A Study of Heavy Oil Solution Gas Drive for Hamaca Field: Depletion Studies and Interpretations

Abstract

Abstract We present results of a systematic study performed to investigate heavy oil solution gas drive process for Hamaca field in Venezuela. The ultimate goal of this study is to provide the data needed for Hamaca reservoir simulation studies. The study included a series of rock and fluid property measurements, and several carefully performed depletion experiments. Supplemental experiments using PVT cells and ideal porous media were also conducted to aid in the analysis of the depletion experiments. For this study, single-phase live oil samples from Hamaca Field were used. Fluid characterization tests were performed, including GOR, viscosity, density, compositional measurements and other PVT experiments. Average properties of the live oil used were GOR of 96 scf/STB, bubble point pressure of 1020 psia at 125 °F and viscosity of 1650 cP at reservoir conditions. In addition, several flow experiments were conducted using the live oil to elucidate so-called "foamy oil" behavior by obtaining rheological data at above and below thermodynamic bubble point. Depletion experiments were conducted at reservoir conditions using field core and oil samples from Hamaca Field. Tests were performed at constant depletion rates from 0.25 psi/hr to 43 psi/hr to cover the flow regimes, which may be occurring at different distances from the wellbore. A CT scanner was used to observe the distribution of gas saturation at various points during the depletion tests. Critical gas saturation values ranged from 8 to 15%. Depletion rate did not have significant effect on the critical gas saturation. Gas-oil relative permeability curves were calculated using a new technique based on effluent density measurements done during the depletion experiment. Calculated gas relative permeability curves appeared to be suppressed, in the range of 10–10 to 10–5. Introduction and Key Elements of the Study: Over the past several years many experimental and theoretical studies have been presented to explain better than predicted oil recoveries in heavy oil reservoirs [1–9]. Several explanations were offered for the higher oil recoveries observed, including sand co-production, reservoir compaction, foamy oil behavior, high critical gas saturation and low gas mobility. Several recent publications present critical reviews and new data. Ref. 1 presents an excellent summary of these studies. High critical gas saturation [3] and low gas mobility [8] are considered to be the primary mechanisms for the high solution gas drive recovery. Data reported on the critical gas saturation varied widely from 3 to 17%. This could be due to variations in the experimental conditions, test parameters, procedures and/or data analysis. Furthermore, prior experiments have been limited in reservoir rock coverage, use of actual reservoir fluids and representative depletion rates, quantification of the gas and oil productions and independent rock and fluid characterizations. This study was initiated to investigate the heavy oil solution gas drive process for Hamaca field in Venezuela with the ultimate goal of providing data requirements for Hamaca reservoir simulation studies. Key elements and objectives of the study were:Use actual reservoir cores and fluids representing different rock typesCharacterize reservoir rocks thoroughly- special core analysis and geological descriptionsObtain live oil PVT data in transient and equilibrium conditionsUse live oil rheological data obtained using capillary tubes and well defined porous media in the pressure regime applicable to depletion testsPerform carefully planned and monitored depletion experiments under reservoir conditions at realistic depletion ratesMonitor gas and oil production rates accuratelyAcquire gas saturation distribution with a CT scanner