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.

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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. .

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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

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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.

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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;

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− 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.

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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

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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.

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Figure 1 - Lateral eterogeneity of the Usinsk Permian - Carboniferous reservoir

Figure 2 - Layered heterogeneity

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Figure 5 - Isoterm maps of the development objects of the PTV zone at 01.01.08

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Figure 6 - Production history of element № 4234

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Figure 7 - The dependense of the CSS efficiency with basic characteristics of stimulated wells

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Well № 7168

Well № 6042

Figure 8 - Results of CSS of vertical wells with radial horizonWell № 6042Well № 7168