A = a = = ideal gas state Subscripts c = critical property i, i, k = component identifications o = reference state A group contribution molecular model is developed for the thermocluding energy of vaporization, pVT relations, excess properties, and activity coefficients. The model is based on the cell theory in which the repulsive forces of molecules are expressed with a modified cell partition function derived from the Carnahan-Starling equation of state for hard spheres. The attractive forces are made u p of group pair interaction contributions. Group and interaction properties have been determined for methyl, methylene, hydroxyl, and carbonyl. Extensive comparisons are School of Chemical Engineering made of predictions of the model with data for pure liquids and their solu-Purdue University tions. West Lofayette, Indiana 47907 1. To develop a comprehensive theory of group contribution for the estimation of various thermodynamic

Numerical simulation of miscible EOR processes requires calculation of the phase equilibria that exist between solvent and oil over the entire solvent/oil composition range. One calculation approach is to tabulate K-values from experimental data and allow the simulator to access the table when necessary. This approach can lead to erroneous conclusions for CO2 miscible processes if there are insufficient tabulated phase equilibrium data to cover all possible compositions, in particular those cases where three phases exist at some point in the displacement. This paper presents a different approach to the problem. If a generalized equation of state (EOS) can match experimental data, then it can be used in a reservoir simulator to calculate the phase equilibria necessary for the prediction of fluid compositions, densities, and viscosities during a displacement process. Previous work has shown how a generalized Redlich-Kwong equation can be used to calculate typical hydrocarbon phase behavior relationships that exist in condensate and black oil reservoir systems. The equation parameters have been modified further for use in hydrocarbon/CO2 calculations over a wide range of CO2 concentrations. Appropriate mixing niles for description of the phase equilibria in CO2/hydrocarbon systems have been developed. Experimental binary vapor/liquid equilibrium (VLE) data have been used to evaluate the constants in the EOS for pure CO2 as well as interaction terms used within the mixing rules. Predictions of phase equilibria then have been made and compared with experimental data for a synthetic multicomponent hydrocarbon/CO2 system and a crude oil/CO2 system. Introduction Current interest in miscible EOR methods has led to the use of compositional reservoir simulators to understand and evaluate performance. An essential pan of such a simulation is a means of predicting the complex phase equilibria possible during EOR processes. However, because of computational time constraints, the numerical complexity of such a technique must be limited. In recent years, a number of relatively simple EOS have been developed and applied to hydrocarbon phase equilibria calculations. One such development is the generalized Redlich-Kwong EOS by Yarborough. This EOS has been applied to many reservoir fluid property calculations with excellent results because of its simplicity. For the simulation of CO2 miscible EOR processes, an EOS must be capable of predicting phase equilibria over a wide range of CO2 compositions. CO2/hydrocarbon mixtures can exhibit complex phase equilibria -e.g., liquid/liquid immiscibility, liquid/liquid/vapor equilibria, and asphaltene dropout. While it is highly unlikely that any simple EOS can provide accurate phase equilibria predictions for all these situations, the generalized Redlich-Kwong EOS referenced previously has been adapted to provide adequate phase equilibria predictions for CO2/hydrocarbon systems over a wide range of conditions. This adaptation involves the use of special parameters to describe pure CO2 above its critical temperature and of modified mixing rules to describe CO2/hydrocarbon mixture behavior. General functions are developed for the parameters involved to permit interpolation (or extrapolation) to other systems and conditions. Literature binary CO2/hydrocarbon VLE data were used to establish these parameters. The generalized EOS was tested through comparisons of predicted phase equilibria with experimental data for a number of binary and multicomponent CO2/hydrocarbon systems. The multicomponent systems include CO2 /synthetic oil and CO2/reservoir oil data presented in this paper. SPEJ P. 308^

Production Co. s% -. wl"+=:"--I Summary.The design of miscible C02 recovery methods and the evaluation of laboratory C02 coreflood and pilot field studies require knowledge of the phase behavior encountered in such processes and the ability to make reliable predictions. Because of the mmplexity of C02/hydrocarbon phase behavior, experimental measurements are necessary as a basis from which to develop an understanding. In this paper, measured phase equilibria and volumetric properties are reported for several west-Texas-resemoir-oiJ/C02, systems. Reservoir oils studied were from the San Andres, Grayburg, and Devonian Chert formations, Both static (singlecontact) and mukiple-contact measurements have been conducted in a visual fluid property cell. Static data cover a wide range of C02 compositions and provide a general understanding of C021reservoir-oil phase behavior. Multiple-contact measurements in which the C02-rich phase is repeatedly contacted with recombined reservoir oil (oil cycling)"simtdaie phase behavior occurring at the flood front, Multiple-contact measurement in which the oil-rich liquid phase is repeatedly contacted with pure C02 (C02 cycling) simulate the phase behavior exhibited by residual 01 near a C02 injection wefl. The mtdtiple-contact data cover narrow compositional paths encountered in displacement processes and serve as a basis for equation-of-state (EOS) evaluation. Phase behatior trends common to afl systems ze discussei

