Automatic Density Control and High Specific Mixing Energy Deliver Consistent High-Quality Cement Slurries
Authors: Stegemoeller Calvin , Allen Tom , Pitts A.J. 
Published: October 2008

Accurate density control and specific mixing energy are two critical requirements for executing a high quality cementing operation. The need for a field mixing system that is capable of mixing thixotropic and high density cement slurries accurately and providing adequate mixing energy is becoming more intense (1) because of increasing emphasis on the quality process and (2) to accommodate changing well requirements.

Of the many parts of the cementing process that may affect quality, the two areas that are addressed in this paper are mixing energy and density control. The paper also discusses a field mixing system that is capable of reliably providing the necessary density control and high mixing energy. Field performance data are presented.

INTRODUCTION

The cement slurry mixing process requires the incorporation of a cement blend into the mixing water to form a homogeneous slurry. During the field mixing process, adequate mixing energy should be applied to fully wet all solid particles and facilitate yielding of the additives and cement such that laboratory mixed and tested slurry properties are approximated. In addition, a control system is required that will accurately proportion the cement and water.

The role of mixing energy in cement slurry mixing process has been investigated for many years. Earlier work3 was initiated to determine why field-mixed slurries extended with bentonite did not perform as laboratory tests had predicted. The result was that the laboratory test equipment and procedures were changed to more closely match the mixing energy found in most field equipment. Later investigations4' 5' G6focused on the effect of mixing energy on slurry properties. These investigations showed that slurry properties (i. e., compressive strength, free water, fluid loss, and rheology) were definitely affected by the amount of mixing energy for these cement compositions.

The relative importance of good density control has been known for many years. For critical liner and production string jobs, the use of batch mixers has been commonplace. One of the reasons for using the batch mixers has been that very close density control, +/-0. 1 lb/gal, could be obtained. Good density control is important because final slurry properties are affected by deviations in density from the design value. Stated another way, slurry properties are dependent on and are a function of slurry density. The functional relationship between properties and density have been investigated and have been found to be generally predictable; some are reported in this paper. With knowledge of these relationships, quantitative significance can be placed on density variations from the design value. Thus, "good" density control can be quantified, job specifications can be more meaningful, and required job equipment can be appropriately selected. Job results should be more predictable and therefore be termed of higher quality.

With specific knowledge of the mixing process requirements, a system has been developed that meets the specific mixing energy and density requirements. This paper elaborates on the slurry property dependence on mixing energy and density and will describe a mixing and control system that meets the developed requirements.