DSpace at VNU: Numerical Simulation of SAGD Recovery Process in Presence of Shale Barriers, Thief Zones, and Fracture System

Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Petroleum S

Publisher: Taylor & Francis

Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Petroleum Science and Technology

Publication details, including instructions for authors and subscription information:

http://www.tandfonline.com/loi/lpet20

Numerical Simulation of SAGD Recovery Process in Presence of Shale Barriers, Thief Zones, and Fracture System

T Q C Dang a , Z Chen a , T B N Nguyen b , W Bae b & C L Mai c a

University of Calgary , Calgary , Alberta , Canada b

Sejong University , Gwangjin-ku , Seoul , Korea c

Ho Chi Minh City University of Technology , Ho Chi Minh , Viet Nam Published online: 19 Jun 2013

To cite this article: T Q C Dang , Z Chen , T B N Nguyen , W Bae & C L Mai (2013) Numerical

Simulation of SAGD Recovery Process in Presence of Shale Barriers, Thief Zones, and Fracture System, Petroleum Science and Technology, 31:14, 1454-1470, DOI: 10.1080/10916466.2010.545792

To link to this article: http://dx.doi.org/10.1080/10916466.2010.545792

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the

“Content”) contained in the publications on our platform However, Taylor & Francis,

our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content Any opinions and views expressed in this publication are the opinions and views of the authors,

and are not the views of or endorsed by Taylor & Francis The accuracy of the Content should not be relied upon and should be independently verified with primary sources

of information Taylor and Francis shall not be liable for any losses, actions, claims,

proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content

This article may be used for research, teaching, and private study purposes Any

substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,

systematic supply, or distribution in any form to anyone is expressly forbidden Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

ISSN: 1091-6466 print/1532-2459 online

DOI: 10.1080/10916466.2010.545792

Numerical Simulation of SAGD Recovery Process in Presence of Shale Barriers, Thief Zones, and Fracture System

T Q C Dang,1

Z Chen,1

T B N Nguyen,2

W Bae,2

and C L Mai3 1

University of Calgary, Calgary, Alberta, Canada

2

Sejong University, Gwangjin-ku, Seoul, Korea

3

Ho Chi Minh City University of Technology, Ho Chi Minh, Viet Nam

This study presents a numerical investigation for evaluating the potential applicability of the steam-assisted gravity drainage (SAGD) recovery process under complex reservoir conditions such as shale barriers, thief zones with bottom and/or top water layers, overlying gas cap, and fracture systems in the McMurray and Clearwater formation The simulation results indicated that the near-well regions were very sensitive to shale layers, and only long, continuous shale barriers (larger than 50 m or 25%) affect the SAGD performance in these well regions In addition, the thief zones had a strongly detrimental effect on SAGD The results also showed that the SAGD recovery process was enhanced in the presence of vertical fractures but horizontal fractures were harmful to recovery Fracture spacing is not an important parameter in the performance of a steam process in fractured reservoirs and extending horizontal fractures will reduce ultimate oil recovery in the SAGD process This article provides a guideline for SAGD operations in complex geological reservoirs

Keywords: fracture system, numerical simulation, SAGD, shale barriers, thief zones

INTRODUCTION

Alberta’s oil sands deposits, with estimated 1.7 trillion barrels of bitumen in place, account for approximately 40% of the world’s bitumen resources (Figure 1) However, an extremely high viscosity of bitumen at reservoir temperature is one of the greatest challenges in using a recovery process At a company with recent advances in horizontal well technology, steam-based in situ recovery methods, aiming at a thermal viscosity reduction, have emerged for exploration of these vast resources (Butler, 2001) The steam-assisted gravity drainage (SAGD) recovery process has opened the door to producing a large number of bitumen reservoirs in Canada

SAGD was first developed by Roger Butler and his colleagues in Imperial Oil in the late 1970s It is a thermal oil recovery process that consists of a pair of two parallel horizontal wells drilled near the bottom of the pay The top horizontal well is used to inject steam, and the bottom horizontal well is used to produce reservoir fluids (Figure 2) The heat from steam is transferred by thermal conduction into the surrounding reservoir The steam condenses and the heated oil flows

to the production well located below by gravity Two types of flows exist during this process

Address correspondence to Wisup Bae, Sejong University, 98 Gunja-dong, Gwangjin-ku, Seoul 143-747, Korea E-mail: wsbae@sejong.ac.kr

1454

FIGURE 1 Oil sands in Alberta, Canada (color figure available online)

One is at the ceiling of a steam chamber and the other is along the slopes of the steam chamber The success of an SAGD project depends on some key factors such as an accurate reservoir description, efficient utilization of heat injected into the reservoir, understanding of displacement mechanisms, understanding of geomechanics, and overcoming various constrains (Doan et al., 1999) Successful field tests have proven that SAGD is a viable technology for in situ recovery

of heavy oil and bitumen (Singhal et al., 1998; Butler, 2001; Boyle et al., 2003)

