•Formation Damage:
–Damage to the productivity of a well resulting from invasion into the formation by mud particles or mud filtrates. –Asphalt from crude oil will also damage some formations. •Common mechanisms for formation damage are: –Mud or drill solids invading the formation matrix, plugging pores. –Swelling of formation clays within the reservoir, reducing permeability. –Precipitation of solids as a result of mud filtrate and formation fluids being incompatible. –Precipitation of solids from the mud filtrate with other fluids, such as brines or acids, during completion or stimulation procedures. –Mud filtrate and formation fluids forming an emulsion, restricting permeability. •Prevention –Formation damage can be minimized by using a Drill In fluid •Drill In Fluids should contain non-damaging polymers, bridging agent •Should have superior regain permeability •May have shale or clay inhibitors •Should be easy to clean up Corrosion
•Corrosion is the destruction of metal through electrochemical action between metal and its environment. •Corrosion can be costly in terms of damage to pipe and well parts and can even result in the loss of an entire well. •About 75 to 85 percent of drillpipe loss can be attributed to corrosion. •Other areas affected by corrosion include pump parts, bits, and casing. •Factors affecting corrosion include: –Temperature. Generally, corrosion rates double with every 55°F (31°C) increase in temperature. –Velocity. The higher the mud velocity, the higher the rate of corrosion due to film erosion (oxide, oil, amine, etc.). –Solids. Abrasive solids remove protective films and cause increased corrosive attack. –Metallurgical factors. Mill scale and heat treatment of pipe can cause localized corrosion. –Corrosive agents. Corrosive agents such as oxygen, carbon dioxide, and hydrogen sulfide can increase corrosion and lead to pipe failure. •Types of Corrosion –Uniform corrosion •Even corrosion pattern over surfaces –Localized corrosion •like corrosion pattern over surfaces –Pitting •Highly localized corrosion that results in the deep penetration of surfaces •Corrosive agents found in drilling fluids include: –Oxygen –Hydrogen sulfide –Carbon dioxide –Bacteria –Dissolved salts –Mineral scale Corrosion -Oxygen
•Oxygen causes a major portion of corrosion damage to drilling equipment. •Oxygen acts by removing protective films; this action causes accelerated corrosion and increased pitting under deposits. •The four primary sources of oxygen are: –Water additions –Actions of mixing and solids-control equipment –Aerated drilling fluids –The atmosphere •If oxygen corrosion is suspected treatment would include adding an oxygen scavengers –Many types of oxygen scavengers exist –Manufactures recommended treatment should be followed in this case Corrosion -Hydrogen sulfide
•Hydrogen sulfide can enter the mud system from: –Formation fluids containing hydrogen sulfide –Bacterial action on sulfur-containing compounds in drilling mud –Thermal degradation of sulfur-containing drilling fluid additives –Chemical reactions with tool-joint thread lubricants containing sulfur •Hydrogen sulfide is soluble in water. •Dissolved hydrogen sulfide behaves as a weak acid and causes pitting. •Hydrogen ions at the cathodicareas may enter the steel instead of evolving from the surface as a gas. •This process can result in hydrogen blistering in low-strength steels or hydrogen embrittlement in high-strength steels. •Both the hydrogen and sulfide components of hydrogen sulfide can contribute to drillstring failures. •Hydrogen sulfide corrosion is mitigated by increasing the pH to above 9.5 and by using sulfide scavengers and film-forming inhibitors. –Sulfide scavengers include Zinc Carbonate, Zinc Oxide and other specialty chemical products –Most film forming inhibitors are amine inhibitors, many are available Corrosion –Carbon Dioxide
•Carbon dioxide is found in natural gas in varying quantities. •When combined with water, carbon dioxide forms carbonic acid and decreases the water's pH, which increases the water's corrosivity. •While carbon dioxide is not as corrosive as oxygen, it can cause pitting. •Maintaining the correct pH is the primary treatment for carbon dioxide contamination. •Either lime or caustic soda can be used to maintain pH. Corrosion –Bacteria
•Microorganisms can cause fermentation of organic mud additives, changing viscosity and lowering pH. •A sour odor and gas are other indicators that bacteria are present. •Degradation of mud additives can result in increased maintenance cost •Microbiocidesare used to control bacteria in drilling environments Corrosion –Dissolved Salts
•Dissolved salts increase corrosion by decreasing the electrical resistance of drilling fluids and increasing the solubility of corrosion by-products. •Some of these byproducts can cause a scale or film to form on the surface of the metal. •Amine filming agents added to the metal will aid in reducing corrosion due to dissolved salts •Mineral scale deposits set up conditions for local corrosion-cell activity. •The continuous addition of a scale inhibitor can control the formation of scale deposits.