spe-78988-ms

Copyright 2002, SPE/PS-CIM/CHOA International Thermal Operations and Heavy Oil Symposium and International Horizontal Well Technology Conference. This paper was prepared for presentation at the 2002 SPE International Thermal Operations and Heavy Oil Symposium and International Horizontal Well Technology Conference held in Calgary, Alberta, Canada, 4–7 November 2002. This paper was selected for presentation by the ITOHOS/ICHWT Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers, the Petroleum Society of CIM, or CHOA and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, the Petroleum Society of CIM, or CHOA, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers, Petroleum Society of CIM, or CHOA is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract This paper presents results of the laboratory investigations leading to development of a novel, radically different waterflooding process, which is gravity stable. The new process is called Toe-To-Heel Waterflooding (TTHW) and uses vertical wells for injection, while horizontal wells are utilized for production. Normally, the system uses a staggered line drive configuration in which the toe of the horizontal producer is close to the line of vertical injectors. The horizontal section of producer is located at the top of oil formation while the vertical injectors are perforated on the lower section of the layer. By using the proposed configuration, a short-distance oil displacement process is achieved. The water/oil segregation causes the injected water to slump, while the horizontal well as a sink causes water to flow upwards; the composite of these two tendencies leads to an advancement of water through almost all the entire pay section, with the water break-through at the toe, followed by an advancement of the displacement front along and under the horizontal leg. A 2-D Hele-Shaw model was used for investigations. The Hele-Shaw model was composed of two parallel plexiglass plates, which were held together at 0.1 mm; these vertical plates form between them a simulated porous medium with a permeability of about 833 D. The rectangular chamber forming the simulated porous medium has the dimensions of 52cm* 21.4cm*0.01cm. The Hele-Shaw model was intended to mimic the vertical section of an oil reservoir and the investigations were designed

specifically for a comparative study of vertical sweep efficiency for conventional waterflooding and toe-to-heel waterflooding. These experiments do not permit an evaluation of areal sweep efficiency. In fact, two toe-to-heel processes were explored: one-phase TTHW and two-phase TTHW; in the last case a blanket of water is intentionally generated at the lower part of layer (by keeping opened the vertical pilot hole of the horizontal producer, while the horizontal leg is closed) and then only the horizontal leg is kept opened (while the vertical pilot hole is closed). More than 20 low pressure (40-80psi) tests were carried out at room temperature. First, some preliminary experiments were conducted in order to establish a base line performance, to provide calibration, or to study the main mechanisms of this process. In these tests, colored water miscibly displaced plain water in a toe-to-heel configuration; no gravity segregation effect between displacing and displaced liquid existed. Also, no mobility effects and two-phase flow along the horizontal leg existed. Then, detailed experiments with five different oils (with viscosity in the range 10 mPa.s to 12,000 mPa.s), and water injection rates of 2.5 ml/hr to 320 ml/hr were carried out. A comparison between conventional waterflooding and TTHW showed that in general for the heavy oils with moderate viscosities, oil recoveries at water-break-through are similar, but at the water-out, at the end of the test, the ultimate oil recoveries were much higher for TTHW; this was a direct consequence of higher vertical sweep efficiencies obtained. Later on, 3-D tests conducted in a real porous medium re-inforced that the concept was sound; the 3-D tests results will be reported in part ll of this series.

Introduction Conventional waterfloods in light-heavy oil reservoirs are limited by three main factors, namely: Rock heterogeneity, leading to the water channelling Gravity segregation (due to oil/water density contrast),

leading to under-riding of the injected water

SPE/Petroleum Society of CIM/CHOA 78988

Laboratory Investigation Of Gravity-Stable Waterflooding Using Toe-To-Heel Displacement: Part L: Hele Shaw Model Results A.T. Turta, C. Ayasse* J. Najman**, D. Fisher and A. Singhal, Petroleum Recovery Institute, Alberta Research Council

2 SPE/PS-CIM/CHOA 78988

Highly unfavourable water/oil mobility ratio, which aggravates the negative effects of the first two factors.

