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Higher-Resolution Subsurface Imaging of Hydrocarbons R&D GRAND CHALLENGES Higher Resolution Subsurface Imaging Jack Neal and Chris Krohn, ExxonMobil Upstream Research Editor’s note: This is the fifth in a series control points. Selection of the right dep- and depositional bars hold the coars- of articles on the great challenges facing ositional model, facies distribution, and est sediments with the most porosity the oil and gas industry as outlined geostatistical analog depends on having while oxbow lakes and floodplain muds by the SPE Research and Development the sharpest, most detailed and accurate can be seen as potential barriers to flow. (R&D) Committee. The R&D challenges image of the subsurface possible—the Now imagine that instead of air between comprise broad upstream business Grand Challenge of Higher Resolution the observer and the geologic feature, needs: increasing recovery factors, Subsurface Imaging. you must see through countless layers in-situ molecular manipulation, Over the past century, the industry of rock with complex physical proper- carbon capture and sequestration, has relentlessly sought ways to improve ties that obscure or muddle your vision produced water management, higher subsurface imaging of hydrocarbons. of the delta below. Further imagine that resolution subsurface imaging of Canadian inventor Reginald Fessen- you have a handful of well penetrations hydrocarbons, and the environment. den first patented the use of the seis- in your delta that stretches over several The articles in this series examine each mic method to infer geology in 1917. A square kilometers and you want to see of these challenges in depth. White decade later, Schlumberger lowered an how produced and injected fluids move papers covering these challenges are electric tool down a borehole in France to between wells. available at www.spe.org/industry/ record the first well log. Today, advances The industry’s goal is to continuous- globalchallenges and allow reader in seismic and gravity data acquisition, ly improve the subsurface images needed comments and open discussion of electromagnetics, signal processing and to better find and produce hydrocarbons the topics. modeling powered by high-performance in reservoirs such as this. Obstacles to computing, and the nanotechnology rev- this goal include remote-sensing limita- olution are at the forefront of improved tions imposed by the physics of the rocks Introduction reservoir imaging. themselves (e.g. energy attenuation with It is hard to read road signs if you have In this paper, we will examine the depth, bed thickness and lateral extent poor eyesight, which is why driver’s challenges of getting higher resolution relative to signal wavelength, and vari- licenses are issued with restrictions subsurface images of hydrocarbons and able rock velocity and density properties requiring that corrective lenses must be touch on emerging research trends and that can scatter or complicate input sig- worn. Likewise, it is hard to find and technologies aimed at delivering a more nal), as well as instrumentation and com- exploit subsurface resources if you can’t accurate reservoir picture. puting power limits. On top of the tech- clearly see your targets or monitor the nology challenges, there are economic movement of fluids in the reservoir. The Problem Statement considerations. If it costs more to acquire Engineers now have powerful tools Hydrocarbon accumulations occur thou- an advanced dataset than you can hope to precisely model subsurface reser- sands of feet below the Earth’s surface to recoup from a development, the tech- voir production behavior, but a precise and the days of finding subsurface hydro- nology might as well not exist. We recog- answer is still wrong if it is derived from carbons through extrapolation of surface nize that absolute accuracy and precision an inaccurate subsurface description. geology are all but gone. Now explora- will never be achieved, no matter how Geoscientists make maps and rock prop- tion is done with remote sensing tools good our technology becomes, since all erty models of the subsurface by inter- that seek to generate sharper pictures measured data are imperfect. The “grand preting images that are produced from with greater detail of the lateral and ver- challenge” is to achieve the truly fit-for- remote sensing data. Analogs from mod- tical changes in subsurface rock layers purpose subsurface image at good eco- ern depositional environments and out- and sometimes the porosity and pore- nomic value. crop exposures guide subsurface data filling hydrocarbons we require. If you Research that addresses imaging interpretation to predict ahead of the bit, have ever seen an aerial photograph of limitations is advancing worldwide on a then postdrill geostatistics are used to fill a delta, you can imagine what a subsur- dizzying array of technologies, but the in stratigraphic details between wellbore face reservoir could look like—channels problem is complex. Sometimes the res- 44 JPT • MARCH 2012 R&D GRAND CHALLENGES ervoir target is undermasking layers of medium highlight not only the