Expanding Cement Application for High Rate Gas Wells in South Sumatra

Abstract

The cement sheath of an oil or gas well is designed to prevent fluid communication between the drilled formations during the productive life of the well through post-abandonment. However, even when cement initially provides good hydraulic seal, zonal isolation can deteriorate after initial placement due to microannuli formation. In many cases, a microannulus can be formed because of wellbore temperature changes, wellbore pressure changes, or set cement bulk shrinkage.Microannuli are considered serious concern in gas wells because, unlike liquid, gas flows easily to surface through very narrow microannuli. Cementing operations which fail to provide adequate zonal isolation may result in loss of isolation which poses a hazard to both personnel and the environment. A cement that has ability to expand post set, can help prevent the formation of microannuli. The concept is that the expanding cement will fill microannuli and ensure good bonding either between the formation and the cement or between casing and the cement. This paper discusses the application of expanding cement in two large gas fields; located in South Sumatra. In those fields, wells are produced using 7-in production tubing, with 13 3/8-in casing and 9 5/8 in liner as production strings. Production strings are constantly exposed to changes in pressure and temperature, both during drilling and during production. These changes can create stresses on the cement sheath and may create microannuli. There is a particular focus on the isolation around 13 3/8-in production string: unlike the 9 5/8-in production liner string, the 13 3/8-in string relies only on cement to isolate the gas zones, without any packer. This string needs to provide isolation is critical to ensure that the well can perform as designed and pose no hazard to personnel and to the environment. The paper also discusses the design steps and considerations, as well as evaluation results.