Maximizing Refrac Treatment Recovery Factors in Organic Shales Using Expandable Liners and the Extreme Limited Entry Process
Authors: Barba Robert , Villareal Mark 
Published: September 2019

With the industry shifting gears toward pad development there has been a significant increase in operator press releases to stockholders expressing concern about fracture driven interactions (formerly called "frac hits") within a drilling spacing unit (DSU) (Triepke 2018). Primary wells (formerly called "parents") (Daneshy 2019) are the initial wells on the pad and infill wells (formerly called "children") are all those that follow on the pad or an adjacent pad. Failure to protect the primary well from infill well fracture driven interactions can result in up to 40% EUR losses in infill wells from asymmetric fractures (Elliott 2019)(Ajisafe et al 2017). Adverse frac interactions between wells in a DSU can be largely eliminated with a combination of primary well refracs and infill well zipper fracs. In the primary well protection process there is a movement away from "preloads" as the overall results from the preloads to date suggest they are not effective in preventing infill well frac asymmetry unless the primary well can be restored to its original stress conditions. A number of operators have announced plans in press releases to increase well spacing in the DSUs to reduce well to well interference. A number of of organic shale operators have also announced performance related reserve write downs according to a March 13, 2019 Simmons Energy report (Harrison and Todd 2019). While in some cases the writedowns were due to changes in pricing expectations, the combination of a known reserve bashing situation and numerous operators still relying on preloads for parent protection raises a red flag. It is highly likely that there is a relationship between DSUs that use preloads instead of refracs for primary well protection and poor overall performance from the DSU. It was proposed in the keynote address at a recent primary-infll frac interaction conference that refraccing primary wells is significantly more effective than preloading them in preventing large infill EUR losses (Elliott 2019) (Figures 1 and 2). Figure (3) has a microseismic interpretation of an infill well assymetric frac offsetting a primary well with no refrac. The stranded hydrocarbons are clearly where there is no microseismic activity. For a DSU with 600,000 BO wells the combination of the 40% infill well EUR loss and the loss of up to two PUDs per DSU can be in the $29 million range so this is hardly an academic exercise.

Figure 1 Depletion Mitigation Opportunities Figure 2 Depletion Mitigation Results Figure 3 Infill Well Asymmetric Frac in Toe Stage with Depleted Primary Well Overlap

Historically, refrac operations in horizontal organic shale wells have had unpredictable production results, with the industry moving toward mechanical isolation following an often painful history that included single stage "pump and really pray" treatments with no diversion to "pump and pray" with chemical or ball sealer diversion. While results from mechanical isolation have been more consistent than these first two methods (Cadotte et al 2018), there is now a lot of discussion on the best mechanical isolation method to use. The two most common isolation techniques are cemented conventional casing and expandable liners. The main advantage of the cemented casing is lower up initial costs, with a $123,000 difference in cost before frac operations commence for a 5000 ft refrac liner. The main advantage of the expandable liner is a larger diameter that allows for 20% to 25% higher pump rates. With the combination of the Extreme Limited Entry (XLE) completion technique and expandable liners the higher treatment rates translate directly into longer stage lengths while still maintaining high cluster efficiency. The resulting lower stage count reduces the overall stimulation cost well below the incremental initial cost of the expandable liner, with a net savings of $446,000 per refrac over the cemented liner option for a 5000 ft lateral. The savings would be higher for longer laterals as the stage number difference will increase.