Current Status and Advances of Gravity Survey in Oil Exploration and Reservoir Monitoring

Abstract

Abstract

No question seismic dominates geophysical exploration. But even seismic may suffers from question marks related to salt structures, volcanic shields, complicated topography, near-subsurface, urban areas, etc.

Gravity as the first geophysical technique used in oil exploration about one century ago still brings another petrophysical parameter – density – to constrain seismic velocity. It's an easy logistics and low cost method. Currently we can perform traditional land survey with up-to-date digital gravimeters with resolution increased by one order to previous stage. Very efficient is a pre-seismic survey that may improve line planning. Many new processing techniques, like new Euler, tilt, edge detection and other are applied to enhance the useful signals in the data and display them in shaded or similar form easily understandable to any geoscientist. Specialized in-house software, or industrial leaders packages like Geosoft, may then transform data into 3D cubes and other 3D space imaging, preferably in conjunction with seismic models.

Detailed gravity and microgravity can characterize near-subsurface inhomogeneities like voids, weathering, karst, lateral lithology etc. to improve statics and velocity models, but also reduce operational hazard.

For large scale scouting surveys airborne gravity is perfectly suitable both on- and off-shore. It can cover difficult areas like national parks, mountains, swamps, jungle or so. Similarly, marine gravity provides enough accurate data, limited however to seismic line survey plan. Significant advance is represented by Full Tensor airborne Gravity Gradiometry used for detailed studies. FTG measures more components of the gravity field and may better highlight extent and edges of target structures. Progress is made in using the satellite data despite much lower areal resolution.

Specific application is 4D gravity. There are projects monitoring gas extraction or injection, water-flooding process and other fluids contacts movement in reservoirs. In recent times it has gained high quality by combining microgravimeters with portable absolute gravimeters to improve reference level control and significantly contribute to IOR/EOR. Also further development of borehole gravimeters is a huge step forward to accurate monitoring of reservoirs.

The case stories will demonstrate to oil companies the impact of advanced gravity on seismic processing and decision making at different stages of oil/gas exploration and development.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016