In recent years considerable effort has been expended on the development of equipment, such as downhole steam generators(3,6), which creates mixture of steam and combustion gases for thermal oil recovery applications. Questions have been raised regarding the effect on reservoir production performance of injecting such mixtures and thus a study was undertaken(l-2) to develop laboratory data and a numerical model capable of stimulating me processes.
The present paper describes the development and application of a fully implicit thermal reservoir stimulation mode1. The model was used to examine parameter sensitivities which allowed determination of factors controlling the laboratory processes and better interpretation of the experimental results, The numerical model has demonstrated the relative importance of gas drive and solubility effects in the improvement over steam-only flooding observed in the steam/gas injection processes, The dominance of thermal effects on the process behaviour is also shown.
A study(1) was undertaken to examine the effects on reservoir performance of injecting mixtures of flue gas and steam as would be created when employing downhole steam generators or like equipment.
A previous publication(2) presented results from unsealed laboratory experiments conducted in a linear system originally saturated with a moderately viscous refined. oil and water. The experiments involved injection of steam/CO2 steam/CO2/N2 and steam/N2 mixtures in the approximate proportions which would result from recombination of steam and the combustion gas products created in raising the steam.
The present paper describes the development and application of a fully implicit numerical simulation model which was written to aid in interpretation of the experimental results. Parameter sensitivity studies were conducted to define the controlling factors in the laboratory processes and several experiments were history matched. The results of these applications of the mathematical model are also included.
Downhole steam generators have several potential advantages over conventional surface generators including (3–4):
reduction in wellbore heat losses.
elimination of stack and flow line heat losses.
Reported(l9–23). Because of the large number of equations and the resultant high computing cost for solution of thermal simulation problems, much emphasis in recent years has been placed on model stability, automatic time step selection and speed of matrix solution methods. Coats(20) concluded on the basis of experience with steamflood modelling that total computer time for thermal simulations decreases with the degree of implicitness. Code intensive variable substitution(20,22) techniques have been employed with success for handling the instabilities which commonly arise in thermal process simulation because of phase disappearance and reappearance. Powerful combinations of direct, sequential and iterative methods have been used for the solution of large scale thermal problems and error controlled time step selection assists in the minimisation of computing time(22).
Weinstein(24) presented simulation results for steam stimulation with natural gas injection in a Cold Lake reservoir. Runs were made to compare the effect of injecting gas before and after the steam. The addition of gas had a marked effect on production performance with average oil rate and oil-steam ratio increased. Gas injection following steam was found to be the best alternative.