Zonal Isolation for Short-Liner Intervals

Abstract The production objectives of any gas well are largely dependent on zonal isolation. Zonal isolation becomes additionally challenging for short-interval cementing. This paper presents the best practices and lessons learnt from short-interval liner cementing in the Abu Butabul field in northern Oman where the operator had a series of short liners in which the perforation interval was located just below a previous casing shoe. The complexity increased by the presence of loss zones in the open hole, which limited the hydrostatic pressure of the cement column. The methods this paper presents not only cover slurry design but the placement practices as well. The main challenge with short liners is the small theoretical slurry volume, which is prone to high contamination from fluid mixing in the pipe alone. Through software simulations, the optimum volume was determined to resist or at least minimize contamination across the zones of interest. Liner overlap length was also increased as was the shoe track. The standoff of the liner was optimized despite the reactive shales in the section, The length of both the overlap and shoe track was based on software simulations, which this paper will present. The slurry height above the liner hanger was also adjusted so as not to compromise the fracturing gradient due to increased hydrostatic pressure. Then the next step was slurry design to maximize rheological and density hierarchies. The additional preventative slurry testing included thickening time selection to allow safe tool release and liner packer setting despite the increase in slurry volume above the top of liner. Design steps to boost compressive strength and slurry formation interaction despite the increased thickening times will also be presented. Lift pressures during pumping were verified against expected design pressures and no indication of loss was observed. The excess considerations were first verified with post-job circulation returns where uncluttered slurry returns were noted above the liner hanger. The final indication of the operation's success was the cement evaluation log, which displayed good cement coverage across the zone of interest. Short liners had been an operator challenge in the field, especially for perforations across formations immediately below the previous shoe. The operator has agreed to carry forward the lessons learnt for cementing similar profiles in the field's Block 60.