Advanced Cement System for Acid Gas Injection Wells

Fakhreldin, Yasser Elmoghazy (PDO) | Al-Sharji, Hamed Hamoud (Petroleum Development Oman) | Al Ruwehy, Abdulah (Schlumberger) | Saadi, Khamis (Schlumberger) | Taoutaou, Salim | Al kalbani, Sultan


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.