Study On The Mechanical Properties And Modification Of Low-density Cement Stone With Floating Beads

Floating beads low density slurry system has a widely application in low voltage leakage easy stratigraphic.Low density slurry will condense and solidify into a cement sheath after it is injected into the annulus formed by the casing and the formation during the cementing process.The cement sheath has the function of protecting the casing and sealing off the formation.Therefore,it is crucial to the downhole production cycle for maintaining the integrity of cement sheath.However,the research on floating beads low density slurry system mainly focuses on the construction site at present and the research on its mechanical properties is not sufficient.The relative mechanical properties of low density floating beads cement slurry were tested and analyzed in the manuscript.The mechanical properties of low density slurry system are improved by choosing the modified materials and modification methods in order to increase the interlayer sealing ability of the low-density cement sheath.The compressive strength and tensile strength of floating beads low density cement in different temperatures and pressures showed that the strength of floating beads low density cement increased with the increasing of curing temperature.Cement sheath integrity experiments showed the interlayer sealing ability of low density cement decreased at 90 ℃.Triaxial tests showed the strain of floating-beads low density cement decreased with the temperature rised.The strain decreased to 0.1%at 90 ℃ and the elastic modulus increases for 579.5MPa at 30℃.The porosity test of low density cement showed that the porosity decreased at 90℃,which was due to the denser structure of floating beads low density cement.X-ray diffraction experiments showed that there was no new phase formed during the hydration process of the floating-beads low density cement matrix.Thermogravimetric analysis showed that the hydration at 90℃ was more severe so the compressive strength and tensile strength were higher.Scanning electron microscopy experiments showed that floating beads can not inhibit the crack propagation as a mitigating material so it was necessary to modify them.Cellulose fibers and carbon fibers were selected as the modified materials.The contact angle analysis showed that hydrophilicity of carbon fibers and cellulose fibers increased after plasma modification and contact angle also decreased after grafted modification compared with plasma specimens.The BET analysis showed that the specific surface area was larger than the untreated one.The fibers were analyzed by infrared spectroscopy and X-ray photoelectron spectroscopy.The groups of-OH,-COOH,-CO and C-O generated on the fiber surface after grafting modification,which were the reasons for the decrease of the contact angle of fibers.Scanning electron microscopy analysis showed that the fiber surface became rough after grafted modification.The chemical reaction on the surface of carbon fiber generated the new groups and it is the main mechanism for grafted carbon fibers.The main mechanism of cellulose fibers were the destruction of the surface lignin and the cellulose internal exposed,which had a better bonding effect with cement matrix.The fluidity and rheology of the cement slurry were reduced with the increase of fiber content added to the floating beads low density cement slurry.The compressive strength and tensile strength were increased first and then decreased.Cement sheath integrity experiment showed that the sealing effect was improved after added grafted ethanol modified fibers.Triaxial experiment indicated the strain of grafted ethanol increase and there were no new phases generated in cement matrix by X-ray diffraction experiments.Scanning electron microscopy experiments showed that the toughening mechanism of carbon fiber based low density cement was fiber bridging.And the main toughening mechanism of cellulose fiber based low density cement was the fiber pulling out.The bonding situation between the fiber and the cement based matrix was improved after modification.