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Depletion of gas fields, even in a tectonically inactive area can induce earthquakes.

Carbon dioxide (CO2) separated from natural gas has been stored successfully below the seabed off Norway for almost two decades. Based on these experiences several demonstration projects supported by the EU and its member states are now setting out to store CO2 captured at power plants in offshore geological formations. The ECO2 project was triggered by these activities and funded by the EU to assess the environmental risks associated with the sub-seabed storage of CO2 and to provide guidance on environmental practices. ECO2 conducted a comprehensive offshore field programme at the Norwegian storage sites Sleipner and Snøhvit and at several natural CO2 seepage sites in order to identify potential pathways for CO2 leakage through the overburden, monitor seep sites at the seabed, track and trace the spread of CO2 in ambient bottom waters, and study the response of benthic biota to CO2. ECO2 identified a rich variety of geological structures in the broader vicinity of the storage sites that may have served as conduits for gas release in the geological past and located a seabed fracture and several seeps and abandoned wells where natural gas and formation water are released into the marine environment. Even though leakage may occur if these structures are not avoided during site selection, observations at natural seeps, release experiments, and numerical modelling revealed that the footprint at the seabed where organisms would be impacted by CO2 is small for realistic leakage scenarios. ECO2 conducted additional studies to assess and evaluate the legal framework and the public perception of CO2 storage below the seabed. The following guidelines and recommendations for environmental practices are based on these experiences.

In this paper we present a review of controlling geological, tectonic and engineering factors for induced seismicity associated to gas depletion in the Netherlands and we place experiences from extensive Dutch geomechanical studies in the past decade in the context of generic models for induced seismicity. Netherlands is in a mature gas production phase, marked by excellent subsurface structural and stratigraphic characterization. Over 190 gas fields of varying size have been exploited. No more than 15% of these fields show seismicity. Geomechanical studies show that, similar to the EGS stimulation phase, largest seismicity is localized on pre-existing fault structures. However, the prime cause for seismicity in gas depletion is differential compaction, whereas in EGS stimulation related pressure build-up and fluid pressure diffusion along the faults form the prime mechanism. On the other hand, our study has a close theoretical analogy to reservoirs where the fluid volumes extracted are significantly larger than the re-injected volumes, and which can result in (differential) reservoir compaction.The observed onset of induced seismicity in the Netherlands occurs after a considerable pressure drop in the gas fields. Geomechanical models show that both the delay in the onset of induced seismicity as well as the non-linear increase in seismic moment observed in the induced seismicity, can be explained using a model of differential compaction, if the faults involved in induced seismicity are not critically stressed at the onset of depletion. The presented model serves to highlight key aspects of the interaction of initial stress and differential compaction in the framework of induced seismicity in Dutch gas fields. It is not intended as predictive model for induced seismicity in a particular field. To this end, a much more detailed field specific study, taking into account the full complexity of reservoir geometry, depletion history, mechanical properties and initial stress field conditions is required.

In this report a number of issues are put forward associated with the Transfer of

This paper presents the results from an assessment of high-capacity storage options in the offshore area of The Netherlands. For deep saline formations, available regional geological maps were combined with fault structures derived from seismic data to reveal the compartmentalization of prospective reservoir formations. Adding knowledge concerning these formations from offshore oil and gas activities and concerning the behavior of gas fields located in these formations, resulted in a shortlist of lowest-risk, potential storage locations with a total theoretical capacity of about 1.5 GtCO2. A high-level risk analysis was performed for offshore gas fields. The largest offshore gas fields add another 350 MtCO2. The development of saline formations and gas fields is outlined; while a gas field can be converted from production to storage in 5-6 yrs, it takes at least 7 yrs to develop CO2 storage in a saline formation that has not been accessed before for hydrocarbon production.

In this paper an integrated workflow is described for risk assessment within CCS. IFPEN, SINTEF and TNO joined forces to define a comprehensive and transparent risk assessment methodology. The tools developed in these institutes are thereby integrated. The workflow can be applied to proposed carbon storage sites. Starting with a qualitative analysis and scenario definition potential risky scenarios are defined. These are subsequently investigated in a quantitative way, either by so-called fast models or by fully developed numerical codes. The scenario analysis includes modelling of the reservoir and caprock behaviour, well integrity evaluation and migration path analysis. The important item of sensitivity analysis is addressed by probabilistic means. Finally, the consequences of potential surface leakage are addressed in this suite, resulting in a clear picture of the risks at a specific site / time. The presented methodology is tested on a real offshore gas field.

In het kader van de Mijnbouwwet 2003 moeten seismische risicoanalyses in het winningsplan opgenomen worden. Bij olie- en gaswinning gaat het om geïnduceerde seismiciteit. In 2003 is door TNO en KNMI in opdracht van de mijnbouwmaatschappijen onderzoek uitgevoerd naar de seismische hazard door geïnduceerde aardbevingen. De resultaten van dit onderzoek zijn samengevat en vastgelegd in het TNO/KNMI rapport ‘Seismisch hazard van geïnduceerde aardbevingen. Integratie van deelstudies.’ (NITG-02-244-B, d.d. 20 december 2004). Eens per 5 jaar wordt de Seismische Hazard Analyse en het rapport ‘Seismisch hazard van geïnduceerde aardbevingen. Integratie van deelstudies’ herzien aan de hand van nieuwe observaties m.b.t. geïnduceerde aardbevingen en additionele onderzoeksresultaten. In 2009 is besloten geen uitgebreide herziening te publiceren omdat de beschikbare data daar geen aanleiding toe gaven. In het huidige rapport wordt de deterministische hazard studie van TNO (NITG 04-171-C, d.d. 7 april 2004) geheel herzien.

In the northern parts of The Netherlands, the exploitation of gas fields has induced small earthquakes. These induced earthquakes have occasionally caused damage to buildings. A workflow was developed in order to evaluate the seismic risk of the exploitation of hydrocarbon fields. A Bayesian analysis was performed to investigate the correlation between the reservoir properties and production specifics of the fields and the occurrence of induced earthquakes. Based on geomechanical and reservoir production key parameters, a probability of that induced seismicity will occur can be assigned to an individual hydrocarbon field. Probable ground motions for average surface conditions were obtained from a probabilistic seismic hazard analysis. Next, the impact of local soil conditions was taken into account by analysing the site response of shallow soil profiles. Finally, the relation of ground motions and probability of damage was investigated, using data on damage during past induced erathquakes. In this extended abstract a concise overview of this workflow is presented.© 2010, Society of Petroleum Engineers.

We present a general framework that enables decision-making when a threshold in a process is about to be exceeded (an event). Measurements are combined with prior information to update the probability of such an event. This prior information is derived from the results of an ensemble of model realisations that span the uncertainty present in the model before any measurements are collected; only probability updates need to be calculated, which makes the procedure very fast once the basic ensemble of realisations has been set up. The procedure is demonstrated with an example where gas field production is restricted to a maximum amount of subsidence. Starting with 100 realisations spanning the prior uncertainty of the process, the measurements collected during monitoring bolster some of the realisations and expose others as irrelevant. In this procedure, more data will mean a sharper determination of the posterior probability. We show the use of two different types of limits, a maximum allowed value of subsidence and a maximum allowed value of subsidence rate for all measurement points at all times. These limits have been applied in real world cases. The framework is general and is able to deal with other types of limits in just the same way. It can also be used to optimise monitoring strategies by assessing the effect of the number, position and timing of the measurement points. Furthermore, in such a synthetic study, the prior realisations do not need to be updated; spanning the range of uncertainty with appropriate prior models is sufficient. © International Association for Mathematical Geosciences 2009.