J.Y. Yuan

Evaluation of Steam Circulation Strategies for SAGD Start-Up

The impact of steam quality, circulation rate and pressure difference between the well pair during SAGD initialization using steam circulation was explored through the use of numerical simulations employing a discretized wellbore model. These operating parameters appear to affect uniformity of reservoir heating, occurrence of steam breakthrough and time required to establish communication between the well pair. The simulation results indicate that, for the given tubing and liner sizes and reservoir properties, relatively lower circulation rates at high steam quality are more favourable for faster initialization and development of uniform temperature between the horizontal well pair. At lower steam qualities, however, higher circulation rates appear more favourable. The use of high steam quality in combination with high circulation rates leads to slower rates of initialization, less uniform heating along the length of the wells, and possibility of premature steam breakthrough at the heel. It was also found that having a higher steam quality in the lower well than in the upper well could lead to faster initialization and more uniform heating between the well pair. A small pressure difference, offsetting the natural hydraulic pressure (50kPa), appears to be more favourable for faster and more uniform initialization.

A Wormhole Network Model of Cold Production in Heavy Oil

High permeability channels (wormholes) are believed to be generated, starting from the wellbore and propagating into the reservoir, during the initial phase of cold production. The d evelopment of wormholes drastically enhances oil production. Understanding the wormhole development pattern, therefore, is critical to the modelling of the fluid flow behaviour and r ecovery rates in the cold production process. We propose that the wormhole growth can be described by the probabilistic a ctive walker (PAW) model, which is a generalisation of the classical random walk model. This simple description may lead to an improved understanding of: the mechanisms i nvolved in cold production, the fluid flow in the area where wormholes are formed (the wormhole zone) and field production data. Previous experimental and theoretical studies ind icate that the wormhole diameter may be a function of distance from the wellbore. We assume that this function follows a power law, slowly decreasing with increasing radial distance. We calculated the mobility of a slurry of sand and oil through the wormhole network. This mobility was used to calculate oil and sand production rates. Furthermore, we related the maximum size of the wormhole zone and its expansion rate to the sand production data in a typical cold production oper ation. This relationship can be useful in determining well spacing. This wormhole network model can also be useful as a tool for analysing field data and for the development of field scale numerical simulations of the cold production process. Cold production and wormhole development Cold production is a non-thermal primary heavy oil recovery process in which sand production is encouraged. It is believed that high permeability channels are formed during cold pr oduction. The main cause of wormhole formation is believed to be the flux of fluids through unconsolidated sand. This flux exerts a drag force strong enough to overcome the forces that hold sand grains together. The sand grains are transported along the wormhole. This kind of flux induced erosion pro cess is local at the wormhole tip as suggested from X-ray co mputed tomography images 1-5 of a wormhole as it grows in a sand pack. In these experiments, the porosity of the sand su rrounding the wormhole tip did not appear to be disturbed. Along the length of the wormhole, the porosity was roughly constant, around 55%. In addition, it decreased sharply, from 55% to 34%, as the wormhole tip was reached. 3 The dimensions of the pixels used in the reconstruction of the CT images were 1.5 mm by 1.5 mm by 8 mm long. In other words, instead of a collapse over a large area, this erosion process causes the surface of the matrix to successively erode at the tip of the wormhole. Experimental studies

Impacts of Initial Gas-to-Oil Ratio (GOR) on SAGD Operations

This study addresses the important role of initial gas-to-oil ratio (GOR) in steam assisted gravity drainage (SAGD) operations. A numerical model using CMG’s STARS was validated through history matching of laboratory experiments conducted at the Alberta Research Council. The impacts of initial GOR on process performance were then studied using field scale numerical simulations. The results indicate that high initial GOR may have beneficial effects, namely, reduction of oil viscosity, and improvement of the oil-to-steam ratio (OSR). A detrimental impact, however, is also shown as the gas impedes the rate of steam chamber growth, and hence reduces oil production rates. Further analysis indicated that this is because of a “dynamic vacuum” effect due to steam condensation at the front of the steam chamber. This dynamic vacuum effect dominates the diffusion process and creates a gas-rich zone at the front of the steam chamber, thereby resisting further growth of the steam chamber and slowing oil production. The same effect occurred when noncondensable gas was co-injected with steam in either live oil or dead oil reservoirs.

Determination of the Relative Permeability Matrix Coefficients

This paper summarizes the recent development of the generalized matrix formulation and its incorporation into a reservoir simulator as collaborative efforts conducted at Alberta Research Council (ARC) and Computer Modelling Group (CMG). The focus of this paper is on how the matrix coefficients, which are the generalization of relative permeability curves, can be determined from experimental data, in two-phase and three-phase systems. We studied different types of experimental conditions available in the literature to obtain these matrix coefficients for two phase flow. These include experiments by Bentsen and Manai and Boubiaux and Kalaydjian. A more general description was derived, leading to the disclosure of intrinsic connections between the above two types, and all other possible types. This allows the choice of various independent pairs of relative permeability curves as input for a reservoir simulator (CMGs STARS model). Selected experiments were then re-simulated with this matrix formulation. The concept was, for the first time, further generalized systematically to three-phase flow. A scheme and detailed formulations have been developed, allowing a reservoir simulator to deal with a matrix formulation for three-phase flow. These relations were also implemented in CMGs simulator STARS.