Usually, rock heterogeneity is reflected in a pronounced vertical stratification, due to a relatively large contrast in horizontal permeability of different layers. On the other hand, the negative effect of gravity segregation is mainly felt when the stratification is not very pronounced, and pay thickness and permeability of the pay zone are relatively large. The effect of gravity segregation on waterflood performance was fully recognized in 1953 when the first mathematical model of water tonguing (under-riding) was published by Dietz1. Initially, Dietz theory was believed to be applicable mostly to thick formations. However, subsequently, Outmans showed that this theory is equally valid for thin oil formations2. Recently, for such horizontal waterfloods, the effect of gravity segregation on waterflood performance in light oil reservoirs was analytically expressed by Ekrann3, who showed that it becomes important for the high permeability formations, for moderate ratios between vertical and horizontal permeability. The recovery decreases with increasing water/oil mobility ratio, and increases with increasing ratio between viscous and gravity forces. The last condition means that from a practical point of view, high water injection rates discourage under-ride; therefore they lead to higher oil recoveries. For a stratified two-layer system, Ekrann found that the most unfavourable stratification occurs when the high permeability interval is located at the bottom, while the most favourable one is when the high permeability interval is located at the top. Ekrann’s theoretical results were close to the laboratory results reported by Craig4. All his calculations were for a case of water/oil density difference of 200 kg/m3. From Ekrann’s paper, one can conclude the following: - Vertical sweep efficiency is less than 40% for homogeneous systems, at water/oil mobility ratios higher than 2. - Vertical sweep efficiency is less than 20% when a high permeability interval is located at the bottom; this is valid for a large range of water/oil mobility ratios (higher than 0.5). - The under-ride phenomenon is not a concern in achieving good vertical sweep for the case of the high permeability interval located at the top. To combat under-riding phenomenon in the conventional waterflooding, in order to achieve a gravity stable waterflood, the first alternative is to inject something similar to the water, but with a density very close to the oil. However, in this project, a second alternative was adopted; in order to achieve a gravity stable waterflood, a new approach to re-engineering of the gravity effect during displacement, using horizontal wells, was developed5. This approach focussed on the novel technology of Toe-To-Heel Waterflooding (TTHW) process. The TTHW process is a Toe-To-Heel (TTH) displacement process. It can be classified as a short-distance oil

displacement (SDOD) process, which is important mainly for heavy oil reservoirs6. Conventional oil reservoir flooding methods (pattern or line drive configurations), as applied during the past 60 years, are long-distance oil displacement (LDOD) processes; mobilized oil typically needs to travel several hundred metres to the production wells. Actually, the oil is “pushed” by the injectant from its original location to the outlet and, most of the time oil flows along the stratification. In spite of being relatively inefficient, these are still economically acceptable in situations with low oil viscosity (<10 mPa.s), and the need to switch to SDOD was not felt so acutely. However, for flooding heavy oil pools SDOD has now become feasible, especially in view of the widespread use of horizontal well technology in recent years. The main disadvantages of the long-distance displacement processes are outlined elsewhere5,6. The most important disadvantage of LDOD processes is the dependence of the displacement front advance on the distribution of properties (mainly permeability and viscosity) along the flow path, from the injection to the production well. This crucial negative effect is eliminated in the SDOD processes, where displacement front advance depends only on the distribution of the same properties (mainly viscosity and permeability) in a region immediately ahead of the displacement front. This region can be narrower or larger, depending upon transmissibility, which is the ratio between permeability and the product of oil viscosity, oil compressibility and porosity; generally, the higher the oil viscosity, the narrower is this region. The SDOD processes are defined as a broad category of displacement types in which the mobility (viscosity) of injectant is important, but it no longer dominates the process. A more significant feature is the short distance travel for any oil particle, before it is produced. The injectant could still flow long distances, as SDOD are specifically designed for high mobility injection fluids, generally with unfavourable mobility ratios between injectant and oil. So, instead of looking for solutions for making mobility ratio more favourable like in LDOD, the SDOD processes tend to reduce its importance. This approach is by far more practical, since for most heavy oil pools even if a mobility ratio of one is attained, injection pressures required to sustain an economically acceptable oil rate would be impractical and may lead to fracturing, which is always undesirable in displacement processes. Steam Assisted Gravity Drainage (SAGD) and Vapor Extraction (Vapex) are also SDOD processes. However, the essential difference between TTH displacement processes, and SAGD and Vapex is that the last two processes are integrally gravity controlled processes, while TTH displacement processes are pressure gradient and gravity controlled, in other words, TTH displacement processes include an active drive component.

