Seismic Monitoring Project Research Articles

Dictionary Learning for Single-Channel Passive Seismic Denoising

Abstract Passive seismic denoising is mostly performed using a simple band-pass filter, which can be problematic when signal and noise share the same frequency band. More advanced passive seismic denoising methods take advantage of fixed-basis transforms, for example, the wavelet, to remove noise. Here, we present an open-source package for data-driven denoising based on adaptively learning sparse transform. Contrary to the fixed-basis transforms, the proposed method belongs to the adaptive-basis transforms. We learn the 1D features embedded in the passive seismic data from all the available waveform data sets without requiring spatial coherency in a data-driven way. Thus, the new method is flexible to apply in any passive seismic monitoring project because of its data-driven and single-channel nature when implemented. Considering the computationally expensive K-singular-value-decomposition (KSVD) in the traditional dictionary learning framework, we suggest applying a fast SVD-free dictionary learning method that can be readily applicable to process massive seismic data during passive seismic monitoring. The proposed method is applied to two synthetic data examples and three real passive seismic data sets to demonstrate its effectiveness in improving the signal-to-noise ratio, and its potential in applications like arrival picking. The open-source reproducible package can be found in the Data and Resources section.

Storing CO2 in geothermal reservoir rocks from the Kizildere field, Turkey: Combined stress, temperature, and pore fluid dependence of seismic properties

As part of a seismic monitoring project in a geothermal field, where the feasibility of re-injection and storage of produced CO2 is being investigated, a P- and S-wave seismic velocity characterisation study was carried out. The effect of axial and radial (up to 42 MPa) stress, pore pressure (up to 17 MPa), pore fluid (100% brine or supercritical CO2) and temperature (21–100 °C) on seismic properties were studied in the laboratory for the two main reservoir formations at the Kızıldere geothermal reservoir. Each (un)confined compressive strength test performed revealed a similar trend: rapidly increasing velocity at low stresses followed by a more moderate increase at higher stresses. The data implied that the stress-dependency of the velocity increased with temperature. Increasing temperatures resulted in decreasing P-wave velocities due to mineral thermal expansion. This temperature-dependency increased with reducing stress levels. The S-wave velocity seems to be more sensitive to changes in pore pressure than the P-wave velocity. On the other hand, the S-wave velocity is less affected by an increasing axial stress compared to the P-wave velocity. By performing multiple nonlinear regression on the velocity dataset, related to a brine-saturated fractured marble, second-degree polynomial trends were found for the P- and S-wave velocity, as a function of temperature, axial stress, and pore pressure, that can potentially be used for predicting velocities at Kızıldere, or other similar, geothermal site(s). For distinguishing between a 100% brine-saturated versus a fully supercritical CO2-saturated fracture, the arrival times of the first arrivals were too close to each other to allow their utilization. The fracture aperture was too small compared to the wavelength of the source signal. However, differences in P- and S-wave amplitudes of the first arrivals were seen, where the supercritical CO2-saturated crack revealed consistently lower peak and trough amplitudes compared to the brine-saturated scenario.

Seismic Velocity Characterisation of Geothermal Reservoir Rocks for CO2 Storage Performance Assessment

As part of a seismic monitoring project in a geothermal field, where the feasibility of re-injection and storage of produced CO2 is being investigated, a P- and S-wave seismic velocity characterisation study was carried out. The effect of axial (up to 95 MPa) and radial (up to 60 MPa) stress on the seismic velocity was studied in the laboratory for a broad range of dry sedimentary and metamorphic rocks that make up the Kızıldere geothermal system in Turkey. Thin section texture analyses conducted on the main reservoir formations, i.e., marble and calcschist, confirm the importance of the presence of fractures in the reservoir: 2D permeability increases roughly by a factor 10 when fractures are present. Controlled acoustic-assisted unconfined and confined compressive strength experiments revealed the stress-dependence of seismic velocities related to the several rock formations. For each test performed, a sharp increase in velocity was observed at relatively low absolute stress levels, as a result of the closure of microcracks, yielding an increased mineral-to-mineral contact area, thus velocity. A change in radial stress appeared to have a negligible impact on the resulting P-wave velocity, as long as it exceeds atmospheric pressure. The bulk of the rock formations studied showed reducing P-wave velocities as function of increasing temperature due to thermal expansion of the constituting minerals. This effect was most profound for the marble and calcschist samples investigated.

Geological Survey of Western Australia: SWAN takes off - a new seismic monitoring project in Western Australia

The Geological Survey of Western Australia, in collaboration with the Australian National University, Macquarie University, the Department of Fire and Emergency Services and Geoscience Australia, h...

