Shaking up exploration

O&G. What are the key challenges for international oil companies (IOCs) to consider when approaching seismic exploration activity?

TK. The three key challenges for international oil companies are reserves replacement, production enhancement and optimizing the recovery factor. Oil is most often found where oil is already found, so satellite appraisal and efficient extraction are key activities requiring accurate seismic data calibrated to known rock physics to get the best estimate of volumetrics. Since one well may be all that a satellite can support, getting the best estimate of reservoir connectivity prior to drilling requires sufficient seismic resolution to identify potential sealing faults. A good depth conversion is also necessary, since that one well is likely to be horizontal, to maximize drainage, and good seismic with reliable velocity-depth control is needed for geosteering the bit.

In terms of fresh exploration, the new basins open to IOCs are few and far between, with the Arctic Ocean possibly offering the last major opportunity. Exploration here must be coupled with a strong commitment to environmental protection, as the Arctic is relatively poorly connected to the rest of the global oceanic circulation and the coastal margins are extremely vulnerable environments. Exploration challenges include a short seismic season, mobile ice, poor weather and a paucity of drill holes to calibrate the seismic data. The situation is somewhat similar to exploration in Norwegian waters north of 62 degrees in the late 1970s and early 1980s. There was plenty of seismic shot but no drilling was allowed and so there was the opportunity to make spectacular interpretation mistakes. Current seismic data is much better quality, but still reflection amplitudes are ambiguous because there are more variables in the subsurface (lithology, fluids, porosity, stresses) than seismic measurements. Good geological models together with rock physics models derived from the most likely analogues are required.

O&G. Maximizing the potential of existing oil fields is as much of a challenge as discovering new sources. How can seismic surveying techniques be used to drive more efficient extraction?

TK. Satellite exploration and appraisal was briefly covered above. More efficient production and overall recovery factor can be addressed through the use of time-lapse seismic, coupled with more traditional well surveillance to interpret production-induced changes in the reservoir. In many fields produced through floating production, storage and offloading (FPSO), it is not feasible to re-enter the wells for production logging campaigns and so down-hole sensors with 4D seismic is the best that can be done. Good resolution, calibrated amplitudes and highly repeatable survey geometries are acquisition requirements, and repeatable processing with fast turnaround is needed to get the most out of the seismic in a timeframe appropriate for making production-related decisions. The seismic data can respond to changes in saturation, phase, pore pressure and even subsidence, so again good calibration to the rock physics is crucial, together with the ability to integrate the seismic results into a reservoir simulation environment. There are many successful case studies where 4D seismic has brought significant benefits in optimizing in-fill well placement, identifying compartmentalization, fluid pathways, pressure connectedness and flow barriers; for once, 4D has been more successful than anticipated, partly because of the power of 3D imaging and modern seismic’s ability to repeat acquisition geometries.

An issue with seismic is the cycle time to commission, acquire, process and interpret data. This makes it especially suitable for guiding campaigns of production or injection drilling activities that have a similar time-scale. Experiments with permanently installed seabed cables, allowing for three-month cycle times, have been carried out in the Norwegian Valhall field. The technical results have been impressive, even considering the complications of a subsiding overburden, and the economic analysis of the benefits to production are awaited with interest.

O&G. Can seismic surveying techniques be used to complement other drilling functions like borehole surveillance to get a more rounded view of reservoirs? If not, is this a challenge that needs to be addressed?

TK. The benefits of 4D seismic is well demonstrated. The concept of instrumenting boreholes with permanent seismic networks in an effort to hear the ‘creaks and groans’ of a reservoir as it is being produced is under evaluation. This form of passive seismic monitoring uses the change in stress that follows production or injection to interpret drainage and connectivity of the reservoir. Accurate location of these passive seismic events also allows interpretation of sub-seismic fault patterns, which often control fluid flow. To take full advantage of this form of monitoring, knowledge of the down-hole pressure regime is needed to drive a reservoir simulator coupled to a geo-mechanical model of the reservoir and the surrounding rocks. While there are increasing numbers of pilot projects taking place in various parts of the world (PDO in Oman is a good example), this is a less mature technology than 4D surface seismic. However, with the permanently installed borehole sensors, it is also possible to acquire repeated VSP surveys using controlled sources to monitor in detail the volume around each instrumented borehole. There is a lot of interest in passive seismic sessions at EAGE meetings and it is clear that this is a rapidly growing area of seismic technology.

O&G. Drilling in remote and hostile environments presents many challenges throughout the operation. What are the particular challenges of seismic surveying in these environments?

TK. I’ve already touched upon marine acquisition in the Arctic, and weather issues are always present when acquiring data in deeper water and higher latitudes. On land there are the perennial challenges of operating in hostile environments, whether in the desert or the jungle. Access difficulties, field logistics and the requirement to minimize the environmental footprint are all challenges that modern seismic is addressing. On the one hand, there is a desire to acquire data with increasing fold and higher spatial density, but irregular geometries imposed by access issues and variable surface characteristics require flexible, post-acquisition grouping of recorded data to minimize noise and focus on signal. This drives up the channel count and increases the logistical challenge. Even in the open desert, dunes, wadis and even former minefields all add to the logistical complexity of keeping seismic production going: a modern land crew-chief must have the logistical skills of an army general as well as an eye for data quality.

O&G. How can companies get the most out of 4D surveying?

TK. In general, to get the best out of 4D seismic, every piece of data acquired in a seismic survey intended to have 4D application needs to be qualified with auxiliary data and/or confidence metrics. At the basic level, this means calibrating the sensitivity of every seismic sensor so that volts can be converted back to pressure or particle velocity in the seismic wave. But furthermore, we need to know where each sensor is located, where each airgun was when it fired, what the water depth is above each gun or sensor, what the pressure pulse from each gun looks like, how the source array signature varies in direction for each shot fired, how rough the sea-surface is and how that affects the downward traveling ghost. As well as knowing where the guns and sensors were for the last shot, we need to know how to get them in the right places at the right time for the next shot, so positioning control is also needed.

The requirements for 4D data also feed back to benefit conventional 3D and even 2D surveys. Just this summer, three deep 2D seismic profiles were acquired over the region of the 2004 Great Sumatran earthquake, to provide crustal researchers with data to help understand the causes of both the largest earthquake in the digital era and the most catastrophic tsunami in modern times. These data were acquired with exactly the same technology as that used for the most sophisticated 4D seismic.

O&G. Increasing the accuracy of seismic survey data is a constant challenge; what new developments do you expect to see in the next five years?

TK. The trends in seismic data acquisition are outlined above – increasing the useful bandwidth at both high and low frequencies, improved amplitude control at both source and receiver, reducing positioning uncertainty, controlling positions in real-time, and characterizing the fundamental seismic measurement with auxiliary data that will place confidence measures on all aspects of that measurement. On the processing side, velocity analysis, multiple attenuation, imaging in complex media, and seismic inversion for impedance contrasts that can be converted to rock and fluid properties will all improve as the reservoir target becomes ever more elusive. Remember that the goal will be both to increase confidence in the reliability of the seismic measurement, and to place accurate bounds on that confidence, so that the interpreter can use his/her skill to predict the stratigraphic trap with the correct risk factor.

EAGE is a multidisciplinary society strongly believing in integration between the various geoscience and engineering disciplines. Dr Kortekaas would like to thank his colleagues on the EAGE Board and in particular Dr Phil Christie for their very extensive input.