SPE/PS-CIM/CHOA 78988 3

A TTH displacement process can be applied as a non-thermal recovery technology, such as Toe-To-Heel Waterflooding (TTHW) or as a thermal technology such as Toe-To-Heel Air Injection (THAI), with its variant, catalytic THAI (CAPRI), for in situ upgrading of the oil. The THAI process was conceived and confirmed as an effective displacement process for in situ combustion, early in 19925,6,7. A schematic a THAI is shown in Figure 1. As can be seen, the oil flows only in a region immediately ahead of the displacement front, within the so-called mobile oil zone (MOZ). A viscosity gradient, due to high temperature in MOZ, does exist. More than 70 laboratory tests and some simulation work have confirmed the concept of thermal TTH displacement is a sound concept8,9,10. The laboratory work so far has focussed mostly on THAI and catalytic THAI (CAPRI). The potential for the application of waterflooding in a TTH configuration was explored only very recently8. During 1996/1998 period, more than 20 Hele-Shaw tests were carried out. These tests showed that the TTHW concept has merit. In this paper these initial semi-quantitative tests results are presented. However, later on, 3-D tests conducted in a real porous medium proved that the concept was sound; the 3-D tests results will form part ll of this series, while the results of detailed simulations will be included in part lll.

Fig. 1: Schematics of Toe-To-Heel Air Injection Process (after CMG) A schematic of TTHW process is provided in Figure 2. As far as gravity segregation is concerned, its negative effect is largely mitigated as a consequence of two opposing effects: downwards flow due to gravity segregation and upwards flow due to the horizontal producer being a pressure sink. Another important feature is the abatement of heterogeneity effect due to the horizontal well acting as a line-sink, covering a considerable length; there is a pressure drop along this line-sink. So, due to a slightly lower pressure drop towards the heel, oil has the tendency to flow within the reservoir along

the horizontal leg (this tendency was strongly influenced by oil-rock properties and horizontal well design.

TTH Waterflooding Process

Brine: Sp.Grav. >1.0

Vertical injector Horizontal producer

Fig. 2: Schematics of Toe-to-Heel Waterflooding (TTHW) process, as resulted from Hele Shaw model tests

Experimental Set-up, Procedure and Materials The experimental set-up is shown in Figure 3a. Two positive displacement pumps regulate injection rates for oil and water. The Hele-Shaw cell is made of transparent plexiglass. A set of lights enables a video camera to continuously record images of water invaded zone. At any time, the ratio of this water invaded area to the total model area yields the vertical sweep efficiency, which is equivalent to the oil recovery for this simplified system, representing a vertical cross section. Figure 3b shows details for the Hele-Shaw cell itself. It is composed of two 2.5 inch thick plexiglass plates 21.4 cm by 52 cm, which are held together by a series of long bolts with a sealing gasket setting the plates 0.1 mm apart. In this way a rectangular chamber (52cm*21.4cm*0.01cm) is formed. The 0.1 mm distance between plates is simulating a porous medium having a (calculated) permeability of 833 D. There are auxiliary outlets, which are used for vacuum, upper injection, or cleaning of the model.


