Mud-to-cement conversion of synthetic-based drilling muds using geopolymers
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When constructing wells ranging from simple land wells to complex deepwater wells, incompatibility between oil-based and synthetic-based muds (OBM / SBM) and Portland cements can lead to poor cementation and loss of cement integrity, which in turn may compromise zonal isolation. An alternative cementitious material based on geopolymers has been developed with improved OBM / SBM compatibility for primary cementing and lost circulation control as well as well abandonment. Benefits of using geopolymers go beyond mere OBM / SBM compatibility: it is in fact possible to solidify non-aqueous drilling fluids (NAF) such as SBM and OBM using geopolymer formulations. This also means that such NAFs can be disposed of in a more cost-effective way, which presents a viable option for environmentally acceptable on-site or off-site disposal of drilling muds and cuttings. In the following, focus will be primarily on the compatibility between SBM and geopolymers, with the understanding that the results obtained for SBM can generally be extrapolated to OBM as well. Geopolymer is a type of alkali-activated material that forms when an aluminosilicate precursor powder (such as fly ash) is mixed with an alkaline-activating solution (such as sodium hydroxide). A novel SBM solidification method was developed by blending varied amounts of geopolymer and SBM. The consolidated mud was named a “geopolymer hybrid cement”. In an effort to develop the geopolymer hybrid system as a novel well cementing material, the solidification method was comprehensively studied with various sources of precursor powders, activators, as well as SBM and OBM formulations. Fresh state properties, such as slurry rheology and thickening time, and hardened state mechanical properties, such as compressive strength (under both uniaxial and triaxial confinement conditions), as well as the self-healing capabilities of the geopolymer hybrid cement were evaluated. Strength testing results showed that geopolymer cement can solidify up to a 60/40 geopolymer/SBM ratio by volume. The incorporation of SBM greatly improved the rheological properties of the geopolymer hybrid, allowing for the otherwise non-pumpable slurry to become pumpable for well cementation and lost circulation control purposes. The laboratory evaluations showed that the geopolymer hybrid cement could meet typical requirements as a well cementing slurry. By changing the amount of geopolymer and SBM in the slurry, the geopolymer hybrid can be deliberately designed with high compressive strength for primary cementation, or with lower compressive strength for lost circulation control. Moreover, geopolymer and geopolymer hybrid cements reveal true self-healing capability, which means that they can recover and even increase their strength after prior yielding. This ability would possibly allow such cements to better adapt to subsurface stress changes acting on abandoned wells, making them better suited for use in permanent barriers in plug and abandonment operations.