NW Natural said Friday that it wants to raise overall rates by 6 percent next year to pay for operations and maintenance of its natural gas distribution system.

The Portland utility says this is the first time it has sought a general rate increase since 2011, a period when natural gas prices have been falling amid a major uptick in production.

New drilling techniques, including fracking and horizontal drilling, have made gas production far more efficient. NW Natural said its average residential customer bills are down 25 percent since 2011. The utility said it expects the proposed increase would generate $40.4 million in new revenue.

NW Natural has 730,000 customers, nearly 90 percent of them in Oregon. The utility said a typical residential customer using 53 therms a month on average would pay $60 more annually under its rate proposal; a commercial customer using 238 therms monthly would pay $180 more each year.

The rate increase is subject to review by the Oregon Public Utility Commission, which will evaluate NW Natural's rationale and could reject or reduce the request. It's a long process that could take most of 2018, meaning the new rates wouldn't go into effect before next fall.

"Typically they ask for more than they expect to get," said Bob Jenks, director of the Citizens Utility Board of Oregon, an organization created by state voters to advocate on behalf of utility customers. In particular, Jenks said NW Natural's new request appears to seek a higher profit margin than Oregon ordinarily allows.

"I suspect this will become a smaller number pretty quickly," he said.

While Jenks said he believes NW Natural does have safety and maintenance investments to make, he said the utility board and utility commission will scrutinize the proposal to ensure they're spending the money appropriately.

NW Natural reported $521.8 million in revenue in the first nine months of the year, up from $442.4 million in the same period of 2016. It reported profits of $34.5 million, up from $30.6 million a year earlier.

Separately, the utility commission said Friday it expects filings from six utilities – including NW Natural – for proposals to reduce rates following the corporate tax cut President Donald Trump signed last week.

Oregon and other states allow regulated utilities to pass on certain costs to their customers; when those costs go down, prices must, too. The new tax law cut the corporate tax rate from 35 to 21 percent.

It's not unusual for regulators to consider competing factors, like the effects of higher utility investments and lower taxes, when reviewing rate proposals like NW Natural's.

This article has been updated with additional comment from NW Natural, noting that 53 therms are typical for its residential customers.

-- Mike Rogoway; twitter: @rogoway; 503-294-7699