Publications

Cement in oil and gas wells is placed in the annular gap between formation and casing to provide zonal isolation. This means, cement sheath should be able to prevent fluid communication between the drilled formations during the life of the well and even after the well is abandoned. However, even when cement initially provides good hydraulic seal, zonal isolation can deteriorate after initial placement due to microannuli formation. Of many causes, they can be formed because of wellbore temperature changes, wellbore pressure changes, or bulk shrinkage. Microannuli are considered serious concern in gas wells because, unlike liquid, gas flows easily to surface through very narrow microannuli. Cement treatments which fail to provide adequate zonal isolation may result in unacceptable gas flow to surface which poses hazard to safety and environment. Cement that has ability to expand, after it sets, can 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 7in production tubing, with 13 3/8in casing and 95/8in 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 a lot of stresses on the cement sheath and may create microannuli. There is a particular concern on the isolation around 13 3/8in production string: unlike the 9 5/8in production liner string, the 13 3/8in 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 safety and environment. The paper also discusses the design steps and considerations, as well as evaluation results.