This paper wae praaerrtadat the SPE 1985 Reservoirsimulation Symposiumheld in Dallas, Texaa, February 10-13, 1SS5.The material is subjectto corractiin by the author. Permission to mpy is restricted to an atwtrect of not more than 300 wade. Write SF'E, P.O. sax S3S836 Rkhafdam Te=s 750SMS36. Telex 7309SS SPE DAL: ABSTRACT fully implicit scheme. It is almost as 1ow as that A new fully implicit formulation for composi-required by an IMPES scheme. Llurrdl > Imuldbur-s iS pI-e>eIIbe U.~a~~e r~~~fi~o~~ing for pressure, saturations and phase compositions, the new formulation solves for pressure, INTRODUCTION overal1 concentrations and "K-values. This change Compositional reservoir simulators are imporof primary unknowns to be solved improves nunerical tant tools for predicting the performance of oi1stability by yielding a more diagonally dominant Jacobian.The use of K-values as primary unknowns recovery methods when oil and gas undergo vigorous interphase mass transfer during the recovery also allows the composition constraints to be process. These processes include (1) nitrogen solved separately from the other equations.The injection into a gas condensate or volatile oil solution of the constraint equations is very effireservoir during the primary production period, and cient because they can be reformulated as monotonic or enriched gas injection during the functions*. The primary unknowns are further clas-~;~a~~~recovery period.Recently, compositional sified into reduced-unknowns and pivotal-unknowns.simulators have become even more important as more The latters are eliminated from the mass consergas condensate and volatile oil reservoirs are vation equations using the fugacity equalities and discovered with deeper drilling and as more constraints.The pivotal-unknowns are selected enriched gas or C02 gas injection enhanced recoverY according to the sensitivity of the equations to projects prove successful. It is also being recogthe unknowns.This selection conforms to the nized that this type of model can be useful in partial pivoting strategy of Gaussian elimination studying "black-oil-type" reservoir with the crude and enhances numerical stability.Finally, a oil being characterized by two hydrocarbon componpartial solution method is introduced to eliminate entsl-2. This importance as well as usefulness has unnecessary calculations of those equations which motivated a continual development of more efficient have met the convergence criteria. and reliable compositional simulators. For a simple system, it is shown that the newThe development can be classified into two formulation results in a more diagonally dominant categories. The first category concerns the Jacobian matrix.Numerical experiments involving development of simple and reliable mathematical one dimensional multicontact-miscibile (MCM) andrepresentations of physical phenomena. Modelling immiscible problems, a two dimensional MCM problem of fluid phase behavior and properties3-8 interand a three dimensional MCM problem are conducted facial tensiong-lo, relat...
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