This paper was prepared for presentation at the 1999 SPE Annual Technical Conference and Exhibition held in Houston, Texas, 3–6 October 1999.
The Quiriquire Deep Field located in the Eastern Venezuela is a large and complex compartmentalized accumulation, with a hydrocarbon column of some 3,500 ft. (3000 ft in the gas cap and 500 ft in the oil rim). The appraised part of the field has an estimated 2.0 TCF of Original Gas in Place both free and associated, and 550 MMBO (50 MMBO from the gas cap and 500 MMBO from the oil leg). There is a very significant variation with depth of the stock tank oil API and the standard gas/stock tank oil ratio within the fluid column both in the gas cap and the oil legs (>50 °API to <15 °API). The multidisciplinary study included a static numerical representation of the geological model, taking into account all available information and integrating the vertical communication among formations and the reservoir compartmentalization as inferred from the available performance/dynamic data (mainly pressures, production tests and fluid types). Given the very limited dynamic information available and the limitations of seismic imaging in this area, a History Matched coarse grid simulation model was used as a Material Balance tool to study reservoir compartmentalization; evaluate volumes in place per compartment; and estimate well counts and overall recoveries for alternative development scenarios. A key part of the model was an accurate representation of the complex PVT behavior, including depth profiles of oil content in the gas, oil API gravity and initial Rs; as well as GOC depth estimates for each compartment. The study supports the blowdown of the gas cap as the most commercially attractive and less risky development strategy. Introduction The Quiriquire Deep Field located in the Eastern Venezuela basin [discovered by Creole (ESSO) in 1952] is a complex compartmentalized accumulation with 3,500 ft of hydrocarbon column (3000 ft in the gas cap and 500 feet in the oil leg). The Quiriquire Deep Field structure is a SW to NE trending thrust anticline (formed during late Oligocene to Early Miocene) 18 Km long and 3.5 Km wide with around 4,300 ft of vertical relief. The anticline is controlled by a back thrust and a steep forelimb and is crossed by a series of steeply dipping normal faults, which in some cases cause pressure seals that compartmentalize the structure (Fig. 1). The target reservoir sandstones range in age from Cretaceous to Late Oligocene. The Hydrocarbon Fluid Column is quite complex. Both the oil leg and the condensate from the gas cap have an important variation of API and gas oil ratio with depth. Several attempts to develop the oil leg in the past have had limited success, mainly due to diminishing API with depth in the oil rim, the complex stratigraphy, and the sometimes poor rock properties (specially in the deeper formations). The Material Balance simulation model built in this study support the blowdown of the gas cap as the most attractive development strategy. Static Model The reservoir targets in Quiriquire Deep are mainly inter-bedded marine sandstones, ranging in age from Upper Cretaceous to Late Oligocene, with porosities rangng from 4 to 12% and permeabilities between 0.1 to 100 md. The main productive formations in Quiriquire Deep (as illustrated in Figure 2) are: Los Jabillos (Late Oligocene); Caratas (Eocene), Vidoño (Paleocene), San Juan (Cretaceous), and San Antonio (Cretaceous), which were correlated using Maximum Flooding Surfaces as markers, as identified in the electric logs adjusted with micropaleontology data. A static model using a geocellular approach was built with the information from 29 wells (Vsh, porosity and Sw), structural maps representing the various formations of the stratigraphic column, and fault planes. The final geocellular model contained over 700,000 cells. Fig. 3 shows a cross section from the static model displaying the Vsh attribute to illustrate the high the degree of heterogeneity that exists.
E. GONZALES P. COLONOMOS I. RUSINEKthis article begins on the next page F F JCPT86-02-07 SIMULATION A new approach for characterizing oil fractions, and for selecting pseudo-components of hydrocarbons E.GONZALEZ, P. COLONOMOS; and I. RUSINEK INTEVEP S.A. ABSTRACT The non-uniqueness of the inversion of physicochemical parameters to obtain molecular weight distributions for heavy oil fractions is discussed. A method to obtain bounds for th-e cumulatives of these distributions from the usually available experimental information is presented. The method can be applied systematically using linear programming techniques, and the bounds obtained are mathematically rigorous due to the properties of Tschebychev systems. The method is also used to calculate bounds for the pseudo-critical properties needed to characterize pseudo-components for equation-of-state use. These bounds provide a way to select optimal lumping schemes (for a given number of pseudo-components) in a systematic manner. Numerical examples for the bounds obtained and for the optimized lumping procedure are presented. Introduction Equations-of-state are used in reservoir engineering to predict the PVT behaviour of crudes when complete experimental measurements are not available and in compositional numerical simulation.Generally, with a sufficiently large number of pseudo-components, a satisfactory description of the heavy fraction of the fluid of interest can be obtained. However, there are strict limitations on the maximum number of components that can be used in numerical simulation work and the original composents have to be lumped into a smaller number of new pseudo-components. One way of determining a characterization for a heavy frac-tion is to take each distillation cut or identified Cn group on a partially extended analysis as a pseudo-component, determining its pseudo-critical parameter via correlations and completing the match by fitting the known experimental data using the binary interaction coefficients as adjustable parameters. Whitson(l) has presented a review of the many correlations available for this purpose.An alternate approach proposed by Whitson(2) assumes a given functional form with adjustable parameters for the dis-tribution of C. groups (Cn molecular weights) in the heavy frac-I . The.-adjustable parameters are used to fit the known average properties for the heavy fraction and for the distillation cuts or identified C. groups. The adjusted distribution together with a set of generalized properties for C. groups up to C,, com-pletes the characterization. The problem of lumping these pseudo-components into a smaller number without losing the predicting power of the equa-tion of state has been addressed by many authors(2-5). This prob-lem has several stages: into how many components should one lump'?, which components should be lumped together?, and how, does one determine the EOS constants for the new lumped pseudo-components? In this paper we address the latter two points-Lee e. t 6!/.(3) have proposed a method in which thermody...
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