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SUB SEISMIC FRACTURE DETECTION

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This report describes another unique approach by enhancing very weak scattered seismic waves generated from active seismic by heterogeneity ,such as open fractures. The side view technique has also been employed in a passive mode recording production induced seismic activity. The method is based on the same principles used in side scan sonar.

The tool has been tested and demonstrated extensively, mainly in Russia but also in China, Indonesia. The SVSL has been applied in more than 100 areas ranging from 10 to 250 sq km, across oil and gas fields and exploration prospects.

The method offers also a 3 D monitoring tool for production changes by down hole seismic stimulation if the latter induces fracturing in carbonates as demonstrated by a recent seismic survey befor and after the seismic stimulation by a downhole tool.

It is suggested that the method has strong potential for Exploration and Production, especially in carbonate provinces .If used in 4D mode it may prove to be a cheap alternative to 3 component time lapse seismic, if the target is detection of spatial distribution of open fractures regions in carbonate deposits or continuous progress of the oil water front in water injection schemes.

It is interesting that a 2005 Russian experiment established that the lunar cycle, causing compaction and decompaction of the reservoir rock , affect the effectiveness of water injection schemes.

A passive seismic SVSL experiment across an carbanote field in real time established the leaking of the injected water during the lunar decompaction cycle!!

The energy of scattered waves seems to be 10 to 100 times smaller than the reflected energy and hidden in the noise. This is why this phenomenon, outside Russia; has not been observed and exploited before like the very visible reflected offset AVO.

It is claimed that the main contributor to the scattered wave energy are the open fracture system making it suitable as a predictive fracture detection system.

Description
  • SVSL field observations are mostly implemented using a 2 position locator. The sensors are placed in an aerial star array of 120 phones and each shot out of a hundred shot patterns are fired sequentially enabling to perform a staggering 10,000 fold coverage. This compares to reflection seismic where on land 6 to 12 fold is common. The whole layout is than moved along station to station. For example a 80 sq km required eight moves.

  • The interpretation uses the basic stress information known from the stress deformed state for the particular geological setting. The reflected images are superimposed over the scattered data set. The latter has a 3D cross line appearance. The data today demonstrated that the method can be applied for static as well as dynamic time spatial determination.

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