Improved Reservoir Imaging by using 4C full Azimuth Node Methodology

The main application areas are still to be explored and utilized in large scale within reservoir characterization and reservoir monitoring where the greatest potential are still to be identified.

It is now a common consensus among the oil companies that most of the large oil fields in the world are discovered. The possibility to find new large field is getting smaller even though a certain small number of such fields could be discovered in immature areas of the world. It has then become a strategy for the oil companies to focus on how to produce more oil from already producing fields (“Enhanced Oil Recovery”- EOR). Effort is being put in research and development for technologies that can increase both the producing life time and the hydrocarbon recovery rate of these fields. This includes more efficient and accurate production technologies and also solutions for better static and dynamic reservoir characterization using seismic methods.

World-wide we currently experience a strong interest in 4D and other EOR methods, not only to enhance the performance of established producing fields but also as a way to manage new fields from the very start of its producing life. This interest stems from an increasing gap between known resources and fossil energy demand - combined with the fact that the projected recovery rate can be disappointingly low. In the North Sea area the estimated recovery rate of clastic reservoirs (high permeability) will typically be around 40%-50% of the theoretical reserves. With the full potential of different EOR measures the goal is to recover 50% and 75% of the oil and gas reserves, respectively. For carbonate fields the estimates has increased from a modest 20% to 37%, mainly due to water injection.

The methods for marine seismic exploration and reservoir imaging have during the last decades been through several stages of adjustments and improvements. The streamer technologies have gone from 2D seismic to 3D seismic and now in the last decade to 4D seismic. In special cases, however, in order to e.g. “see” through/map gas invaded zones, so-called multi-component or 4C which measures pressure PP and converted shear PS waves, was demonstrated as proven for the last decade.

The methodology for recording multi-component seismic (4C) data is to place the sensors on the seafloor, rather than towing them behind a vessel. Each sensor group or point receiver contains both hydrophones and geophones for recording of pressure (PP) and the 3 orthogonal particle motions. This methodology, called 4C, “multiwave” or OBS, has been in the market for about a decade, driven by the desire to record better and more accurate seismic data. Up to now one could say that OBS data in general has not yet lived up to expectations, and if high quality acquired data is possible, dedicated processing and interpretation are lagging behind. Ocean bottom cable (OBC) method has been so far the most dominant and the node methodology has got increasingly higher interest in the market among oil companies. New trends of node applications have its fundament in the unique node design 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.

The general idea was first to satisfy data quality. There is no doubt that the planted node design present the best possible coupling .To avoid severe degradation of the signal, the sensors has to be rigidly positioned into the more consolidated sediments and should avoid any crossfeed from the rotation of the sensor assembly .This is essential for the shear waves. With a high degree of flexibility the present system is able to acquire data with the highest possible quality in all kinds of challenging areas. Operating a cable-less system is, of course, advantageous in areas with a lot of infrastructure (obstructions) on the surface and on the seafloor, areas that are environmentally fragile, or in ultra deep waters beyond 1000- 2000m – where working with a cable is very challenging.

The much wider application of the node technology beyond the proven imaging through “gas clouds”, was clearly demonstrated on the Cantarell field in Mexico. This survey acquired by SeaBed in 2003/2004 for Pemex is so far the world’s largest 4C-3D project ever performed (approx. 230km2). The Cantarell field offshore Mexico is the world’s largest offshore producing field. This was a full azimuth node survey using 250 nodes and the area was covered with approximately 1,500 Node positions. The Cantarell field is densely populated with platforms, subsea structures and pipelines and there was lot of vessel activity in the area. These challenging conditions demonstrated the operational benefits by using a cable-less Node-based system.

The final reservoir imaging results are, of course, what the client is really paying for and these results were presented by Mr. Marco Vazques, Pemex at the SEG conference in Houston, November 2005. Striking PP and PS images were shown. The direct comparison of the improved imaging with the PP node data compared to the old OBC data was shown. Frequency content, reflector continuity and imaging resolution are well improved.

The best PP wave reservoir imaging results were obtained with the nodes and with higher frequency content in the data. The 3D shape of the structures is well defined without influence of the direction of acquisition as compared to previous OBC seismic.

There is no doubt that the combination of well planted nodes and full azimuth acquisition geometry has contributed a lot to the final imaging results of the PP wave data with increased resolution and more accurate imaging than previously acquired data. In addition, the PS converted wave data contributed surprisingly also to add geological structural information in some areas with higher resolution than even the node PP wave data and penetrating down to 5-6km in depth. This means that PS converted wave data have a much wider range of applications than just to be used as a special “prediction” tool when the PP wave data is severely hampered by PP transmission effects.

Another application which has achieved increasing attention the last years is the use of nodes for complex imaging beneath salt pillows combined with ultra deep waters like in Gulf of Mexico. In this case the full azimuth with nodes planted at 2000-3000m water depth is may be the only methodology which can solve the problem. Azimuth is everything!
A few pilot demonstration projects have been performed successfully and a large survey has recently been executed by BP in the Gulf of Mexico. This market segment is expected to grow rapidly in the next 2-3 years.

Finally, there is an increased focus on highly repeatable 4D seismic services, seen 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 that makes nodes particularly suited for 4D use, and why this business area may grow into a future cornerstone for the node methodology:

i ) significant lower installations costs where no trenching/burying is needed than with permanent buried cable installations.

ii) complete flexibility in obstructed area and easy relocation of nodes at any time in the oilfield’s life-cycle.

iii) easily maintainable and fault-tolerant with no system degradation over time .

iv) modular node architecture makes new system adaptations relatively easy and cost effective where the system can evolve without major re-investments.

Time lapse 4D seismic data represents a huge potential for reservoir management, but is still immature. A few programs have has been carried out, especially in the North Sea region. They have either been based on buried seabed cables or towed surface hydrophone cables. Based on the new market trends, the oil companies are increasingly demanding more accuracy in the acquired seismic data as well as more operational flexible and lower cost solutions. The node methodology is now a hot topic in 4D projects world wide. An important impact to this new trend is definitely the very encouraging results obtained from the improved imaging of the Cantarell Field of both the PP and PS data which really pushed the technology a big step forward. Presently one can extrapolate these results to envisage that the node methodology will have a 25-50% share of the total 4D seismic market. The future 4D seismic market is expected to exceed 50% of the total seismic market within a few years.