TitleOn the Validation of PVT Compositional Laboratory Experiments

AuthorsF. Samaniego-V., UNAM; B.J.L. Bashbush, Geoquest; G.A. Leon, Pemex; U.C. Mazariegos, B.A. Corona, P.P.F. Castillo, UNAM

SourceSPE Annual Technical Conference and Exhibition, 26-29 September 2004, Houston, Texas

ISBN978-1-55563-151-2

Copyright2004. Society of Petroleum Engineers

PreviewAbstract The validation of compositional PVT results can be understood as a systematic correction of the inherent errors of these experiments, in a way that the true characteristics of the fluids are preserved. When carrying out a compositional simulation study (flow in the reservoir, pipeline flow, optimization of separation conditions, etc.), five steps must be taken: 1. Collect representative samples; 2. Carry out careful laboratory experiments - the basic procedure for compositional systems gas condensate and volatile oil, is the constant volume depletion (CVD); 3. Validation of the PVT results from step 2; 4. Perform a regression analysis on the validated results of step 3, to fit the parameters of an equation of state (EOS); 5. Simulate the process. This study presents original procedures for the validation (checking of consistency or quality) of compositional PVT results obtained from the CVD experiment. A review of the literature indicates that programs for the validation of these experiments have not been presented. This paper covers this gap. The present work discusses three programs developed by the authors to validate PVT compositional CVD laboratory results. The findings of this study clearly indicate that performing step 4 of the above procedure, without a careful work regarding step 3 can lead in many cases to very serious errors in the regression analysis of step 4 and consequently, in the simulation results of step 5. In general the validation procedure of this study consists of the two main steps as follows; 1. Correct the main PVT inconsistencies, like negative equilibrium constants, crossing of the equilibrium constants curves, graphed in terms of log Ki vs. pressure, and kinks or jumps of these curves. The corrections change the gas phase composition of the components that show inconsistencies and then, the computer program is run again to check whether the new Ki values present inconsistencies; 2. The second step consists of checking that the summation of the gas mole percentages of all components results in a 100 value. The main graph to adjust these summations is that of log Ki vs. the boiling temperature Tb of each component. Three different approaches for this process are discussed, with advantages and disadvantages clearly stated. The first is a conventional manual procedure for the two main steps of the process. The second is a semi-automated procedure that automatically performs the first validation step. It was developed to study PVT reports where the detected error level in the measurements was high, which rendered the first procedure difficult to use, because of the many hours needed to complete the analysis. The third procedure is an enhanced program, where a first version for a fully automated validation procedure is presented. From the many hydrocarbon mixtures analyzed, an example is presented for a gas-condensate, which clearly that serious errors in the simulation results can occur if a proper validation study of the PVT compositional results is not carried out. Introduction The basic PVT experiment for compositional fluids, gas condensate and volatile oils, is the constant volume depletion (CVD) experiment, designed to provide volumetric and compositional data, for reservoirs producing by pressure depletion1-5. As a result of drilling deeper and hotter formations, the number of variable composition reservoirs has increased considerably during the last few decades 6,7. These reservoirs are often found at temperatures near the critical temperature of the hydrocarbon mixture they contain. Fig. 1 shows a graph of depth vs. temperature for a group of variable composition reservoirs, including several critical point fluids. These fluids are characterized by having considerable amounts of intermediate hydrocarbons (C2-C6 ). Table 1 of reference 3 present the compositional analysis of 10 of these reservoir. The temperature and composition of these fluids are the reasons for obtaining large retrograde condensations from the gases and high shrinkage in liquid volumes from the oils.