Analysis Of The Slurry Key Gas Blocking Properties And The Slurry Gas Blocking Ability Evaluation Method

Gas migration is a difficult problem with which all natural gas wells will confront. Gas migration after cementing not only leads to zonal isolation failure, but also affects safe exploration and development of oil and gas field. Gas block ability of slurry is an important factor that affects early gas channeling. Studies show that the essence of early gas migration is the the physical process of gas percolation through time-variable-plastic porous media. There is not any classical theory to describe the law of gas migration through slurry matrix. Domestic and foreign scholars encounter bottlenecks in evaluating gas block ability. The evaluation level still remains using a rough qualitative comparison to analysis gas block ability.In order to form a scientific evaluation method, basing early gas migration, studying from material properties of cement, proposed three key gas blocking properties of slurry: slurry permeability in gas kick dangerous time, SGS transition time, volume shrinkage before initial setting. Then the development of the three key gas blocking properties was analyzed by gas migration experiment, SGS experiment, volume shrinkage experiment. Basing on these analyses, gas migration model was established and slurry volume shrinkage evaluation method was proposed. At last, the accuracy of the evaluation method was examined in Φ177.8mm liner cementing in DN2-5well and DN2-7well.After laboratory research and cementing practice, following conclusions and understanding were drawn:(1) Slurry permeability in gas kick dangerous time reflects the gas blocking ability; SGS transition time affects gas kick probability; volume shrinkage causes the decrease of pore pressure. So they are the key gas blocking properties of slurry.(2) Water to cement ration and the type of fluid loss additive affect the gas blocking ability of slurry. With the increasing w/c, slurry permeability increases, but threshold pressure decreases. When the SGS is equal to48Pa, the threshold pressure of film forming fluid loss additive is the highest of all, the polymeric colloid fluid loss additive is the second, terpolymer loss additive and cellulose fluid loss additive are the lowest. But when the SGS is equal to240Pa, the gas blocking ability of6type fluid loss additive has a little difference. The volume expansion of expanding cement happens after final set, which has little relation with early gas migration. The liquid silicone and latex slurry system can reduce the cake permeability and significantly improve the gas blocking ability of slurry.(3) Water to cement ratio and retarder are important factors which affcet the SGS transition time. The higher the water-cement ratio is, the longer the SGS transition time will be. Retarder has some negative impact on SGS transition time. The hydroxycarboxylic acid high temperature retarder has more significant negative impact on SGS transition time. The nature reason of the impact of retarder on SGS time is that the retarder slows the cement hydrationrate which extends the plastic state time of slurry. Thickening time is a dynamic property of slurry, while SGS transition time is static property of slurry. The two do not rmeflect the SGS increasing in same hydrating time and have no direct relation.(4) Basing on the home and abroad cement shrinkage/expansion device, three problem "continuous measurement of the cement volume change","test of cement initial set and final set","continuous measurement of cement pore pressure change" were solved, the high temperature high pressure cement shrinkage/expansion device was developed. The high accuracy device has functions of continuous measurement of the cement volume change, test of hydrating thermal discharge, test of cement pore pressure change.(5) Water to cement ratio and expanding agent are important factors which affect the volume shrinkage before initial set. With the increase of water to cement ratio, the plastic stage time extends, the initial set and final set prolongs, the volume shrinkage before initial set increase. Lattice type expanding agent promotes cement to expand by crystallization pressure through later stage crystallization, which can’t improve early cement shrinkage. Gassing agent KQ-B can compensate cement volume decrease by chemical gassing, delay pore pressure decrease, when the slurry is in plastic stage. Even pressure is50MPa, Gassing agent KQ-B can significantly reduce the cement volume shrinkage before initial set and increase the gas blocking ability, when the dosage of KQ-B is more than2%.(6) Cement practice in two typical high pressure gas DN2-5well and DN2-7well Φ177.8mm liner cementing showed that the forecast results of slurry gas blocking ability coincided with the log interpretation and gas leakage test. The accuracy of comprehensive gas blocking ability evaluation method is high.