CO2 flooding strategy to enhance heavy oil recovery

CO_(2) flooding is one of the most promising techniques to enhance both light and heavy oil recovery.In light oil recovery,the production pressure in CO_(2) flooding in general keeps constant in order to maintain the miscibility of injected CO_(2) and crude oil;while in heavy oil recovery,a depleting pressure scheme may be able to induce foamy oil flow,thus the oil recovery could be further enhanced.In this study,different pressure control schemes were tested by 1-D core-flooding experiments to obtain an optimized one.Numerical simulations were conducted to history match all experimental data to understand the mechanisms and characteristics of different CO_(2) flooding strategies.For the core-flooding experiments,1500 mD sandstone cores,formation brine and a heavy oil sample with a viscosity of about 869.3 cp at reservoir condition(55℃ and 11 MPa)were used.Before each CO_(2) flooding test,early stage water-flooding was conducted until the water cut reached 90%.Different CO_(2) injection rates and production pressure control strategies were tested through core-flooding experiments.Experimental results indicated that a slower CO_(2) injection rate(2 ml/min)led to a higher recovery factor from 31.1%to 36.7%,compared with a high CO_(2) injection rate of 7 ml/min;for the effects of different production strategies,a constant production pressure at the production port yielded a recovery factor of 31.1%;while a pressure depletion with 47.2 kPa/min at the production port yielded 7%more oil recovery;and the best pressure control scheme in which the production pressure keeping constant during CO_(2) injection period,then depleting the model pressure with the injector shut-in yielded a recovery factor of 42.5%of the initial OOIP.For the numerical simulations study,the same oil relative permeability curve was applied to match the experimental results to all tests.Different gas relative permeability curves were obtained when the production pressure schemes are different.A much lower gas relative permeability curve and a higher critical gas saturation were achieved in the best pressure control scheme case compared to other cases.The lower gas relative permeability curve indicates that foamy oil was formed in the pressure depletion processes.Through this study,it is suggested that the pressure control scheme can be optimized in order to maximize the CO_(2) injection performance for enhanced heavy oil recovery.

UNCONDITIONALLY OPTIMAL ERROR ESTIMATES OF THE BILINEAR-CONSTANT SCHEME FOR TIME-DEPENDENT NAVIER-STOKES EQUATIONS

In this paper,the unconditional error estimates are presented for the time-dependent Navier-Stokes equations by the bilinear-constant scheme.The corresponding optimal error estimates for the velocity and the pressure are derived unconditionally,while the previous works require certain time-step restrictions.The analysis is based on an iterated time-discrete system,with which the error function is split into a temporal error and a spatial error.The τ-independent(τ is the time stepsize)error estimate between the numerical solution and the solution of the time-discrete system is proven by a rigorous analysis,which implies that the numerical solution in L^(∞)-norm is bounded.Thus optimal error estimates can be obtained in a traditional way.Numerical results are provided to confirm the theoretical analysis.

SUPERCONVERGENCE ANALYSIS OF LOW ORDER NONCONFORMING MIXED FINITE ELEMENT METHODS FOR TIME-DEPENDENT NAVIER-STOKES EQUATIONS

In this paper,the superconvergence properties of the time-dependent Navier-Stokes equations are investigated by a low order nonconforming mixed finite element method(MFEM).In terms of the integral identity technique,the superclose error estimates for both the velocity in broken H-norm and the pressure in L2-norm are first obtained,which play a key role to bound the numerical solution in Lx-norm.Then the corresponding global superconvergence results are derived through a suitable interpolation postprocessing approach.Finally,some numerical results are provided to demonstrated the theoretical analysis.