Foam cement offers a versatile and economical means of cementing wells with low density, high strength material. Several common applications of foam cement are available, and these include placing strong cement across weak or highly fractured zones without placing strong cement across weak or highly fractured zones without losing circulation; filling vugs or porous thief zones; or simply as an inexpensive light-weight filler slurry.
The various aspects of slurry design, including their methods of determination, are discussed. It was found that thickening times could be determined directly on the foam cement in existing equipment with only minor modifications; the fluid loss and the rheology could be measured directly with no modifications; and the compressive strengths were determined in a special cell to prevent water invasion from the curing chamber. Since these parameters are determined by the nitrogen content, their relationships to density are also discussed.
Job procedures for pumping foam cement are discussed in two methods, i.e., constant nitrogen rate (variable foam density) and constant density (variable nitrogen rate). Included in this discussion is the description of an improved atomizer for generating foam cement. From yard tests utilizing field equipment, it was found that this device improved the foam stability by means of better disbursement of the nitrogen in the cement slurry. The computer program which calculates the surface gas injection rates to achieve the desired density at specified depths is discussed and evaluated. Case histories are then presented to illustrate the effectiveness and versatility of foam cement as well as its relative ease of application. The case histories deal with repairing casing leaks and lost circulation problems in weak formations.
In primary cementing, many situations arise in which conventional weight slurries of 11 lb/gal (1320 kg/m3) or greater cannot be used due to low fracture gradients in the open hole section of the well. The types of slurries used to cement these wells have had to rely on hollow fly ash spheres or hollow glass balloons to provide low density and sufficient compressive strength to hold casing when set. These materials have two major drawbacks. First, a definite pressure limitation before crushing; and secondly, high relative pressure limitation before crushing; and secondly, high relative cost due to the large amounts of the material added to the cement.
With the development of efficient liquid nitrogen vaporizing units, a third type of light weight additive can be used –– nitrogen. The other requirements for use of this material are a foaming agent and a foam generator "T". The design of the cement slurry remains the same, except for the inclusion of the foaming agent, so that the physical properties such as thickening time, compressive strength and fluid loss can be controlled with traditional additives. The results are a cement slurry containing a compressible additive to give light weight, high strength cement at a relatively low cost.
In addition to the slurry design, the uniform disbursement of nitrogen (or air) into the cement is also an important factor. Uniform disbursement of very small bubbles of gas greatly diminishes the mobility of the gas in the slurry, i.e., a stable foam results. When a foamed slurry sets under these conditions, a true light weight cement is produced. Therefore, the foam generator "T" design is important and becomes a major factor in a successful job operation.
The design of foam cement slurries is for the most part the same as designing an unfoamed slurry. A thickening time and compressive strength are necessary as well as fluid loss and rheology data.