Experimental study of genesis of a geologically activated and self-sealing cementing material for deep wellbore plugging and abandonment under acidic conditions

Abstract

Harsh subsurface conditions pose a widespread challenge for permanently plugging and abandoning (P&A) oil and gas wells. A new, cement-free plugging material is proposed and demonstrated using nature-inspired geochemical processes to generate stable and resilient, rock-like material that is ideally suited to acidic geofluid, high-pressure, and high-temperature (AG-HP-HT) conditions. These conditions accelerate hydration and carbonation reactions that turn granular ultramafic raw materials into competent rock, dubbed here as “Geologically Activated Cement” (GAC). The experiments firstly demonstrate the generation of GAC in an acidic environment, using a pressure of 13.7 MPa and temperature of 180 °C as typical representative values relevant to P&A operations in high temperature reservoirs. The results show that within hours to days, and hence relevant to P&A operational timeframes, a solid, rock-like material can be produced from Mg2SiO4–CO2 mixtures. The reaction products are analyzed by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and Energy-dispersive X-ray spectroscopy (EDS). These analysis shows the rock to be comprised of unreacted olivine particles that are bound together by magnesium carbonate. Because of the presence of unreactive olivine, the reaction is set in motion again by exposing the olivine to CO2-enriched fluid by thermally damaging (cracking) the sample. The damaged material is then subjected to flow-through of carbonic acid for 8–14 hours. Under these conditions, GAC experiences a 75% reduction in permeability over the hours immediately following damage. Control experiments using class H cement experience steady to slightly increasing permeability under the same conditions and over a similar time frame. We therefore show that GAC can self-seal cracks within a few hours when subjected to damage and subsequent flow of acidic fluid under HPHT conditions. This self-sealing behavior provides the potential for a resilient cementing system over geological timescales.