Sea of change

Offshore exploration presents some very unique challenges, combining a variety of mapping and sampling techniques to try to form a more accurate picture of the seabed. Evidence of how difficult it can be to get right is borne out in official figures, which show exploration drilling success has been declining in recent years. Can new advances in technology help exploration teams ride a new wave of success?

David Monk is currently 1st Vice President of the Society of Exploration Geophysicists, and Director of Geophysics at Apache Corporation, where he has worked since 2000, and has technical responsibility for worldwide seismic activity. After obtaining his PhD in Physics, in 1979, Dr Monk worked for two years in West Africa and was involved in all aspects of seismic acquisition in Land and Swamp environments. Since then he has lived in England, Norway and the USA, and worked across the globe. He has been awarded six US patents, and has published over 60 papers.

Dave Ridyard graduated from Durham University in England, with a degree in Applied Physics in 1980. His career started at GSI, before founding his own company, QC Tools, which was acquired by Input/Output in 1994. Throughout 25 years in the seismic industry, Ridyard has pioneered the development of innovative 3D and 4D seismic equipment and services – primarily in the arena of marine data acquisition. In January 2006, he joined EMGS as Vice President and Product Champion, with a mission to bring seabed logging into the 3D age.

Eivind Berg started his research career with the SINTEF Group in Trondheim. The main focus of his work was towards seismic inversion methods and their application within detailed and quantitative reservoir description. In 1997, together with a colleague, he launched seabed seismic service company SeaBed Geophysical AS in the development of the next generation of node-based technology for seabed applications. The new autonomous node technology was demonstrated on the Cantarell Field in Mexico, so far the largest 4C survey (240km2) ever executed. Berg is Technical Director at SeaBed Geophysical AS.

O&G. Seabed exploration is particularly challenging because surveying comprises of both seismic and sampling techniques. What key technologies are now helping to make this process more accurate and efficient compared to a few years ago?

DR. The big enabling technologies, from an efficiency standpoint, are coming from outside the oil and gas business. Low cost and low power electronics, such as flash memory chips for recording, are allowing us to design seabed equipment that is highly reliable and can be packaged for efficient, cost-effective operations. The accuracy of electromagnetic recording is slightly different: in this area, we are doing something that is unique. Most of the improvements in data fidelity, such as high sensitivity-low noise sensors and digitization circuitry, are coming from our own in-house innovations.

DM. Controlled Source EM (CSEM) has exploded as a commercially viable methodology in the last couple of years having been in restricted to academia before then. While currently restricted to deep water, there are technical developments which will make this a viable technique for shallow water soon. In a perverse twist, compared to EM methods, shallow water Ocean Bottom Cable seismic has been relatively easy to acquire from a technical standpoint, it is the very deep water application of bottom cable seismic that has been difficult until recently.

SB. The ocean bottom cable (OBC) method has been so far the most dominant and the node methodology has gained increasingly higher interest in the market among oil companies. The important limitation was the cost efficiency of having an adequate sampling of the total vector field .The new trend is for unique node designs based on an autonomous system where there a no cables between the units or to a surface facility. The system is based on a small, low-mass sensor package, connected to a control and data acquisition unit, which contains a processor, power supply, high accuracy clock, data storage medium and telemetry system.

With the cableless nodes, the receiver groups have a sparse geometry and the fine sampling is given by the shot density. One node can “see” the complete reservoir with a well sampled vector field in every azimuth in increasing offset, but without multiplicity

O&G. What are the main risks involved in offshore exploration and what kinds of solutions are helping to minimize these?

DR. Despite the tremendous advances made in exploration technology over the past 20 years, exploration drilling success rates have continued to decline to very low levels (15-20 percent on average). As a result of this, current global consumption of hydrocarbons is far outpacing replenishment. Whilst the industry is pursuing new hydrocarbons in ever more challenging regions, poor success rates are fundamentally linked to a reliance on indirect evidence. Traditional exploration workflows place a large emphasis on increasing the probability of finding hydrocarbon accumulations, rather than detecting hydrocarbons directly. For example, techniques such as petroleum system modeling, and the evaluation of source rocks, migration pathways, seals and structures are essentially aimed at establishing whether hydrocarbons can exist, rather than whether they do exist. The application of reservoir-scale electromagnetic (EM) sensing techniques to the exploration workflow now puts a greater emphasis on detecting hydrocarbons directly before drilling. This will clearly boost drilling success rates. Rutt Bridges (SEG President 1997-98) describes this development very well. He said: "Seabed logging appears to offer a rare scientifically independent ‘second opinion’ on the validity of hydrocarbon plays. It has been a long time since the industry has had an approach independent of seismic, and it comes at a good time.”

