reservoir depths between 2965 and 2995 m. The loading of a high density brine column within the wellbore prevents reservoir gas from flowing inward through the perforations. Because the well is an open system, it is sensitive to variations in hydrostatic pressure in the reservoir. Our speculative hypothesis is that the pressure front caused by overpressure at the remote wellhead and propagated through the aquifer of the reservoir reached SDM-1 and moved the brine column up, raising the water table in the well. Following the level change within the hole borehole, the GWC in its immediate vicinity was also increased as determined by the seismic data.

While we know that parts of our interpretation are highly speculative, we have not been able to find any other plausible explanation. Nonetheless, it seems evident that the observations are related to remote drilling, an effect that is unexpected and may be important to other drilling activities.

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