��

��

OIL

WA

TER

VCR

RUSKA PUMPS

Video-camera

Vertical pilot hole

Oilcollector

Fig. 3a: Laboratory set-up for TTHW investigation

Fig. 3b: Hele Shaw plexiglass cell The wells, which are 5/16 inch in diameter, are drilled directly into the plexiglass walls, being connected with the chamber of the cell by a series of ports placed every 1 cm (Figure 3b). During TTHW tests, the cell shown in Figure 3b is used upside-down, such that the horizontal well (HW) is located at upper part of “layer”; HW has a length of 34 cm, and between its toe and the shoe of the vertical well there is a space of approx. 10 cm. The vertical well is located at approx. one inch from the edge of the model. The volume of the cell is 10.8 cc, that of horizontal well is 40.8 cc and that of the vertical well is 16.5 cc. Given the low volume of the cell compared to that of wells, the water break-through may be masked and the production of oil can be delayed. Because of this, tests are considered semi-quantitative and oil recovery is calculated based on water invaded area. Given the configuration of the system – the porous medium is mimicked by the space confined by the two closely spaced vertical plates – the oil recovery is equal to the

vertical sweep efficiency for this simplified system. Therefore, conclusions from this section refer mainly to the vertical sweep efficiency. No information on horizontal sweep efficiency was obtained from these tests. Given the ratio 52/21.4=2.4, between the length and the height of the model (“layer”), it can be considered that the tests described mimic a thick layer. Five different oils, with viscosities in a broad range, 10 mPa.s to 12,000 mPa.s, were used in these investigations. The density contrast was in the range 0.05 g/cc to 0.184 g/cc, while the injection rate was in the range 2.5 cc/hr to 320 cc/hr. For each oil, the TTHW process result was compared to a conventional, vertical injector-vertical producer test, conducted at the same injection rate. The tests were performed at room temperature and with an injection pressure in the range of 40-80 psi; the exit pressure was very close to the injection pressure and in some cases it was the atmospheric pressure. Preliminary Experiments These preliminary tests were designed to explore whether the TTH displacement is feasible when no viscosity gradient (due to temperature distribution) exists. Also, they served as a basis of comparison for further tests and allowed us to understand the fundamental mechanisms. In Figure 1, the TTH concept was demonstrated for thermal processes; the horizontal section of horizontal well is located at the bottom layer, while the injection is through the vertical well, using the upper perforations. Using the same lay out, a non-thermal TTH process was tried; a heavy liquid (glycerol) with a density of 1220 kg/m3 and viscosity of 1500 mPa.s was displaced by a light liquid, plain water with a higher mobility. The water was dyed with green food color to improve the visualization of invaded zone. An injection rate of 320 ml/hr was used. As seen in Figure 4, a direct channeling of the water did not occur. Instead, a partial anchoring of the water body took place in the toe region, and a high recovery of glycerol was achieved. Therfore, it was demonstrated that when using a pair of liquids with low-density contrast, it was possible to achieve a stable TTH displacement. Afterwards, a TTH displacement for a high-density contrast pair of liquids was investigated by displacing water by gas in the same model, the same configuration and the same injection rate, used previously. This time, the displacement process was not stable. As seen in Figure 5, the gas segregated and formed a gas cap at the upper part. Later on, downward displacement of water occurred; when gas injection rate was higher than the critical rate, gas coning somewhere along the horizontal well took place. Finally, a basic displacement of water by water (same density and viscosity) was performed this time in the full set up for TTHW (Figure 6), i.e. with the horizontal section of horizontal well located at the top of the layer and the injection through


the vertical well (all thickness perforated). The above conditions of unity mobility ratio and equal density are ideal from the point of view of long–distance oil displacement (LDOD) processes, and it may be viewed as a calibration test. There are only two forces dominating the advance of the water; the drive force – horizontally, and the action of the horizontal section as a line-sink - vertically upwards. As a result, displacement front is curved as shown in Figure 6. Given the ideal conditions of same density and viscosity,

Figure 4: Low–density contrast displacement;

displacement of glycerol by green water in a TTH configuration.