SB. OBS methods have the potential to solve a variety of seismic imaging problems or to reduce the main risks involved in conventional streamer operations. The acquisition of shear wave information was expected to add value but it has not yet completely lived up to its promise, primarily due to bottlenecks in processing and interpretation.

Another application which has achieved increasing attention over the last few years is the use of nodes for complex imaging beneath salt pillows combined with ultra deep waters, like in the Gulf of Mexico. The full azimuth with nodes planted at depths of 2000-3000 meters may be the only methodology which can solve the problem.

Finally, there is an increasing focus on highly repeatable 4D seismic services in conjunction with EOR. Node-based 4C solutions seem to be very well suited for this due to its repeatable acoustic coupling and positioning accuracy. But there are many other reasons why nodes are particularly suited for 4D use:

• Significantly lower installation costs where no trenching/burying is needed and lower costs than for permanent buried cable installations.
• Complete flexibility in obstructed areas and easy relocation of nodes at any time in the oilfield’s life-cycle.
• Easily maintainable and fault-tolerant with no system degradation over time.
• Modular node architecture makes new system adaptations relatively easy and cost effective where the system can evolve with time without major re-investments.

O&G. How do techniques differ in acquiring multi-component data from the seabed and what are the advantages?

DM. Multi-component data is seismic data, acquired through generation of an acoustic signal (not usually at the sea floor, but close to the sea surface), and detection of the subsequent reflections of the acoustic energy from the subsurface using devices which are sensitive to sound. Because sound can travel in two different ways through the subsurface (but not through water), MC detectors can be used to detect both types of energy. EM exploration utilizes electromagnet energy rather than sound, and allows remote measurement of an entirely different rock property. More information about different rock properties help us infer exactly what is contained in the rock several miles below the sea floor.

DR. The seabed offers a favorable environment for electromagnetic sensors. Seawater attenuates EM signals, so getting our sensors on the seabed maximizes our ability to record the weak signals that are returned from deep targets. Furthermore, EMGS’s approach of using autonomous recording nodes gives us great flexibility in survey design. We can use the same equipment for detailed, target-oriented surveys that we use for frontier scanning applications. This gives us great flexibility to deliver a wide range of differentiated services to our customers. Ultimately, by logging the seabed with electromagnetic sensors, EMGS is providing resistivity information on a reservoir scale. Given that resistivity has always been the industry's primary indicator of hydrocarbons, then the advantages speak for themselves.

SB. In principle, there is no doubt the quality of multi-component data can be superior to that of conventional data acquired near the sea surface. It is the benefits of high multiplicity and areal density of traces, enabling advanced processing techniques that compensate for some of the limitations of surface shooting.

Node technology started with the concept of having the best possible quality data from the sea bottom. This quality concept was kept by assuring the best possible coupling of the sensor to the sea bottom. The advantage is that the planted sensor is linked by a short cable to the recording unit, maintaining the full isotopic conditions of the sensor. Optimization of the deployment by ROV(s) and only one boat are the new trend making this technology more cost effective.

O&G. How can better data results be achieved by combining seismic and electromagnetic surveying techniques?

EB. Geophysical methods do play a major part in the exploration and production of hydrocarbons. Recent developments in four-component (three-component geophone and pressure) ocean bottom seismic data – including permanent sensors, 4D seismic and marine controlled source electro-magnetic (EM) data – are helpful in the mapping and production of untapped parts of the reservoir.

Seismic information is only one technique and should be associated with other techniques and well information to gather important additional data, for example, passive seismic, electromagnetism and micro gravity. Well observations are precise but spatially sparse. Seismic is indirect and fairly imprecise but has good spatial coverage. Reservoir management should be supported by 3D representations of lithologies and brine/oil/gas saturations of the reservoir. In the future, the node technology presently used with the Seabed seismic could successfully be extended for more effective combined operations, which could include EM and passive seismic.

DM. Seismic data (P or MC) images the structure of the subsurface and potentially with MC, some of the lithology. From this, we can infer the possible presence of hydrocarbon but there is absolutely no guarantee, so there is risk before drilling unless it was been previously drilled. EM data adds another aspect because it measures a different attribute of the subsurface. We know that in general when reservoir rock contains hydrocarbon there is a change in the resistivity of the rock. If we can see structure, lithology and resistivity generally associated with hydrocarbon, then we have additional risk reduction prior to an initial well. However, they are still no guarantees as resistivity changes are not only associated with the presence of hydrocarbon.

