Time-lapse CSEM: how important is survey repeatability?

Abstract

Summary An important concern for time-lapse studies using the controlled-source electromagnetic (CSEM) method is repeatability of acquisition parameters for the base and monitoring surveys. We consider a challenging case when variations in source and receiver positions, conductivity of seawater etc. will lead to differences in the recorded EM fields that exceed EM response due to production-induced changes in the reservoir resistivity. We show that even in that case, 4D response can often be clearly resolved if the base and monitor EM datasets are inverted to produce 3D resistivity distributions. More precisely, for a synthetic model of the Snøhvit gas field, changes in the inverted resistivity maps caused by such non-repeatability will be at least 10 times smaller than the time-lapse differences in the reservoir resistivity. By contrast, measurement errors and poor knowledge of background resistivity may reduce the resolution of 4D CSEM to a much stronger degree. Analysis of field CSEM data from the Wisting oil field supports our conclusion about strongly relaxed repeatability requirements when 4D effects are established by examining inverted resistivity volumes rather than by looking at raw EM data.

Time-Lapse CSEM Monitoring: Correlating the Anomalous Transverse Resistance with SoPhiH Maps

“…CSEM time-lapse studies can be sorted into two main groups: data-domain studies, where CSEM datasets from base and monitoring surveys are directly compared [36,37], and model-domain studies, where a comparison is made by using the recovered models for each survey [16,18]. The latter approach's main advantage is that it avoids or minimizes the need for strict repeatability requirements when analyzing the 4D response in the data domain.…”

“…In the present work, we extend the findings of [18] in monitoring the effects of CSEM due to changes in resistivity resulting from water saturation changes within the reservoir. Other effects will not be considered, as they have been thoroughly examined by [16].…”

“…Reservoir monitoring and surveillance are the new frontier for the CSEM method [10,11]. Several synthetic studies have shown the CSEM time-lapse sensitivity in order to solve for resistivity changes associated with fluid substitution [12][13][14][15][16]. However, to date, only one 4D survey has been reported in the literature [17].…”

New Concept Ocean-Bottom Multiphysics (OBMP) Nodes for Reservoir Monitoring

“…We assume that the resistivity changes are only associated with the fluid substitution within the reservoir's boundaries (Figure 3a), while the background resistivity remains unchanged. Other superficial effects are not considered as they have been thoroughly investigated by [2].…”

“…Ongoing development efforts have demonstrated the potential to enhance cost-effective deep-water reservoir-monitoring applications. Indeed, several pioneering feasibility studies (e.g., [1][2][3]) have revealed that time-lapse (4D) CSEM can play a role in improving our knowledge of the reservoir structure, fluid flow, and fluid saturation changes when appropriate acquisition and advanced integrated interpretation workflows are applied.…”