Mountain Fuel Supply Co. on Friday asked the state Public Service Commission for an $11.95 million increase in its Utah natural gas rates.

That would translate to a 3.13 percent increase, or about $18 per residential customer per year.The increase, which Mountain Fuel wants to go into effect Feb. 1, was requested in the utility's "pass through" filing. The filing allows the company to adjust its rates semiannually to reflect changes in gas supply costs.

Mountain Fuel representatives said the increase is needed to offset a shortfall in its "191 account." This account is used to balance gas costs against revenue collected from customers. If the account balance shows a surplus, it reduces rates the following year; if the balance shows a deficit, it increases the following year's rates, utility representatives said.