Foamed-Cement Characterization Under Downhole Conditions and Its Impact on Job Design

Abstract

Summary The engineering properties of foamed-cement slurries were studied in detailwith an original methodology based on new equipment. With this laboratoryequipment, foamed cement can be mixed and characterized at pressures andtemperatures representative of field conditions and a pressures andtemperatures representative of field conditions and a densities as low as 400kg/m3. The pressure and shear field under which a foam is generated are foundto have a strong effect on bubble-size distribution (BSD), permeability, and toa lesser extent, compressive strength. Data on thicken time, fluid loss. andthe effect of temperature on foam properties are also presented betterunderstanding of foamed-cement properties allows optimization of job design sothat prescribed pressure properties allows optimization of job design so thatprescribed pressure and density profiles in the column are achieved both duringcirculation and after placement. Critical design parameters are identified andtheir relative influence is shown through a case study. Introduction Among the various systems used to cement oil wells, foamed cement presentsunique possibilities. First, it offers useful properties for densities rangingfrom the density of a base slurry to below that of water. Mechanical andphysical properties of a foamed cement vary with density, but chemicalproperties (e.g., thickening time and durability) are unaffected and remain thesame as those of the base slurry. Therefore, the base-slurry composition can beoptimized, depending on the properties desired for the final foamed cement, regardless of the density aspect. The reason is simply that the gas added toreduce the density of the system is inert and does not affect the slurry'schemical properties. Foamed-cement properties depend on many parameters, which can be dividedinto two main groups. The first group contains all the parameters that affectthe BSD, namely the foam quality and stability, the mixing procedure, andpressure. These parameters directly influence the mechanical properties of thesystem. The second group includes the composition of the base slurry: the brandof cement, the nature of the additives, and the density of the base slurry. These parameters more directly influence the physicochemical properties of thefoam (thickening time, fluid loss, physicochemical properties of the foam(thickening time, fluid loss, durability, etc.). Other specific foamed-cement properties that make it unique are itscompressibility and unusual rheology. One can take advantage of theseproperties for cementing long strings where very tight density control isrequired. The design of a foamed-cement job requires a good understanding of therelative influence of the various parameters. In the past, designs were basedon the hydrostatic profile in the well at the end past, designs were based onthe hydrostatic profile in the well at the end of the job, and the mainparameters were the number of stages and the nitrogen ratio. New software hasbeen developed that allows us to describe the hydrostatic and density profilesduring the entire circulation. These results made it possible to optimizedesigns further. Having an accurate backpressure adjustment when the foam iscirculated to surface is important. A well-designed cap slurry should be pumpeddown the annulus to compress the foam and to prevent nitrogen from migrating tosurface. prevent nitrogen from migrating to surface. This paper presentsexperimental results obtained from original equipment with which foamed cementwas prepared under conditions close to field conditions. We discuss how thevarious parameters affect the foamed-cement properties. We then use a fieldcase parameters affect the foamed-cement properties. We then use a field caseto demonstrate an efficient design methodology for foamed cement. Laboratory Study Mixing foamed cement comprises two steps: mixing of a base cement slurry byuse of standard methods and foaming this slurry after the addition ofsurface-active agents. No laboratory equipment able to foam a base slurry in areproducible, homogeneous, and easy way has been described yet. Montman et al.did present a mixing cell, but the sampling of the foamed cement by present amixing cell, but the sampling of the foamed cement by decompression into acuring cell modified its structure. Most of the laboratory studies on foamed cement have been performed withfoams generated and cured under ambient conditions performed with foamsgenerated and cured under ambient conditions The base slurry and the properamount of foamer and stabilize are poured in a closed vessel and stirred athigh speed. Foam quality is a function of slurry volume and vessel volume(assuming that the foam fills the entire volume). Unlike a conventional slurry, foam density is affected strongly by any change m pressure and to a lesserextent by temperature variations. Therefore, a foam generated under ambientconditions cannot easily be cured at higher pressure or temperature before itsproperties are tested. In this section, we present an experimental method formixing and studying foamed present an experimental method for mixing andstudying foamed cement under high pressure. Mixing. The study of foamed cement under wellbore conditions should improveour knowledge of its properties, specify the range of application of thissystem, and ultimately improve the design of foamed-cement jobs. To performthis study, we had to develop new laboratory equipment, including a mixer andseveral cells. The foamed-cement generator has two functions: to mix the foam and tocirculate it in a closed circuit that contains the mixer and the cell in whichthe cement will be analyzed. Mixing is ensured by a paddle rotated by amagnetic drive. An effective system was developed to circulate the foam (Fig.1). In this system, a reciprocating piston pushes the foam in the circuit whenit moves downward and transfers the foam from the upper (C) to the lower part(D) of the cell when it moves upward. The piston is moved by the injection ofpressurized air into a compartment at the upper pan of the mixer (Part A or B). Foam is transferred by means of a hole in the piston and a check valve. Whenthe piston moves downward, the check valve obstructs the hole and the pistonpushes the foam into the circuit. When the piston moves up, the check valveremains open, allowing the foam to flow through the hole. At the beginning of the experiment, the circuit is filled with the properamount of base slurry and surfactant to achieve the desired proper amount ofbase slurry and surfactant to achieve the desired quality. Then nitrogen isintroduced at the desired pressure. The piston is moved back and forth. Theflow of the nitrogen-based piston is moved back and forth. The flow of thenitrogen-based slurry mixture through the hole of the piston is enough toensure perfect mixing of the foam. Then the test cell is isolated from thecircuit perfect mixing of the foam. Then the test cell is isolated from thecircuit and the foamed cement cured at the desired temperature. Fig. 2 is a schematic of the cell used to cure the foam. The cell containsthree cubic molds to allow study of compressive strength. In the other mold, asample can be cured to allow study of its per-meability. The cell can withstandoperating conditions of per-meability. The cell can withstand operatingconditions of 150 degree C and 10 MPa. Surfactant Selection. A simple way to evaluate a surfactant composition forpotential use as a foaming agent is to proceed as follows. Pour some surfactantinto 100 cm3 of the water that will be used for the base slurry. This watershould also contain all the additives that will ultimately be used in thecement. Then foam the mixture. SPEPE P. 297