Flow behavior of nano-silica solutions in cement cracks/fractures

Abstract

One of the most prevalent causes of failure in oil and gas wells is the formation of cracks in the wellbore cement. The cracks can lead to loss of zonal isolation and fluid leakage to the surface. Compared to cement slurry which is difficult to squeeze into the cracks due to its high-solid content, nanoparticle (NP) gels have gained significant attraction for sealing fractures/cracks in oil and gas wells. It is highly desirable to know the flow behavior of NP solutions in the cracks. This study investigated the flow of nano-silica solutions through narrow rectangular cross-sections simulating sealant flow in cracks of oil well cement sheath. Pressure gradient was tested at 4 flow rates for various aspect ratios of the rectangular cross-sections. For a given cross-section of conduit, pressure drop increased with flow rate. For a given conduit height, the pressure drop decreased with conduit width due to increased flow area. Pressure drop increased as the conduit height reduced. These characteristics of nan-particle solution flow are like those found for conventional fluids. The pressure gradient of nano-silica solution flow in a conduit of a given rectangular cross-section is nearly proportional to flow velocity at tested flow rates, indicating laminar flow conditions in the tested flow conditions. The proportionality factor (rate of increase) increases as the aspect ratio of the cross-section decreases (slot becomes narrower). This is because a system with a lower aspect ratio has four boundaries that generate a 2-dimensional velocity profile, while a system with a higher aspect ratio has essentially two boundaries that generate a 1-dimensional velocity profile. More boundaries mean more frictional resistance to fluid flow. Hydraulics models for conventional fluid flow through conduits of rectangular cross-sections under-predict the pressure gradient of nano-silica solution. The discrepancy/difference is over 90% in pressure gradient prediction in general. The hydraulics model presented by Guo et al. (2022) with aspect ratio is better than the classical hydraulics model with hydraulic diameter. However, both models fail to predict the pressure gradient along the conduits with slot-like cross sections. The excessive pressure drop is attributed to the interactions between the nano-particles and conduit walls. Further investigations are required to gain a thorough understanding of nano-silica flow.