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SPE 115125
First Results of Cyclic Steam Stimulations of Vertical Wells with Radial Horizontal Bores in Heavy Oil Carbonates Stanislav Ursegov, Alexander Bazylev, and Evgeny Taraskin, PECHORNIPINEFT Ltd.
Copyright 2008, Society of Petroleum Engineers This paper was prepared for presentation at the 2008 SPE Russian Oil & Gas Technical Conference and Exhibition held in Moscow, Russia, 28–30 October 2008. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers 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 SPE copyright.
Abstract. This work shows the first experience of cyclic steam stimulations of
vertical wells with radial horizontal bores in the deep heavy oil carbonates of the Usinsk
Permian – Carboniferous reservoir.
In the end of 2006, more than 30 radial horizontal bores of approximately 100 m
length each were drilled in 8 vertical wells, 5 of which were then stimulated by steam
injection.
Radial drilling in low productivity carbonates without the following cyclic steam
stimulations were ineffective because it did not resulted in significant increase in the
well productivity.
Integrating radial drilling and cyclic steam stimulations proved highly effective by
reducing oil viscosity. The average growth of oil rate was about 15.0 tones per day.
Introduction. Because of the rapid extraction of light oil, the problem of active
development of heavy oil and bitumen reserves is becoming one of the most pressing
for the petroleum industry in the new millennium.
Heavy oil and bitumen relate to unconventional sources of hydrocarbons which
world's resources have already exceeded the resources of light oil and gas and
equivalent to 600 - 800 billion tons of oil. It is expected that by 2025 unconventional
sources of hydrocarbons will provide more than 20 % of the world oil production [1].
According to various estimates, Russian recourses of heavy oil and bitumen
range from 30 to 75 billion tones.
2 SPE 115125
The largest Russian development experience of heavy oil is gained in the Komi
Republic located in the North – East corner of the European part of the country near the
Timan Mountains and the Pechora River. Here there are two major Russian heavy oil
fields: Yarega and Usinsk Permian – Carboniferous reservoir with the total amount of
oil initial in place of over 1 billion tones which are under the industrial development for
dozens of years.
Reservoir Geological and Physical Characteristics. The Usinsk Permian –
Carboniferous reservoir is the largest field in the Timan -Pechora region according to its
amount of oil initial in place (730 million tones) and characterized by the following
features complicating its development process:
• High lateral heterogeneity (figure 1) – there are three different facial zones,
the reservoir is complicated by bioherms and karst identified by three-dimensional
seismic and vertical seismic profiling;
• Layered heterogeneity (figure 2) – the reservoir is stratified by 13 pay
intervals united in 3 development objects;
• Fluid flow heterogeneity is due to the presence of abnormally permeable
zones (natural fractures, caverns, karst cavities) with dozens of mkm2 permeability
which are called “supercollectors”. The proportion of such layers is more than 20 – 30
% of the volume of the reservoir, and its permeability of 2 - 3 orders exceeds the
permeability of porous matrix blocks containing the main oil reserves;
• Abnormal oil rheological properties – the reservoir oil has high viscosity (≈
710,0 mPa*s) and non-Newtonian characteristics. Laboratory studies of oil recovery
mechanism confirmed by production data analysis showed that at the initial reservoir
temperature (+ 210C) only the oil reserves concentrated in “supercollectors” may be
extracted [2].
In order to intensify the development process of the reservoir, different thermal
methods of oil recovery were tested such as hot water flooding in the PTV-1 and PTV-2
areas from 1982 until 1998, the initialization of the in-situ combustion process in the E-1
area in 1984 – 1985, and steam injection in the PTV zone since 1992 until now. .
SPE 115125 3
Current Conditions of Reservoir Development Process. At the present time,
in the reservoir there are 1098 vertical wells drilled with a density of 6.25 ha a well,
drilling reached approximately 40 % of the reservoir reserves.
In the process of thermal recovery, only 15 % of the reservoir reserves located
mainly in the PTV-3 area is involved, the rest area of the reservoir is being developed
by the natural water flooding regime.
