Fluid frontier

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The DeepLook-EM crosswell acquisition system directly measures the resistivity of the reservioir between wells up to 3 280 feet apart.

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Time lapse DeepLook-EM crosswell resistivity surveys can illustrate large water sweeps.

Enhanced crosswell reservoir imaging is enabling superior insight into the region's EOR projects. Matthew Scotland met Colin Hulme, Schlumberger's technical director of deep reading, at MEOS 2009.

Traditionally, logging of reservoirs would be conducted with borehole measurements that investigate from a few inches to several feet into the reservoir around the wellbore wall. While this provides a detailed picture of the close surroundings, it is too simplistic to determine fluid properties at the reservoir scale, and seismic acquisition does not provide the resolution required to do this effectively.

Now Schlumberger believe they have come up with a solution to fill the gap, with its recently launched DeepLook-EM enhanced crosswell reservoir imaging and monitoring system.

The technology, which is part of its Deep Reading portfolio, uses similar electromagnetic (EM) physics as conventional induction logging, but differs in that the transmitter and the receiver are separated and run in different adjacent wells. These may be up to 1000m apart depending on the objective, and the transmitter signal levels are four orders of magnitude greater than those in conventional induction logging tools.

"With conventional wellbore logging, you form an evaluation by lowering sensors into the wellbore and measuring the physical properties of the rock and fluids. What we are interested in is identifying where the hydrocarbon is, how much there is, and more importantly will it flow and by how much," says Martin Isaacs, wireline marketing communications manager, Schlumberger.

"With the standard investigation we are only scratching the surface of the wellbore by a few inches to a few feet. Within a reservoir you will have wells a long way apart - several hundred metres to a kilometre or more - so the question we want to answer is what you have in between."

The gap that falls in between the high-resolution but finer scale information from borehole logs, and the large scale but coarser resolution data collected from seismic, is what Schlumberger calls the deep reading gap.

"What we are developing is a portfolio of technologies to fill the deep reading gap, in order to give us information about the reservoir at the reservoir scale," explains Colin Hulme, technical director Deep Reading, Schlumberger.

"One of the most exciting tools we have developed is the DeepLook-EM. It essentially works by measuring the physical resistance between the wells. We can then gain, in a similar way to a CAT scan, what is going on inside the reservoir."

Launched at the Middle East Oil and Gas Show (MEOS) 2009, DeepLook-EM can deliver resistivity profiles between wells up to 1km apart and in time lapse mode, the movement of fluids within the reservoir can be accurately tracked.

For example when using an enhanced oil recovery (EOR) programme such as water alternating gas injection, the DeepLook-EM will be able to take measurements over staggered periods of time to give an assessment of how the water was moving through the reservoir, and whether it was being injected in the best way to enhance hydrocarbon recovery.

"We will take a survey after injection has commenced and any changes that occur will give an indication of how effective the sweep has been," says Isaacs.

"What we have done essentially is split the existing tool into two parts and run them in separate wells, but with much stronger transmitter signals and much more sensitive receivers. It is a very powerful tool, and it is the first time you will be able to see images of fluid movement between the wells at the reservoir scale, so it is unique," adds Hulme.

ADCO case study

A few years after the discovery of a giant carbonate reservoir in Abu Dhabi, peripheral water flooding was initiated to maintain reservoir pressure. Recently, however, it appears that the complexity of the reservoir has led to an uneven sweep.

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