Generalized Phase Mobilities In Gravity Drainage Processes

Adequate numerical prediction and forecasting of reservoir performance rely on the knowledge of relative permeabilities. In gravity driven processes the flow is complex, consisting of co-current and counter-current flows. This work describes the characteristics of gravity driven flow and provides generalized permeabilities (or mobilities) for such flow. The results can be used in improved numerical simulation of gravity drainage processes. BACKGROUND The. standard multiphase relative permeability approach used in existing reservoir simulators is sufficient for describing steady state processes for either coor countercurrent flow. However, in the case of complex dueedimensional flow that combines both coand counter-current components, such as that in gravity drainage processes, transport coefficients have more complex form and should be addressed through matrix formulation. Furthermore, in the case of non-steady state counter-current flow in gravity drainage, the effective mobility becomes a more complex ftmction of steady state relative permeabilities and the staudard approach no longer represents the physics of the process accurately. During the last decade, new matrix formulation of phase mobilities has been extensively discussed in the literature. The theory is mature and its implementation is long overdue. The new momentum equations are a natural extension of the standard formulation with proper representation of crossinfhrences between flowing phases. The theory provides improved flow description as compared to the standard relative permeability concept in modelhng complicated nonsteady state processes such as gravity drainage. This is due to: 1. correct description of multiphase flows, 2. easy incorporation into existing multiphase simulators, 3. being a natural extension of existing standard relative permeability theory.

Impacts of Gas on SAGD: History Matching of Lab Scale Tests

This study addresses the importance of initial GOR in SAGD heavy oil recovery operations. By history matching two laboratory scale experiments—one with dead oil and the other with live oil—we corroborated the theoretical and numerical prediction that gas would accumulate at the front of a steam chamber. This gas accumulation could slow down oil production as well as heat loss to the overburden. It is suggested that monitoring gas production during SAGD field operations may be critical for the investigation of impacts of gas, and for developing strategies for performance improvement. 33 cc/min, except during the first 10 min. During the first 10 min, steam was circulated into the injection and production wells, with the steam injection rate being at an average value of 68 cc/min. The experiment lasted 450 min. In the live oil experiment, the initial cell pressure was 2,184 kPa and the initial cell temperature was 22° C. The sandpack porosity was 36.3%. The initial oil saturation was 89%. The live oil was prepared by saturating dead oil with methane at a GOR of about 8 m 3 /m 3 . The steam was slightly superheated and injected at an average rate of 35 cc/min. During the first 10 min, steam was circulated in both the injection and the production wells. The experiment lasted 450 min.

Impacts of Gas in SAGD: History Matching of Lab Scale Tests

This study addresses the importance of initial GOR in SAGD heavy oil recovery operations. By history matching two laboratory scale experiments—one with dead oil and the other with live oil—we corroborated the theoretical and numerical prediction that gas would accumulate at the front of a steam chamber. This gas accumulation could slow down oil production as well as heat loss to the overburden. It is suggested that monitoring gas production during SAGD field operations may be critical for the investigation of impacts of gas, and for developing strategies for performance improvement. 33 cc/min, except during the first 10 min. During the first 10 min, steam was circulated into the injection and production wells, with the steam injection rate being at an average value of 68 cc/min. The experiment lasted 450 min. In the live oil experiment, the initial cell pressure was 2,184 kPa and the initial cell temperature was 22° C. The sandpack porosity was 36.3%. The initial oil saturation was 89%. The live oil was prepared by saturating dead oil with methane at a GOR of about 8 m 3 /m 3 . The steam was slightly superheated and injected at an average rate of 35 cc/min. During the first 10 min, steam was circulated in both the injection and the production wells. The experiment lasted 450 min.

Benefit of Wettability Change Near the Production Well in SAGD

In a SAGD process, what are the benefits of changing formation wettability near and surrounding a production well from water-wet to oil-wet? Are these benefits, if any, strictly near-wellbore effects or are they reservoir scale? Based on our experimental experiences we investigated these issues numerically. The study indicated that: (1) changing wettability near the production well appeared to have more than near-well effect; (2) in a water-wet reservoir, an oil-wet zone near and surrounding the production well would noticeably shorten the fluid communication time; (3) it would increase oil production rate, at least during early stage of production; (4) water appeared to be partially blocked by the oil-wet zone and (5) the larger the oil-wet zone, the more apparent these effects were. INTRODUCTION The concept of enhancing oil production by changing the wettability surrounding the production well from water-wet to oil-wet in a water-wet reservoir is described in a recent US patent. Its applications were demonstrated in lab experiments conducted at Alberta Research Council. In order to examine whether this concept is applicable to field scale SAGD operation, we conduct this numerical study using CMG’s STARS. Our studies include: (1) History match the lab test results to validate the numerical model and to learn about possible key parameters; (2) Study the potential impacts of altering wettability near production well on SAGD operation using field scale numerical simulations. HISTORY MATCH OF EXPERIMENTAL RESULTS The lab experiments were done in a 2D visualization cell. A schematic diagram of the rectangular model, 60 cm wide, 21 cm high and 3 cm thick, is shown in Figure 1. The model was packed with sand having a permeability of 300 Darcy and a porosity of about 35%. It was saturated with water. The water was then displaced

Measurement of Slurry Viscosity In Cold Production

The falling ball viscometer method was used to measure the viscosity of bitumen and sand mixtures. The viscosity of the slurry was measured over a range of concentrations, but with emphasis on higher concentrations, a regime which is believed to be representative of sand concentration in wormholes during cold production of heavy oil. Non-Newtonian behaviour was observed at very high sand concentration.