Shale players in the US are increasingly concerned about damage to legacy wellbores from hydraulic fracturing jobs at new sites nearby.

"We are seeing it more and more," MicroSeismic president Peter Duncan said at a well interference forum in San Antonio this week.

As the shale plays have evolved from exploration to full-field development, operators have opted to space wells closer and closer to each other in order to boost inventory and asset value.

However, over time, greater scrutiny has been placed on tighter well spacing after several operators reported that interference from infill wells, sometimes called child wells, had negatively affected production from older legacy wells, sometimes called primary or parent wells.

Concho Resources recently took a loss in the second quarter as it credited too-tight well spacing for poor well performance at its Dominator project in the Permian basin.

Duncan said interference from infill wells could cut the primary well's estimated ultimate recovery by 20% to 40%.

Conversely, interference could also cause the primary well to "rob" child wells, leading to as much as a 20% reduction in production at the infill wells compared to the primary wells.

However, Duncan said frack jobs can also reactivate existing faults that can in some cases shear the wellbore. In particular this has been seen in compressive environments such as China and the Vaca Muerta play in Argentina, but US players are also becoming worried about wellbore damage.

"Because of the amount of pressuring up of the reservoirs we're seeing even in the US, in the Delaware, in the Eagle Ford, I am becoming more aware of an increasing concern about wellbore damage," Duncan said.

"Not necessarily shearing it off but causing constrictions in the well."

Interference has been believed to be behind a host of other problems, including twisted pipes and sand damage to pumps, according to Ali Daneshy, president of Daneshy Consultants.

"We keep thinking that if we drill more wells and put more fractures, and bigger fractures, then it is better," Daneshy said.

That is true for a short time after the frack job, he said, but “the long-term effect is not positive. I think we are too aggressive in our frack jobs, I think the spacing between these clusters of fractures is getting too short."

Abraxas Petroleum, which has operations in the Permian and the Bakken, is one operator that has taken steps to mitigate the negative effects of well interference.

"We find that if you don't defend your wells, at least in the Bakken and the Delaware for sure, you're likely to have wellbore fill in your lateral that will necessitate an expensive coil-tubing cleanout," the company's vice president of engineering Peter Bommer said at the conference.

Bommer noted that there were "significant" interactions between two wells targeting the Wolfcamp formation in the Delaware sub-basin of the Permian that were spaced 330 feet apart.

"Our pressure performance was very similar between the legacy and the infill well, so the legacy well was hugely energised by this interaction," Bommer said.

"We see that the interaction resulted in a higher water-to-oil ratio that did not recede."

Bommer said Abraxas has focused on defending its primary wells and trying to design its stimulations to confine the energy of the frack.

The company has been using a method of "pushing back" on interference by injecting water into legacy wells while frack jobs take place at infill wells.

To achieve this, Abraxas has installed pressure transducers on its legacy wells to feed back one-second data in real time.

"We monitor and when we start to see these interactions we pump against them and can at least get fluid moving the right direction," Bommer said.

The process has succeeded at 40 legacy well sites so far, he added. The process has also proved economic.

One six-well Bakken project cost less than $50,000 per well, he said. "It was very successful and we were happy to spend that money and avoid multi-hundred thousand dollar cleanouts on these legacy wells," Bommer said.

Experts speaking at the forum called on players to monitor their wells in real time so they can react quickly at the first signs of well interference.

"Frack hits are probably impossible to eliminate entirely. We see these at such distances from the parent well that there is probably no way to avoid them completely," Duncan said.

"So instead, manage them, monitor them, try to minimise the effects, but then go on to model to see what the effects of the proximity of these wells is going to do for you economically."