Mud-to-cement conversion of non-aqueous drilling fluids using alkali-activated fly ash

Abstract

Driven by the needs for reduced waste disposal and a sustainable environment, many efforts have been made to recycle coal combustion products. Fly ash, for instance, has been well-researched and successfully re-used in various industries and fields. Despite the potential for various applications, only 1% of the recycled fly ash has been directed to the oil and gas industry. The present study evaluated the feasibility of using alkali-activated fly ash (AAF) as an alternative cementitious material in well cementing applications.The key finding of this study is that AAF slurries provide improved compatibility with oil-based and synthetic-based drilling muds (OBM/SBM) in comparison to Portland cement slurries. Benefits of using AAF go beyond mere OBM/SBM compatibility: it is in fact possible to incorporate large amount of non-aqueous drilling fluids (NAF) such as SBM and OBM into AAF formulations while maintaining necessary well cement characteristics. This also means that NAF-associated waste 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 present work, a novel mud-to-cement conversion method was achieved by blending varied amounts of AAF and SBM. The consolidated AAF/mud mixture was named “AAF hybrid cement”. In an effort to develop the AAF hybrid system as a novel well cementing material, fresh state properties, such as slurry rheology and thickening time, and hardened state mechanical properties, such as compressive strength, were comprehensively studied. The laboratory evaluations showed that the AAF hybrid cement could meet the typical characteristics and requirements of a well cementing slurry. By changing the relative amount of AAF and SBM in the slurry, the AAF hybrid can be deliberately designed with high compressive strength for primary cementation, or with lower compressive strength for lost circulation control.