challenge part of the challenge. Accurate place- salt, thrust sheets, or volcanics. It may to achieve higher resolution, but also ment of a target can be even more criti- be more than 30,000 feet below the sur- concerns on depth conversion. Diffract- cal and correct depth placement is espe- face, hidden within a producing field, or ed, attenuated, and multiply reflected cially important for exploration drilling have physical properties that are near- signal energy can result from the com- in areas of overpressure. Increasing com- ly indistinguishable from surrounding plex physical property structure of rock putational power, multicomponent and rocks. The problems generated by imag- layers to degrade subsurface imaging, but rich azimuth data collection, and algo- ing through a complex and unknown a higher resolution of the image is only rithm efficiency are bringing the indus- try forward toward a more accurate and detailed Earth model that uses all of the signals possible in seismology; even bet- ter seismic data acquisition and process- ing technologies are needed. Only a minuscule fraction of the subsurface can be directly observed: the *HW portion generally less than 12 inches in diameter that is penetrated by boreholes. 0RUH Directly observed, that is, if cored, oth- erwise indirectly observed through well logs. Well logs can sense a short distance into the formation, depending on the character of the signal/receiver setup, but the tradeoff is depth into the forma- tion at the expense of vertical resolu- tion. Moving just a few meters away from IURP the wellbore puts reservoir imaging back into a remote sensing problem with limi- tations for our ability to illuminate and view the region. Generally, sources and \RXU receivers must be placed far away at the surface and energy propagated a long distance to the target and back. Corre- spondingly, there is a large drop in reso- lution. Compare this situation with the &RUH medical imaging problem, which allows X-ray sources and detectors to be used With reservoirs becoming increasingly complex, around the patient. In some cases, geo- you need the most accurate information you can physical sources and receivers can be get to better understand your reservoir. placed much nearer to the zone of inter- est along wellbores, but then it like try- Weatherford Labs helps you get more from your core by combining an unsurpassed global team of geoscientists, ing to see through a key hole; the source engineers, technicians and researchers with the industry’s and receiver locations may or may not most comprehensive, integrated laboratory services adequately illuminate the region that we worldwide. From core analysis, sorption, geochemistry want to image. and isotopic composition to detailed basin modeling and Reflection seismology is the most comprehensive data packages, we provide you with real reservoir rock and fluid information that hasn’t been ZHDWKHUIRUGODEVFRP used remote sensing method in geophys- distilled by a simulator or iterated by software. ics. Seismic data delineates subsurface velocity and density variations, but if ™ We call it “The Ground Truth ” – giving you the accurate rock properties are favorable, they can be answers you need for better reservoir understanding. You’ll call it a better return on your reservoir investment. To learn used to distinguish hydrocarbons versus more, contact [email protected]. formation water in a reservoir. Repeated 3D surveys over the same area can track fluid movement through time (4D). How- ever, 4D surveying has its challenges: More_Core_4.4375x7.5_4C.indd 1 1/4/2011 8:45:11 AM 46 JPT • MARCH 2012 R&D GRAND CHALLENGES topic for research and development. The understanding of these rocks as reser- voirs is still in its infancy compared with conventional reservoirs. Therefore, addi- tional research into resolving controls on unconventional resource producibil- ity remains a challenge. Reflection Seismology What progress is being made toward delivering more accurate and higher res- olution subsurface images? Seismology has earth physics challenges to overcome. Signals sent into the ground are reflected, refracted, diffracted, mode converted, and attenuated as they pass through lay- ers of the earth. At greater depth, signal attenuation reduces resolution. Under Fig. 1—Processing speed in flops (floating point operations per complex geology, distortion of the wave- second) through time of the top 500 published computers and the field can give false images. The past few sum total global capacity. years have seen a flurry of innovation and image improvement, due largely to time to acquire, process, and accurately has not reached broad application. There advances in computing speed, which con- align with past surveys, limits in coverage is a great need for improved technolo- tinues to follow Moore’s Law by averaging caused by production facilities, and lim- gies that image reservoirs and pore-fill- a doubling of processing speed every year its in resolution due to earth physics and ing fluids at high resolution within pro- up to the current record holder that is illumination.
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