GAS

Figure 5: High–density contrast displacement; displacement of water by gas in a TTH configuration.

the displacing liquid front profile is related only to the pressure drop along the horizontal leg of the horizontal producer. As seen in Figure 6, contrary to our expectations, a direct channeling of the injected water through the horizontal leg did not take place, and more than 60% of the resident water was displaced. The advance of the injected water was

faster in the upper part of the layer, while some advancement also occurred at the bottom of layer. Figure 6: Displacement of water by green water in a TTH configuration.

In Figure 2, water invaded zone for a real TTHW process was illustrated. Although water viscosity is slightly lower and density of water is slightly higher than those for oil, which from the point of view of LDOD processes is very unfavorable, in the TTH system, in general, it led to a recovery higher than that obtained for a more favorable case of same density and viscosity fluids. This shows that gravity segregation plays an important role in these processes. There are three forces dominating the advance of the high mobility injected water: the drive force – horizontally, the action of the horizontal section as a line-sink - vertically upwards, and oil/water gravity segregation - vertically downwards. When the water first enters the “porous medium” the oil flow dominates the whole process and the water tends to flow directly towards the toe; afterwards, as more water has been injected, the oil/water segregation - vertically downwards - is more and more active and causes the formation of a “nose” of the water body directed downwards. The shape of the water body, as shown in Figure 2, is typical for a TTHW process. The displacement front is loosely anchored to the horizontal section of horizontal well, and although the front is not perfectly vertical, the water body advance is still relatively efficient from the point of view of sweep, which increases in time. Results of the Main Tests The results are presented in Tables 1 to 5 in order of increasing oil viscosity. In Figures 7 to 10 the shapes of water invaded zone are shown. In Figures 7-a to 7-b for a 10 mPa.s oil, the shape of water invaded zone at the water break-through for the conventional waterflood (Figure 7-a) is compared to the shape of water invaded zone at the water break-through for the TTHW process (Figure 7-b).

Vertical injector Horizontal producer

WATER

GAS


In Figures 8-a to 8-b, for the 780 mPa.s oil, these shapes are shown for TTHW at the beginning (Figure 8-a), and at watering out (Figure 8-b). As seen in Figure 8a, at the beginning of injection the fingering is quite extensive, but it is still possible for the water body to have horizontal and downwards advancement, so that the final vertical sweep is relatively good, as seen in Figures 8-b.

Table 1: Pembina Oil

Oil viscosity, mPa.s 10

Oil density, kg/m3 851.7

Brine-oil density contrast, kg/m3 168

Brine injection rate, ml/h 20

Flood Type

Well

Types

%Recovery

at brine

break-through

%Recovery

at watering

out

Conventional VI-VP 22 46

TTHW VI-HP 93 96

Legend: VI = Vertical injector VP = Vertical producer HP = Horizontal producer Table 2: Dunsmore Oil


Oil density, kg/m3 918



Flood Type

Well

Types

%Recovery

at brine

break-through

%Recovery

at watering

out


TTHW VI-HP 24 58

Table 3: Court Oil





Flood Type

Well

Types

%Recovery

at brine

break-through

%Recovery

at watering

out


TTHW VI-HP 12 54

Table 4: Lindberg Oil

Oilviscosity, mPa.s 1200



Brine injection rate, ml/h 2.5

Flood Type

Well

Types

%Recovery

at brine

break-through

%Recovery

at watering

out


TTHW VI-HP 21 67

Table 5: Bodo Oil

Oil viscosity, mPa.s 12,000

Oildensity,kg/m3 988.1


Brine injection rate, ml/h 2.5

Flood Type

Well

Types

%Recovery

at brine

break-through

%Recovery

at watering

out

Conventional VI-VP - -

TTHW VI-HP 14 32


Table 6: TTHW Process – Effect of Brine Injection Rate on Oil Recovery

Light oil Pembina oil


Oil density, kg/m3 851.7


Injection

Rate

ml/hr

%Recovery

at brine

break-through

%Recovery

at watering

out

60 78 84

20 93 96

Fig. 7a: Pembina oil: Conventional waterflooding. Shape of water invaded zone at water break-through

