Theoretical and Experimental Investigation of Isothermal Compositional Grading

Abstract

A centrifuge system capable of producing a potential difference across a live oil column equivalent to 1,000 ft of gravitational head was designed and tested. Initial tests on a simple ternary system yielded results similar to equation-of-state (EOS) models. A blackoil sample from a Gulf of Mexico field, Bullwinkle J2-RB sand, exhibiting compositional gradients was segregated in the centrifuge. Experimental results from the centrifuge were similar to field values, indicating that the large variation in composition observed for this field may be attributed to gravitational segregation alone.From the results of these two sets of experiments and the subsequent analysis of the graded fractions, we can conclude that significant compositional variation of reservoir fluids not near their vapor/liquid critical points can be caused by gravity alone. The grading phenomenon is sensitive to the saturate/aromatic balance of the oil. Existing EOS models did not correctly predict the compositional variations in fluids like the Bullwinkle J2-RB because pseudocomponents generated from volatility distributions have a fixed saturate/aromatic character. It is subtle changes in the saturate/aromatic balance that drive the grading phenomena for these fluids.

Numerical Investigation of Gravitational Compositional Grading in Hydrocarbon Reservoirs Using Centrifuge Data

“…As the setup was discussed in detail elsewhere (Ratulowski et al 2003), we will only reiterate certain aspects of the system relevant to our modeling. The geometry of the pressure vessel during centrifugation and select properties of the setup are provided in Fig.…”

“…Additionally, the presence of nearby conductivity variations (such as the existence of salt structures) or changes in the overburden thickness may create the temperature differential needed to make thermal effects (i.e., convection) dominant. There is abundant information in the open literature on examples of graded fluid columns and analyses of the forces thought to be responsible for these observed gradings (Ratulowski et al 2003;Schulte 1980;Hoier and Whitson 2001;Riemans et al 1988;Creek and Schraeder 1985;Montel and Gouel 1985;Ghorayeb et al 2003;Whitson and Belery 1994). The approaches described in most of these papers are to compare sector/field data with equilibrium or steady-state gradient models of varying complexities.…”

“…This paper focuses on the numerical modeling aspects of the work reported by Ratulowski et al (2003), with additional experimental data and model-based analyses of those results using a gravity equilibrium algorithm. This manuscript is divided into an experimental section (to provide calibration data where the grading mechanisms of interest are isolated) and a modeling section.…”

“…To our knowledge, the amount of laboratory experimental data available on hydrocarbon isothermal gravitational grading, under temperatures and pressures typical in the subsurface environment, is scarce. The majority of what is available in literature pertains to model binary and ternary systems (Ratulowski et al 2003;Chang et al 1983;Firoozabadi et al 1996).…”

“…Comparisons of measured data and calibrated model show that the EOS model qualitatively and, in many cases, quantitatively described the observed equilibrium fluid grading behavior of the fluids tested. First, equipment was calibrated using synthetic fluid systems as shown in Ratulowski et al (2003). Then real reservoir fluids were used ranging from black oils to condensates [properties ranging from 27°API and 1,000 scf/stb gas/oil ratio (GOR) to 57°API and 27,000 scf/stb GOR].…”