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Petroleum Development Oman, PDO, is planning to improve ultimate recovery of condensate from a retrograde condensate gas field by reducing the rate of reservoir pressure decline. This shall be accomplished by re-injecting into the reservoir some of the produced gas and all of the acid gas extracted from the sweetening process. The composition of the injected gas will vary over time, from 15% CO2 and 3% H2S to 56% CO2 and 10% H2S. These combinations of CO2 and H2S can cause the wells cement to deteriorate. Portland cement tends to strongly degrade once exposed to such acid gases by reacting with calcium hydroxide formed from hydrated calcium silicate phases. As carbonates are dissolved in a low pH environment, the cement-carbonation products will not act as a self-plugging agent / s in the cement sheath. The resulting decrease of compressive strength and increase of permeability could lead to loss of zonal isolation and casing corrosion. These requirements led PDO to investigate and trial CO2-resistant cement to enable zonal isolation and ensure long term containment of the reservoir fluids. The nominated new technology cement system was trailed in a deep gas well which penetrated a reservoir which has high concentrations of CO2 and H2S at a super critical condition. The CBL/VDL log which was run after well completion showed excellent results. The well-cement quality shall be re-logged prior to any zonal shutoff work-over or well decommissioning. This paper will discuss the design, execution, and evaluation of the first acid gas resistant cement in PDO in one of the high profile gas well in South of Sultanate of Oman.
Petroleum Development Oman, PDO, is planning to improve ultimate recovery of condensate from a retrograde condensate gas field by reducing the rate of reservoir pressure decline. This shall be accomplished by re-injecting into the reservoir some of the produced gas and all of the acid gas extracted from the sweetening process. The composition of the injected gas will vary over time, from 15% CO2 and 3% H2S to 56% CO2 and 10% H2S. These combinations of CO2 and H2S can cause the wells cement to deteriorate. Portland cement tends to strongly degrade once exposed to such acid gases by reacting with calcium hydroxide formed from hydrated calcium silicate phases. As carbonates are dissolved in a low pH environment, the cement-carbonation products will not act as a self-plugging agent / s in the cement sheath. The resulting decrease of compressive strength and increase of permeability could lead to loss of zonal isolation and casing corrosion. These requirements led PDO to investigate and trial CO2-resistant cement to enable zonal isolation and ensure long term containment of the reservoir fluids. The nominated new technology cement system was trailed in a deep gas well which penetrated a reservoir which has high concentrations of CO2 and H2S at a super critical condition. The CBL/VDL log which was run after well completion showed excellent results. The well-cement quality shall be re-logged prior to any zonal shutoff work-over or well decommissioning. This paper will discuss the design, execution, and evaluation of the first acid gas resistant cement in PDO in one of the high profile gas well in South of Sultanate of Oman.
Iron sulfide is a common scale-formation in sour-gas wells that restricts tubular diameter, reducing well productivity. Compared to other scales, iron sulfide has unique risks associated with chemical removal. For example, due to the corrosiveness of hydrochloric acid (the most common chemical agent for both sulfide and carbonate scale removal), damage to the completion metallurgy at elevated temperature limits its applicability. Another main concern related to the use of acid for iron sulfide removal is the rapid generation of H 2 S byproduct and the risks associated with production of this toxic gas to the surface.Owing to H 2 S toxicity and the resultant elevated corrosion risk, new chemical solutions are needed for high-temperature FeS scale dissolution with low H 2 S generation. This study describes the development and characterization of a powerful noncorrosive solution for iron sulfide removal based on a chelating agent. Testing shows the fluid dissolution capacity under varied temperatures, scale-surface area, treatment fluid volume, and exposure time. Tests are also included showing the comparative benefits in dissolution capacity compared to other commercially used products such as diethylenetriaminepentaacetic acid (DTPA) and Tetrakis (hydroxymethyl) phosphonium sulfate (THPS). Finally, the mild-pH of the new chemical solution provides significantly lower corrosion rate.This work describes an altogether new family of chemicals for sulfide scale, providing high dissolution capacity, low corrosion rates, and limited generation of toxic H 2 S.
Hematite and hausmannite Ore ground are the most common material for weighting cement, it most adequately fulfills all the requirements and achieves the highest effective specific gravity, they usually dry blend with oil well cement to prepare high-density cement slurries. Cement sheath made of Portland cement and high-density additive of metal oxide such as Hematite and or hausmannite Ore ground decomposes much faster than cement sheath made of net or low Portland cement when it exposes to hydrogen sulfide (H2S) and or wet carbon dioxide (CO2) during the Well-Life. Hematite and hausmannite Ore ground's pre-densified cement sheath breaks down into many compounds through series of chemical reactions often involve an energy source that breaks apart the bonds of compounds. It decomposes into metal sulfide and or carbonate and calcium carbonate with a small concentration of iron sulfide as a result of the metal oxide and or Portland cement content sulfidation and carbonation reactions. Cement sheaths made of net, high, or low-Portland cement without high-density additive of metal oxide decomposition rate is less than cement sheath made of Portland cement with a high-density additive of metal oxide when it exposes to H2S and or CO2. H2S alone does not drastically decompose cement sheaths made of net, high, or low-Portland cement without high-density additive of metal oxide. H2S has limited impact on these cement sheath type as it reacts with iron(III) hydroxide [Fe(OH)3] in which is a part of 10% of the cement hydration products. Carbonic acid reacts with 90 % of the cement hydration products (calcium silicate hydrate and lime) and degrades cement sheath thoroughly. Despite that net cement-sheath does not drastically decompose when it exposed to H2S alone, it does when expose to CO2 alone and the decomposition rate increase in the as H2S concentration increase; unfortunately, it is not a case of if, it is a case of when cement sheaths with or without a high-density additive of metal oxide decompose due to the Carbonation and or Sulfidation reaction under wet CO2 and or sour environments. The cement sheath deterioration not only prevents well production or injection rate by reducing the inner diameter of the tubing and restrict access to the well with surveillance equipment for data acquisition but also damage the well-integrity and allow hydrocarbons along with H2S and CO2 to break through to the surface eventually. Immiscible gas injection is the worst case scenario ever, Portland cement sheath whether it contain a heavyweight additive of metal oxide or not, low-Portland or high Portland cannot sustain the acid gas, and it will lose its integrity. These wells have to fail in a short period due to the cement sheath deterioration. This paper discusses the durability of different oil well cement formulations under H2S / wet CO2 environments and demonstrates why some metal oxide containing cement more stable than others under sour CO2 reservoir downhole conditions.
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