Ultra-low Density Cementing Compositions
Authors: Ripley H.E. , Harms W.W. , Sutton D.L. , Watters L.T. 
Published: March 2010

Introduction
Until recently, attempts to improve extra-low density cement grout slurries (1000 to 1300 kg/m3) suitable for oil and gas well cementing have accomplished little except to define a disappointingly low strength-density ratio and confirm the low density limit for useful compressive strengths. Commonly available porous, light-weight fillers which yield good strength-density ratios at surface conditions seldom provide the desired benefits under high pressure conditions. The use of water-extended admixtures results in little or no Useful strength at slurry densities below 1300 kg/m3, and the amount of gas entrainment needed for low-density foam or cellular grout is often impractical at high pressures.
This paper describes the use of a new light-weight admixture which maintains low density, even at high pressures, and has a relatively low mixing water requirement. Slurries using this new material have greatly improved strength as compared to other types of low-density slurry systems. Final compressive strengths for slurries with densities of only 1050 kg/m3 easily exceed 3.5 MPa. This new admixture consists of small (10-100 mm) diameter inorganic high-strength microspheres (HSMS). It can be used with any type of cement, survive under hydraulic pressures approaching 50 MPa and withstand geothermal well temperatures.
To date, proven applications range from cementing conductor pipe and casings where fast strength development is
(Figure in Full Paper)
The trapped saturations pictured for the composite core (Core b) tends to be lower than the averages of the individual cores, perhaps reflecting a length effect even after 4 pore volumes of injection. The trapping behaviour of Core D at higher water saturation does, however, begin to mirror the dependence exhibited by Core C.

Conclusions
Miscible recovery of methane from full-diameter carbonate reservoir cores is inferior to the recoveries obtained using a homogeneous consolidated sandstone. The ultimate recovery of methane decreases as water saturation increases, thus indicating the increased trapping of "pockets" of gas by the immiscible water phase. In the homogeneous core, increases in the displacement rate improved efficiencies by decreasing dispersion. In most carbonate cores, more efficient recovery occurred at the lower rates because mass transfer from poorly connected pores controlled recovery. This dependence on rate was only investigated for dry cores. The methane recoveries obtained using a three-core composite core were found to be superior to those obtained with the constituent single cores, indicating that displacement results from single cores are conservative due to length effects. It was possible to rank the relative displacement efficiencies of the various cores, even though there was an extremely wide range of litho-facies present. In a carbonate reservoir, the injection of a miscible fluid following waterflooding may not necessarily lead to reconnection and total recovery of the trapped hydrocarbon phase.

Acknowledgments
The authors thank Hudson"s Bay Oil and Gas and the Petroleum Recovery Institute for permission to present the material for the carbonate cores and Berea core, respectively.
The authors gratefully acknowledge the contributions of I.