Annular gas pressure, also known as sustained casing pressure (SCP), is a common problem and potential threat to the safety of personnel and equipment, as well as to the environment. Improved means of primary cementation and life-of-the-well simulations show promise for preventing future annular gas-flow problems. However, there has been little success in eliminating SCP, once developed, without jeopardizing the economic life of a well.
Possible solutions to SCP in the form of remediation include bullheading cement, injection of zinc bromide brines into the well's annulus, or use of expensive resins to seal the annulus. A proposed solution that can compensate for the drawbacks of the above options and has shown promise on small-scale physical models is palletized alloy-metal. This solution involves placing palletized alloy-metal into the well's annulus, heating the alloymetal above its melting point, and then allowing the alloy-metal to cool. These steps form a continuous alloy-metal plug in the well's annulus.
Data and conclusions documented in this paper are from full-scale pipe-in-pipe and pipe-in-sandstone geometry models having the following scope:Geometry of 5 1/2×8 1/2-in. pipe-in-sandstone and 10 3/4×13 3/8-in. pipe-in-pipe configurationsModel length limited to 15 ft and deviated at 30° from verticalAlloy-metal pellets placed and activated with water-based drilling fluid present in annuliTemperature limited to 200°F
Results are also documented by dissecting the models and photographing the exposed cross-sections. Information gathered in this testing will help with the field introduction of this technology.
Introduction
SCP is a growing problem among offshore wells, leading to expensive shut-ins and remediations on many wells. Poor primary cementing, inadequate cement coverage, gas/water influx during or after cement placement, mud-cake shrinkage, and the development of stress-induced microfractures and microannuli are all cited as potential factors in SCP. The time between well completion and the onset of SCP can indicate the more likely causes.
While there are several suspected culprits for SCP, pressure and temperature cycles are high on the list. Casing growth and contraction that result from production cycles and stimulation operations can de-couple the bond between the cement and casing. These forces can also stress-crack the cement. Both scenarios can create small pathways for high-pressure, lowvolume communication of annular gas to the surface, but the inaccessible nature of these pathways limits remediation options.
Early onset mechanisms can include the following:Gas-cutting of the cementGas movement through a free-water channelCasing/tubing connection leaksInadequate cement coverage
Late onset SCP can result from the following:Channels of bypassed mudStress cracks in the annular cement sheathShrinking or drying of the mud filter-cakeCasing/tubing connection leaks
These factors are well documented, appearing frequently in past research. The following sections summarize a few main causes.
