Cementing the Conductor Casing Annulus in an Overpressured Water Formation
Authors: Griffith James , Faul Ronnie 
Published: October 2008

This paper presents a technique for cementing a 20- or 24-in. OD conductor pipe offshore, in deep water, in the presence of pressurized water flow, which constantly threatens to wash cement away from the well bore. At 1,000 ft below the mudline (BML), a highly pressurized flow of water prevents the use of conventional cementing methods and products to cement conductor pipe.

The principal challenge to slurry designers is the narrow window of slurry density that will work in the conditions described in this paper. For example, the fracture gradient of the formation might limit the cement slurry weight to only ll ib/gal, while formation pore pressure will wash out any slurry of less than IO.5lb/gal. The conductor pipe featured here was cemented with a lightweight, foamed slurry with good compressive strength; the compressible nature of the foamed slurry helps control water flow. A single cement dry-blend can be used for all densities needed.

The method described in this paper includes the use of three key fluids:Foamed drilling fluid sweeps. The well bore for the conductor pipe is drilled with straight seawater as a drilling fluid, so on a regular basis, 200-bbl stages of foamed sweeps are used as drilling fluid to bring cuttings out of the well bore.Settable spotting fluids. These fluids have low gel strength, and are composed to provide fluid-loss control. The settable fluid is used because it will set in 7 to l0 days if it is bypassed by the cement slurry pumped. Heat of hydration yielded by the cement slurry speeds setting time significantly.Foamed cement slurries. The combination of settable spotting fluid and cement slurry provides good zonal isolation of the water-flow interval.

This paper presents and illustrates the procedures necessary to cement a conductor pipe through a zone of high-pressure water flow successfully.

Introduction

Operators in offshore areas require special techniques to cement geologically young formations that occur less than 2,000 ft BML and are not well consolidated. Makeup of these formations is largely a product of erosion from the continental shelf (Figs. I and 2, Page 6 and 7). Consequently, conditions there hold the potential [or (I) overpressured water flow formations, and (2) the existence of weak formations that can fracture and cause loss of cement/mud returns to the sea floor.

Some water sands may be overpressured so that they cause saltwater to flow into the cement sheath (Fig. 3, Page 7). Prevention of this flow is critical, since remedial steps to stop flow and repair damage are far more expensive than proper preventive steps. Containment of the overpressure problem is further complicated by weak (low fracture gradient) zones that can fracture and cause loss of mud and cement returns.

The challenge presented by this situation is to cement the conductor casing annulus in an overpressured water formation. Solving this problem depends on the following:hole preparation before running the casing and cementing the annulusproper slurry weight to help control formation influx and help prevent formation fracture