The effective subsidence capacity concept: How to assure that subsidence in the Wadden Sea remains within defined limits?

Abstract

Subsidence caused by extraction of hydrocarbons and solution salt mining is a sensitive issue in the Netherlands. An extensive legal, technical and organisational framework is in place to ensure a high probability that such subsidence will stay within predefined limits. The key question is: how much subsidence is acceptable and at which rate? And: how can it be reliably assured that (future) subsidence will stay within these limits?To address the issue for the Wadden Sea area, the concept of ‘effective subsidence capacity’ is used. To determine the ‘effective subsidence capacity’, the maximum volumetric rate of relative sea-level rise, that can be accommodated in the long term, without environmental harm, is established first. The volume of sediment that can be transported and deposited by nature into the tidal basin where the subsidence is expected, ultimately determines this ‘limit of acceptable average subsidence rate’. The capability of the tidal basins to ‘capture’ sediment over the lunar cycle period of 18.6 years is the overall rate-determining step. Effective subsidence capacity is then the maximum average subsidence rate available for planning of human activities. It is obtained by subtracting the subsidence volume rate ‘consumed’ by natural relative subsidence in the area (sealevel rise plus natural shallow compaction) from the total long-term acceptable subsidence volume rate limit.In the operational procedure for mining companies, six-years-average expectation values of subsidence rates are used to calculate the maximum allowable production rates. This is done under the provision that production will be reduced or halted if the expected or actual subsidence rate (natural + man induced) is likely to exceed the limit of acceptable subsidence. Monitoring and management schemes ensure that predicted (6-year average) and actual (18.6-year average) subsidence rates stay within the limit of acceptable subsidence rate and that no damage is caused to the protected nature. A GPS based early warning system is used for early detection of unexpected behaviour. In support of SSM (State Supervision of Mines, the government regulator), TNO-AGE (an independent government advisory group) applies an independent Bayesian statistical analysis of all data, as they become available, to calculate the probability of scenario's under which future subsidence will exceed the defined limits. It is external to the operator's annual measurement and control loop and ensures that preventive actions can be taken in time in case such scenarios emerge.Regular communication keeps the authorities and the general public informed on the use of the effective subsidence capacity to demonstrate that the actual average subsidence rate stays strictly within the defined bounds and that, from a scientific point of view, there is no reasonable doubt that damage to the tidal system will not occur now or in the future.

Production induced subsidence and seismicity in the Groningen gas field – can it be managed?

“…It illustrates that initial uncertainties, prior to the start of production, are perhaps as large as a fac- tor of two (up or down) relative to the predicted expectation case. This has meanwhile been confirmed for a number of other fields in the Netherlands (de Waal et al, 2012). Major potential contributors are uncertainties in subsurface models, unknown influx of bottom and lateral aquifers, core damage, non-linear rock compressibility, the difference in loading rate between laboratory and field, the very large change in loading rate at the start of production, the in-situ stress state, reservoir burial history, salt flow, the relation between subsidence and compaction etc.…”

“…Validating this explanation for field application is therefore problematic. The newly developed rate-type compaction model explained the difference between the initial field observed and the labderived compressibility for Groningen and explained the observed reduced and/or delayed subsidence above a number of other depletion-drive oil and gas reservoirs (de Waal, 1986;de Waal and Smits, 1988).…”

“…Such an amount of subsidence requires the design and implementation of proper mitigation measures in a near sea-level country like the Netherlands (de Waal et al, 2012). The country would largely flood without the presence of dykes and active pumping and the predicted sea-level rise from global warming increases the challenge.…”

From checking deterministic predictions to probabilities, scenarios and control loops for regulatory supervision

“…The large changes over time in the predicted maximum subsidence suggest an initial uncertainty, prior to the start of production, of at least a factor of two (up or down). This has been confirmed for several other fields in the Netherlands (De Waal et al, 2012). It can be caused by knowledge gaps, uncertainties in subsurface models, unknown influx of bottom and lateral aquifers, core damage, nonlinear rock compressibility, the difference in loading rate between laboratory and field, the very large change in loading rate at the start of production, the in situ stress state, reservoir burial history, salt flow, uncertainties in the relation between subsidence and compaction, etc.…”

How to connect societal concerns on subsidence to expert knowledge

“…Step 0 brought on a 14 year national public debate pointed at subsidence threatening ecological values. The technical work related to steps 2 and 3 took a couple of years, resulting in a "type F" threshold (De Waal et al, 2012) related to net sand budgets in tidal basins (step 4) and a number of limit values (step 6), some of which have to be periodically updated (i.e. local sea level rise scenarios are to be updated every five years).…”