Reports by LOUISE S. DURHAM
EXPLORER Correspondent

'Geology 101' Gets New Applications
The Tried and True Still Works

Cecil's Log Plainly Is Gassy

See related story:
'Old-Timey' Geology Pays Off

Gas lift is a common practice in old Gulf Coast fields. For many of these fields, the availability -- or lack thereof -- of field gas to run compressors, pump engines, etc., is all that stands between production and shut-in.

A number of longtime operators who are picking over old fields are finding not only little stringer gas sands to meet field needs but substantial pay zones as well, using what appears to be a rather simplistic, relatively inexpensive well log that's been around for more than 30 years.

It's a cased hole log comprising three tools -- gamma ray, density and neutron curves (GDN) -- and it's found a lot of hydrocarbons for a lot of folks since it was initially developed in 1970, according to veteran exploration geologist Alan Pennington.

In fact, it's been dubbed the "gas-finding log" by a number of users.

It's also frequently referred to as the "Cecil Eicke log," in deference to the now-retired founder and owner of United Surveys (US) in Richmond, Texas, who developed the log with help from an electronics engineer at US, who continues as an on-site expert today.

"We've run it for everybody, both big and small," said Eicke, who spent his entire career in the wireline business. "There was a lot of trial and error in developing it, and we've done a lot of fine tuning over the years."

The GDN log appears deceptively simple: the sand shows up on the gamma ray, and gas presence is indicated by a reversal and ultimate crossover of the neutron and density curves. An old open hole log for comparison and verification helps but is not essential, according to Eicke.

Where the old log without the density-neutron curves showed interesting places, the GDN lets the owner go in for relatively little expense to verify gas, Pennington noted.

Operators have used the US log for a variety of applications over the years to:

• Locate gas behind pipe to aid in recompletion evaluation.
• Evaluate abandoned wells for possible re-entry.
• Help define depleted zones and moved water contacts in producing zones.
• Detect oil zones in some instances.

"It's as good a tool as any in the industry, cased-hole-log-wise, to indicate reserves remaining in the well," said Richard Lee, managing member of Masters Petroleum. "It also shows a gamma ray reversal where a zone has been swept, so you know to stay away.

"We've used it on close to two dozen wells up and down the Texas Gulf Coast with incredible accuracy," Lee said.

"For a company like Masters that makes its living re-exploring mature oil and gas fields," Lee said, "a log like this is invaluable."

Good for Oil, Too

There are a number of more sophisticated, expensive cased hole evaluation logs available, such as the pulsed neutron, or thermal neutron decay (TDT) logs. For those companies who need the information a TDT provides, such as porosity and water saturation, it's likely worth the expense.

In fact, a petrophysicist with a major may be inclined to take this path and bypass the GDN because it is qualitative and not quantitative.

"The GDN doesn't tell you porosity, but it does tell you there is gas effect," Pennington said. "It was developed for a niche market that doesn't need TDT or pulsed neutron and can get along fine with a less expensive version.

"It's the difference in a Mercedes and a Ford Taurus," he said. "Both get you there; it just won't be quite as in style."

Like a number of other operators who have used the GDN, Lee noted it's a head-scratcher as to just how the log works. Pennington says, simply, "nobody runs a cased hole density log like they do."

Although known and valued primarily as a gas finder, the GDN is also a pretty nifty tool to find oil.

"We've run this log on 30 or 40 old wells the last couple of years," said Richard O'Donnell, president Houston Petroleum Company, "and we've found a lot of both oil and gas with it.

"With oil, it doesn't necessarily give a crossover," O'Donnell said, "but you can see where the density pulls in and the neutron pulls out. When they get close, or begin to kiss, it indicates you've got some hydrocarbons there."

It worked for Lee.

"We recently ran the log in a well at Red Fish Reef, where we saw a possible oil zone overlooked by Exxon," he said. "We perfed, and it came in flowing 200 bopd at 1,350 psi and no water.

"We've experimented with a lot of different logs," Lee said, "and I find it amazing that with all the very expensive, sophisticated tools out there, it's the old tried and true proven technology that's so often the best."

Pennington offers a personal endorsement for the myriad oil finders picking over innumerable old fields today:

"You should run this log in every well before you plug it," he said. "If you can find a little gas sand that will make a few hundred million, that's a lot of money -- and this thing really has made a lot of money for a lot of people."