The current number of production wells is 607, injection wells - 27. In the
reservoir, 22 stationary and 7 mobile steam generators are set and operated with a
gross output of about 12.0 thousand tons of steam a day.
As shown in Figure 3, in the recent years the current oil production stabilized at
1.5 million tones a year. The water cut was 82 %, the average oil rate of a production
well - 7.3 tons a day, the average steam injection rate of the reservoir - about 4.3
thousand tons a day.
By 2008, the cumulative oil production reached 53.3 million tons, the oil recovery
index - 7.3 %, the cumulative liquid production - 162.4 million tons. The inadequate
compensation of liquid production by fluid injection (only 25.2 %) led to the depletion of
the reservoir and the drop of the average reservoir pressure from 14.3 to 10.5 MPa.
According to the latest calculations using the different decline models, the
average initial recoverable reserves in the whole reservoir is 75.7 million tones, in the
PTV zone - 16.0 million tons, in the area of the reservoir being developed by the natural
water flooding regime - 60.4 million tons.
The forecasting ultimate oil recovery index with the 98 % water cut of the whole
reservoir is estimated by 9.3 %, the PTV-3 area - 23.0 %, the area of the reservoir
being developed by the natural water flooding regime - 8.1%.
Steamflooding. Currently, to improve the ultimate oil recovery of the reservoir,
the steam flooding and cyclic steam stimulation methods are being employed.
Steam flooding is implemented in the PTV zone covering about 520 hectares.
The area has the amount of oil initial in place of 62.4 million tones and 249 drilled wells
including 50 injection wells.
The steam injection was initiated in August 1992. By 2008, the cumulative oil
production of the PTV zone reached 11.5 million tons including 7.5 million tons
4 SPE 115125
extracted during the steam flooding period. From 1992 to 2008, the oil recovery index
calculated by the material balance method rose from 6.5 to 18.5 %.
At 01.01.08 in the PTV zone 20.4 million tons of steam were injected. The
amount of the cumulative additional oil production is estimated at 2.5 million tons. Thus,
the cumulative steam oil ratio is about 8.0.
Figure 4 shows the average temperatures of pay intervals of the PTV zone at
01.01.08. On the same picture, the initial temperature of the reservoir was also
presented. As it can be seen from Figure 4, the most heated layers of the PTV zone are
currently pay intervals 6 and 7+8 of the middle development object. Temperature of this
object ranged from +36.70C (pay interval 6) to +38.50C (pay interval 7+8). In the lower
and upper development objects the most heated pay intervals were 5 and 9
respectively which contacted with the middle development object.
As it is clear from the maps of isotherm constructed for 3 development objects
using the data gathered in 2007 (Figure 5), the main area of the PTV zone (53.5 %)
was heated by +30.50C. The area heated by +500C was 25.6 %, by +700C - 11.1 %, by
more than 1000C - 2.15 %.
In 2007 to improve the efficiency of steam flooding and reduce the steam oil ratio,
the non-stationary regime of steam injection was carried out in 11 elements of the PTV
zone providing a temporary stop of injection wells for a period of 2 to 6 months.
Figure 6 shows the production results of an element of the PTV zone where the
effect of the non-stationary regime of steam injection has been most evident. The
restriction of steam injection from January to March 2007 into the well 4234 and stop of
the well for further two months helped to reduce water cut of production wells in this
element almost by 10 % (from 56 % in December 2006 to 46% in June 2007). The
average oil rate of production wells increased by 1.5 tons per day (from 8.1 tones per
day in December 2006 to 9.6 tonnes per day in June 2007).
In 2007, in all elements, which took place a restriction or a temporary stop of
steam injection, the technological effect was received. The total value of this effect was
estimated at 20.4 thousand tons of additional oil, representing almost 15 % of the 2007
total additional oil production from steam flooding.
