Special Cement System and Cementing Techniques Improve Zonal Isolation in South Oman Fields

摘要

A cluster of fields in the South of Oman consists of heterogeneous platform carbonates at ~4500m depth embedded in salt. These salt-sealed reservoirs have no aquifer support so ultimate recovery will be maximized by injecting miscible gas. During drilling operations, the drilling fluid density has to be high enough to balance the high-pressure carbonate stringers whose pore pressure can be almost lithostatic. When circulating to a lighter completion brine however a huge drawdown is applied to the wellbore causing a microannulus to form between the liner and cement sheath. It is important to close this gap to avoid the miscible gas flowing behind the casing and hence impairing reservoir management. This gas is expected to break through in the higher permeability zones and gas shutoff remedies will be impossible if the casing to formation annulus is not sealed. The increase in ultimate recovery from miscible gas injection will be significantly reduced if gas shutoff in the higher permeability layers is impossible. A special cement was trialed which expands over time to close this micro-annulus. Other cementation challenges include the narrow margin between formation pore and fracture pressures, the wide range of formation permeability, the extremely sour nature of the reservoir fluids and the saltsaturated heavy mud. Multiple cement logging runs were made to evaluate this cement at various times and with various fluid densities in the borehole. The first three logs were made within twenty-one days of pumping the cement; all before the slow cement expansion occurred. The well bore fluid during the first log was mud with a slightly lower density than that used to displace the cement. The log was repeated with a surface pressure applied, and repeated again with a muchlighter completion brine giving a 61000 kPa differential to the liner. The micro annulus expected from this pressure change is over 180 micron. This third log showed all the bond apparent the second log had disappeared. The fourth cement evaluation during a workover was made twenty-one months later, again in the much lighter brine. This final log showed the cement had expanded and the bond was restored.