| Sudan oilfield has low formation pressure gradient with easy leakage, and the underground crude oil is heavy oil, with absolute viscosity of above100(0.1Pa.s). Therefore, in the cementing operation, the slurry system generally uses microsphere low-density cement and the high temperature resistant cement of silica sand. The application of this cement has the following main issues:(1) Cementing quality and cement height of microsphere low-density slurry wells fail to meet the requirements of the design and packer formation;(2) The high temperature resistant cement of silica sand has insufficient stability and intensity, and fails to meet the requirements of repeated steam flooding. Packer and cementing quality decreased significantly after the thermal recovery;(3) Among the69wells in the statistics,34wells use microsphere low-density slurry, and31of which vary greatly in cement height with the design requirements, and the other3wells have poor cementing quality; in the8thermal recovery wells, those with general silica sand slurry have decrease in cementing quality after thermal recovery;Aiming at these issues, it is necessary to study the non-microsphere low-density slurry system and the new high temperature resistant slurry system. This study adopts a combination of theoretical and experimental methods, as well as the product development and field application, and the main results achieved are as follows:(1) It established the relationship of power-law fluid particle settling velocity, sedimentation time and Reynolds number. The relationship can be quantitatively analyzed. When the particle density and the slurry density are certain, a reasonable choice of particle size and control of slurry rheological properties can well meet the requirements of the slurry stability. When the particle diameter is less than45um, K>(0.1Pa.sn), n<0.7, and particle settling of lcm is greater than8h of sedimentation time, the slurry is in a stable state;(2) Triangle composed by three particles of unequal diameters, and considering the impact of the wall, is the basic unit of particle accumulation. Multilevel filling of particles is in line with the actual situation of different particle sizes of cement materials and added materials. Studies have shown that the particle size range of Level I filling is (0.03-0.33) d50, the particle size range of Level Ⅱ filling is (0.01～0.15) d50, and the corresponding volume porosity decreased more than63%and90%;(3) The fundamental basis for and requirements of selecting high quality micro silicon are:content of SiO2≥97%, and other impurities such as Al2O3shall be less than0.5%Granularity of70%of the micro silicon shall be within0.1-0.5mm or particle size of micro silicon within0.1～10μm in even distribution Specific surface>22m2/g.(4) Density of micro silicon low-density slurry in study is1.35～1.40, with compressive strength of (60℃×24h)>14Mpa, API water loss<30ml, pulping capacity increased by40%, as well as good stability, leak-proof and anti-gas channeling ability. Micro silicon high temperature resistant slurry can be maintained at the temperature of250℃,2to3times throughput, with no significant change in compressive strength, and has50～80%increase in strength compared with general silica sand cement;(5) The application of integrated measures of improving the displacement efficiency of the slurry system on the site has achieved good results. According to the wells in the tracking test, among the30micro silicon low-density slurry wells,29are high-quality wells and qualified wells, and their cement height meets the design requirements. Among the thermal recovery wells,20are micro silicon high temperature slurry wells, all with qualified cementing quality, including15high-quality wells. After several rounds of thermal recovery, cementing quality of these wells has no significant change. |
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