He listened for hissing or rumbling nearly 2 miles below the earth, easing his microphone down the natural gas well known as SS 25. Mitch Findlay heard something that caught his attention.

Findlay, the owner of a leak-detection company, was testing the well for his client, the Southern California Gas Co. It was November 1991, nearly 25 years before the well above Porter Ranch ruptured and spewed roughly 94,000 metric tons of methane into the atmosphere.

It was a “distant noise,” Findlay noted in the typewritten log filed with state regulators. But the thermometer attached to the mic didn’t sense any dramatic cooling — a good sign. The sound, he explained during a recent interview, could have come from moving water or another well in the oil and gas field.

“Put together, they are not alarming,” Findlay, a retired 20-year oil and gas consultant, said of the results from the two leak testing methods.

But annual temperature surveys and the occasional noise test gave an incomplete picture of the well’s integrity, he and other experts say. Another common way to detect leaks, a pressure test, was performed on the well just twice since 1989, according to state records.

California regulators do not require natural gas companies to test the pressure of storage wells, despite the industry’s endorsement and widespread use of the leak-detection method.

“These wells should be pressure tested every five years,” Findlay said. Pressurizing the space between interior tube and the surrounding casing of the well, the area called the annulus, “would have found the leak,” he believes.

In Texas and Kansas, states with modern underground gas storage regulations, the well indeed would have been required to undergo pressure tests at least every five years.

“It’s a common industry practice and the best-regulated states all require it,” said Adam Peltz, an attorney in the Environmental Defense Fund U.S. Climate and Energy Program.

Casings expand and contract like balloons, and can crack under heavy pressure. Officials said SS 25 was leaking from a breach in its 7-inch diameter metal casing. The more than three-month-long leak emitted an unprecedented amount of greenhouse gas, sickened residents and forced the relocation of thousands of families.

To make sure a well is gas-tight, contractors typically seal the annulus, increase the pressure to a certain threshold and monitor it for 30 minutes. Some minor leakage is considered normal. But a loss of more than 10 percent of pressure would be reason for concern, experts say.

“Pressure testing detects whether or not the casing is holding pressure. Period,” said Richard Haut, the director of energy production at the Houston Advanced Research Center.

Temperature tests could detect a casing leak, but they also are used to detect leaks within the well’s interior components, experts say. Gas cools as it moves from a high-pressure area to a low-pressure zone, so a thermometer lowered into a well could detect gas escaping.

Regulations coming under new scrutiny

When the state approved the Aliso Canyon storage facility, regulators required annual testing of each well’s integrity. To fulfill the requirement, they’ve accepted temperature tests. SS 25 showed no significant problems in its temperature surveys each year from 1989 through 2014, according to records from the state Division of Oil, Gas & Geothermal Resources.

California requires some injection wells (such as those for wastewater ) to undergo pressure testing every five years, but regulators make an exception for natural gas storage wells.

Critics contend the pressure-testing exemption is an example of long-standing coziness between regulators at the California Department of Conservation and utility managers. The federal government mostly leaves states to regulate underground gas storage.

“DOGGR, until this point, has been incredibly docile. They presume a company will work in a responsible manner and fix wells when they spot a problem,” said Gene Nelson, a physical sciences professor at Cuesta College in San Luis Obispo.

A set of state emergency regulations took effect early this monththat requires operators to check each gas wells’ pressure daily and report any anomalies to the state. But the temporary rules stop short of full pressure testing.

The permanent regulations will be changing soon, said Jason Marshall, chief deputy director of the Department of Conservation. The reforms could require pressure tests every five years, according to a draft released in January. Last updated in 1984, the underground controls were already under review before the leak struck in October.

Exemptions for natural gas storage wells are “past history,” Marshall said in an interview. “We’re fixing the past.”

Pressure testing isn’t a cure-all

While important, pressure testing isn’t a panacea, said Paul Bommer, senior lecturer in the Department of Petroleum Engineering at the University of Texas, Austin. He remembered pressure testing a well successfully, and practically the next day corrosion ate through casing and caused a leak. At older wells — SS 25 was originally drilled in 1953 — the most revealing test would be a “casing inspection log,” Bommer said. Operators can measure the thickness and condition of a well’s interior with ultrasound and other technologies.

That type of testing is expensive. In a 2014 regulatory filing, SoCalGas said it would cost $390,000 per well for a weeklong process, including pressure and corrosion tests. Pressure testing alone requires specialized equipment and operators.

The Gas Co. did not respond to questions about pressure testing. It is unclear if the company performed any pressure tests on SS-25 that were not reported to the state. In an earlier statement a spokeswoman said it monitors pressure gauges weekly.

“SoCalGas operates in compliance with relevant regulations, which are set up to ensure the public safety,” company spokeswoman Melissa Bailey wrote in an email.

On Wednesday, state regulators listed all the tests SoCalGas must conduct to resume injecting gas at Aliso Canyon, including pressure testing the wells.

This story has been amended to reflect the proper category of injection well California requires to be tested.