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Abstract The presence of overpressured shallow sediments in deep water in the Gulf of Mexico has resulted in numerous drilling and cementing problems, These overpressured zones are prone to uncontrolled flows of massive amounts of salt water, especially after cementing operations. Uncontrolled flows from such sands have led to total loss of the well in several cases, This paper focuses on recent technical advances in cementing across these water flow zones. The theories of critical hydration period (CHP) of cement slurries and critical wall shear stress (CWSS) in the wellbore are investigated as mechanisms for water entry and migration, The CHP isdefined as a period of time that begins when the slurry is no longer capable of transmitting hydrostatic pressure that overbalances the pore pressure of the formation and ends when the slurry has developed enough cohesive strength to preventthe entry and flow of reservoir fluids. The CWSS represents the lower limit of the CHP. A formula for mathematically deriving this lower limit is given in the paper. Best practices for cementing in areas of shallow sediments prone to water flow are discussed, including the use of specially formulated right-angle-set (RAS) cement slurries and compressible foamed cements. Laboratory data for a foamed RAS system is presented. Finally, case histories that describe the successful implementation of this technology in wells with known water flows are given. Background Areas where shallow salt water flows have been encountered in the Gulf of Mexico are primarily on the transitional slopebetween the continental shelf and the deep abyssal environment. This translates into water depths ranging from 1,000 to 7,000 feet, although isolated flows have been reported in water as shallow as 500 feet. Geology. The sands responsible for the flows are commonly referred to as turbidites (Fig. 1) because they were deposited by turbidity currents. Turbidity currents1 are tongue-like masses of debris laden water that flow down-slope beneath clearer water by virtue of their greater density. These currents are generated by slumping along slopes on which rapid edimentation occurs. This slumping may occur when semifluid deposits become unstable on inclined surfaces. Turbidites are typically deposited as graded sediment beds with the coarsest particles being on the bottom. Successive turbidity currents passing over previous deposits may lay down additional graded beds without disturbing the relatively fine upper surface of the previous layers. This results in undercompaction and thus overpressuring of the sediments. Formation Pressures. To date, most of the overpressured sands have been encountered between 500 and 2,000 feet below the mudline. The pore pressure in these sands, as referenced to the kelly bushing, ranges from 9.0 to 9.6 Ib/gal EMW, but is typically around 9,3 Ib/gal EMW. Pore pressures are determined through the use of pressure while drilling sensors as well as from the density of the kill mud required to control the flow. Information on the location and thickness of the sands can be obtained from shallow seismic data, MWDILWD data, offset data and static flow checks with an ROV. |
