Extreme Limited-Entry Perforating Enhances Bakken Completions
Authors: Carpenter Chris 
Published: October 2018

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 189880, “Mining the Bakken II—Pushing the Envelope With Extreme Limited-Entry Perforating,” by Paul Weddle, SPE, Larry Griffin, SPE, and C. Mark Pearson, SPE, Liberty Resources, prepared for the 2018 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 23–25 January. The paper has not been peer reviewed.
This paper presents the evolution of a Bakken advanced completion design with the added enhancement of extreme limited entry (XLE) perforating. With this strategy, an operator has consistently stimulated more than 11 perforation clusters per stage. The high number of active clusters, or fracture-initiation points, has been measured directly with radioactive tracers and fiber-optic diagnostics and is validated through improved production relative to offset completions.
XLE
The technique, as the name suggests, pushes the level of perforation friction past 1,500 psi. Additional sources of pressure variations have been identified and can be summed up as the fracture-entry pressure. Fracture-entry pressure, defined as the pressure from immediately outside the perforations to the fracture tip, includes, but is not limited to, near-wellbore tortuosity, net pressure, stress shadowing, and fracture extension.
To advance the goal of evenly diverting stimulation fluid to all the fracture-initiation points (clusters), the designed perforation friction must moderate perforation-/cluster-level pressure variations in the fracture-entry pressure. To achieve this, the magnitude of the sources of variability should be quantified and addressed with the completion and perforating design. Additionally, one must account for the dynamic fracturing process, within which most of the fracture-entry-pressure parameters, as well as the perforations, are changing as the stimulation progresses.
The primary sources of variability to be considered when designing for XLE are as follows:
Minimum horizontal stress variability along the lateral; 90% of a Bakken lateral is within a 750-psi range.
Near-wellbore friction variations, stage to stage and perforation to perforation. Stepdown tests demonstrate a P50 of 625 psi for starting near-wellbore friction.
Stress shadowing between active clusters. The magnitude of stress shadowing is dependent on formation properties, completion-design parameters, and cluster spacing. For design purposes, it is assumed to be approximately 200 psi.
Fracture-extension-pressure variability based on changes to net pressure.
Perforation-friction initial-condition variability may limit ability to reach design pump rate. Perforation diameters at all possible orientations must be considered, along with the number of perforations open vs. the number shot.
Perforation friction changes during a fracturing stage of approximately 500 psi caused by rounding and erosion of perforations and possible loss of clusters because of proppant settling in the liner.
For the Bakken, a minimum 2,000-psi perforation friction is targeted at the beginning of the job, with the expectation that, as perforations round and erode, it will remain greater than 1,500 psi at the end of pumping to account for all possible fracture-entry pressure variations.