Fig. 7b: Pembina oil: TTH waterflooding. Shape of water invaded zone at water break-through

Fig. 8a: Court oil: TTH waterflooding. Shape of water invaded zone at the start of water injection

Fig. 8b: Court oil: TTH waterflooding. Shape of water invaded zone at the watering out

Fig. 9a: Lindberg oil: TTH waterflooding. Shape of water invaded zone at water break-through


Fig. 9b: Lindberg oil: TTH waterflooding. Shape of water invaded zone at watering-out

Fig. 10a: Bodo oil: TTH waterflooding. Shape of water invaded zone at water break-through

Fig. 10b: Bodo oil: TTH waterflooding. Shape of water invaded zone at watering-out The water invaded zones for the heaviest oils with viscosities of 1200 mPa.s and 14,000 mPa.s, at water break-through and watering out are shown in Figures 9-a to 9-b, and 10-a to 10-b, respectively. There are important differences in the size of

these zones; also, the fingering is more accentuated for the heavier oil. However, it is seen that for both oils, the fingering is more pronounced close to the injection well compared to the region far away from the injection well, where the water zone becomes more compact. At this time, the mechanisms leading to this phenomenon are not understood. While interpreting these results it should be noted that the injection rate was not the same for all the tests: it was 20 ml/hr for the 10 mPa.s and 112 mPa.s oils; 10 ml/hr for the 780 mPa.s oil and 2.5 ml/hr for the 1200 mPa.s and 12,000 mPa.s oils. Watering out was deemed when a water cut of 95% was attained. From Tables 1 to 5 and Figures 6 to 9, the following conclusions can be drawn: At water break-through oil recovery of the TTHW process

was close to that of conventional waterflooding. An exception to this was observed during light oil tests (10 mPas viscosity oil), for which break-through oil recovery was much higher for TTHW process. This conclusion seems to indicate that for relatively light-heavy oils once onset of water fingering occurs, the main finger advances extremely quickly, such that the value of distance from the injection point to the outlet becomes unimportant. In fact, the distance between the vertical injector and the vertical producer was at least 4 times higher than the distance between the shoe of vertical injector and the toe of horizontal producer.

Oil recovery at watering out, compared to conventional

waterflood, increased as follows: 46% to 96% for the 10 mPa.s viscosity oil, 27% to 58% for the 112 mPa.s viscosity oil, 23% to 54% for the 780 mPa.s viscosity oil, and 14% to 32% for the 12,000 mPa.s viscosity oil.

The unfavorable effect of water/oil mobility ratio was

reduced, but not totally eliminated as witnessed by the difference in the vertical sweep efficiency for the most viscous oils (Figures 9 and 10)

The main unswept zone is located above the horizontal

section of horizontal well; smaller unswept zones are found at upper corner, between the vertical well shoe and the horizontal well toe and close to the vertical well shoe in the lower section, mainly for more viscous oils. This last finding reveals that TTHW, unlike conventional waterflooding, would leave some undisplaced oil close to the injection point.

The direct effect of injection rate on oil recovery for the 10 mPa.s oil is shown in Table 6. There is a slight increase in oil recovery (both at water break-through and watering out) when a low injection rate is used. Also, for the same viscosity ratio, higher density contrasts will lead to high oil recovery. This was verified for the case of 1200 mPa.s oil; higher sweep