SPE 115125 5
Cyclic Steam Stimulations of Vertical Wells. From 1993 until 2007 in the
reservoir 381 of cyclic steam stimulations (CSS) of 238 production wells were
conducted with 1.7 million tones of steam injected and 1.48 million tones of additional
oil produced. The obtained steam oil ratio estimated at 1.12 characterizes the CSS
technique as one of the best technological activities in the reservoir.
The CSS effectiveness essentially depends on the basic characteristics of
stimulated wells (productivity, water cut, reaction on steam flooding) (Figure 7).
If a well’s water cut is more than 75 % the necessary level of CSS profitability
does not achieve regardless of a well’s productivity. CSS is profitable if a nonreacting
well’s water cut is less than 25 % regardless of its productivity.
The reaction on steam flooding increases the efficiency of CSS of a stimulated
well with a low productivity (less than 30 m3/day/MPa) and an average water cut (less
than 75 %).
The effectiveness of CSS of a well with an average water cut depends on the
well’s water production mechanism: for an average productive well (the productivity
index is more than 30.0 and less than 100.0 m3/day/MPa) or a high productive well (the
productivity index more than 100.0 m3/day/MPa) with a stable water production the
effectiveness is usually higher than for the wells with the same productivity and having
the increasing water production.
Taking into account the direct impact of geological characteristics of the
stimulated wells on the results of CSS, it is necessary to conduct some additional
technological activities aimed at reducing water production and increasing productivity
in the wells with a water cut is more than 75 % and a productivity index is less than 30
m3/day/MPa.
Based on the analysis of the conducted CSS in the reservoir, the following basic
geological criteria and technological requirements were developed compliance with
which could provide the necessary profitability of the technique:
− well’s water cut - no more than 75 %;
− well’s productivity index − at least 75 - 80 m3/day/MPa;
− steam injection rate - 100 - 120 tons per 1 m of the well’s perforated net
pay thickness;
6 SPE 115125
− putting a well into production after the injection and soaking periods should
occur at the well bottom hole temperature of 120 – 1500C;
− number of cycles for each stimulated well - at least three;
− considering the trend of injecting heat flow move up, to achieve a greater
vertical sweep efficiency, it is recommended to inject steam into the lower part of the
perforated interval.
Before CSS of the wells with an average water cut of more than 75 %, the
following preparation works should be done:
− geophysical identification of water saturated intervals and working
intervals;
− isolation of highly water saturated intervals.
In the wells with the productivity index of less than 75 m3/day/MPa, the additional
perforation of new productive intervals followed by acidizing should be conducted
before CSS.
Despite the high efficiency of the CSS technique in the reservoir, the main
drawback of this technique in the system of vertical wells is the low ultimate oil
recovery.
Pilot Tests of Vertical Wells with Radial Horizontal Bores. In 2006 and 2007
in the reservoir, the first pilot tests of using radial horizontal bores drilled in 8 vertical
production wells were conducted.
The radial drilling execution process consists of that using coiled tubing ending
with a high-pressure joint and a hydraulic nozzle are pushed through pre-drilled holes in
casing into the productive formation. The hydrodynamic impact of the high-speed water
jet flowing from the nozzle destroys the formation. The average diameter of this radial
horizontal bore is of approximately 50 mm.
Theoretically, from the perspective of fluid mechanics, radial drilling gives clear
and positive forecast for the deepening of perforation channels. Such work provides an
increase in the effective radius of wells in all types of formations. The deep penetration
into the formation by radial horizontal bores should lead to a sharp decline of flow
resistance in a Skin-zone around a well and significantly increase a well’s drainage
area.
SPE 115125 7
In the reservoir, a Skin-index of drilled wells in most cases has a negative value,
reflecting the lack of a Skin-zone, thus artificially increasing the length of the existing
flow channels (natural fractures and karst cavities) can not have a significant impact on
improving the quality of the formation perforation. Consequently, in the reservoir the
effect of only the creation of radial horizontal bores will be insignificant, but in
combination with the following CSS, the effect should be substantially higher.
