A new light weight blended cement has been developed in Canada for solving a series of operational problems for cementing across low pressure formations where foam cements are usually applied. The slurry density can be varied from 1,100 to 1,400 Kg/m3
This system imparts good slurry properties such as uniform cement column density, zero free water, non-settling, reliable rheology, fluid loss and thickening time control behaviors. During the transition from liquid to solid state, the system develops stable gel strength, high shear bond strength, and high compressive strength even at low temperatures.
References and illustrations at end of paper In field applications, the system reduces significantly equipment and manpower requirements when compared with foam cementing job operations, and can eliminate the need for a stage collar. No cement fall- back has been seen after implementing more than 25 jobs. Case histories including an on-site computer job record are discussed.
Light weight cements are solutions for cementing across rock formations which are fractured or having a low fracture gradient, highly permeabable, vuggy or cavernous. These weak formations usually have very low pressure that they may not be able to support the hydrostatic pressure from a collumn of normal cement slurries.
Depending on the severity, the well cement.ing condition can be treated as a Loss Circulation problem which can be approached and handled systematically(1). A familiar parameter reflecting the capability of a weak formation to withstand a mud column is called Equivalent Mud Density (EMD). Recently, a few new solid free gelling fluids have been successfully field tested which can moderately increase the EMD of some weak formations(2) and therefore allow the use of some exteuted cement systems. These relatively light weight systems are blended cements containing various types of fly ash and clayey gel materials which could have the slurry density range from 1,500 to 1,700 Kg/m3.
In Canada, some of the formations could have EMD's much less than 1,500 Kg/m3 and in some cases required a slurry having a density less than that of water. To enable cementing in such cases ultralow-density such as Foamed Cements have been developed and can be successfully designed and implemented(3).
However, a lot of operational problems arise due to the use of nitrogen gas as an extender. The generation and placement of quality foam with changing slurry rates, nitrogen rates and pressure is difficult. Quite often, nitrogen gas breaks through the cement to the' surface, cement fall back occurs, and consistent cement column density cannot be ensured. In addition, foamed cement jobs are equipment and manpower intensive and require significantly long rig time.
This paper presents a new light weight blended cement system which does not use any gas as an extender to reduce cement slurry density. Most importantly, the system was developed not by following a quick and easy approach which furtherextends the classical extended cements.
Classically, cement slurry density can be successively reduced by adding more water, pozzolans and other fly ashes, clay (Bentonite) and silicate gels.