Drilling and Production

Oct. 1, 2000
The quest for a true formation water saturation Sw has been the "holy grail" of the oil and gas industry since the first electrical measurements in a wellbore were taken by the Schlum-berger brothers in 1927.

The saturation game: Seeing beyond casing and away from wellbore

The quest for a true formation water saturation Sw has been the "holy grail" of the oil and gas industry since the first electrical measurements in a wellbore were taken by the Schlum-berger brothers in 1927.

From oil-in-place estimates through prod-uction performance and total reservoir recovery calculations, this value is probably the most widely used parameter throughout the life of a reservoir.

Most will say that the one mathematical relationship exemplifying the oil and gas industry the most is Archie's Equation. Yes, the same one that dominated your university petroleum education, and the one yielding - you guessed it - water saturation, Sw.

Swn = aRwn Rt

The relationship hasn't changed since its formulation by Gus Archie. The only changes have been in the way we measure and process the quantities (formation properties) involved in the relationship.

Seeing through metal

In open-hole environments, Sw is easily obtained with resistivity measurements. In cased-hole environments, nuclear measurements take over the workload, with pulsed neutron-capture (PNC) and pulsed neutron-spectral (PNS) tools being the tools of choice.

There are always limitations with any measurement device. PNS tools may be affected by water-to-oil ratios, cement, and other environmental effects.

PNC tools show quantitative measurement limitations at low formation porosities (< 10-15 % in some publications) and low water salinity (< 30,000 ppm in most publications).

Depths of investigation (DOI's) of nuclear tools range anywhere from 8-in. with carbon-oxygen logging, to 18-in. with sigma logging techniques.

Of course, this depends on wellbore annular distance from the tool to the formation face, cement integrity, and formation flushed-zone properties. Vertical resolution is dependent on transmitter-receiver spacing.

In cased-hole environments, any quantitative measurement of water saturation has historically been, at best, interpreted with some degree of skepticism.

The comment has even been made that less than 1/3 of all the producing reservoirs in the world have the necessary formation parameters to yield good quantitative cased-hole nuclear measurements - a bold statement, but one shared by production and reservoir engineers around the world.

With this in mind, another alternative is needed for better water saturation measurement.

Better money option with new wells

During early industry boom periods, the object was to find as much oil with as many wells as possible. At that time, measurement technologies were in the growing stages, and it was cheaper and quicker to simply drill wells as fast as you could.

Where spacing regulations were nonexistent, the end result was a landscape of derricks and wellheads all the way to the horizon.

As time progressed and the industry matured, technology started to take over, reducing the number of wells needed to develop a newly discovered field. But the over-riding philosophy, "find as much oil as fast as possible," still persisted. The result was a large number of fields discovered and developed hastily.

Lots of potentially productive reservoirs were simply overlooked and hidden behind casing, while drilling to the well-known deeper productive zones.

This strategy worked for a long time. As exploration costs began to grow and fewer discoveries emerged, more economic alternatives were sought - such as returning to old producing fields, shooting seismic and redefining older reservoirs. New, high potential strata were found with this technique in producing reservoirs all-around world.

The evaluation method of choice, after more detailed seismic information, was to cut windows in existing casing and drill into these potentially high productive zones with newly developed directional drilling techniques and while-drilling measurement tools. This was an effective, but expensive and time-consuming alternative.

Resistivity alternative

Measurement technologies continued to develop over the years, letting the industry peer behind this steel barrier more effectively. Through-casing resistivity measurement tools recently left the concept phase. Two service providers now market through-casing resistivity measurements.

The concept for a fundamental electrical measurement inside steel casing has been around for almost 20 years, with patents dating back to the late 1980s.

ParaMagnetic Logging, Inc., now owned by Baker Hughes, registered the original patents on the concept. Since then, numerous articles and revised patents have been written and registered on the concept.

Two-part measurement evaluates leakage

The fundamentals of the measurement include a calibration phase and an actual measurement phase. This is different from traditional logging techniques where a calibration is performed prior to the run on surface, or downhole before logging commences.

This two-part measurement decreases the effective tool logging speed. With frequent pre-defined logging stoppages to calibrate and take measurements, an effective speed of 120 ft/hr can be obtained.

Once stopped at a measurement point, a known current is sent down the casing. A base casing resistance is measured to distinguish the casing effect from the very small current leakages into the formation, later measured between different electrodes along the tool itself. This current leakage into the formation is then used to determine formation properties.

Initial estimates of DOI range from slightly less than 10 ft to more than 30 ft, at vertical resolutions up to 4 ft. This benefit alone should be worth the time and effort to run the tool in a well.

Think of the value of knowing Sw, Shc, 30 ft away from the wellbore. Tertiary recovery methods can be more effectively monitored and managed. Reservoir fracturing can be better estimated. The list goes on.

Immediate ROI

This technology has the capability to rejuvenate previously uneconomical older fields and boost total recovery. Most of the immediate benefit will be on land, but offshore marginal fields (and platforms) could see some benefit from this measurement technology. Interpreting production drop-off and/or cessation in existing wells is possible with this technology.

We've just seen the tip of the iceberg. New techniques and applications will emerge as the technology catches on and more operators get comfortable with the measurement.

Smaller operators may benefit the most in the short term. The recent mergers within the ranks of the major oil companies have resulted in a sea of high potential properties being offered up for sale.

The smaller companies with cash in their pockets from the past year's increase in oil prices have been purchasing a lot of these properties.

A few extra exploration dollars spent on evaluating these properties with technology, such as this will let them see a little deeper into their new purchase and maybe find some nice surprises.

Editor's Note: For additional information, see "Through Casing Resistivity Tooltrademark to Locate Bypassed Oil" by Vail, Momii, and Woodhouse, American Oil & Gas Reporter, October 1995.