Study On The Effects Of Formation Pressure Decrease On Gas Well Productivity In Sandstone Gas Reservoir

The rock and fluid medium of the sandstone gas reservoir are regarded as the study object in this thesis.By integrate rock deformation experiment,theory study and practice,a quantitative evaluation theory used to analyse the effects of formation pressure decrease on gas well and producing water gas well deliverability has been founded.Non-linear elasticity deformation rock porosity and permeability have been measured by simulating effective pressure increase experiment.Thanks to rock volumetric strain and rock frame volumetric strain defined, the theoretical formulas including the linear elasticity deformation and non-linear elasticity deformation rock volumetric strain,rock porosity,rock permeability considering tortuosity change, bound water saturation, average capillary radius,average capillary pressure,capillary pressure curve, gas relative permeability,water relative permeability and gas/water relative permeability curve have been developed in order to analyze quantitatively them.By means of defining generalized pseudopressure with specific permeability for single phase gas flow and pseudopressure with specific permeability for gas/water two-phase flow,and considering the formation pressure decrease, gas well non-steady and pseudostationary deliverability equation have been developed for single phase gas Darcy, high speed non-Darcy, low speed non-Darcy considering kickoff pressure gradient and low speed non-Darcy considering slippage effect flow;producing water gas well non-steady and steady deliverability equation have been established for gas/water two phase gas Darcy and high speed non-Darcy flow; these deliverability equations have been applied to analyse the effects of formation pressure decrease on gas well and producing water gas well deliverability in the example gas reservoir.After introducing specific inertial coefficient,the new methods including single-shot steady law, systematic well testing, modified isochronal test for gas well deliverability, modified isochronal test for producing water gas well deliverability have been proposed and applied to evaluate deliverability in the example gas reservoir.The theoretical formulas of the rock porosity which have been developed by means of the rock deformation experiment and the petrophysical parameter change study supported the view-point of“porosity change is definite”.The method of“trial calculation & iteration”, used to obtain the rock non-linear elasticity deformation constants from the experimental data,provided the theoretic foundation in order to calculate quantitatively the rock non-linear elasticity deformation and make the data conversion from the surface conditions to reservoir conditions come true.The theoretical formulas including bound water saturation, capillary pressure, gas relative permeability and water relative permeability helped to forecast exactly capillary pressure curve and gas/water relative permeability curve of formation pressure decrease.The defining generalized pseudopressure with specific permeability for single phase gas flow and pseudopressure with specific permeability for gas/water two-phase flow took the direct and indirect influential factors of formation pressure decrease into account and made the developed gas well and producing water gas well deliverability equations accord with field practice.The specific inertia coefficient defined made the factor A and B of turbulent flow equation become constants,so the definition provided the theoretic foundation so as to evaluate the gas well and producing water gas well deliverability of formation pressure decrease.The coincidence rate between rock porosity & permeability calculated and measured is preferable;The evaluation results of the effects on gas well & producing water gas well deliverability indicate that the deliverability is descending during the formation pressure decrease and that the descending feature is accorded with field production performance.The gas well & producing water gas well deliverability of considering formation pressure decrease with the new evaluation method is slightly higher than the normal evaluation method,so the results are more reasonable.