Dissolvable frac plugs (DFPs) can eliminate milling out, enable longer lateral wellbores, and reduce operational risks. Although DFPs have been successfully used in many cases, uncertainty about their degradation has also been known. For example, some DFPs have not degraded under certain conditions even though many of the conditions were similar to those under which successful degradation occurred. We report the degradation behaviors of dissolvable materials used in DFPs to understand the reasons for the uncertainty and to share our knowledge about how to deploy DFPs under various conditions to maximize the benefits of DFPs.
The two major dissolvable materials for DFPs, polyglycolic acid (PGA) and a dissolvable Mg alloy, were chosen for the degradation study. The effects of various factors, such as temperature, salinity, and chemicals added to the fracturing and pump-down fluid, were investigated to understand the degradation behaviors comprehensively. The temperature range was 50 – 150 °C (120 – 300 °F), which covers the bottom hole temperature range in most unconventional areas. The salinity dependence study considered species such as sulfate and bicarbonate in addition to chloride because these are also commonly found in actual water sources. Other chemicals studied were biocides, friction reducers, and scale inhibitors, which are often added to fracturing and pump-down fluids.
The study results were as follows. (1) PGA had a simpler temperature dependence than the Mg alloy. The degradation rate of PGA was proportional to temperature, whereas the Mg alloy had a degradation rate peak and showed a sharp rate decrease at high temperature in fresh water. (2) The degradation rate of PGA was stable and was not affected by salinity level, ionic species, degradation byproduct, and chemicals. (3) Mg alloy degradation had high salinity dependence, as expected. Each ionic species (chloride, sulfate, bicarbonate) had a different effect on the degradation and a thorough analysis of water is needed to predict Mg alloy degradation accurately. (4) The Mg alloy degradation rate was affected by chemicals. Certain chemicals, such as biocides and friction reducers, may decrease the rate in fresh water.
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