throats, whole mud may invade the formation, depending on mud solids size; Use bridging agents to block large opening, then mud solids can form seal. For effectiveness, bridging agents must be over the half size of pore spaces / fractures. Bridging agents (e.g. calcium carbonate, ground cellulose). Depending on the mud system in use, a number of additives can improve the filter cake (e.g. bentonite, natural & synthetic polymer, asphalt and gilsonite). Maintain wellbore stability Source: Chemical composition and mud properties must combine to provide a stable wellbore. Weight of the mud must be within the necessary range to balance the mechanical forces. Wellbore instability = sloughing formations, which can cause tight hole conditions, bridges and fill on trips (same symptoms indicate hole cleaning problems). Wellbore stability = hole maintains size and cylindrical shape. If the hole is enlarged, it becomes weak and difficult to stabilize, resulting in problems such as low annular velocities, poor hole cleaning, solids loading and poor formation evaluation In sand and sandstones formations, hole enlargement can be accomplished by mechanical actions (hydraulic forces & nozzles velocities). Formation damage is reduced by conservative hydraulics system. A good quality filter cake containing bentonite is known to limit bore hole enlargement. In shales, mud weight is usually sufficient to balance formation stress, as these wells are usually stable. With water base mud, chemical differences can cause interactions between mud & shale that lead to softening of the native rock. Highly fractured, dry, brittle shales can be extremely unstable (leading to mechanical problems). Various chemical inhibitors can control mud / shale interactions (calcium, potassium, salt, polymers, asphalt, glycols and oil – best for water sensitive formations) Oil (and synthetic oil) based drilling fluids are used to drill most water sensitive Shales in areas with difficult drilling conditions. To add inhibition, emulsified brine phase (calcium chloride) drilling fluids are used to reduce water activity and creates osmotic forces to prevent adsorption of water by Shales. Minimizing formation damage Source: Skin damage or any reduction in natural formation porosity and permeability (washout) constitutes formation damage skin damage is the accumulation of residuals on the perforations and that causes a pressure drop through them. Most common damage; Mud or drill solids invade the formation matrix, reducing porosity and causing skin effect Swelling of formation clays within the reservoir, reduced permeability Precipitation of solids due to mixing of mud filtrate and formations fluids