efficiency was obtained for 23% salinity case, as compared to the 3% salinity case. For this reason, in a TTHW process, using injection brines with the highest feasible salinity is recommended. This is totally opposite to conventional waterflooding, where higher water density (due to higher salinity) leads to more water under-ride and harms performance. This constitutes a significant difference between TTHW and conventional waterflooding. The tests showed that the position/tilting of displacement front is determined by the combined influence of contrast of density and viscosity for the displacing/displaced fluid, injection rate, and the relative location of the vertical injector and horizontal section of producer. Also, they showed that TTH displacement process applications lead to high oil rates from the very beginning (no waiting period). The oil recovery increases substantially, especially when the density contrast is relatively high (high brine salinity and/or low oil density), low injection rates are used and water/oil mobility ratio is not extremely unfavourable. Discussion of Results. Limitations The main limitation of Hele Shaw tests is that they give us only semi-quantitative results. Moreover, the permeability is extremely high, and capillary effects do not exist. Also, ratio of length to height of the model is approx. 2.4, which is very low and can be associated with only very thick oil formations. From this point of view the results are generally very optimistic. The Hele Shaw tests showed that in heavy oil pools, the fingering phenomena are pronounced close to the injection well; this fingering disappears far from the injection well where water invaded zone becomes ‘compact’. At the end of a flood, the zone around the injection well is less swept, compared to zones more remote. That this effect would also occur in a real porous medium is still to be demonstrated. In a TTHW process, the whole oil reservoir volume situated above the horizontal section of the producer will remain unswept, and this may force operators to locate the horizontal section as close to the overburden as possible. Such application would also impose a minimum limit on the pay thickness. Conclusions 1. The Toe-To-Heel Waterflooding (TTHW) process was investigated in a Hele Shaw laboratory model, which mimicked a vertical section of a simulated porous medium. The semi-quantitative results of the investigations showed that the process achieves a good vertical sweep efficiency, better than that for conventional waterflooding. 2. Because, on the one hand, the fact that the results were semi-quantitative, and on the other hand, the horizontal (areal) sweep efficiency was not determined, it remained to be confirmed that the process has a sound foundation. 3-D tests in real porous media were therefore necessary for validation. Later on, they were conducted and confirmed the soundness of

the process. These results will be presented in Part ll of this series of papers on TTHW.

References 1. Dietz, D.N. : "A Theoretical Approach to the Problem of Encroaching and Bypassing Edge Water." Konikl. Ned. Akad. Wetenschap (1953) Proc. B56, 83. 2. Sandrea, R. and Nielsen, R, Dynamics of Petroleum Reservoirs Under Gas Injection, Gulf Publishing Company, 1974 3. Ekrann, S.: "Gravity and Vertical Sweep Efficiency", In Situ, 17(2), 183-199, (1993) 4. Craig, F.G.: The Reservoir Engineering Aspects of Waterflooding, SPE Monograph, 1993. 5. Turta, A.T.: “Toe-To-Heel Oil Displacement” Short Paper Presented at Canadian International Petroleum Conference, June 4-8, 2000, Calgary. 6. Turta, A.T., and Singhal, A., K.: “Overview of Short-Distance Oil Displacement Processes” Paper presented at SPE/PS - CIM International Conference on Horizontal Well Technology, November 6-8, 2000, Calgary 7. Greaves, M, and Turta A T.: “Oilfield In-Situ Combustion Process” U.S. Patent No.5,626,191, May 6,1997. Canadian Patent No. 2,176,639, August 8, 200. 8. Greaves, M., Saghr. A.M. Xia, T.X., Turta, A. and Ayasse, C: “THAI – New Air Injection Technology for Heavy Oil Recovery and In-Situ Upgrading” Journal Of Canadian Petroleum Technology, March 2001, Vol 40, No.3. 9. Xia, T.X. and Greaves, M: “Upgrading Athabasca Tar Sand Using Toe-To-Heel Air Injection (THAI)”. Fourth International Conference and Exposition on Horizontal Well Technology, November 6-8, 2000, Calgary. 10. Coates, R. and Zhao, L.L.: “Numerical Simulation of THAI Process” Canadian International Petroleum Conference, June 12-14, 2001, Calgary, Canada 11. Ayasse C. and Turta A T.: “Toe-To-Heel Oil Recovery Process” USA Patent 6,167,966, January 2, 2001. Canadian Patent 2,246,461, June 18, 2002.

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