The offshore EM challenge

Abstract

Exploration for hydrocarbons in offshore settings is both challenging and expensive. The chance of an exploration well finding hydrocarbons remains relatively low despite significant improvements in seismic acquisition and processing methods over the past decades. This is partly due to seismic data not being a strong fluid indicator. There are numerous examples of discoveries where seismic data revealed no indications of hydrocarbons, only a structural or stratigraphic closure. In other cases, flat-spots and bright-spots turn out to be caused by a residual 5-10% gas saturation (i.e. non–commercial reserves). Obviously, any means to increase the chance of discovery will be of great benefit for those involved in oil and gas exploration. In 2000, Statoil tested the concept of using marine controlled source electromagnetic (MCSEM) induction for direct hydrocarbon detection in deep water offshore settings (Eidesmo et al., 2002, Ellingsrud et al., 2002). They found that the method was suitable for detection of reservoirs with high (i.e. commercial values) hydrocarbon saturation levels of 60-70% or more. This led to the establishment of ElectroMagnetic GeoServices (emgs), the first company to successfully utilize MCSEM (called Sea Bed Logging by emgs) for direct hydrocarbon detection in offshore settings. Shortly after emgs was founded, two other industry competitors started to provide similar services for marine hydrocarbon prospecting using MCSEM. These are Offshore Hydrocarbon Mapping (OHM) and AGO/Schlumberger. Both utilize mobile horizontal electric dipole sources and a set of electromagnetic field sensors deployed on the seafloor similar to that used by emgs. Less than three years after the establishment of these companies, more than 100 surveys have been conducted world wide and many of these have been verified by wells. The concept of using electromagnetic (EM) data for direct hydrocarbon detection is now proven and is already identified by many as a game changer in the petroleum industry and potentially the most important technology since the introduction of 3D seismic data a couple of decades ago. Large E&P companies such as Statoil, ExxonMobil, and Shell have already started to implement the technology, whereas smaller companies are still struggling to evaluate the potential of MCSEM and how it can be used in their exploration strategy. This creates a window of opportunity for aggressive start-ups and first movers such as Rocksource, the first independent E&P company to establish EM technology as the fundament for their exploration and production strategy by being expert users of MCSEM. Although the use of MCSEM soundings for direct hydrocarbon detection is proven successful, it is not obvious for the industry how to develop and use the technology to its full extent. This paper discusses some of the aspects related to the future needs for equipment design, survey planning, 3D/4D modelling and inversion, joint inversion with seismic and geological data, and finally, integration of MCSEM data into an E&P company workflow. These are important aspects that must be addressed if the full potential of offshore EM technology is to be reached and if the technology is to be brought into more complex settings and potentially into production for reservoir monitoring purposes.