Drilling Fluid Additives for Wellbore Strengthening and Reservoir Protection

Abstract

The objective of the research was to optimise drilling fluid additives for wellbore strengthening, preventing lost circulation and avoiding drilling fluid induced formation damage. Industry standard testing, such as HTHP fluid loss tests following API 13B, yield limited insight into important areas such as wellbore strengthening and formation damage. Therefore, new testing methodologies were developed and evaluated. These provided new insight into important areas for designing and evaluating drilling fluids and drilling fluid additives for wellbore strengthening and reservoir protection. Key conclusions were that exposing particles to mechanical wear significantly impacted the relative performance of materials used for preventative treatments. Oil-based fluids were found to create a high-degree of internal formation plugging, whereas water-based fluids more predominantly isolated the wellbore pressure from the pore-pressure though an external filter-cake. Inclusion of cellulose based fibres where the D90 value value ⪞ 3/2 the median pore size was shown to reduce internal plugging and reduce formation damage, in both water-based fluids and oil-based fluids. Particle degradation studies showed that CaCO3 degraded rapidly for particles > 23 μm and that the most wear resistant particles were selected cellulose-based materials. Combinations of fine CaCO3 and slightly coarser cellulose mixtures were found to be effective for creating low-permeability filter-cakes and preventing formation damage. For preventative treatment in drilling conditions with large differences between the matrix pore-size and the aperture of natural or induced fractures, a dual mode particle size distribution was found to be effective in both laboratory studies and field applications. In such situations, the fine mode of the PSD provided low filter-cake permeabilities when the particles followed an Andreasen distribution with a packing factor of around 0.08-0.10. Natural and induced fractures were most effectively sealed when granular cellulose particles made up the coarse mode of the PSD and these particles were sized similar to or slightly larger than the fracture aperture.