Theoretical model for relationship between well spacing and oil production by CO₂ immiscible flooding in the extra-low permeable reservoir

Establishment of well spacing model according to the anticipated oil production has important significance to determine reasonable well spacing density.Based on non-linear seepage theory of extra-low permeable reservoir, consideringthe effect of CO₂ to reduce oil viscosity and the oil phase starting pressure gradient change, one-dimensional mathematical model for the seepage by CO₂ immiscible flooding was established.By the application of Buckley-Leverett displacement theory and seepage theory, the theoretical model for the relationship between well spacing and oil production by CO₂ immiscible flooding in the extra-low permeable reservoir was built and the well spacing calculation software was developed for the example calculation and theoretical chart drawing.The results show that the well spacing is maximum with consideration of both viscosity and starting pressure gradient changes of the oil phase.In the early stage of displacement, the oil phase starting pressure gradient has significant effect on the well spacing while oil phase viscosity has small effect on the well spacing.In the later stage of the displacement, the effect of the oil phase viscosity on the well spacinghas gradually become stronger.Well spacing becomes smaller with the increase of the anticipated production.The less the anticipated production becomes, the faster the well spacing increases.The well spacing becomes smaller with the increase of the initial viscosity of the formation oil and initial starting pressure gradient of the oil phase.Moreover, the theoretical model for the relationship between well spacing and oil production has offered a rapid calculation method for the well spacing calculation on the basis of anticipated production and provides the criteria for the well network deployment in oilfield.