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.
The Abu Dhabi Company for Onshore Oil Operations (ADCO), operated under the ADNOC umbrella of companies, has implemented water injection for the lower units of the reservoir. A multi-year and multi-measurement monitoring plan has been established to determine the performance of the pilot study, including deep reading technologies, such as Schlumberger’s DeepLook-EM.
According to Schlumberger, what was missing was measurement of the flood movement in the areas between wells.
Therefore the company set about using the DeepLook-EM transmitter and receiver to monitor the flow between a triangular pattern of three observation wells.
By providing reservoir-scale resistivity images between three boreholes from 240m to 300m apart with time-lapse surveys, Schlumberger was able to image the fluid displacement across the reservoir, and the data was then fitted into a model and survey images.
The images from six months after the baseline indicated successful water flooding from the injector well to observer wells 1 and 3, showed that a large volume of water swept through a preferential east to west flow direction.
Although water has not migrated to the pair of observer wells 2 and 3, further DeepLook-EM surveys at six-month intervals will verify the direction and coverage of water movement, and according to the company enable fine-tuning of injection management to avoid both water override and ineffective sweep.
“The beauty with the crosswell EM is that we can measure between these wells and we can see the water movement at the reservoir scale, and this can be done in a matter of months rather than waiting several years, which has a very high value to customers,” explains Hulme.
“It gives them the opportunity to modify their injection pattern and the ability to update their prediction model on what is going to happen.
“We can understand how the sweep is moving in the reservoir; is it staying in the reservoir compartment, or as often happens in Abu Dhabi, is the water riding up over the reservoir into non-reservoir compartments – knowing this is really important,” he continues.
“This [ADCO project] is a really great study. We have actually done more time lapses since those published ones.” tremendous results
Early results from the ADCO pilot study are consistent with the expected flow behaviour, and the monitoring of the sweep from water injection via crosswell EM has proven to be effective and successful in the field.
“As the crosswell EM monitoring studies continue at six month intervals, we plan to incorporate more geological data in the inversion process by building higher resolution models. This should further improve the already obtained resolution by a further 1-2% of the well spacing,” reads a Schlumberger report. The crucial point is that the results actually represent something that is meaningful, as Hulme points out.
“If we take the measurements, we must be able to convert it into something that makes sense – this is the critical point. You want to know before you go to the expense of conducting a survey that you have a high probability that you will get a result that gives meaningful scenarios, and we have a high success rate of 18 out of 18 surveys, with really tremendous results” he concludes.
