| There are a lot of oil and gas reservoirs covered by salt-gypsum at home and abroad, if we want to develop these blocks, we must broach the salt-gypsum. However, salt-gypsum cementing has been a world-recognized problem for a long time, what’s more, this problem has not been fundamentally solved by years of research.Thick-walled casing and grout combination cementing is a necessary and ordinary cementing measure during the drilling. In the salt-gypsum cementing operations, especially in deep salt-gypsum, thick salt-gypsum and other complex wells cementing, use conventional grout may bring following problems:firstly, In the cementing phase, due to the concentration effect, the salt in formation dissolved in the grout will make grout thickening or gelling. Secondly, Salt dissolved in cementing slurry can expand wellbore, and cause sidewall irregular, impact drilling operation.The third, in the the grout hardening phase, continued dissolution of the salt strata, not only affects the rate of hardening of the grout, but also affect the strength of the hardened. It will also form a solubility cracks in the cement sheath and salt paste layer, may provide channels to underground oil, gas and water channeling.The fourth, because of salt dissolution persists, the strength of cement formed by the. conventional slurry is low, the cementation quality is poor, it can not effectively resist the external squeeze pressure caused by creep of the salt layer formation, the external squeeze pressure can lead to thick-walled casing deformation, the downhole operation can not be continued, the thick-walled casing can even been broken and collapsed, the service life of the oil and gas wells will be shortened.The fifth, NaCl and CaSO4are all strong electrolytes, their affection on cement slurry property is complex, the affection mainly as cement slurry dispersed, density increased, accelerated, retardation and so on, these would cause filter loss and densify time can not be controlled. Besides, when the salt content is high, the cement hydration process to consume part of the water, making the slurry liquid phase concentrated salt oversaturated and crystallized out.The crystals will produce a certain impact on cement strength.In order to ensure the quality of the salt-gypsum cementing, according to the salt-gypsum features, it is necessary to proceed from the following aspects to build a slurry system:1. Slurry has a high density and high temperature resistance, because the salt-gypsum buried deep, relatively high temperature, so the slurry should add high-temperature resistant materials and weighting agent. 2. Slurry has good rheological, a good the rheology is the an important guarantee of the cement slurry smooth pump from the ground into the wells and filling sufficiently. So, in the construction of the slurry system, rheology is one of the key factors. Salt and calcium in salt-gypsum dissolved in the slurry causes the slurry thickening or gelling, is not conducive to pumping. So we request the slurry is salt-resisting and calcium-resisting.3. Slurry has a small filter loss, if the filter loss is to much may cause the salt-gypsum swelled, hole diameter reduced, at last, the slurry is difficult to pump. At the same time salt dissolved into the slurry can cause slurry system performance mutation. So,the filtrate reducer is very important, it determines the safety and success of the whole cementing. An salt-resisting filtrate reducer is needed.4. Cement paste with high strength. Salt in salt-gypsum dissolved in slurry make the strength of cement been low, cementing quality can be affected. So, we should add adaptable intensifier into the slurry.5. Slurry densification time is adjustable, salt and calcium have accelerating and delayed coagulation effect on slurry, it affect the densification time and inversion time. Therefore, screening out suitable accelerant and extender is necessary.6. Cement has expansion after hydration, generally speaking, after solidification of slurry, the volume will occur, large or small contraction, reduced quality cement wall, resulting in gas channeling, affecting the cementing quality and even cause accidents, resulting in cementing failure, so it is necessary to select a bulking agent.Combining these slurry system design ideas, referencing to cement slurry experimental methods in API specification and other relevant standards, and applying our own design and evaluation methods to select expanding agent, we confirm main agents of the high density NaCl supersaturated slurry after a large number of select experiments,such as:CG88L(salt-resisting filtrate reducer), CF44L(dispersant), H21L(extender),CX66L(antifoamer),Exp-1(expanding agent), MX(intensifier),Tips(intensifier and gas block agent).In the salt-gypsum cementing process, for different strata burst pressure, we need to design commensurable density of the slurry to ensure well wall in a stable situation, and achieve the best sealing results. When we design the slurry system, we should make the density adjustment simple and conveniently. Through a series of laboratory experiments, we find that we can adjust the density of slurry by adjusting the ratio of the weighting agent. While the ratio of water, filtrate reducer and other agents should be adjusted too. In the end we construct1.90～2.40g/cm3slurry formulations: 1.90g/cmJ cement slurry formulation:100percent (Class G cement)+49%water+10%CG88L(salt-resisting filtrate reducer)+20%NaCl+1%CF44L(dispersant)+0.5%H21L(extender)+0.5%CX66L(antifoamer)+1%Exp-1(expanding agent)+10%MX(intensifier)+1.5%Tips(intensifier and gas block agent);2.0g/cm3cement slurry formulation:100percent (Class G cement)+50%water+13%CG88L(salt-resisting filtrate reducer)+20%NaCl+1%CF44L(dispersant)+0.5%H21L(extender)+0.5%CX66L(antifoamer)+1%Exp-1(expanding agent)+10%MX(intensifier)+1.5%Tips(intensifier and gas block agent)+15%weighting agent;2.10g/cm3cement slurry formulation:100percent (Class G cement)+53%water+14%CG88L(salt-resisting filtrate reducer)+22%NaCl+1%CF44L(dispersant)+0.5%H21L(extender)+0.5%CX66L(antifoamer)+1%Exp-1(expanding agent)+10%MX(intensifier)+1.5%Tips(intensifier and gas block agent)+52%weighting agent;2.20g/cm3cement slurry formulation:100percent (Class G cement)+53%water+13%CG88L(salt-resisting filtrate reducer)+22%NaCl+1%CF44L(dispersant)+0.5%H21L(extender)+0.5%CX66L(antifoamer)+1%Exp-l (expanding agent)+10%MX(intensifier)+1.5%Tips(intensifier and gas block agent)+80%weighting agent;2.3g/cm3cement slurry formulation:100percent (Class G cement)+57%water+15%CG88L(salt-resisting filtrate reducer)+23%NaCl+1%CF44L(dispersant)+0.5%H21L(extender)+0.5%CX66L(antifoamer)+1%Exp-1(expanding agent)+10%MX(intensifier)+1.5%Tips(intensifier and gas block agent)+118%weighting agent;2.40g/cm3cement slurry formulation:100percent (Class G cement)+50%water+15%CG88L(salt-resisting filtrate reducer)+24%NaCl+1%CF44L(dispersant)+0.5%H21L(extender)+0.5%CX66L(antifoamer)+1%Exp-1(expanding agent)+10%MX(intensifier)+1.5%Tips(intensifier and gas block agent)+150%weighting agent;(PS:If the temperature exceeds93℃, we should change0.5%H21L into0.5%H63L (high-temperature extender), and add35%fine silica to reduce the strength attenuation in the high temperature)After a comprehensive performance evaluation for above different density of slurry. we can find that:at60℃, slurry have a good rheological property.it meet on-site pumping requirements, at90℃,HTHP filtration are less than50ml. compressive strength are all more than14MPa.The slurry has a certain capacity expansion and wall cementing property, calcium salt resistance performance is good, the flushing fluid and good compatibility barrier fluid at different temperatures adjustable thickening time, to achieve a good sealing effect is to delay the well life.From the evaluation results, the constructed high-density oversaturated NaCl salt slurry system to meet the needs of salt-gypsum cementing. |
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