| Heavy oil,as an unconventional oil and gas resource with large reserves,requires special recovery technology to improve oil recovery.At present,steam huff and puff recovery technology is most widely used.However,most relevant data of steam huff and puff wells show that with multiple rounds of injection of high-temperature steam,the number of steam channeling wells in the block increases linearly,and the steam channeling path covers a large area of adjacent wells,resulting in long shut down time of oil wells,which seriously affects the recovery and wellbore life.Annular cementing system is the weak link of wellbore,and cement sheath is characterized by low strength and high brittleness,which is easy to be damaged.Cement sheath is characterized by low strength,high brittleness and easy to be damaged.To clarify the failure mechanism and conditions of annulus cement sealing system for steam huff and puff wells under high-temperature cycle conditions,and on this basis,propose optimization methods for parameters such as mechanical properties of cement paste and corresponding construction parameters,which have important academic value and engineering significance for maintaining the integrity of annulus cement sealing system,ensuring production safety and prolonging the life cycle of oil wells.In this paper,the annulus cementing system of steam huff and puff well is taken as the research object,and the meso damage model of cement sheath is established to study the meso fracture development and mechanical property degradation of cement sheath under high temperature cycling conditions;The influence of high temperature cycling on the plastic zone expansion of cement sheath and the size of interfacial micro annulus is studied by the thermal cycling plastic damage strain calculation model of cement sheath;Based on the failure modes and conditions,the optimization indexes of mechanical parameters and construction parameters of cement sheath to meet the integrity of annulus cementing system of steam huff and puff well are proposed respectively.The main research contents are as follows:(1)Taking three kinds of cement stones commonly used in steam stimulation Wells as the research object,laboratory experiments such as X-ray diffraction,SEM scanning electron microscope and X-CT scanning were carried out to analyze the change rule of phase composition,microstructure and meso-crack distribution of cement stones of different systems after high temperature cycling.Combined with the mechanical properties experiment of cement stones after high temperature cycling,The effect of hydration products and internal microstructure of cement on macroscopic mechanical properties of cement was studied.The compressive strength of cementite increases first and then decreases,and the increase of strength is due to the improvement of the phase composition and microstructure of cementite with sand after the temperature increases from 70℃ to 350℃.On the one hand,the strength reduction is due to the weakening of the microstructure of the cement stone caused by the continuous high temperature,on the other hand,the circulating thermal stress makes the cement stone appear meso and macro cracks.The strength deterioration of cement is consistent with the damage of meso structure.(2)The meso damage discrete element model of cement sheath is established.The thermal mechanical stress of cement sheath under high temperature cycling is restored by applying a two-dimensional ring shear servo boundary,and the initiation and propagation of meso cracks in cement sheath are simulated.On this basis,the meso damage variables about the number and length of micro cracks are proposed,and the described damage law can fully match the macro strength degradation process of cement sheath.The influence of different mechanical parameters on the deterioration of cement sheath strength is analyzed: with the increase of heating rate,elastic modulus of cement sheath,Poisson’s ratio and thermal expansion coefficient,the deterioration of cement sheath strength is accelerated,and microcracks are concentrated from the inner wall to the outer wall.The heating rate mainly affects the length of microcracks;The elastic modulus of cement sheath mainly affects the number of microcracks,and the damage trend increases significantly when it exceeds 10 GPa;Poisson’s ratio and thermal expansion coefficient have little influence on the meso damage of cement sheath;The cracks produced by the cement sheath with heat shrinkage property are short and few,distributed unevenly throughout the cement sheath body,and the damage degree is low.The change with the heat shrinkage coefficient is not obvious.When thermal mechanical stress acts together,the damage accumulation and strength deterioration of cement sheath in the cycle process are obviously intensified.(3)Considering that the plastic zone formed in the heating stage is the main reason for the failure of the interface seal,a finite element model for the expansion of the plastic zone of the cement sheath during a single rise and drop and an elastic-plastic damage model for the interface of circulating high-temperature cementing are established.The plastic damage strain proposed can refine the transition process of the cement sheath inner wall from elasticity to plasticity,accurately judge the interface contact situation during the thermal stress cycle and calculate the corresponding micro annulus size,The effects of different mechanical parameters on the integrity of the cementing interface are analyzed: with the increase of steam injection temperature and the elastic modulus of cement sheath,the interfacial tensile stress increases gradually when the temperature drops,and the size and amplitude of the micro annulus generated increase gradually;Poisson’s ratio and thermal expansion coefficient have little influence on the integrity of the cementing interface.When the cement sheath has thermal shrinkage property,the size of the interface micro annulus exceeds 40μm at the cooling stage.Moreover,other parameters cannot be adjusted to compensate for the influence of thermal shrinkage properties.The thermal expansion performance of cement sheath shall be controlled by comparing the integrity failure degree of cement sheath body and interface.(4)Based on the strength decline of high-temperature circulating cement sheath and the formation conditions of micro annulus at one interface,combined with its conventional macro mechanical failure conditions,an evaluation process for the integrity of steam huff and puff well annulus sealing system is proposed: optimization of anti circulation performance based on meso damage mechanism;Parameter matching optimization based on failure mechanism of cement sheath;Parameter matching optimization based on interface failure mechanism.Finally,taking a case well as an example,a cement slurry system suitable for actual heavy oil thermal recovery conditions is proposed,which provides a theoretical basis and optimization method reference for ensuring the integrity of annular cementing system of steam huff and puff wells. |
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