The SAGD technique has many advantages over other thermal methods such as conventional steam flooding methods SAGD overcomes the shortcomings of steam override by using only

FIGURE 2 Steam-assisted gravity drainage technology (color figure available online)

gravity as the driving mechanism, which leads to stable displacement of oil and potentially high oil recovery In addition, the heated oil remains hot and movable as it flows toward the production well, whereas in conventional steam flooding, the oil displaced from the steam chamber cools down and consequently the oil-phase viscosity increases as the oil flows to the production well (Chen

et al., 2008) The SAGD process is made more thermally efficient by maintaining a liquid pool that surrounds the bottom production well and preventing the escape of steam from the steam chamber However, Farouq Ali (1997) and Singhal et al (1998) pointed out some limitations of SAGD

as follows: (1) the theory pertains to the flow of a single fluid; (2) only steam flows in the steam chamber, oil saturation being residual; (3) the heat transfer ahead of the steam chamber to cold oil is by conduction only; (4) sand control may be necessary; (5) there is a hot effluent/high water cut production; (6) frequent changes in operating regimes and high operating costs occur; and (7) deterioration of production at late stages occurs This article presents a comprehensive evaluation of SAGD’s performance in presence of continuous shale barriers, discontinuous shale barriers, bottom and top aquifers, and gas cap layers In particular, the effect of a fracture system

on SAGD operation is also described

STATEMENT OF THE PROBLEM

The success of SAGD has been mostly demonstrated by numerical simulation with homogeneous reservoir models However, this process is very sensitive to reservoir heterogeneity; therefore, it

is necessary to have a comprehensive understanding of the effects of reservoir heterogeneity on SAGD performance for wider and more successful implementation

The efficiency of the SAGD process is significantly decreased in the presence of shale barriers

or thief zones such as bottom and top aquifers, overlying gas caps, or fracture systems Thus, the first attempt of this research is motivated by the need for an improved SAGD process in heterogeneous reservoirs Such an improvement is crucial to broaden the applications of SAGD and unlock vast discovered heavy oil/bitumen resources worldwide

In addition, the performance of SAGD is compared in different geological areas including the McMurray formation and the Clearwater formation This comprehensive comparison will allow

us to fully evaluate the effect of reservoir properties on the SAGD process in hostile conditions

DESCRIPTION OF A SYNTHETIC RESERVOIR MODEL

The advanced thermal reservoir simulator, STARS, developed by the Computer Modeling Group Ltd (Calgary, Alberta, Canada), was used to construct a reservoir model and evaluate the perfor-mance of the SAGD process A synthetic reservoir model that represents two generic formations

in the Alberta oil sands was selected for this research The main reservoir properties of two formations are shown in Table 1

In order to reflect the reservoir heterogeneity, the formation consisted of clean sands and shaly sands that contained some thin shale lenses The bitumen viscosity of the McMurray formation was much higher than that of the Clearwater formation The producers, with a length of 900 m, were located at the bottom of the reservoir and the injectors were 5 m above the producers The horizontal spacing between well pairs was 50 m The steam injection pressure was set at 2,500 kPa for the McMurray formation and 3,600 kPa for the Clearwater formation

In the first 4 months, we specified a line heater in the grid cells that contained the wellbores instead of using steam circulation through both the left and right injection and production wells The heat flux was determined by the amount of latent heat in which 400 m3

/day of 0.95 quality steam was delivered to the reservoir A steam trap control is important in SAGD as well as

TABLE 1 Typical Reservoir Properties of McMurray and Clearwater Formations

Reservoir Parameter

McMurray Formation

Clearwater Formation

Vertical permeability (K v ), D 3 1.5 Permeability ration (K v =K h ) 2 2

Reservoir pressure, kPa 1,800 2,900

Bitumen viscosity at reservoir temperature, cP 2,000,000 60,590 Rock compressibility, 1/kPa 7E 06 9.6E 06 Formation heat capacity, kJ/m 3 K 2.39EC06 2.35EC06 Rock thermal conductivity, J/m.d.C 6.6EC05 6.6EC05 Oil thermal conductivity, J/m.d.C 1.15EC04 1.15EC04 Water thermal conductivity, J/m.d.C 5.3496EC04 5.3496EC04 Gas thermal conductivity, J/m.d.C 139.97 139.97

fast-SAGD to prevent or reduce steam production from the reservoir This steam trap control should result in keeping all of the latent heat generated by the steam inside the reservoir and producing only bitumen and condensed hot water In this study, the operating constraint at the production wells imposed a maximum temperature difference between the saturation temperature corresponding to the pressure of the fluids and the temperature in the wellbore equal to 5ı

