Separating the Oil from the Water
EM Detecting Fluids on the Rocks
Widely available and ever-popular seismic technologies are great for detecting subsurface structures and identifying potential reservoirs.
But when it comes to determining the type of fluid contained in the reservoir, seismic methods just don’t measure up.
As a result, there’s increasing industry interest in using electromagnetic (EM) survey methodology to try to reduce the risk of drilling dry holes given EM’s potential to identify fluid content in the rocks prior to drilling. Just count the cost savings if hydrocarbons are determined to be absent in a structure without ever turning the drill bit.
Non-invasive EM technology for the oil and gas industry evolved within the hallowed halls of academia in North America and Europe during the 1980s, generating some interest on the part of a few companies. Yet EM long-languished in the dark corners of the oil industry toolkit for the most part.
That began changing a few years ago.
“EM is becoming quite a key component to a lot of companies’ exploration efforts,” said Richard Kellett, senior geophysicist in exploration at Pioneer Natural Resources Canada. Kellett, who earned a doctorate in marine EM in the late 1980s noted: “It’s always been in the background with maybe 2-3 percent of company geophysicists having exposure to EM methods, and now a lot of companies are scrambling to get staff and learn more about them.”
EM applications have the capability to differentiate between low resistivity water-saturated reservoirs, and high resistivity hydrocarbon-containing reservoirs.
“When the pore fluid within the rocks changes from hydrocarbons to water, it is accompanied by a change in the rock’s physical properties, and this significantly impacts electrical resistivity,” said Anton Ziolkowski, technical director at Edinburgh-headquartered MTEM.
“A change in electrical resistivity of as much as three orders of magnitude may occur when oil is replaced by brine in a reservoir,” he said. “Yet this has little effect on acoustic impedance.”
The overriding reason that surface EM methods have long been overlooked in the oil industry is that resolution of conventional EM data has been low compared with seismic resolution.
“That’s the nature of EM,” Kellett said. “It’s based on the physics of diffusion rather than wave propagation, so it’s always going to be lower resolution than seismic.”
Seismic pulses travel long distances with little loss of resolution. Today, novel signal processing techniques and high precision multi-channel recording systems have significantly increased both the frequency bandwidth and the dynamic range of EM data, Ziolkowski noted.
Two categories of EM methods are used for hydrocarbon exploration -- passive EM and active EM -- according to Jason Robinson, MTEM’s vice president for North and South America.
The MTEM (multi-transient electromagnetic) method is an active application, which creates a localized EM field, as opposed to the passive, or magnetotelluric (MT) method, which looks at local distortion in the earth’s natural magnetic field to estimate subsurface resistivity.
The active MTEM method can be used both onshore and offshore in contrast to another increasingly popular active EM approach called controlled source electromagnetic (CSEM), which is limited to deep water. Another active method -- DC resistivity (electrical resistivity imaging) -- has very limited depth penetration, which restricts it to shallow targets.
EM first became a major industry buzzword in the late 1990s when a major financial publication featured a front-page story about ExxonMobil’s reportedly successful operations using its proprietary CSEM technology dubbed remote reservoir resistivity mapping (R3M).
“That was our sort of ‘going public’ after many years of industry work,” said Len Srnka, senior research adviser at ExxonMobil upstream research company. “There’s a 25-year history inside our company pursuing this (EM).”
Statoil is another big-name devotee of CSEM technology. Its method is often called Sea Bed Logging, and the company has played a key role in commercializing CSEM.
CSEM tows an electric source in the marine environment only, emitting a continuous monochromatic signal tuned to detect the target reservoir, according to Robinson. Some of the radiated energy becomes an airwave, traveling through the water to the water/air interface, then along this interface and back through the water to the receivers, and contaminating the received signal.
CSEM currently is confined to deep water where the water column attenuates up-going and down-going energy.
Shell became seriously involved in using CSEM beginning in 2004, according to Mark Rosenquist, senior staff geophysicist in the E&P research division.
