Investigation of a Long-Term Cement Deterioration Under a High-Temperature, Sour Gas Downhole Environment

Abstract

Abstract The indications of cement deterioration at one of the greatest gas condensate fields in Pannonian basin (Croatia) was the motivation for research study carried out in order to investigate the possible involved mechanism of impairment. After almost 15 years of hydrocarbon production form naturally fractured carbonates, some wells begun to suffer of plugging across the perforated zone with unidentified debris, which was accompanied with water cut increase. This paper describes the laboratory evaluation of the actual environmental downhole impact (high reservoir temperature, BHST > 180°C, sour gas: 22% CO2 and 150 ppm H2S) to the cement, as well as debris identification. The work indicated that oilwell cement under hostile downhole conditions after long-term exposure could be deteriorated loosing of compressive strength and its integrity. The dominant mechanism of cement deterioration is caused by CO2 corrosion process in the form of carbonic acid leaching. The CBL log data verified the cement disintegration behind 7-in. casing after 15 years of well production period.