Cleaning shale debris from the well bores is an effective way to boost flow rates from old open hole Devonian shale gas wells, research on six West Virginia wells begun in 1985 has shown.
Officials involved with the study say the Appalachian basin could see 20 year recoverable gas reserves hiked by 315 bcf if the process is used on a wide scale.
Columbia Natural Resources Inc., Charleston, W.Va., planned to reenter and clean out 20-40 Appalachian basin wells in 1990, at a cost of $30,000-40,000/well, after reviewing the study data.
Columbia hopes to increase production by amounts similar to those of six open hole wells it and Gas Research Institute (GRI), Chicago, cleaned out in the 1985 research project. Officials say cleaning out the well bores allows them to interact with the more gas permeable zones.
Total production from those six wells, all in Southwest West Virginia, improved by an overall average of 437%, to 2.6 MMcfd from 595 Mcfd.
Columbia and GRI originally had planned to core, log, and test 11 naturally producing open hole wells in the area to determine reservoir characteristics and geologic controls that allowed them to produce at fairly high rates.
When researchers reentered the wells in late 1985, however, they found all open hole sections filled with shale debris. So Columbia cleaned the debris from six of the wells in Wingo and Wayne counties, W.Va., and ran slotted tubing in each to keep the producing interval open.
David Hill, project manager for GRI's Devonian shale research program, said three of those wells were tested before and after cleanout, and all three showed an increased level of stimulation.
"None of the wells showed any signs of problems prior to the cleanout operations," Hill said. "In fact, they appeared to have typical production profiles for the area.
"This suggests that many of the open hole completed wells may benefit from cleaning out the well bore.
BENEFIT ANALYSIS
Columbia arranged with S.A. Holditch & Associates Inc., College Station, Tex., to analyze the economic benefit of cleaning out Devonian shale wells, considering cleanout and operating costs, royalties, and taxes.
Holditch forecast well performance before and after cleanout using long term production and precleanout and postcleanout reservoir descriptions from transient pressure tests.
Mian Ahmad, Columbia's manager of reservoir engineering, said the 1985 study tentatively showed cleanouts are a viable workover method for old open hole Devonian shale wells.
"Our conclusion was highly dependent on the wells sustaining the increased performance observed just after cleanout," Ahmad said.
In 1990, Columbia supplied Holditch production figures on the wells for the 4 years after cleanout operations, which showed increased flow rates had been sustained.
"Each well is performing as well as or better than our original forecasts," said Holditch Vice Pres. David Lancaster. "in fact, postcleanout production rates do not appear to be declining as steeply as we predicted. The wells are declining at about the same rate they were prior to cleanout."
CLEANOUT RESULTS
Columbia has begun using a rotary rig for the cleanouts. Jack Watson, director of production, said the rigs have reduced problems with hole caving and tool sticking and have reduced operations time to about 40 hr/well.
"Our initial results have been encouraging, with 75% of the cleaned out wells proving economic," Watson said.
About 40% of present Devonian shale wells in the Appalachian basin have problems with debris in the well bores, said GRI's Hill. Of those, only about half are potential cleanout candidates.
"The other half can't be reached by a rig because roads, houses, and trees have sprung up around them," Watson said.
GRI estimates there may be 1,500-3,000 such wells in the basin that are cleanout candidates. One condition, a GRI official said, is they must be wells with no produced water. Assuming 1,500 Devonian shale wells can be cleaned out with results similar to those recorded by the original study wells, Holditch predicts an increase in 20 year recoverable gas reserves of 315 bcf, or about 210 MMcf/well.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
