Experiments and Analysis of Heavy-Oil Solution-Gas Drive
- A. Sahni (Chevron Petroleum Technology Co.) | F. Gadelle (Chevron Petroleum Technology Co.) | M. Kumar (Chevron Petroleum Technology Co.) | L. Tomutsa (Lawrence Berkeley Laboratory) | A.R. Kovscek (Stanford U.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2004
- Document Type
- Journal Paper
- 217 - 229
- 2004. Society of Petroleum Engineers
- 5.5 Reservoir Simulation, 5.4.2 Gas Injection Methods, 4.6 Natural Gas, 5.3.1 Flow in Porous Media, 5.3.4 Integration of geomechanics in models, 4.3.4 Scale, 4.3.3 Aspaltenes, 5.6.4 Drillstem/Well Testing, 5.2.1 Phase Behavior and PVT Measurements, 4.1.5 Processing Equipment, 5.5.2 Core Analysis, 1.6.9 Coring, Fishing, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating
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We provide a general framework for interpreting heavy-oil solution-gas-drive experiments. Evolution of the gas phase below the thermodynamic bubblepoint is investigated with dimensionless scaling groups, mechanistic modeling, and experiments. Carefully planned and monitored depletion experiments are conducted to evaluate pre- and post-critical gas-saturation behavior. In-situ phase saturations are measured with X-ray computerized tomography (CT) scanning. These data are used in conjunction with differential-pressure measurements along the core to obtain phase mobilities. It is demonstrated that most laboratory heavy-oil-depletion experiments show nonequilibrium effects that depend on the oil-phase viscosity and depletion rate. 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). Examination of field-pressure gradients and flow rates suggests that dispersed gas flow may occur close to the wellbore or near wormholes, if present.
Introduction and Study Objectives
It is widely reported that many heavy-oil reservoirs have shown higher oil production and recoveries than normally would be expected from conventional reservoir engineering principles.1 While sand coproduction and reservoir compaction may contribute to the increased production rates, the role of the solution-gas-drive mechanism is also of great significance. Several experimental2-11 and theoretical studies12-19 have been conducted to understand this phenomenon.
While there are summary and critical review papers, 1,20 to the best of our knowledge there is no documented work that provides an overall framework for interpreting heavy-oil solution-gas-drive experiments and relates the experiments to field-depletion rates.
Accordingly, the objectives of this paper are as follows:
To 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.
To further our understanding of process mechanisms by conducting carefully monitored depletion experiments under reservoir conditions.
To interpret the pre- and post-critical gas-saturation behavior of experiments and contrast the behavior with past experimental studies.
To discuss implications of conducting experiments at pressure-depletion rates and pressure 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 is broken into three steps: (1) bubble nucleation and growth, (2) bubble coalescence and the start of bulk gas flow at the critical gas saturation, and (3) two-phase flow of gas and oil. Note that the gas may be dispersed under certain conditions in the third step.
A micromodel investigation (Fig. 1) of the process provides insight into the pore-level physics of these steps.21 Micromodel observations indicate that bubbles in viscous mineral oils exhibit behavior that is analogous to their counterparts in light oils. Time scales for bubble growth and coalescence are, however, much larger, probably because of increased oil-phase viscosity.
In this section, we discuss how the evolution of the gas phase below the thermodynamic bubblepoint depends on fluid properties and depletion rates. Dimensionless scaling groups are used to interpret laboratory experiments carried out with different fluid systems and depletion rates. Then, a mechanistic model of diffusion-dominated bubble growth is used to quantify the effect of oil viscosity and depletion rates on gas-phase formation.
Dimensionless Scaling Groups.
Consider a single-phase liquid having initial volume V o and pressure P o in a closed system at a constant temperature T and at time t=0.
Suppose that at some time t>0 corresponding to a pressure po, a new gas phase is formed, and its volume is Vg.
The total fluid volume at any time is
where Bo is the oil formation volume factor accounting for the compressibility of the oil and Vsc o is the initial volume expressed at standard conditions.
The gas-phase volume is written
Equations 3 and 4
where Rs is the solution gas/oil ratio (GOR), V gdsc is the volume of dissolved gas at standard conditions, and K is a constant denoting the slope of the solution-gas curve. Svrcek and Mehrotra22 verified the generality of Eq. 4 for heavy oils.
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