California shuts 2nd gas storage site on new leak

PG&E's largest underground natural gas storage field in San Joaquin county, California, has been shut so that its wells can undergo the same rigorous testing as those at the Aliso Canyon storage field following the discovery of a low-level gas leak last month.

This means two of the state's largest gas storage facilities are now unavailable as summer demand builds for gas-fired power plants.

An expanded inspection regime now required under emergency gas regulations from the California Department of Conservation's division of oil, gas and geothermal resources (DOGGR) found detectable gas emissions at PG&E's 81 Bcf (2.3bn m³) McDonald Island gas storage facility near Stockton, California, in mid-June.

Emissions described as "low level" from 10 wells were identified during a daily inspection that is part of new emergency gas regulations issued in February by DOGGR in the wake of a four-month gas leak at SoCal Gas' Aliso Canyon storage field near Los Angeles.

Data from flyovers at McDonald Island, located in a remote area of the San Joaquin delta, showed a minor gas release ranging from 263kg/hr to 763 kh/hr, "similar to or only slightly above background levels at natural gas storage facilities," California oil and gas supervisor Kenneth Harris said in a letter to PG&E on 30 June. Emissions at Aliso Canyon peaked at more than 60,000kg/hr before the leak was stopped, DOGGR said.

Given the number of leaking wells at McDonald Island, DOGGR directed PG&E to test all 81 wells and to stop all injection and withdrawal activity. Two wells have already been isolated from the gas reservoir. Because of minimal venting, the leak may be underground, allowing gas to seep through the sand layer and some wells, according to DOGGR.

No safety, environmental or reliability issues have been identified because of the minor leak, but the field was shut "out of an abundance of caution," said PG&E's Nick Stimmel. "Regardless of the size, we take this seriously and we are going to do this work to find the source."

PG&E may complete initial temperature and noise logs at each well next week. DOGGR ordered PG&E to file a plan to detail how it will conduct the initial tests and what corrective action will be taken. Further meetings will determine how remaining well integrity tests will be conducted.

McDonald Island is one of three gas storage facilities owned by PG&E. In addition to the Los Medanos and Pleasant Creek facilities, the utility also owns a 25pc stake in the Gill Ranch field. Combined, the four sites have a total of 105 Bcf in working gas. Total injection capacity is 635mn cf/d and withdrawal capacity is 2.18 Bcf/d.

McDonald Island accounts for 73pc of PG&E's daily withdrawal capacity.

PG&E performed noise and temperature inspections at McDonald Island in October 2015. "We moved up the annual testing," Stimmel said.

Similar testing was completed at Pleasant Creek and Los Medanos earlier in 2015.

PG&E also utilizes storage at four independent facilities with an additional 105 Bcf in working gas and injection capacity of 1.8 Bcf/d and withdrawal capacity of 2.7 Bcf/d

SoCal Gas has plugged 67 of the 114 wells at Aliso Canyon as it works to conduct advanced integrity tests on a small number of wells in an effort to return the storage field to partial service by late summer to avoid potential blackouts as California power demand peaks.

Daily inspections at Aliso Canyon also detected a small gas leak on 2 July which was quickly repaired.