Experiments and Analysis of Heavy Oil Solution Gas Drive

Abstract

We provide a general framework for interpreting heavy oil solution gas drive experiments. Evolution of the gas phase below the thermodynamic bubble point is investigated using dimensionless scaling groups and mechanistic modeling. The role of initial solution gas oil ratio, oil viscosity and depletion rate on the early growth of the gas phase is discussed. Factors leading to the coalescence of gas bubble clusters and development of bulk gas flow are illustrated with examples, along with an empirical correlation for critical gas saturation (Sgc). Two carefully planned and monitored depletion experiments are conducted to evaluate the pre and post-Sgc behavior. These experiments are CT-scanned to obtain in-situ saturations, that are used in conjunction with differential pressure measurements along the core to obtain phase mobilities. Implications of conducting experiments at pressure depletion rates and gradients substantially higher than field rates are discussed. Introduction and Study Objectives It is widely reported that many heavy oil reservoirs have shown higher oil production and recoveries than would be normally expected from conventional reservoir engineering principles [1]. While sand co-production and reservoir compaction may contribute to the increased production rates in many cases, the role of solution gas drive mechanism is of great significance. Several experimental [2–11] and theoretical [12–19] studies have been conducted to understand this phenomena. Some of the key findings are summarized in Appendix A and B, respectively. While there are some good summary and critical review papers [1,20] on what has been done, to the best of the authors' knowledge there is no documented work that provides an overall framework for interpreting heavy oil solution gas drive experiments and relates them to field depletion rates. The key objectives of the current paper are as follows:Use mechanistic modeling and dimensionless scaling groups to relate pore level mechanisms of bubble nucleation and growth to macroscopic observations of heavy oil solution gas drive.Perform carefully planned and monitored depletion experiments under reservoir conditions, having the following salient features:–CT scanning for in-situ saturation measurement–Overburden pressure to maintain suitable stress–An accurate collection system, especially for gas production.Interpret pre and post-Sgc behavior of experiments conducted and put it in context with past experimental studies.Discuss implications of conducting experiments at pressure depletion rates and gradients substantially different from those observed in the field. Framework for Interpreting Heavy Oil Solution Gas Drive Experiments The mechanism of gas evolution during primary depletion maybe broken into three elements:Bubble nucleation and growthBubble coalescence and start of bulk gas flow (at the critical gas saturation)Two-phase flow of gas and oil. Note the gas may be dispersed under certain conditions.

The Effect of Pressure-Decline Rate and Pressure Gradient on the Behavior of Solution-Gas Drive in Heavy Oil

“…In many of the previous studies (Kumar et al 2002;Sahni et al 2004;Talabi et al 2003), pressure-decline rate and pressure gradient were related through production rate; higher production rates led to higher pressure-decline rate and pressure gradient. In the work of Talabi et al (2003), the effect of these factors is lumped in a dimensionless group called the depletion index.…”

The Effect of Pressure Decline Rate and Pressure Gradient on the Behaviour of Solution Gas Drive in Heavy oil

“…In many of the previous studies 2, 18,19 , pressure decline rate and pressure gradient were related through production rate; higher production rates led to higher pressure decline rate and pressure gradient. In the work of Talabi et al 19 the effect of these factors were lumped in a dimensionless group called depletion index.…”

Gas Recharging Process Study in Heavy Oil Reservoirs

“…It is widely reported [29][30][31] that primary production of heavy oil reservoirs has demonstrated higher oil recovery than expected based on conventional reservoir engineering principles.…”

“…However, in the case of heavy oil reservoirs [29][30][31] , due to higher viscosity, after the gas is formed it tends to remain trapped longer within the oil as a result of the low viscosity and density 37 , high compressibility and reduced gas mobility. A two-phase gas in oil dispersion (foamy oil) forms within the oil, the foamy oil flow under solution gas drive results in a higher primary recovery, lower produced gas oil ratios and slower-pressure decline rate within the reservoir.…”