Preparation of Expanding Oil-well Cements.pdf
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1、F.Agzamov1, N. Karimov2, K. Akchurin2 1Ufa State Petroleum Technological University 2Research and Production Enterprise “Azimut” PREPARATION OF EXPANDING OIL-WELL CEMENTS The report substantiates the expediency of employment of expanding oil well cements for well cementing. It considers the principl
2、es of preparation of cements with different binding base and expanding additives. The mechanism of sulphoaluminate and oxide expansion is shown. It studies the effect of different factors on the expansion value and kinetics. It presents the results of experimental investigation and oil well cement c
3、ompositions. GENERAL PRINCIPLES The problem of high-quality well cementing and formation isolation is one of the most crucial and complicated points in the whole cycle of the well construction. The presently used conventional materials cannot provide reliable sealing of the annular behind the casing
4、 string. It is caused by heightened water loss, low sedimentation stability, contraction and shrinkage strain during solidification. The above factors lead to formation of channels able to conduct formation fluid both along cement stone body and its contact zones (cement stone formation, cement ston
5、e casing string). References 1, 2 show that portland cement stone solidified in the annulus as well as against low permeable formation has a worse bond with formation and casing as compared to the stone solidified in an interval of permeable and especially water- saturated formations. The formation
6、of fluid conducting channels in the cement stone is also promoted by operations carried out inside the casing string. We have conducted calculations that show that the clearance forming between the cement stone and casing string can reach 1 mm depending on the value of pressure drop in the casing as
7、 well as on pipe type-size and material. We consider that one of the promising directions to improve quality of well cementing sealing is employment of expanding oil-well cements that provide stress contact of the cement stone with the limiting surfaces. In our opinion the optimum expansion is 2 to
8、4% and contact pressure should be 2.5 to 3.0 MPa. Besides it is important to take a differential approach to kinetics of volumetric deformations of the cement stone. It is necessary that the main part of expansion should take place after pumping cement slurry into the annulus. Besides, the expansion
9、 should begin after the formation of the cement stone structure starts. If the expansion takes place during the cementing process when the suspension is in a liquid state it will not produce a positive effect on the quality of formation isolation. Too late expansion, i.e. after formation of rigid cr
10、ystalline structure can cause fracturing and negatively effect the cement stone porosity. It is especially dangerous is there is possibility of free deformation. The cement stone solidification in a limited space, which is characteristic of wells, will cause generation of stress contact with the lim
11、iting surfaces. In this case big inner stresses can appear decreasing the cement stone resistance to impact loads. The foregoing proves the importance of making a correct cement composition to obtain the best quality of well cementing. PREPARATION OF EXPANDING OIL-WELL CEMENTS Preparation of expandi
12、ng cements is based on introduction of additives forming new substances of a bigger volume when they interact with water or solidification products. To obtain the cement expansion crystalline pressures of calcium hydrosulphoaluminate 3CaO . Al2O3 . 3CaSO4 . 31H2O (three-sulphate form) or 3CaO .Al2O3
13、 . CaSO4 . 12H2O (mono-sulphate form) are used. For this compound crystallization it is necessary that the solution should contain ions of Ca2+, Al3+, SO42- and pH12. Gypsum and gypsum containing substances can be used as an expanding additive. It is possible to use a mixture of gypsum with high-alu
14、mina binder or high- alumina slag or anhydrous calcium sulphoaluminate. The resultant cements are quick- solidifying, have late expansion, work at the temperatures of 20 to 80 0C are employed in the construction industry. Cements with oxide expansion are developed that are used for well cementing. T
15、heir expansion is provided due to formation of low soluble magnesium and calcium oxides that remained in the cement in a free state during clinker roasting. The high roasting temperature causes low chemical activity of the above oxides that is why at a temperature lower than 50 0C they hydrate slowl
16、y. It causes inner stresses after the cement stone has lost ability to plastic deformation. That is why the expansion is accompanied by fracturing and stone strength decrease. The hydration rate of CaO and MgO can be controlled due to the roasting temperature and dispersion degree of the additives.
17、In this connection it is reasonable to prepare expanding cements with CaO and MgO by mixing portland cement with pre-roasted additives. To prepare special cements chromate-aluminate additives are perspective. The expansion is provided by formation of high-chromate form of calcium hydroaluminate (3Ca
18、O . Al2O3 . 3CaSO4 . 31H2O) and accompanied by increase of the solid phase by 4.6 times as compared to C3AH6. According to this pattern the expansion of sulphaferrite cements takes place only due to formation of calcium hydrosulphaferrites. They differ in slowed-up structure formation at normal temp
19、eratures. The expansion during solidification is characteristic of conventional portland oil-well cements mixed on mineralized liquids. The expansion is caused by negative contraction accompanied by crystallization of salts from the liquid during the stone solidification. Generalizing it we can note
20、 that the expansion nature is in crystallization pressure of the solid phase that is not able to take its place in the porous space of the solidifying system. At the same time there is contradiction. The matter is that during hydration of all the clinker minerals the volume of solidification product
21、s increases as compared to the volume of the solid phase undergone the reaction. But even in this case the volume of the products providing expansion is less than pore volume formed in the stone during full hydration of the cement. It means that possibility of expansion is determined by that extent
22、to which the space previously occupied by the liquid phase will be filled with the solid phase. The possibility of crystal intergrowth in pores is determined by relation of pore size and liquid phase oversaturation value. The condition of crystal growth is expressed by the equation rRT V C C m = 2,
23、1 0 2 ln , (1) since 1,2- surface tension, Vm - molar volume, R - universal gas constant, T - temperature, r pore radius. It follows from this equation that crystalline hydrates of the expanding additive will intergrow in pores in the size of )/ln( 2 0 2, 1 CCRT V r m . (2) If oversaturation C/Co is
24、 constant, the smaller Vm the bigger probability of pore intergrowth with new crystalline hydrates. As noted before to provide the effective expansion to the expanding additive hydration it is necessary to form the frame of the solidifying cement. If the formed frame consists of pores with the radiu
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