C

RESULTS AND DISCUSSION

Shale Barriers

Heterogeneity plays a critical role in understanding steam chamber growth at the actual field scale and within simulations It is important and necessary to understand the factors determining growth rates and areal propagation Unfortunately, most numerical simulation investigations have been conducted with homogeneous systems, so these studies cannot be applied directly to pro-vide accurate, reliable predictions for a field-type system During the last two decades, several researchers have attempted to evaluate the effect of reservoir heterogeneity on steam chamber development for the SAGD process One of the first to present their research on this topic was Joshi and Threlkeld (1985) Through experiments at the laboratory scale, Yang and Butler (1992) studied the effect of a shale barrier length (short and long horizontal barriers) for both top steam injection and bottom steam injection cases With a top steam injection, the presence of a short horizontal barrier has no effect on the general performance and a long horizontal barrier decreases the production rate, though not as much as expected in some configurations They also concluded that the heated bitumen above the barrier may not be produced even though it is hot because

of the steam pressure holding up the oil at the bottlenecks to the flow Additionally, Yang and Butler (1992) showed that long shale barriers can cause a difference in the advancement velocity

of the interface above and below the barrier This difference is reduced by the drainage of heated bitumen through conduction above the barrier

Pooladi-Darvish et al (2002) proposed a better way to investigate the effect of shale barriers in complex geological characterizations by using a stochastic model based on geostatistical methods

to represent the shale distribution Chen et al (2008) conducted a numerical simulation study on

the stochastic of shale distribution near the well region and above the well region They stated

in their conclusion that the SAGD performance was affected adversely only when the above-well region contained long, continuous shales or a high fraction of shales

Recently, Ipek et al (2008) conducted numerical studies of interbedded shales in SAGD The purpose of this research was to determine the potential of pressure cycling as a method of enhancing the reservoir permeability Le Ravalec et al (2009) conducted a numerical investigation and showed that the influence of shale baffles depended upon their locations relative to the well pairs Shin and Choe (2009) constructed a two-dimensional homogeneous model and tested the effect of shale barriers that were located in the above- and between-well pairs

The effect of reservoir heterogeneity on SAGD performance was studied by including ran-domly distributed, discontinuous or continuous, thin shale lenses Shale is characterized by low permeability, typically in the range of 10 6

to 10 4

mD The effects of shale barriers have been investigated in many case studies (Yang and Butler, 1992; Pooladi-Darvish et al., 2002; Chen

et al., 2008) and different depending on the location, size, and volume of the shale layers

Shale Barriers between Injector and Producer

First, the effect of discontinuous shale barriers in the horizontal direction was evaluated; the size of shale barriers varied from 5 to 30 m Steam cannot perfectly propagate in a reservoir when

a shale barriers exist; thus, the cumulative oil recovery continuously decreases as the size of the shale barriers increases (Figures 3 and 4) The shale barriers had a great effect on the amount of oil recovery in the Clearwater formation

Figure 5 shows the effect of shale barrier orientation on cumulative oil recovery in the McMurray formation The numerical simulation indicated that the thermal efficiency would be significantly decreased in the presence of vertical shale barriers due to the fact that a steam chamber cannot perfectly develop in the sideway as shown in Figure 6 As a result, the SAGD performance is higher in the case of horizontal shales as compared to vertical shale barriers In

FIGURE 3 Effect of discontinuous shale barriers in the horizontal direction on cumulative oil recovery in the McMurray formation (color figure available online)

FIGURE 4 Effect of discontinuous shale barriers in the horizontal direction on cumulative oil recovery in the Clearwater formation (color figure available online)

FIGURE 5 Effect of discontinuous shale barrier orientation on cumulative oil recovery (color figure available online)

FIGURE 6 Effect of discontinuous shale barrier orientation on the steam chamber (color figure available online)

addition, an increase in shale barriers in both cases led to an increase in cumulative steam-oil ratios (CSOR; Figures 7 and 8)

The effect of continuous shale barriers in the horizontal and vertical directions was compared and is shown in Figure 9 The existence of continuous shale barriers in the vertical direction is the worst-case scenario for SAGD operation; it prevents the steam chamber from forming in the sideway and, as a result, the CSOR is the highest and the cumulative oil recovery is the lowest among three cases Figures 10 and 11 indicate the dominant effect of continuous shale barriers

on the cumulative oil recovery in the McMurray and Clearwater formations Oil recovery is much lower in the presence of lengthy continuous shale barriers

FIGURE 7 Effect of discontinuous shale barriers in the horizontal direction on CSOR (color figure available online)

FIGURE 8 Effect of discontinuous shale barriers in the vertical direction on CSOR (color figure available online)

FIGURE 9 Effect of continuous shale barriers in the horizontal and vertical directions on CSOR and cumulative oil recovery in the McMurray formation (color figure available online)

FIGURE 10 Effect of continuous shale barrier size on cumulative oil recovery in the McMurray formation (color figure available online)

FIGURE 11 Effect of continuous shale barrier size on cumulative oil recovery in the Clearwater formation (color figure available online)