GEO ExPro

A Revolution in Resolution

The need to maximise recovery has spelt success for a seismic technique which hugely increases reservoir knowledge.
This article appeared in Vol. 5, No. 4 - 2008

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Seismic signals are sent from a borehole to a number of adjacent wells, and both direct arrivals and reflections from horizons above and below the source and receiver positions are processed to produce a wealth of high resolution data about the reservoir and surrounding subsurface. Image: Z-Seis Mark McCallum is Director of Business Development GeoScience for Z-Seis, a company focussed exclusively on the acquisition and processing of Crosswell high resolution seismic. Photo: Jane Whaley Z-Seis is a specialist seismic company concentrating exclusively on the acquisition and processing of high resolution seismic between wells, known as Crosswell imaging, which can provide up to 100 times the resolution of conventional surface seismic. "Surface seismic provides a view of the earth from above, but our imagery gives us far more data from within the reservoir," explains Mark McCallum, Director of Business Development with Z-Seis. "It gives us valuable extra information and fills in the unknown parts of the puzzle."

The technique uses a seismic source emitting a high frequency (100 - 2,000 Hz) sonic ‘chirp', which is lowered into a well, with the receiver array placed at the base of one or more adjacent wells. By gradually moving the source up the borehole, a fan of data is obtained, which is enhanced by changing the position of the receivers and repeating the process a number of times. Intricate data processing then produces both 2D and 3D images and models of the subsurface.

Useful throughout field lifecycle

"Crosswell imaging is useful throughout the exploration and production lifecycle," Mark says. "For instance, in the early exploration stages it can be used to create a detailed characterization of the reservoir from the outset, as well as to identify pools missed by previous seismic surveys. It lets the geologist build up a very detailed image of the reservoir, allowing accurate placement of wells, which can ultimately reduce the total number required."  

"During primary recovery the technique assists in further accurate reservoir delineation, and also helps target the highest porosity and the best potential producing zones. For example, in a survey we undertook recently in Mexico, we compared our results with conventional seismic, and discovered that what had previously been interpreted as a porous zone was shown to be an area of small faults, which were effectively compartmentalising the reservoir and reducing, rather than increasing, productivity. Detecting these potential challenges is particularly important in the fractured reservoirs of the Middle East."  

This type of high resolution seismic is also increasingly being found to be a cost-efficient method for reservoir management and monitoring and for developing models.  

Stepping inside the reservoir

Crosswell seismic is rapidly developing as an important tool in enhanced oil recovery operations (EOR), particularly those using CO2 and water or steam flood, as it provides a series of detailed pictures of operations over time, checking for the efficiency of the flood between wells. Imaging the reservoir during EOR also ensures that these expensive flooding fluids are used efficiently, reducing the overall costs of EOR activities.  

"The technology is useful right through to field abandonment stage," adds Mark. "It helps predict the reservoir's life through an advanced understanding of its characteristics, and provides a mass of information about the subsurface between wells, which assists the search for bypassed oil and new reserves, before a field is finally closed-in."  

"In the US at the moment there is a big uptake in the exploration for unconventional hydrocarbons, for stacked sands and coalbed methane, for example. To efficiently exploit these reserves, it is vital to fully understand the reservoirs and plays and their interaction, and Crosswell seismic imaging is becoming an integral part of this process. The significant increase in interest in the product in recent years has been driven by high prices and a reduction in the level of discoveries, as well as greater awareness and understanding of the technology.  

"Our advertising line is ‘Resolution so high, it's like being there' which sums it up well," Mark McCallum adds. "To me it's like searching for a house; when you finally find one you really like, you want to step inside it and look around. Crosswell seismic lets you do that to the reservoir."

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