While oil and gas companies work to drain reservoirs of every possible barrel by drilling thousands of infill, or child, wells, they have found many of those infill wells communicate or “hit” parent wells. The effect often leads to decreased production in parent wells—up to 30%, according to Schlumberger—declines in EUR or, in some cases, damaged well equipment.
Suffice it to say, frac hits are a problem that has the industry’s attention. It was telling that a primary theme at February’s annual Hydraulic Fracturing Technology Conference and Exhibition (HFTC) in The Woodlands, Texas, was on parent/child well interactions—at least 10 SPE papers addressed the topic.
So far, the causes of well interaction have been identified as ones of proximity and volume.
“More infill drilling, more stages, high sand [per] lateral foot completions, tighter cluster spacing—these are among the reasons frac hits are becoming a persistent problem,” wrote Yvonne Scherzer and others from Endeavor Resources in their study of frac communication in the Midland Basin.
Meanwhile, Tao Xu, et al., of Schlumberger found that, normalized by total proppant and lateral length, parent wells in the Delaware Basin located less than 244 m (800 ft) from a child well saw a 91% chance of experiencing a frac hit. The chances fell the farther apart the wells were spaced, but the odds were still high—81% for 366-m (1,200-ft) and 488-m (1,600-ft) spacing, and 79% for well spacing greater than 488 m.
The development trend of infill wells shows no signs of slowing down. Since 2011 the number of new parent wells drilled in the Midland Basin has declined by about 53%, while the number of new child wells has increased by 450%, according to Xu. In the Delaware Basin, new parent wells during the same period have declined by 36%, while new child wells have grown by 150%.
Fortunately, it appears the industry is beginning to get a grasp on solutions. One of the key indicators of an impending frac hit is a pressure increase on the parent well. Several of the presenters at the HFTC suggested real-time pressure monitoring systems on parent wells.
Other successful applications companies reported include refracturing parent wells, applying far-field diversion to child wells and wider well spacing.
Mary Garza, completions engineer for the Eagle Ford at Noble Energy, discussed her company’s efforts to refrac two parent wells with diversion following the completion of an infill well. Garza said the efforts resulted in a 40% improvement in EUR in one of the parent wells but a breakeven EUR in the other.
It’s not likely frac hits can be eliminated completely—the rapid growth in the number of infill wells and tighter well spacing make it nearly impossible—but the industry seemingly has a firm grasp on their causes. And like any other challenge in the unconventional world, any solution needs to come with scalability. While only select wells might be eligible for refracs, pressure monitoring and diverter systems are gaining in popularity.
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