“We’ve put it to good use in a number of basins around the world,” he said, “mostly looking for deep water turbidites where it seems to be an awfully good fit.
“In shallow burial turbidites, if they do have hydrocarbons in them they have a big resistivity contrast with the surrounding sediments,” Rosenquist said, “that makes them stand out and give a really strong CSEM response.
“In particular, we like it in thrust belts where the main risk is you have a blown trap because of faulting or seal failures at the crest of thrusts,” he noted. “CSEM is just great for that because it’s almost like an on/off switch.
“If you have high saturation hydrocarbons it gives a really strong response, and with low saturations you get almost nothing back,” Rosenquist noted. “It’s really sensitive to hydrocarbon saturation, so when that’s the major risk, it’s the ideal tool.”
It Matters Because...
You’re likely thinking this is all well and good but wondering where it fits in with your land prospects where a tool with some hydrocarbon sensitivity could go a long way toward zeroing in on the pay zone(s).
As a result of EM technology evolving and kicking into high gear in the oil patch, you now have options.
The MTEM system, which is often referred to as “logging before drilling,” is a relatively new option garnering lots of attention -- and kudos -- these days. Like EM in general, it originated in academia, specifically the University of Edinburgh.
“It’s one technology that’s separating itself from the others in terms of really advancing,” Kellett said. “They’re taking a unique approach to processing the EM signal, and they’re listening to other people with experience, for example the mining and environmental industries.
“They have three parallel processing workflows to satisfy immediate demands, and they’re approaching it from a very technical strong point of view,” Kellett noted. “It’s got a lot of potential.”
Here’s the blueprint, according to Robinson.
The MTEM method works by injecting a series of pulse-coded electrical transient signals into the ground and measuring the voltage response between pairs of receiver electrodes along the recording line at different offsets, or distances, from the source. The acquisition geometry entails multiple source and receiver positions.
The impulse response of the earth is extracted at the receiver, and signal processing techniques are used to produce a resistivity cross section. Zones identified as highly resistive may indicate the presence of hydrocarbons.
Real Time Appraisal on site provides both QC and initial processing results within hours of recording.
Land MTEM has no airwave problem. In the marine environment, the method is suited for shallow water applications down to 300 meters because of its capability to remove the airwave.
“Onshore case studies show delineation of hydrocarbon reservoirs both laterally and with depth,” Robinson said. “We often can image the reservoir when seismic can’t see anything, plus the system is capable of identifying stacked resistors and placing them at their correct depths.”
MTEM land applications include:
- Exploration: wildcat and high-grading.
- Field appraisal.
- By-passed pay I.D. in mature fields.
- Gas/water flood monitoring.
- 4-D monitoring.
- Fresh water detection.
The company has reported results from a number of case studies.
“A Real Time Appraisal of the data acquired over a gas storage reservoir in southern France indicated the depth and lateral extent of a subsurface resistor,” Robinson said, “which correlates with known gas in a relatively shallow sandstone reservoir having simple structure.
“The MTEM data were processed blind with no input from the client,” he noted, “yielding an exact match with the seismic and log data provided after analysis.”
A tar sands project in northern Alberta clearly demonstrated the technology is positioned to play a role in today’s hot arena of unconventional reservoirs, which often are difficult to evaluate via seismic and traditional EM methods.
“MTEM is being used routinely in tar sands,” Kellett noted. “It works because the oil sands are very thick, resistive and close to the surface.”
A sampling of the Alberta project results include:
- Ability to see the resistors in the subsurface.
- Inversion distinguished hydrocarbons from glacial deposits.
Len Srnka of ExxonMobil envisions a bright future for EM technology.
“I think the future is one of expansion of the technology into pretty well all of the upstream -- certainly offshore -- and especially the deepwater offshore,” Srnka said. Almost all the applications today are in exploration, and we expect that to roll through the rest of the upstream in both development and production.”