The first ex-Enron executive to go to prison may get a change of scenery as early as Tuesday to testify in the first criminal trial to emerge from the energy company's 2001 collapse.

Prosecutors asked U.S. District Judge Ewing Werlein on Monday to grant immunity to former Enron Corp. (search) treasurer Ben Glisan Jr. (search) for crimes he may have committed other than the conspiracy count to which he pleaded guilty a year ago. The judge was expected to approve the prosecution request before Glisan appears to testify.

Glisan could begin testifying late Tuesday. Prosecutors plan to first present an FBI agent after defense attorneys finish questioning a Securities and Exchange Commission (search) lawyer who investigated Enron in 2002 for a Senate committee.

Glisan is a key witness in the conspiracy and fraud trial of four former Merrill Lynch & Co. (MER) executives and two former midlevel Enron executives regarding an alleged sham sale of three barges to the brokerage in December 1999.

Prosecutors contend the sale was really a loan because Glisan's former boss, then-finance chief Andrew Fastow (search), verbally promised Enron would find another buyer for or buy back Merrill Lynch's interest in the barges by mid-2000. The defendants contend Enron was never obligated to do so.

A partnership Fastow created, LJM2, bought the barges at what prosecutors say was an agreed-upon premium in June 2000.

Earlier in the trial, which entered its third week Monday, prosecutors presented jurors an e-mail Glisan wrote in May 2000 to others on Enron's barge dealmaking team that said they had to follow through on the promise:

"To be clear, (Enron) is obligated to get Merrill out of the deal on or before June 30. We have no ability to roll the structure," the e-mail said.

Glisan didn't initially cooperate with prosecutors, so he began serving a five-year sentence immediately after pleading guilty. But he later testified before a grand jury in Houston.

He was indicted in May 2003 on two dozen counts of fraud, conspiracy, money laundering and others for participating in various schemes orchestrated by Fastow.

Those included pocketing a $1 million return from an investment of more than $5,000 in one of Fastow's myriad shady partnerships and schemes, some of which fueled the energy company's collapse.

After jurors were released Monday, Werlein heard arguments about whether the jury should see a March 2001 e-mail written by one of the defendants, former Merrill executive James A. Brown, that refers to the barge deal.

Regarding an unrelated deal, Brown wrote, "We had a similar precedent with Enron last year, and we had Fastow get on the phone with (defendant Daniel) Bayly and lawyers and promise to pay us back no matter what. Deal was approved and all went well."

Brown's lawyer, Lawrence Zweifach, said the e-mail was prejudicial and taken out of context. Prosecutor Kathryn Ruemmler said it was "powerful evidence" that a verbal promise was made in the barge deal. Werlein has yet to rule, and prosecutors plan to try to present the e-mail to jurors Tuesday.

In addition to Brown, the barge defendants are former Merrill Lynch executives Daniel Bayly, Robert S. Furst and William Fuhs; former in-house Enron accountant Sheila Kahanek; and former Enron finance executive Dan Boyle.