Kyubo Noh

mCSEM inversion for CO2 sequestration monitoring at a deep brine aquifer in a shallow sea

Carbon dioxide injection monitoring in offshore environments is a promising future application of the marine controlled-source electromagnetic (mCSEM) method. To investigate whether the mCSEM method can be used to quantitatively monitor variations in the distribution of the injected CO2, we developed a mCSEM inversion scheme and conducted numerical analyses. Furthermore, to demonstrate the monitoring capability of the mCSEM method in challenging environments, we used a deep brine aquifer model in shallow sea as an injection target. The mCSEM responses of the injected CO2 in the deep brine aquifer were severely decayed and heavily masked by the air wave due to the proximity of the free space. Therefore, the accurate computation of small mCSEM responses due to the injected CO2 and the proper incorporation into the inversion process are critically important for the mCSEM method to be successful. Additionally, in monitoring situations, some useful a priori information is usually available (e.g. well logs and seismic sections), and the proper implementation of this to our inversion framework is crucial to ensure reliable estimation of the distribution of the injected CO2 plume. In this study, we developed an efficient 2.5D mCSEM inversion algorithm based on an accurate forward modelling algorithm and the judicious incorporation of a priori information into our inversion scheme. The inversion scheme was tested with simplified and realistic CO2 injection models and successfully recovered the resistivity distributions of the injected CO2, although it still required the presence of a considerable amount of the injected CO2. Based on these inversion experiments, we demonstrated that the mCSEM method is capable of quantitatively monitoring variations in the distribution of injected CO2 in offshore environments. We developed an efficient 2.5D mCSEM inversion algorithm based on an accurate forward modelling algorithm and the judicious incorporation of a priori information into our inversion scheme. We demonstrated the successful recovery of resistivity distributions of the injected CO2 from the deep brine aquifer model in the shallow sea.

Computation of Apparent Resistivity from Marine Controlled-source Electromagnetic Data for Identifying the Geometric Distribution of Gas Hydrate

요약: 해양 전자탐사의 겉보기 비저항은 해수층으로 인해 지표탐사와 그 정의가 달라지게 되며, 이를 적절히 계산할 수있는 알고리듬의 개발은 해양 전자탐사의 출발점이 될 수 있다. 이를 위해, 1차원 층서 가스 하이드레이트 수치모형과 해수층과 그 하부의 반 무한매질로 이루어진 수치모형에서 계산한 전자기적 반응을 비교분석하였다. 겉보기 비저항을 계산하기 위해서는 실수와 허수 성분 보다는 진폭과 위상을 사용하는 것이 더 적합하였으며 해양 전자탐사 반응의 민감도를정량적으로 분석하여, 근거리 영역에서는 위상이 원거리 영역에서는 진폭 성분이 더 안정적인 결과를 주는 것을 알았다.또한 위상과 진폭의 선택기준으로써 유도상수의 값을 제안하였다. 이러한 분석을 토대로 격자 탐색법(grid search)을 사용하여 겉보기 비저항을 계산하는 수치알고리듬을 개발하였다. 개발된 알고리듬을 이용하여 1차원 층서 가스 하이드레이트수치모형의 다양한 변수를 변화시켜가며 겉보기 비저항을 계산해봄으로써 알고리듬의 타당성을 검증하였다. 마지막으로,계산한 겉보기 비저항 값을 이용한 가스 하이드레이트 부존양상 정보의 도출가능성을 살펴보았다. 동해 울릉분지의 가스하이드레이트 부존양상을 모사한 2차원 가스 하이드레이트 수치모형에서 계산된 자료의 겉치레 단면도는 가스 하이드레이트 부존양상 정보 추출이 가능함을 보여주었다. 주요어: 해양 전자탐사, 겉보기 비저항, 가스 하이드레이트, 부존양상Abstract: The sea layer in marine Controlled-Source Electromagnetic (mCSEM) survey changes the conventionaldefinition of apparent resistivity which is used in the land CSEM survey. Thus, the development of a new algorithm,which computes apparent resistivity for mCSEM survey, can be an initiative of mCSEM data interpretation. First, wecompared and analyzed electromagnetic responses of the 1D stratified gas hydrate model and the half-space model belowthe sea layer. Amplitude and phase components showed proper results for computing apparent resistivity than real andimaginary components. Next, the amplitude component is more sensitive to the subsurface resistivity than the phasecomponent in far offset range and vice versa. We suggested the induction number as a selection criteria of amplitudeor phase component to calculate apparent resistivity. Based on our study, we have developed a numerical algorithm, whichcomputes appropriate apparent resistivity corresponding to measured mCSEM data using grid search method. In addition,we verified the validity of the developed algorithm by applying it to the stratified gas hydrate models with various modelparameters. Finally, by constructing apparent resistivity pseudo-section from the mCSEM responses with 2D numericalmodels simulating gas hydrate deposits in the Ulleung Basin, we confirmed that the apparent resistivity can provide theinformation on the geometric distribution of the gas hydrate deposit.Keywords: mCSEM, apparent resistivity, gas hydrate, geometrical distribution