Typically, in the production wells 4 radial horizontal bores with an average length
of 100 m each were drilled. In the formation the radial horizontal bores were located
with a shift of 900 around a vertical wellbore, in a vertical plane the distance between
the perpendicular pairs of radial horizontal bores were changed in the interval from 15
to 45 m.
The radial horizontal bores were recommended to drill in vertical wells with the
low productivity characteristics to increase the volume of the reservoir heated by the
following CSS. According to this advice, in 5 wells with radial horizontal bores the
following CSS were conducted. At the same time, in the other 3 wells, radial horizontal
bores were drilled without the following CSS.
As anticipated, drilling radial horizontal bores in the wells with low productivity
without the following CSS were not technologically successful. Radial horizontal bores
did not change the negative Skin-index value of stimulated wells.
Thus, drilling radial horizontal bores in the wells of the reservoir only leads to an
increase in the effective radius of the wells, but not in a significantly change of the main
negative component of the wells’ low productivity - a small oil mobility.
Drilling radial horizontal bores in combination with the following CSS in the wells
with low productivity and average water cut less than 75 % provided high technological
efficiency. The total additional oil production of CSS of the wells with radial horizontal
bores in 2007 increased up to 16.3 thousand tons or 2.7 thousand tons per well, with
an average growth of oil rate equals to 15.1 tones per day and the duration of the effect
– more than 180 days. The steam oil ratio of such wells was near 2.6.
The greatest effect was received in well №№ 6042 and 7168, of which an
amount of 10.8 thousand tons of additional oil were produced or 5.4 thousand tons per
8 SPE 115125
well, with an average growth of oil rate and the duration of the continuing effect at
01.01.08 equal to 20.4 tons per day and 270 days respectively (figure 8).
The obtained high effect was primarily due to the involvement by radial drilling
and following steaming earlier undeveloped formation intervals with low productivity
and high oil saturation. Radial horizontal bores are high permeable channels which can
be used to distribute injecting steam over long distances from the vertical wellbore,
which substantially increases the heated volume of the reservoir and provides an
increase of oil recovery.
In 2008 – 2011 in the reservoir in addition to continuing the implementation of
CSS of wells with radial horizontal bores, drilling deviated wells and horizontal wells
with the smart completion with the following steam injection are planned.
Conclusions: 1. World experience of heavy oil production is growing by the development of
the unique geological structure and highly largest reserves Usinsk Permian –
Carboniferous reservoir in the European North – East of Russia, the Komi Republic.
2. The development of low productivity formation intervals is an objective
necessity and is now using targeted delivery of steam using radial horizontal bores.
3. Experiences gained so far permits to approve the high efficiency of CSS of
production wells with radial horizontal bores.
References:
1. Paul B. Future Energy. How the New Oil Industry Will Change People,
Politics and Portfolios. John Wiley & Sons. 240 pages. (2007).
2. Ruzin L.M., Ursegov S.O. Development of thermal methods of heavy oil
production in the Usinsk Permian – Carboniferous reservoir. // Oil industry, 2005, № 2,
p. 82 - 84.
Authors' biographies: Stanislav Ursegov:
Major - petroleum engineer, candidate of technical science, SPE member.
Position – Department head of PechorNIPIneft Ltd.
SPE 115125 9
Alexander Bazilev:
Major - petroleum engineer.
Position - Principal engineer of PechorNIPIneft Ltd.
Evgeny Taraskin:
Major - petroleum engineer.
Position - Principal engineer of PechorNIPIneft Ltd.
10 SPE 115125
Figure 1 - Lateral eterogeneity of the Usinsk Permian - Carboniferous reservoir
Figure 2 - Layered heterogeneity
SPE 115125 11
12 SPE 115125
SPE 115125 13
Figure 5 - Isoterm maps of the development objects of the PTV zone at 01.01.08
14 SPE 115125
Figure 6 - Production history of element № 4234
SPE 115125 15
Figure 7 - The dependense of the CSS efficiency with basic characteristics of stimulated wells
16 SPE 115125
Well № 7168
Well № 6042
Figure 8 - Results of CSS of vertical wells with radial horizonWell № 6042Well № 7168