Author: Erik Engelcke
Refracs in horizontal shale wells have evolved significantly in recent years and are today most successfully performed by way of the casing-in-casing method (1). Perhaps the most compelling reason for implementing a refrac program is the prospect of mitigating the parent – child well performance degradation effect. It is widely accepted that early horizontals were understimulated compared to today’s methods; but does frac intensity tell the whole story? Perforating designs have also evolved significantly over the last decade with most operators now favoring geometric limited entry or extreme limited entry perforating schemes to maximize cluster efficiency. As casing-in-casing refracs are typically designed much like an initial completion, the perforating design is commonly based on modern initial completion practices. QReFrac offers a method for optimizing perforating designs to overcome the intra-stage stress contrasts, which are magnified in refrac subject wells that had low cluster efficiency during the initial completion.
An underappreciated challenge for constructing perforating designs for refracs is achieving optimal cluster efficiency when perforating previously unstimulated sections and depleted sections of the lateral for treatment in the same refrac stage. Let’s recall the central theme of the limited entry technique, as first described in the work of Lagrone and Rasmussen and subsequently widely implemented in vertical completions (2). The objective of limited entry perforating is to limit the entry of the treatment fluid into the lower stress zones, when treating multiple zones with significant stress contrasts in one stage, by designing higher perforation friction for the low stress zones and lower perforation friction for the high stress zones.
Perhaps the same way that the frac design evolution has come full circle to today’s slickwater and sand designs, limited entry might find its way back to its original spirit. In order to design a limited entry perforating scheme for a casing-in-casing refrac, a minimum horizontal stress log of the lateral must be used to design the appropriate number of perforations for each cluster. An important design consideration is an appropriate assumption of which clusters were previously stimulated and which clusters were not stimulated during the initial completion.
Quantico’s QReFrac service offers E&Ps a new, cost effective AI tool for refrac design optimization. By using our patented QLog technology, we generate sonic log predictions (not merely an MSE-based log) for refrac subject wells from the data gathered during the initial drilling of the well. This adds a valuable input for refrac candidate selection and design. When examining the likely cluster efficiency of the initial completion, actionable insights can be gained for designing an effective limited entry refrac. It is not unthinkable that perforation designs, much like frac designs did, could come full circle and evolve from modern limited entry to an “engineered geometric completion”. While the practice of grouping clusters in like-rock, commonly referred to as an engineered completion, aims to increase cluster efficiency by minimizing stress contrast, our method optimizes a geometric completion by designing each cluster’s entry friction with respect to the geomechanical properties at its location in the lateral. When contemplating modern perforating designs, it is easy to forget that the number of perforations per cluster (or pay zone), not the flow rate per perforation, was the primary “design knob” for the technique that was referred to for over a half of a century as limited entry (3).
1. R. J. Cadotte, Brittany Elbel, and Neil Modeland. 2018. Unconventional Multiplay Evaluation of Casing-in-Casing Refracturing Treatments. Presented at the SPE International Hydraulic Fracturing Technology Conference and Exhibition held in Muscat, Oman, 16 - 18 October 2018. SPE-191451-18IHFT-MS
2. Lagrone and Rasmussen. 1963. A New Development in Completion Methods-The Limited Entry Technique. Journal of Petroleum Engineers. 15 (07).
3. Economides and Nolte. 2000. Reservoir Stimulation, 3rd Edition (3rd ed.). Wiley.