Marine CSEM surveying for hydrocarbon exploration was commercialised in 2002 by EMGS, a spinoff company from Statoil (GEO ExPro 01/2004, 05/2005, 04/2007). Since then, significant improvements have been made in data acquisition, processing and interpretation. The innovative technology has been applied worldwide and in addition to EMGS, several other companies now supply EM services.
Rocksource appears, however, to be the only oil company explicitly saying that their business model is to fully integrate EM in their basin and prospect evaluations. Their approach makes sense.
More information
Controlled-source electromagnetic (CSEM) technology in an offshore setting is loosely analogous to seismic in terms of theory and acquisition. However, while acoustic energy reacts to differences in density and velocity, electromagnetic energy reacts to differences in resistivity. Also, an important difference is that electromagnetic energy propagation for low frequencies is more by diffusion than by wave propagation, something that must be considered when handling the data. Illustration: Rocksource“The seismic method gives information about the acoustic impedance (velocity/density) of the geological layers, while EM gives complimentary information about the resistivity. In other words, EM provides another key parameter characterising the subsurface and should therefore be welcome amongst explorationists,” Hesthammer says. “The additional parameter provides the geoscientist with information directly relevant for hydrocarbon exploration, and the value of the data far exceeds the cost of acquiring them. No one would consider a log analysis that does not include the resistivity log.”
“But is also important to be aware of the fact that, while EM data are extremely useful, they do not represent perfect information. Just like seismic data there will be uncertainties associated with their interpretation. All the tools we use in exploration have their strengths and weaknesses and it is very important that we understand the uncertainties and limitations of each.”
When CSEM (Seabed Logging) technology for marine hydrocarbon detection was introduced more than six years ago, it was marketed as a tool that could pinpoint hydrocarbon accumulations before drilling wells. While such an approach may still be valid Hesthammer prefers to think differently about how the EM data should be used.
Derisking prospects
Jonny Hesthammer with the Norwegian oil company Rocksource advocates the use of EM for the purpose of improving the decision making process. The company has already declined to participate in a number of farm-in opportunities because the integrated analyses of EM data did not support the drilling of a well. That may have saved the company a lot of money. Photo: Halfdan Carstens“Although it is possible to identify leads using just CSEM scanning surveys, Rocksource’s main focus is on how the CSEM technology can help us to change the risk profile of prospects that have already been mapped seismically. The idea is based on the concept that adding EM surveys as an additional step in the prospect evaluation process will actually improve efficiency by reducing the number of dry wells, optimizing rig capacity by drilling the right wells, and reducing finding cost per barrel. This leads to improved value creation from exploration.”
The Rocksource team have developed a proprietary state-of-the-art analysis approach to successfully deal with EM data in both simple and complex settings. Their workflow starts by screening areas and defining basins and plays in which the technology will work. The next step is to identify prospects that are suitable for derisking with EM. Given the relatively low cost of the surveys, a series of prospects are then tested and the results are used to rank the prospects prior to committing to expensive well decisions.
“We aim to identify prospects where the environmental conditions are suitable for our technology and where it can impact the prospect chance of success – both positively and negatively.”
Saving money
The basic principle behind CSEM is that hydrocarbon-filled sedimentary rocks have higher resistivity than water-saturated rocks. A complicating factor using CSEM is that different lithologies have a wide spectrum of resistivity values, in some cases an order of magnitude greater than that associated with hydrocarbon filled sediments. This is why modelling and subsequent detailed analyses are of crucial importance. Illustration: Rocksource“CSEM represents a tremendous technology with a huge value potential,” Hesthammer says, stressing that “it is very important to use it correctly. It’s not a black and white technology providing you with a definitive answer, but it can significantly change your risk profile. The extent of that risk modification depends on the quality of the interpretation of the data. You always have to do extensive analyses, including detailed modelling based on proper G&G input.”
Hesthammer is particularly concerned about understanding what Rocksource have defined as false-positive and false-negative interpretations. The EM measures resistivity in the subsurface, not the presence of hydrocarbons. The industry has already experienced failure cases in which an EM anomaly turned out to be caused by something else than hydrocarbons, resulting in a dry well, and – vice versa – the lack of an EM anomaly was interpreted as evidence of no hydrocarbons being present, whereas a subsequent well demonstrated the presence of oil or gas.
Proper processing, coupled with integration of all the available data reduces the chance of getting erroneous interpretations from the EM data. “Rocksource has already declined to participate in a number of farm-in opportunities because the integrated analyses of EM data did not support the drilling of a well, although EM anomalies were observed. One of these prospects has later been drilled and proven dry. We believe that these decisions have saved us a lot of money,” Hesthammer says, clearly illustrating his point.
The future
And the future? “To date, CSEM has only been used in exploration. The value of CSEM technology for appraisal and monitoring purposes in production settings may exceed the value in exploration. One day we will have systems for permanent EM monitoring of reservoirs as we produce them, allowing us to track and optimize production” Jonny Hesthammer believes.
