UPDATE 1-Natgas wells remain offline in Colorado after fire

* Encana brings 31 of 35 wells back online

* Black Hills' 98 wells remain shut

July 2 (Reuters) - Up to one hundred natural gas wells remain shut in northern Colorado to safeguard against wildfires that have spread across the state over the past week, though some are returning to service, operators said on Monday.

The Bureau of Land Management identified areas potentially under threat in the Piceance Basin in the northwest of Colorado, forcing Black Hills Exploration and Production to shut 98 wells and Encana Corp to shut 35 in recent days.

Encana said that 31 of its 35 shuttered wells returned to service on Monday, with just four "low producing" wells still offline. Since Friday, Encana lost about 99 million cubic feet of production in total, representing a tiny fraction of the Piceance Basin's 2 billion cubic feet per day output.

It is unclear how much gas Black Hills was forced to shut.

"We continue to work with the Bureau of Land Management, as well as local authorities, to monitor the fire, and the affected wells will stay shut-in until the area is deemed safe," a Black Hills spokeswoman said on Monday.

Last week's wildfire was the most destructive blaze in state history, forcing 35,000 people from their homes. It is one of dozens of fires being fought across the United States as high temperatures, drought and erratic winds set many areas alight.