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In the in-situ conversion of low purity shale oils, extremely high temperature will result in strength retrogression of the set cement. To solve this problem, study was conducted on the macro-properties and microstructures of an aluminate cement before and after aging 650 ℃. The aluminate cement is a cement modified by micro silica compounded sodium hexametaphosphate. It is found that at 50 ℃, sodium hexametaphosphate can significantly reduce the permeability of a set aluminate cement, and the compressive strength of the set cement is significantly increased. After treatment at 650 ℃, the aluminate cement modified with 5.0% sodium hexametaphosphate has the highest compressive strength, which is 47.19 MPa, while the aluminate cement modified with microsilica compounded sodium hexametaphosphate has strengths that are declining. The hydrational products of the aluminate cement before and after treatment at 650 ℃ have experienced significant changes, from C3AH6 and AH3 to C12A7 and CA, respectively. Among these products, C3AH6 and AH3 are thermally degraded in temperatures between 180 ℃ and 400 ℃, and because of the changes of the crystal form, the permeability of the set cement is increased. Micro silica accelerates the formation of C2ASH8 by the set cement at 50 ℃, and this is beneficial to the improvement of the microstructure of the set cement. Since C2ASH8 will degrade after treatment at 650 ℃, the sizes of the pores in the set cement will also increase. [ABSTRACT FROM AUTHOR]

In the Sulige gas field in Changqing Oilfield, the Liujiagou formation is a low pressure formation, the low density cement slurries presently used in this area have poor pressure resistance; the cement slurries under high pressures will have their density increased and rheology deteriorated. In field operations with these cement slurries, the operation pressures were high, generally causing the cement slurries to be lost into the formations, and the cementing quality is poor because of less cement slurries in the annulus. To solve this problem, a new cement slurry with low friction low density was formulated. This new cement slurry is pressure resistant and has the property of preventing itself from being lost into formations. Using the close packing theory, particles of 4 sizes were selected. A light material with excellent pressure resistance and other appropriate additives were selected to formulate the target cement slurry. The density of the new cement slurry is adjustable between 1.25 g/cm3 and 1.35 g/cm3, the Fanning friction coefficient was reduced by about 50%. The new cement slurry has good pressure resistance and leaking preventive property. Study on the agitation and mixing techniques shows that mechanical agitation and mixing can be used, 99% of the properties of the bulk sample and those of the small sample coincide with each other, ensuring the qualification and efficiency of the cement mixing. In field operations in the Sulige gas field, four wells were cemented with success, no loss of cement slurry has ever happened, and the cementing job was qualified. During the cementing, the operational pressure was significantly reduced, and the job quality was obviously improved. This laboratory study and the field application provide a strong technical support to cementing wells penetrating low pressure formations into which cement slurries are easily lost. [ABSTRACT FROM AUTHOR]