INSIGHTS TO CONSIDER FOR IMPROVED PLUGGING PRACTICES IN THE APPALACHIAN BASIN

The Appalachian oil and gas basin has a history of over 150 years of drilling and producing hydrocarbons. This legacy has left oil and gas wells across the region needing to be plugged to mitigate leakage and reduce methane emissions. Long term isolation of plugged wells is important to prevent hydrocarbon fluid migration in the subsurface and to the surface. Code requirements (e.g. PA Chapter 78.71, API Recommended Practice 65-3) and common practices in the region, which includes Pennsylvania, Ohio, West Virginia, and New York, prescribe placing Portland cement in hydrocarbon-producing intervals and non-producing intervals with a gel composed of no less than 4% bentonite and water. The bentonite gel is placed in between the Portland cement intervals and should support the cement layer as it hydrates to fully isolate the wellbore. Our experimental results show that this mixture has a low viscosity and no yield stress and allowed the cement to fall through it, which makes proper placement of the cement in the required interval unlikely. The additional cost to remediate leaking plugged wells far exceeds the cost to plug a well. Ensuring that wells are plugged correctly and permanently for the long-term is essential to reduce emissions and prevent environmental issues.

The rheological properties of bentonite mixtures in water are also affected by typical oilfield brines if the concentration is high enough. Our studies show that the brines decreased the viscosity and yield stress of the bentonite and water plugging mixtures and reduced the ability to support a cement column in high enough concentrations. Experiments carried out with different concentrations of NaCl and CaCl₂ and different concentrations of sodium bentonite based plugging materials, indicate the critical concentration of these brines before the plugging material properties are adversely affected. Guidance and recommendations for field application of bentonite plugging materials in these environments will be discussed and provided.

Additional conditions of the wellbore environment are currently being included in experiments and recent results will be presented to provide additional recommendations for plugging best practices.