Sustained Anulus Pressure in NoC Wells. Prevention and Remediation Using Polymeric Isolating Materials

Abstract

Sustained Annulus Pressure (SAP) is a common issue that affects shale plays wells worldwide. The cement sheath barrier deteriorates due to extremely high hoop stresses generated in the cement annulus during fracture treatments. These stresses are approximately one order of magnitude higher than the cement maximum tensile stress. Once cement sheath fails, gas or oil migrates from high-pressured subsurface formations through vertical cracks along the annulus. Pressure at the wellhead can reach values above the installation maximum rate which persistently rebuilds after bleed-down operations. The presence of SAP means that well integrity has been compromised generating serious safety and environmental risks for operators. After conducting several finite element simulation studies, that demonstrated the failure hypothesis, a research project was conducted to design an isolating material that would withstand service conditions generated during well termination and its overall life cycle. A polymeric sealing ma terial with high deformation capacity, consequently low Young's modulus, high toughness and bonding properties was developed. The material was tested following API recommended practices and ASTM standards. A procedure to prevent SAP in new wells was proposed. Five well of different Vaca Muerta shale fields were isolated combining a cement slurry batch with the polymeric system pumped in tandem. Also, a repair procedure for wells showing SAP was proposed and successfully tested using a similar polymeric sys tem. This procedure consisted of injecting a 14.8 ppg formulation in the annulus in a pump and relief (P&R) manner. The pilot test is currently under evaluation. Wells isolated with the polymeric system have not presented SAP after more than six months, nor have the two remediation jobs conducted so far. Annular pressure of new and repaired wells is being monitored to determine treatment effectiveness.