The well at fault for the historic Aliso Canyon underground natural gas storage facility leak had extensive corrosion along the casing that got worse as the pipe traveled deeper underground and included a 19-inch long split hundreds of feet below the surface, according to a preliminary report on the cause of the failure.

Blade Energy Partners, the consulting group researching the failure for state regulators, found that the severity of well casing corrosion increased the deeper into the ground the casing joints were located, with the most severe corrosion at Joint 22.

The group's recently released metallurgical protocol for the analysis showed that Joint 2, which extended between about 20 feet and 61 feet below the surface had "little corrosion," while Joint 14, which was from about 518 feet and 560 feet below ground showed "shallow corrosion." By Joint 20, which was between about 770 feet and 811 feet, there was "a large area of deep corrosion."

The consultant pulled a total of 24 casing joints from the well, and Joint 22 was found to have fractured. Based on a visual inspection, there was not only a circumferential fracture around the joint but also a 19-inch long split that intersected the circumferential crack. The cracking and splitting was accompanied by bulging and wall thinning caused by corrosion, the consulting group said in a report published by the California Public Utilities Commission.

Blade Energy Partners also found "oblique, shallow crack-like features," along with gouges, tong marks, slip marks and possible erosion and corrosion marks on some of the joints, indicating mechanical damage had affected the materials.

Aliso Canyon facility operator Southern California Gas Co. first discovered the field was leaking in October 2015, and the utility permanently sealed the breached well in February 2016. California regulators did not clear the company to restart injecting gas into the facility until July 2017, and even then, only at limited capacity.

State regulations in place at the time the leak started required a mechanical integrity test, also known as a pressure test, every five years, and the company was also doing noise and temperature tests to check for infrastructure problems annually, according to California Department of Conservation spokeswoman Teresa Schilling.

The well had passed the state-required tests as of October 2014, but those tests alone are not able to show the well casing thickness or leak potential, Schilling said.

"Since the … leak, a battery of tests and monitoring procedures are now required to — in addition to determining the well casing's thickness — focus on preventing a failure rather than leak detection," Schilling said.

Blade expects to have a final report on the root cause of the well failure in late November 2018, the consulting group said.

SoCalGas, a subsidiary of Sempra Energy, is the largest natural gas distribution utility in the U.S. and serves 21.6 million customers in central and southern California.