Combining electromagnetic and seismic methods to monitor carbon dioxide sequestration

该项目涉及开发一种地球物理策略，使用地震和电磁（EM）方法来监测埋藏在海床以下数百米的油藏中二氧化碳的封存情况，例如北海。这项研究的动机来自于CASE的合作伙伴，石油地质服务公司（PGS），他们希望在该地区开发一种既高效又技术可靠的服务。当二氧化碳被注入含盐含水层或以前含有碳氢化合物的储层时，它会占据岩石的部分孔隙空间，并改变岩石的性质。对地震响应的影响是，早期注入少量气体会显著降低声阻抗。此后，地震反应基本不变。早期CO2注入对储层岩石电阻率影响不大，但随着CO2注入量的增加，岩石电阻率呈指数增长。因此，电磁方法具有监测储层中储存的二氧化碳量的潜力。地震反射资料的偏移反演技术已经成熟。本项目重点研究电磁数据。该项目的目标是：1。将观测到的地震和电磁异常的震级与注入气的数量、气饱和度和储层深度联系起来，并确定在存在噪声的情况下异常的可探测性。2. 基于爱丁堡正在开发的一种新的射线理论方法，开发一种自上而下的建模方法来反演地下电阻率的合成电磁数据。这是全新的，将地球脉冲响应的峰值与已知的地下射线路径联系起来。“3。应用同样的方法将电磁异常与地下电阻率变化联系起来。4. 将所开发的反演方法应用于实际瞬变电磁资料。工作计划1所示。模型。建立储层的三维（3D）物理和岩石物理模型，该模型嵌入在变深度水层以下的背景介质中。电阻率模型为各向异性，垂直分量大于水平分量（3个月）。2. 通过向储层孔隙流体中添加气体（1个月），模拟储层地震、电磁（电阻率）和岩石物理性质的变化。3. 地球物理监测使用PGS提供的“Nucleus”建模包对储层进行地震和瞬变电磁测量模拟（3个月）。4. 可行性研究确定观测到的地震和电磁异常的震级作为注入气体量的函数。如图所示，我们预计早期注气的地震异常很大，此后变化很小，而电磁数据应该显示早期天然气的小异常，然后随着天然气的增加，异常逐渐变大。需要使用PGS（6个月）提供的实际噪声测量来估计在带有噪声的实际数据中计算出的异常。5. 基于爱丁堡正在开发的一种新的射线理论方法，开发一种自上而下的建模方法来反演地下电阻率的合成电磁数据。这是全新的，将地球脉冲响应的峰值与已知的地下射线路径联系起来。（1岁）电磁时移合成数据反演应用同样的方法将电磁异常与地下电阻率变化联系起来。(6个月)。7. 实际瞬变电磁数据的反演将所开发的反演方法应用于英国地质调查局爱丁堡办事处的实际瞬变电磁数据。(6个月)。

SUMMARY Project description This project is concerned with developing a geophysical strategy, using seismic and electromagnetic (EM) methods, for monitoring sequestration of carbon dioxide in reservoirs buried hundreds of metres beneath the sea floor in areas such as the North Sea. The motivation for this research comes from the CASE partner, Petroleum Geo-Services (PGS) who wishes to develop a service in this area that is both operationally efficient and technically sound. When carbon dioxide is injected into saline aquifers, or into reservoirs that previously contained hydrocarbons, it occupies part of the pore space in the rock and changes the rock properties. The effect on the seismic response is a dramatic decrease in acoustic impedance with the early injection of a small amount of gas. Thereafter the seismic response is essentially unchanged. The resistivity of the reservoir rock is affected only slightly by early CO2 injection, but as the quantity of CO2 increases, the rock resistivity increases exponentially. Electromagnetic methods therefore have the potential to monitor the quantity of CO2 stored in a reservoir. The migration and inversion of seismic reflection data is now mature. This project focuses on the electromagnetic data. The objectives of the project are: 1. To relate the magnitude of the observed seismic and electromagnetic anomalies to the quantity of injected gas, the gas saturation, and the depth of the reservoir, and to determine the detectability of the anomalies in the presence of noise. 2. To develop a top-down modelling approach to invert the synthetic electromagnetic data for subsurface resistivities, based on a new ray-theoretical approach being developed in Edinburgh. This is totally new and relates the peak of the earth impulse response to a known subsurface 'ray path.' 3. To apply the same approach to relate the electromagnetic anomalies to subsurface changes in resistivity. 4. To apply the developed inversion method to real transient electromagnetic data. Workplan 1. Model-building. Build a three-dimensional (3D) physical and petrophysical model of a reservoir embedded in a background medium below a water layer of variable depth. The resistivity model will be anisotropic, with the vertical component of resistivity greater than the horizontal (3 months). 2. CO2 Sequestration Simulate changes in the seismic, electromagnetic (resistivity) and petrophysical properties of the reservoir by adding gas to the pore fluid in the reservoir (1 month). 3. Geophysical Monitoring Simulate both seismic and transient electromagnetic surveys over the reservoir using the 'Nucleus' modelling package provided by PGS (3 months). 4. Feasibility Study Determine the magnitude of the observed seismic and electromagnetic anomalies as a function of the quantity of injected gas. We expect large seismic anomalies for early gas injection and very little change thereafter, while the EM data should show small anomalies for early gas and then progressively larger anomalies with increasing gas, as indicated in the figure. The detection of the computed anomalies in real data with noise needs to be estimated using realistic noise measurements provided by PGS (6 months). 5. Inversion of EM synthetic data Develop a top-down modelling approach to invert the synthetic electromagnetic data for subsurface resistivities, based on a new ray-theoretical approach being developed in Edinburgh. This is totally new and relates the peak of the earth impulse response to a known subsurface 'ray path.' (1 year) 6. Inversion of EM time-lapse synthetic data Apply the same approach to relate the electromagnetic anomalies to subsurface changes in resistivity. (6 months). 7. Inversion of real transient EM data Apply the developed inversion method to real transient electromagnetic data in PGS's Edinburgh office. (6 months).