A Study On The Complex Phase Behavior Theory Of Multiphase Fluid In Deep Gas Condensate Reservoirs And The New Method To Improve Gas Well Productivity

The theoretical study on complex phase behavior of multi-phase fluid in gas condensate reservoirs and the study on new methods to improve gas well productivity have been the key subjects in oil &gas reservoir exploitation, rather tough and profound. As more and more high-temperature and high-pressure gas condensate reservoirs were explored, the influence of formation water on fluid phase behavior and the exploitation of such gas reservoirs cannot be ignored any more. Moreover, large reductions in well productivity can occur in gas condensate reservoirs during pressure depletion due to the buildup of condensate saturation ring or water blocking of inverse water imbibitions near the wellbore. So, in this thesis, the subjects of the research about oil/gas/water PVT phase behavior experiments and theory considering formation water of gas condensate reservoirs were proposed, including the research of the new methods to remove plugging mentioned above. This will result in great significance on optimizing the process of the exploitation of HT/HP gas condensate reservoirs and on the phase equilibrium theory of multi-phase fluid containing polarity substance.By integrating experiment study with theory and application study, the following works have been done:(1) Employing PVT equipments (RUSKA-PVT and DBR-PVT) of the Key State Lab of Oil &Gas Reservoir Geology and Exploitation, the real phase behavior of gas condensate reservoirs considering the existence of formation water were investigated, including the content of condensate water in gas under HT/HP conditions, the effect of formation water on dew point pressure, P-V relation and constant volume depletion experiments of gas condensate fluid. The phase behavior of the mixture of formation water and methanol were tested. Also, the compatibility tests were conducted to ensure the injection of methanol does not cause salt out.(2) Using the long core flooding equipment from Canada Hycal corp. in Key Lab, dry gas huff and puff with methanol plug and nitrogen huff and puff had been performed for gas condensate reservoir, which provided the experimental base to search the new method to remove the plugging of condensation and water blocking, so as to improve the gas well productivity during the middle and latter exploitation period of deep low permeability gas condensate reservoirs.(3) Based on the K evaluated verifying method of phase stability of Nelson, the stability judgment criterion for three phases (gas-liquid-liquid) was set up, and the three phases flash calculation model was established combining material conversation law by applying Equation of State (EOS) to describe the phase behavior of gas and liquid.(4) The thermodynamics model was established for hydrocarbon mixture containing polarity, which combining cubic EOS (PR, LHSS) with G~E activity coefficient model. And the activity coefficient model of UNIFAC was used here.(5) Applying the multi-phase equilibrium theory presented in this paper, the modelingmethod of the three-phase PVT experimental and the P-T phase diagram for condensate gas reservoirs containing formation water were investigated. The calculation of potential content of condensate water in HT/HP gas reservoirs and the phase mechanism of condensate water produced were also investigated systematically, as well as the method to judge whether there exists mobile water from the above analysis. Eventually, the vaporized phase behavior of formation water near-wellbore region and its effect on well gas productivity were studied.(6) The thesis provided an insight into the phase mechanism of gas huff 'n' puff injection technique in removing the liquid dropout accumulation, and a gas injection sample was tested. On the other hand, the characteristics of well gas productivity loss resulted from condensate liquid accumulation was studied, as well as the mechanism of increasing well productivity by gas injection. Furthermore, the research on the effectiveness of the methanol injection to the phase behavior of gas condensate fluid was performed.Through the work mentioned above, some achievements and conclusions are as follows:(1) The solubility between gas and formation water ought to be considered under HT/HP gas condensate reservoirs. The existence of formation water makes the dew point pressure decreased and liquid dropout increased. When the pressure drops, the producing condensate water increased in the constant pressure depletion tests considering formation water. The P-T phase diagram of gas condensate containing formation water can image the real phase behavior characteristics well, compared with the one not considering formation water.(2) With the methanol injection, the vapor pressure of formation water increases, which will do well to the vaporizing of the water. Water blocking can be removed by methanol injection, by which the gas injection pressure can be decreased. And the long core gas huff 'n' puff injection experimental results indicate that it is feasible to inject methanol slug before gas injection to remove water blocking and gas puff 'n' huff can remove the accumulated condensate liquid.(3) The thermodynamic model based on the EOS of PR and LHSS combining GE activity coefficient model (UNEFAC) can describe the phase equilibrium of hydrocarbon mixture containing polarity well.(4) The potential water content in gas reservoirs under formation condition and the produced mechanism of condensate water can be forecasted successfully by applying multi-phase equilibrium theory. Furthermore, some new views are attained and the dual condensation phenomena mentioned in some documents does not occur during the period of isothermal pressure drop.(5) The effectiveness of the amount of gas injection, gas component and injecting time on the result of gas huff 'n' puff injection was evaluated. As the near-wellbore pressure drops below the dew point pressure, retrograde condensation occurs leading to the accumulation of liquid phase in the near wellbore region, forming a ring, which progressively reduces the gas deliverability. The study shows that huff 'n' puff is a viable option for reducing the liquid blockage in the near-wellbore region and improves the gas well productivityeffectively.(6) The results of study on the phase behavior of mixture of methanol injection and gas condensate show that with the increase of methanol volume injected, bubble point phase state characteristic can be formed from dew point phase state characteristic for condensate gas system. So, during injecting methanol to remove water blockage near-wellbore, the volume of methanol injection should be appropriate or be optimized.