GEO ExPro

The Growing Role of Geophysics

The use of geophysical methods in petroleum exploration continues to expand and is an integral discipline in the discovery and production of oil and gas deposits.
This article appeared in Vol. 6, No. 1 - 2009

Advertisement

Larry Lines, SEG President at the 2008 President’s reception. Dr. Lines is Associate Director and Professor, Department of Geoscience at the University of Calgary with major contributions in seismic imaging, inversion and reservoir characterization. Photo: Deb Bertossa We have seen a remarkable quarter-century of technological expansion in geophysical methods and this growth is continuing. New exploration frontiers and the exploitation of complex and unconventional reservoirs are being made possible by recent advances in processing and acquisition methods. Obtaining detailed images below thick layers of salt in the deep ocean, understanding how reservoirs have evolved over time, and integrating disciplines through all stages of exploration and production are just a few examples.

Reducing risk

The assessment of reserves and resources through modern geophysical methods reduce uncertainty of the four main risk factors, namely the structure, reservoir, seal, and charge.  

The precision and accuracy of interpretation of reservoir structure, the depth, shape and lateral extent has been greatly improved through 3-D seismic techniques. Seismic attributes such as amplitude variation with offset (AVO) have become an effective way to address reservoir risk. Analysis of differences in the reflection signal strength can yield reservoir information such as lithology and fluid content. These advancements play a role in quantifying and risking different factors that go into the reserve estimation process (volumetrics, saturation, permeability, and recovery factor). While historically, engineers and geologists have been responsible for reserve estimation, geophysicists now play a critical role.  

Gas-chimney technology, combined with techniques for accurately mapping faults, help assess the seal, reservoir charge, migration patterns, and detect shallow gas hazards prior to drilling. Gas chimneys or clouds can be found in the overburden above or even below some hydrocarbon reservoirs. These phenomena are the result of seepage of gas over geologic time or the migration of hydrocarbons from the source. The associated effects in seismic data were often filtered out as noise, now new technology attempts to utilize the information.  

4-D seismology

Fred Aminzadeh, last year’s SEG President at the 2008 President’s reception. Dr. Aminzadeh is a special adviser for dGB-USA and has authored many books and articles on cutting edge geophysical technology. Photo: Deb Bertossa Reservoir characterization requires the integration of many different sources of information. Logs and cores provide fine scale reservoir characteristics to 3-D seismic surveys to help define the entire reservoir extent. Dynamic reservoir characterization, through the use of 4-D (sequential time-lapse 3-D) surveys, shows what the reservoir looks like and how it is changing through time. This is one of the most powerful tools now used in reservoir characterization showing anomalies in reservoir changes that once were considered unresolvable. As part of the ‘instrumented oil field', permanently installed seismic monitoring systems that yield changes over time will help the EOR (enhanced oil recovery) process and be invaluable for planning infill drilling in producing fields, as well as help revitalize oil fields.

Unconventional reservoirs

Unconventional reservoirs provide important and growing reserves to our energy supply. Resolution of fractures and permeability pathways are significantly smaller than seismic wavelengths. Extremely high frequencies are needed to directly resolve fractures; however, recent studies indicate that time-lapse differences can be used to image fracture development at the "subseismic level". Emerging microseismic techniques might help with the imaging of fractures and unconventional reservoirs, such as tight shales, coalbed methane, and heavy oil fields. Microseismic geophones continuously record energy from natural vibrations in the subsurface or from field operations such as fracture stimulation jobs and steam injection in heavy-oil production.  

Fractures can also be correlated with structure and are often parallel to the axis of an anticline. Gas-filled fractures lower the seismic compressional (P) wave velocity, creating amplitude anomalies. Shear (S) wave splitting analysis and amplitude variation with offset as well as curvature related attributes and azimuthal velocity calculations are some of the seismic methods that are used to detect fracture orientation.  

The success of the Barnett play in Texas, and other shale and tight sand plays across the U.S., have benefited greatly through these new geophysical technologies.  

Sub-salt, -basalt imaging

Sub-salt reservoirs across a major discovery offshore Brazil. See GEO ExPro no.5, 2008. Illustration: HRT/CGGVeritas Improvements are still needed in the imaging of geologic strata below salt and basalt layers. Sonic-velocity problems confound the positioning of reflections below geometrically complicated structures such as an irregular salt body. Here, the overburden acts like a complex lens, severely distorting the seismic wave paths, disrupting illumination, and damaging the images. To obtain a proper image of the target, a wide variety of different azimuthal directions becomes necessary. Wide-azimuth (WAZ) and muti-azimuth 3-D surveys are becoming more important in solving these imaging problems. Further improvements might come from surveys that use very large offsets (exceeding 4 km) to shoot underneath these otherwise impermeable boundaries.  

When combined with advanced subsurface velocity model building and more accurate imaging techniques, such as prestack depth migration, will lead to successful exploration of the deepwater frontiers. The recent major oil discoveries offshore Brazil and in the deepwater Gulf of Mexico are, in part, attributed to such advances in geophysical technology.

Integration and education

Seabed logging, cableless acquisition systems, acquisition of seismic while drilling, borehole gravity, and other emerging geophysical technologies will all help reduce exploration risk. However, the integration of disciplines from the start of exploration through production is the only way to ‘get the job done'. Geophysicists alone cannot do the job. The fusion of various data types requires the intelligent integration of disciplines and this must be done starting in the beginning stages of exploration and production.  

In order to make this integration work, geoscientists as a group need to have some reasonable knowledge of other disciplines. The integration of disciplines must start with interdisciplinary education. Our medical schools can provide a model. They train generalists before going to a specialty. The education system is already starting such general-purpose ‘geoengineer' programs that will help bridge disciplines.  

If we are serious about energy independence and security, we need to invest in education early on, starting with high school aged students. Early training about energy, the technology, and the people that produce it will help improve the industry's image.  

The more senior members of SEG and the energy industry have a large reservoir of wisdom and experience. Through what we call "geomentoring", this knowledge base can be channeled to those who can benefit the most. Mentoring can cover a large spectrum that would include technical issues, career issues, how to become active in professional societies, how to publish, decide on a career path, define and acheive success, among other things. If this program is successful, we will have a self-perpetuating fountain of geophysical knowledge that will hopefully spill over into the entire energy industry.

Advertisement

Related Articles

GEO Science Explained Worldwide

The Next Generation Subsurface Modelling

Modelling reservoirs using a 3D grid is a common tool for both the geologist and the reservoir engineer. As models increase in structural and stratigraphic complexity the process of creating these grids has become cumbersome. In many cases the results are not satisfactory to either the geologist or the engineer.
Stephen464 thumb