A Study of Cement-Pipe Bonding

Abstract Special cementing problems concerning gas storage wells and tubingless completion wells have indicated the need for better pipe-cement bonding. This presentation stresses some of the problems involved and possible solutions. The application of a resin-sand coating at specific points on the casing will give a greater adhesion at the cement-pipe bond to fluid intrusion and loading is increased. Laboratory and field results as well as acoustic logs emphasize the improvement in bonding when a roughness factor is introduced. Comparative bonding data both with and without mill varnish on the pipe are also evaluated. Introduction The two types of bonds to be considered in this presentation are shear and hydraulic bond between cement and pipe. Shear bond is defined as the bond that mechanically supports pipe in the hole, and is determined by measuring the force required to initiate pipe movement in a cement sheath. This force when divided by the cement-casing contact surface area, yields the shear bond in psi. Hydraulic bond blocks migration of fluids in a cemented area and is determined by applying pressure at the pipe-cement interface until leakage occurs. The hydraulic pressure when leakage appears at either end of the specimen is defined as the bond failure pressure in psig. Gas bond tests were determined using compressed air or nitrogen as the pressuring medium. Fig. 1 illustrates the sample configuration generally used for determining these bond strengths in the laboratory. Dimensionally, the specimens can be of any size without significant effect. Hydraulic bonding is of primary importance, particularly in isolating zones in a producing or injection well, as most cementing jobs provide adequate mechanical support (shear bond) to hold the pipe in place. Shear bond should not be overlooked, however, in determining WOC time prior to subsequent drilling or completion work. It is necessary to consider the effect of various casing conditions and completion techniques if optimum bond is to be obtained and maintained. Some of the primary considerations in planning a more successful cement job from the casing-bonding standpoint are:pipe surface finish;drilling fluid;thermal and stimulation stresses; andcasing equipment. Pipe Surface Finish Hydraulic and gas bond were directly affected by the pipe surface finish against which the cement was placed. Types of casing investigated were steel and plastic as used in field applications. Hydraulic bonds were determined on steel pipe with the following surface finishes: new mill varnish, mill scale (chemical removal of varnish), sandblasted, used (rusty) and resin-sand coated. Two types of plastic pipe used were filament wound and centrifugally cast with both smooth and rough external surfaces. Table 1 shows the hydraulic, gas, and shear bond strength measurements of cement to steel and plastic pipe with varying external surface finishes. As may be seen, the rougher the external pipe finish, (Fig. 2) the higher the hydraulic bond using liquids or gas. These bond strengths indicate the importance of preparing pipe surfaces prior to placement of cement in the well. As an example, mill varnish exhibits the lowest bond strengths compared with other types of finishes, with time dependency after setting being noted in new mill varnish pipe below 140F. Table 2 illustrates the reduction in bonding of cement to pipe that occurs at about two days with this type of surface finish. Acoustic cement bond logs were run on a test well to verify results obtained in physical bond measurement, and correlation was obtained in that acoustic logs indicated a corresponding reduction in bond from two to five days after cementing. It is evident that this low physical bond strength period would be a critical time for running acoustic logs or completion practices on new mill varnish pipe, especially where the temperature is below 140F. JPT P. 157ˆ