Seismic monitoring project turns focus to Alaska

Seismic monitoring project turns focus to Alaska

The role of rock physics for the Enfield 4D seismic monitoring project

The Enfield rock physics model was constructed to enable 4D feasibility studies and interpretation of the 2007 Enfield 4D seismic monitor survey. The rock physics model links reservoir static and dynamic parameters to impedances, using log data from five wells in the field, laboratory core measurements taken from cores on Enfield and neighbouring fields, and theoretical rock models from the literature. The reservoir is modelled by a sand-shale mix: sand properties are described using a modified critical porosity model whereas shale properties are generated from log data averaging. The dynamic properties in the model include saturation and pressure. Saturation is modelled using Gassmann’s formula assuming homogeneous mixing. The reservoir sand velocity-pressure relationship is described by an empirical model fitted to dry core plug measurements. An assessment of the effect of uncertainty is included for both the saturation and pressure elements of the model. The resultant rock physics model was used before the acquisition of the seismic monitor survey to assess the likelihood of detecting a 4D seismic signal only 7 months after production start-up. Our modelling results indicate that the strong pressure build-up around the water injectors would result in a detectable 4D seismic signal and this prediction is confirmed by the successful 4D seismic monitor data acquired in 2007. The rock physics model has been validated against the 4D monitor data and is being used to quantify the 4D interpretation, linking the observed 4D response back to predicted pressure and saturations changes in the field.

Seismicity characteristics of a potentially active Quaternary volcano: The Tatun Volcano Group, northern Taiwan

The Tatun Volcano Group (TVG) is located at the northern tip of Taiwan, near the capital Taipei and close to two nuclear power plants. Because of lack of any activity in historical times it has been classified as an extinct volcano, even though more recent studies suggest that TVG might have been active during the last 20 ka. In May 2003 a seismic monitoring project at the TVG area was initiated by deploying eight three-component seismic stations some of them equipped with both short-period and broadband sensors. During the 18 months observation period local seismicity mainly consisted of high frequency earthquakes either occurring as isolated events, or as a continuous sequence in the form of spasmodic bursts. Mixed and low frequency events were also present during the same period, even though they occurred only rarely. Arrival times from events with clear P-/S-wave phases were inverted in order to obtain a minimum 1D velocity model with station corrections. Probabilistic nonlinear earthquake locations were calculated for all these events using the newly derived velocity model. Most high frequency seismicity appeared to be concentrated near the areas of hydrothermal activity, forming tight clusters at depths shallower than 4 km. Relative locations, calculated using the double-difference method and utilising catalogue and cross-correlation differential traveltimes, showed insignificant differences when compared to the nonlinear probabilistic locations. In general, seismicity in the TVG area seems to be primarily driven by circulation of hydrothermal fluids as indicated by the occurrence of spasmodic bursts, mixed/low frequency events and a b-value (1.17 ± 0.1) higher than in any other part of Taiwan. These observations, that are similar to those reported in other dormant Quaternary volcanoes, indicate that a magma chamber may still exist beneath TVG and that a future eruption or period of unrest should not be considered unlikely.

4D monitoring in Angola and its impact on reservoir understanding and economics

Seismic monitoring surveys have been acquired on Girassol and Jasmim fields, offshore Angola, as part of the Total-operated Block 17 Seismic Monitoring Project. The 4D processing and interpretation workflow we have developed has allowed us to exploit some essential 4D information very soon after acquisition. This key dynamic information was used with traditional monitoring tools to constrain the reservoir model history match. The 4D information has been a key factor in fundamental reservoir management decisions, such as drilling a development well on a poorly drained area that should access 10 million barrels of additional reserves.

Time-lapse seismic surveys in the North Sea and their business impact

In a time-lapse 3-D seismic (TL3D) project, a “monitor” seismic survey is compared to a “base” survey. Ideally, the base survey is a 3-D data set acquired before production from the field started. Normally, the monitor survey is acquired after several years of production. The observed differences between the base and the monitor survey can be interpreted in terms of changes in fluid saturation and pressure. When embarking on a seismic monitoring project, it is important to keep the following questions in mind: In this article, we will answer these questions and illustrate our solutions with two examples. ### What field information might technically be obtained? Fluid drainage information from TL3D may be used directly to locate fluid fronts in each major flow unit. It may also be used to derive lateral variations in permeability of the reservoir and sealing capacity of faults. These variations can cause areas of bypassed oil and blocks that are not being drained. In general, it will help to understand the observed production from the field. It is a very powerful method to map production-related changes in reservoir fluid saturation and pressure away from the wells. TL3D very often reveals some flow behavior that had not been predicted, even by history-matched flow models. This information can be critical to future reservoir development. ### What impact will it have on reservoir management? Time-lapse seismic may be used to optimize the number and location of infill wells to access untapped reserves or to accelerate production. Ideally, this will minimize infill costs and maximize recovery. It may also indicate the need to shut off thief zones to avoid premature water or gas breakthrough. It will …

Assessing the Feasibility of a Time-Lapse Seismic Reservoir Monitoring Project

Summary We discuss a method to assess if a particular seismic survey may be suitable as a base survey for a time-lapse seismic monitoring project, and to predict if anticipated changes in the reservoir's acoustic properties could be interpreted on a repeat survey. Our approach is to generate a simulated repeat survey using a realistic noise realization estimated from the candidate base survey. This repeat survey contains seismic changes that have been modeled by integrating rock and fluid property data, as well as results from the flow simulator. The method can be used to reduce the risk of shooting a repeat survey on which meaningful seismic changes cannot be interpreted. It can also help in deciding the proper timing for an eventual reshoot. Furthermore, the method is valuable for assessing data quality before quantitative interpretation studies. We show an example from the Middle East illustrating a survey that is unfavorable as a base survey. Furthermore, we show examples from the North Sea where application of the method was key for deciding upon a repeat survey and upon the proper timing of a possible repeat survey.