The present invention relates generally to methods of coating solid particulates used in wellbore fluids. In particular, the present invention relates to methods of coating solid particulates used in wellbore fluids using the sol-gel process.
Wellbore fluids contain a variety of solid particulates as weighting agents, loss circulation materials, bridging agents, and lubricating beads. These solid materials have different purposes in the fluid system, but all of them typically need to be modified at some point to improve performance. For instance, weighting agents can be modified to be more compatible with a base fluid, to reduce abrasiveness, or to reduce sedimentation. Loss circulation materials can be treated to prevent disintegration while circulating in the wellbore. Surface treatment of lubricating beads can also be done to reduce their coefficient of friction.
Improving the properties of these solid particulates can be carried out by modifying the surfaces of the particulates through coating. Several methods are known for coating solid particulates, such as using a crosslinkable polymer material to coat the surface, grafting a silanated material, and dry blending with organic dispersants.
In drilling fluids, weighting agents are widely used to control formation pressures, prevent formation caving, and facilitate the pulling of dry pipe. For many years, barite has been the standard weighting agent in the drilling fluid industry. Dwindling supplies and increased consumption of premium barite, however, has led to significant price increases and a reduction in specific gravity in most commercial grades of barite. Other minerals, such as ilmenite and hematite, have been used in the field, but their relatively high hardness can give rise to abrasion/erosion in tubular and surface equipment.
Moreover, weighting agents are commonly added to drilling fluids to give sufficient density to the fluid to hinder inflow of oil or gas into the wellbore from the downhole formation. Sedimentation of weighting agents from the drilling fluids or “sag” is one of the biggest challenges within the drilling fluid industry as it can lead to well control problems, lost circulation, and/or hindered running of casing/liner.
Sag is influenced by a variety of factors related to drilling fluid conditions such as low shear conditions, drill string rotations, drilling fluid formulations and properties, and mass of weighting agents. For drilling fluids utilizing particulate weighting agents, differential sticking or a settling out of the particulate weighting agent is known to occur.
Ilmenite and hematite are two minerals that are used in the field. Their higher density compared to barite, however, impacts both the rheology of the fluids and the settling rate of the weighting material. Because of the mass of the weighting agent, various additives, such as gelling agents and rheology modifiers, are often incorporated to produce rheology that will be sufficient to allow the wellbore fluid to suspend the material without settlement or sag under static and dynamic conditions. The addition of these materials, however, greatly affects fluid viscosity.
Thus, there is a continuing need for improved methods and compositions for coating solid particulates used in wellbore fluids. In particular, there is a need for methods and compositions to reduce abrasiveness and prevent sedimentation of weighting agents.