Summary This paper describes the effects of gas solubility on the properties of oil-based drilling fluids. Prediction methods for gas solubility in oil-based muds were tested by experimental work and found acceptable. Expansion of discrete mixtures of mud and gas are predicted. A blowout simulation program was written and used to predict the effects of a kick on the surface-observable indicators (pit gain, well flow). While the bulk of this paper emphasizes the properties of oil-based muds, a comparison with water-based muds under similar conditions is made. The general conclusions are applicable to both types of muds. It is concluded that pit gain is the most reliable indicator of a kick during drilling in either oil- or water-based muds. It is recommended that a pitlevel measurement system be designed that will detect a pit gain of less than 5 bbl [0.795 m3] in the whole mud system. Introduction Normally, gas or gas-condensate mixtures enter the wellbore only through production equipment under controlled conditions. However, during the drilling of a well, an unexpected over pressured zone may be encountered such that a gas or oil influx can occur. This condition, known as a "kick," can be very dangerous to the crew, rig, and ancillary equipment and can produce a blowout. The events surrounding a kick or a blowout are usually poorly documented because of the highly charged atmosphere and lack of time to record the event. Detailed data on a blowout are very rare. Gas and gas-condensate solubilities in oil and in water are topics that have been studied extensively by reservoir and production engineers. Their work has produced equations of state (EOS's) that will adequately describe reservoir fluid properties under differing pressure and temperature regimes. One such equation of state is called the Amoco Redlich-Kwong equation of state (ARKES).1 Results of this development were used heavily in this paper. Multiphase flow pressure drop models have been developed in the past.2 These models commonly use slightly simpler EOS's or data correlations to predict the PVT behavior of a fluid. Since production is a dynamic phenomenon, they account for viscosity effects in the fluid and migration of gas bubbles in the wellbore fluid. The work reported here is an application of multiphase flow and PVT models to the interaction of drilling fluids with produced gas or gas condensate during a kick. Gas Solubility in Oil and Water Gas Solubility in Oil. Calculation of gas solubility in complex hydrocarbon mixtures at elevated temperatures and pressures requires an advanced EOS such as the Redlich-Kwong EOS3:Equation 1 For pure components, parameters a and b can be transformed into dimensionless parameters Oa and Ob:Equation 2 and 3 Generalized correlations were developed for Oa and Ob by Yarborough.1 These are incorporated into ARKES, which is applicable to a large variety of components over a wide temperature range. As discussed by Yarborough,1 ARKES parameters for C7+ boiling-point cuts are assigned by use of these correlations together with API nomographs and knowledge of the average C7+ molecular weight and specific gravity. To apply the EOS to mixtures, a and b parameters characteristic of particular mixture compositions are derived from pure component parameters through use of the following mixing rules.Equations 4 and 5 Nonzero values are employed for the unlike-pair interaction parameter, Cij, for nonhydrocarbon/hydrocarbon pairs and nonhydrocarbon/nonhydrocarbon pairs. Gas Solubility in Water. The solubilities of hydrocarbon gases in water are very small compared with the solubilities of the same gases in oil. In most of the work discussed here, gas solubility in water could be safely ignored; however, it was not. In all the calculations described, gas solubility in water has been estimated from literature values.4 The amount soluble is less than 1% of the amount soluble in oil at the same temperature. Gas Solubility in Oil. Calculation of gas solubility in complex hydrocarbon mixtures at elevated temperatures and pressures requires an advanced EOS such as the Redlich-Kwong EOS3:Equation 1 For pure components, parameters a and b can be transformed into dimensionless parameters Oa and Ob:Equation 2 and 3 Generalized correlations were developed for Oa and Ob by Yarborough.1 These are incorporated into ARKES, which is applicable to a large variety of components over a wide temperature range. As discussed by Yarborough,1 ARKES parameters for C7+ boiling-point cuts are assigned by use of these correlations together with API nomographs and knowledge of the average C7+ molecular weight and specific gravity. To apply the EOS to mixtures, a and b parameters characteristic of particular mixture compositions are derived from pure component parameters through use of the following mixing rules.Equations 4 and 5 Nonzero values are employed for the unlike-pair interaction parameter, Cij, for nonhydrocarbon/hydrocarbon pairs and nonhydrocarbon/nonhydrocarbon pairs. Gas Solubility in Water. The solubilities of hydrocarbon gases in water are very small compared with the solubilities of the same gases in oil. In most of the work discussed here, gas solubility in water could be safely ignored; however, it was not. In all the calculations described, gas solubility in water has been estimated from literature values.4 The amount soluble is less than 1% of the amount soluble in oil at the same temperature.

The use of continuous vis4-vis discrete descriptions for the C7+ portion of a live oil is examined for a calculation especially sensitive to the nature of the C7+ description, specifically, liquid dropout near an upper dew point of a live oil + gas mixture. Conservation-of-mass failure in the semicontinuous flash problem, a recognized formal shortcoming of continuous thermodynamics, is shown to lead to a previously unexplained bias that occurs in saturation calculation results.