Maintaining cement integrity is a difficult-solving problem during oil exploration and development in the world, especially with the current trend of high cost but low oil price in the industry. The loss of cement integrity can result not only in severe operational difficulties, possibly leading to the loss of the well, but also in dramatic environmental damage. The micro-annulus at the well cement sheath interface may result in loss of cement integrity. The micro-annulus can be potential flow channels, which might be propagated by the fluid pressure during subsequent operations such as hydraulic fracturing. The micro-annulus propagation has already been investigated in casing centered wellbore, however, this scenario differs from that in casing eccentric wellbore.
To stimulate the micro-annulus propagation scenario in casing eccentric wellbore, coupled 3-D poroelastic finite element models embedded with cohesive zone were developed. The behaviors of cement were described by an extended Drucker-Prager model, where the parameters were from a series of tri-axial tests. The traction-separation law describes the emergence and propagation of the interface. In sensitivity analysis, casing eccentricity, in-situ stress and the size of the initial micro-annulus were considered and studied as key elements which influence micro-annulus growth. Through the analyses of the key elements mentioned above, the result indicated that the size of the initial micro-annulus and casing eccentricity are the main elements to micro-annulus propagation. This study gives a better understanding of cement integrity, and helps to optimize the structure of the wellbore.
Cement sheath is the key element to maintain longevity for the life of the oil well. The main effects of cement sheath are to support casing and provide effective zonal isolation. The loss of cement sheath integrity leads to many problems, such as sustained annular pressure, could result in well abandoned or high remedial costs. However, maintaining cement sheath integrity is not always the case in real life oilfield practice. Over 8000 wells in the Gulf of Mexico currently exhibit sustained casing pressure. And with the development of technologies, the oil and gas industry is exploring and producing oil and gas from more extreme environments, such as deepwater, pre-salt, high-pressure and high-temperature reservoirs. All these facts pose a severe challenge to cement sheath integrity.