DR. The combination of seismic and electromagnetic methods is crucial. Today, most of our customers treat the results of seabed logging as an almost binary data point: “Do we have hydrocarbons in place or not?” In reality, there is great richness in the six components of EM data (i.e. X, Y, Z components for both electric and magnetic fields). In order to extract maximum value and create the best overall picture of the reservoir, all of this data must be co-interpreted with the seismic data to understand reservoir parameters such as saturation, permeability, lithology etc. We have made great strides in this area, but we have a long way to go. We envisage that geophysicists will eventually develop techniques that actually co-mingle acoustic and electromagnetic data, going beyond side-by-side interpretation. This technique has been applied in the borehole petrophysics domain for several decades and will open up new opportunities and applications for explorationists.

O&G. With technology now making more raw data available, what kind of functions should solutions comprise to offer the best tools for interpreting and utilizing captured information?

DR. The first step is to get seabed logging data into the interpreter’s workstation, and allow him or her to work with all the data sets in a tightly integrated manner. That part is easy. The hard part is to adapt the exploration workflows to make use of the new measurements that seabed logging offers. Other questions relate to where seabed logging fits within the workflow. Should seabed logging be introduced before 2D or 3D seismic, or after? There is no consensus on this yet. One thing we are sure of is that the earlier seabed logging can be introduced into the exploration workflow, the better. Another opportunity that seabed logging offers is interactive exploration. In many cases, preliminary interpretation of seabed logging is available within hours of acquisition. Some customers are acquiring coarse grids of seabed logging data and then using the data to design more detailed studies of the targets identified, before the vessel leaves the survey area. Putting the right tools and personnel on the vessel could really accelerate this process and reduce risk in the exploration phase.

DM. Multi-attribute interpretation systems which allow simultaneous interpretation of many different attributes of the subsurface will become required. We have seen some of this with different mode seismic data but, as yet, there is no simultaneous seismic, EM interpretation capability that is easily available to those trying to integrate these very different measurements

SB. Very high quality seabed 4C-3D data is possible with node technology. Due to economic restraints, it has been generally considered up to now that seabed acquisition does not produce adequate data density, but this has to be slightly reconsidered. Processing and interpretation are still lagging behind but striking images of the Cantarell oil field in the Gulf of Mexico have been produced using state-of-the-art processing. We can imagine how much more can be achieved by using more recent adaptations of node-oriented algorithms in the processing.

For compressional waves, OBC data is of better quality than streamer data because of better multiple attenuation, multi-azimuth and high fold coverage, although 3D OBC is still more of a multi 2D type .

O&G. Do you think there is likely to be a bigger surge in seabed exploration and seismic technology in the near future? What challenges still lie ahead ?

DM. Yes. Integration of both methods and the application of EM to shallow water are the current big challenges. At present we are still in the “boom” phase of one of the industry cycles. One of the biggest challenges is to make new technologies mature to production levels while the research money is still available in this phase of the cycle. Funding for product development during the “bust” phase is always difficult, and if a new technique has not matured then it is likely to get dropped until the next boom which may be many years away. .

DR. There already is a surge in demand! Those of us who have survived several boom and bust cycles are always nervous of extrapolating trends too far into the future. However, it seems clear that hydrocarbon supplies are finite, and demand is ever upwards. It’s also clear to our customers that including reservoir-scale resistivity measurements in the exploration workflow is bringing tremendous added value in terms of ranking known prospects, and finding new hydrocarbons. Given the accelerating demand for EMGS’ services, our biggest challenge is to grow in a disciplined way, and to be careful to apply seabed logging only where it provides real value to the oil company. Many of our customers want to apply seabed logging to every prospect. But, seabed logging is like every other oilfield technology, in that it has its limitations. To ensure that seabed logging provides results, we accurately model every prospective survey before acquisition commences.

SB. The static and dynamic characterization of petroleum reservoirs are critical for optimal production and recovery. More effective exploration methods are necessary to achieve this and we are facing the challenge of developing new geophysical breakthroughs that can identify undrained hydrocarbons in mature fields and reduce the exploration risk.

Improvement in understanding how changes in the reservoir influence seismic data is currently one of the research topics in Norway. Laboratory measurements and simulation of acoustic properties of reservoir and rocks under realistic physical conditions play an important role. The full range of reservoir properties influence the fluid flow and seismic response. Improved modeling techniques for fluid flow and wave propagation in anisotropic multi-component and multi-fluid phase solids, possibly with fractures, need to be developed. New seabed seismic and perhaps EM imaging and mapping techniques, and inversion for parameters in the reservoir, will improve data interpretation and reservoir management. 3D visualization tools provide an important technology for synergies between reservoir geology, reservoir simulation and geophysical interpretation.