Critical Mixing Parameters for Good Control of Cement Slurry Quality
Authors: Vidick Benoit 
Published: December 2007

The cement mixing procedure can be split into (1) a mechanical process thatincludes the wetting of the powder and the defloccuration and process thatincludes the wetting of the powder and the defloccuration and homogenization ofthe resulting suspension and (2) a physicochemical process that includes thedissolution of some cement phases, the formation process that includes thedissolution of some cement phases, the formation of supersaturated solutions, and the precipitation of cement hydrates.

The mechanical process has beenvalidated by inert suspensions of silica and barite and by cement slurries ofvarious reactivities and physical characteristics (particle size distribution). The physicochemical process was investigated systematically by looking at theprecipitation kinetics, inert suspensions, and finally cement slurries. Therelevant mixing parameters were found to be the residence time of the slurry inthe mixer parameters were found to be the residence time of the slurry in themixer and the rotational speed. Physical properties, such as plastic viscosityand fluid loss, are related to these two parameters by the mixing energy. Properties related to cement reactivities, like the yield value, are Propertiesrelated to cement reactivities, like the yield value, are affected very littleby the rotational speed but are highly dependent on the residence time in themixer. The influence of these two parameters on the response to differentadditives (dispersants, retarders, and fluid-loss agents) is presented and acomprehensive physicochemical mixing is proposed. proposed.

Introduction

Mixing conditions are known to affect the consistency of fresh concretecement paste. But very few studies have paste. But very few studies havethoroughly investigated the chemical and physical phenomena that occur duringthe physical phenomena that occur during the mixing of oilwell cement and theirlong-term effect on cement slurry properties. For laboratory mixing, the APIrecommends the use of a precise procedure to mix an oilwell cement slurry. Butlittle attention has been devoted to determining whether the slurry obtainedwith this method is well mixed and the effects of changing some parameters inthis procedure.

In this paper, cement is first considered as a powder and the differentphysical processes that occur during mixing are processes that occur duringmixing are presented. Next, the cement hydraulicity is presented. Next, thecement hydraulicity is taken into account and integrated into the wholeprocess. From this research, parameters critical to obtaining a well-mixedpowder in the laboratory are identified and discussed. A physicochemical modelof cement mixing is also presented. The second part of the study investigatesthe influence of these parameters on some key slurry properties. parameters onsome key slurry properties.

Experimental Procedures

The cement slurries were prepared by mixing Class G cement and tap waterwith a propeller-type mixer. The volume of mixed propeller-type mixer. Thevolume of mixed slurry was kept constant at 600 mL for all tests. The mixerblades were weighed after every 10 tests and changed when weight loss exceeded5 %. The mixing energy (see the next section) was changed by varying either therotational speed of the blade (500 to 12,000 rev/min) and/or the duration ofmixing (15 to 50 seconds). After mixing, the rheological properties weresometimes measured or the properties were sometimes measured or the slurry wasconditioned in an atmospheric consistometer rotating at 150 rev/min for 20minutes at different temperatures. All physical tests following cement slurrymixing and conditioning were performed according to API Spec. 10. A sieve testwas performed to check for the presence of performed to check for the presenceof aggregates in the slurry. Immediately after mixing, the slurry was pouredthrough a sieve. The choice of sieve size can be based on the postulation thatthe slurry is deflocculated after API mixingi.e., that each particle isisolated from the others. Therefore, a 450-um sieve was selected to allownothing to remain on the sieve after API mixing. The percentage of the slurryremaining on the sieve indicates the amount of aggregates in the slurry.