Use of Biodegradable Materials During Refracturing of a Horizontal Well
Authors: Carpenter Chris 
Published: March 2016

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 171015, “Eco-Friendly Biodegradable Materials for Zonal Isolation of Multiple Perforation Clusters During Refracturing of a Horizontal Well: Case History From Marcellus Shale Hydraulic Fracturing,” by David Arnold, Anastasios Boulis, and Francisco Fragachan, Weatherford, prepared for the 2014 SPE Eastern Regional Meeting, Charleston, West Virginia, USA, 21–23 October. The paper has not been peer reviewed.
One approach to refracturing involves the use of mechanical diverters such as perforation balls and rock salts, which may be useful for one stage; however, cessation of pumping operations results in these diverters falling out of the perforations or dissolving. In this paper, a new approach is presented that uses degradable materials. These materials can be tailored to plug perforations on a temporary basis but degrade over time depending on formation temperature.
Introduction
There are several operational approaches to refracturing wells, some of which require sealing off existing zones and losing the previous-fracturing-treatment production, while others require installing sophisticated downhole tools. These include sliding-sleeve systems, coiled-tubing tools, and perforation-squeeze systems (please see the complete paper for details of these approaches). A simpler approach, considered a proven method of sealing perforations and zones, is the use of mechanical diverters. Traditionally, this is accomplished by pumping ball sealers or degradable balls in the fracturing fluid to seat on the perforations, diverting fluid away from the sealed perforations. Operationally, the perforation-ball method is very easily deployed, needing only a ball gun on the surface. These materials can also be pumped at designated times during the treatment. The main drawback of the method is the multizonal nature of wells. Constant pressure is needed to maintain the balls seated on the perforations; if at any point the treatment needs to be shut down (i.e., for equipment maintenance), these balls will unseat.
French et al. (2014) presented the results of refracturing treatments that used perforation balls and biodegradable-ball sealers as a means of temporary perforation-cluster isolation. While the materials were operationally successful, there were challenges and concerns related to their use. According to that paper, “In a horizontal well, achieving efficient seating action can be especially challenging, given that the ball sealers may settle to the bottom of the casing during the pumping of the treatment; this is especially true when the stimulation treatment suffers any form of interruption or drop in the velocity below critical flow. … Furthermore, if the hydraulic fracturing operation is temporarily halted [because of] operational or mechanical issues, then much of the previous ball diversion effect that was achieved could be subsequently lost.” There are other mechanical diversions that are commonly used such as rock salt, benzoic-acid flakes, and other solid substances that will enter the perforations and near-wellbore fractures, allowing a bridging effect. The drawback of these materials is that they are short-lived and will degrade, possibly before all fracturing treatments have been completed.
Ideally, a method of achieving zonal isolation and diversion that incorporates the simple nature of mechanical diverters should be applied that will enter the perforations and that will allow for shutdowns when necessary and will feature a lifespan that is long enough to permit the entire well to be completed.