Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods
The paper proposes a new approach for using a combination of radioactivity logging (neutron and density logging), which allows determining a set of gas reservoir parameters (nature of saturation, true porosity, gas saturation) taking into account influence of the pressure and temperature conditions...
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Kulyk, V.V. Bondarenko, M.S. 2016-06-23T17:17:58Z 2016-06-23T17:17:58Z 2016 Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods / V.V. Kulyk, M.S. Bondarenko // Геофизический журнал. — 2016. — Т. 38, № 2. — С. 106-119. — Бібліогр.: 29 назв. — англ. 0203-3100 https://nasplib.isofts.kiev.ua/handle/123456789/103763 550.832.5 The paper proposes a new approach for using a combination of radioactivity logging (neutron and density logging), which allows determining a set of gas reservoir parameters (nature of saturation, true porosity, gas saturation) taking into account influence of the pressure and temperature conditions of occurrence. Analysis of the ways of averaging the neutron-apparent porosity and the density-apparent porosity for obtaining the true porosity was made. Method of determination of gas saturation, which uses the same combination of radioactivity logging as in determining the true porosity of gas reservoirs, was developed. The results presented in the paper, were obtained over a wide interval of depth (up to 10 km). Application of developed approaches for determination of petrophysical parameters of gas reservoirs is demonstrated by the example of cased coalbed methane well. Запропоновано новий підхід до використання комплексу радіоактивного каротажу - нейтрон-нейтронного (ННК) і гамма-гамма каротажу (ГГК), який дозволяє визначати сукупність параметрів газових колекторів (характер насичення, істинна пористість, коефіцієнт газонасиченості) з урахуванням впливу термобаричних умов залягання. Виконано аналіз варіантів усереднення позірних пористостей за ННК і за ГГК при отриманні істинної пористості. Розроблено спосіб визначення коефіцієнта газонасиченості, який використовує той же комплекс, що й при визначенні істинної пористості газових колекторів. Приведені результати отримано для широкого інтервалу глибин (до 10 км). Застосування розвинутих підходів до визначення петрофізичних параметрів газових колекторів продемонстровано на прикладі обсадженої метановугільної свердловини. en Інститут геофізики ім. С.I. Субботіна НАН України Геофизический журнал Научные сообщения Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods Идентификация газовых резервуаров и определение их параметров с помощью комбинации методов радиоактивного каротажа Article published earlier |
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Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods |
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Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods Kulyk, V.V. Bondarenko, M.S. Научные сообщения |
| title_short |
Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods |
| title_full |
Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods |
| title_fullStr |
Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods |
| title_full_unstemmed |
Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods |
| title_sort |
identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods |
| author |
Kulyk, V.V. Bondarenko, M.S. |
| author_facet |
Kulyk, V.V. Bondarenko, M.S. |
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Научные сообщения |
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Научные сообщения |
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2016 |
| language |
English |
| container_title |
Геофизический журнал |
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Інститут геофізики ім. С.I. Субботіна НАН України |
| format |
Article |
| title_alt |
Идентификация газовых резервуаров и определение их параметров с помощью комбинации методов радиоактивного каротажа |
| description |
The paper proposes a new approach for using a combination of radioactivity logging (neutron and density logging), which allows determining a set of gas reservoir parameters (nature of saturation, true porosity, gas saturation) taking into account influence of the pressure and temperature conditions of occurrence. Analysis of the ways of averaging the neutron-apparent porosity and the density-apparent porosity for obtaining the true porosity was made. Method of determination of gas saturation, which uses the same combination of radioactivity logging as in determining the true porosity of gas reservoirs, was developed. The results presented in the paper, were obtained over a wide interval of depth (up to 10 km). Application of developed approaches for determination of petrophysical parameters of gas reservoirs is demonstrated by the example of cased coalbed methane well.
Запропоновано новий підхід до використання комплексу радіоактивного каротажу - нейтрон-нейтронного (ННК) і гамма-гамма каротажу (ГГК), який дозволяє визначати сукупність параметрів газових колекторів (характер насичення, істинна пористість, коефіцієнт газонасиченості) з урахуванням впливу термобаричних умов залягання. Виконано аналіз варіантів усереднення позірних пористостей за ННК і за ГГК при отриманні істинної пористості. Розроблено спосіб визначення коефіцієнта газонасиченості, який використовує той же комплекс, що й при визначенні істинної пористості газових колекторів. Приведені результати отримано для широкого інтервалу глибин (до 10 км). Застосування розвинутих підходів до визначення петрофізичних параметрів газових колекторів продемонстровано на прикладі обсадженої метановугільної свердловини.
|
| issn |
0203-3100 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/103763 |
| citation_txt |
Identification of gas reservoirs and determination of their parameters by combination of radioactive logging methods / V.V. Kulyk, M.S. Bondarenko // Геофизический журнал. — 2016. — Т. 38, № 2. — С. 106-119. — Бібліогр.: 29 назв. — англ. |
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| fulltext |
V. V. KULYK, M. S. BONDARENKO
106 Геофизический журнал № 2, Т. 38, 2016
1. Introduction. The subject of investigation
is a gas reservoir in open and cased wells taking
into account the pressure and temperature con-
ditions of occurrence (PT-conditions).
Porosity, gas saturation and permeability are
the main petrophysical parameters of gas reser-
voirs. Important parameters are also bulk density
of the rock; weight and volume fraction of shale
and clay minerals; hydrogen indices of rock,
shales, clay minerals and interstitial fluids [Inter-
pretation…, 1988].
Radioactive logging methods are universal
methods for determination of parameters of con-
ventional and unconventional gas reservoirs in
both open and cased holes. The density logging
(for determination of the bulk density), neutron
logging (for determination of the neutron poros-
ity) and gamma ray logging (as neutron logging
correction for the hydrogen content in clay min-
erals) are effective enough methods for obtain-
ing the majority of petrophysical parameters of
reservoirs [Golovatskaia et al., 1984; Serra, 1984;
Nuclear..., 1990; Ellis, Singer, 2008].
Features of the application of these radioac-
tive logging methods (density logging, neutron
logging and gamma ray logging) for investiga-
tion of gas reservoirs are considered in [Alger,
Dewal, 1969; Golovatskaia et al., 1984; DasGupta,
УДК 550.832.5
Identification of gas reservoirs and determination of their
parameters by combination of radioactive logging methods
© V. V. Kulyk, M. S. Bondarenko, 2016
Institute of Geophysics, National Academy of Sciences of Ukraine, Kiev, Ukraine
Received December 30, 2015
Presented by the Editorial Board Member R. I. Kutas
Запропоновано новий підхід до використання комплексу радіоактивного каротажу
— нейтрон-нейтронного (ННК) і гамма-гамма каротажу (ГГК), який дозволяє визнача-
ти сукупність параметрів газових колекторів (характер насичення, істинна пористість,
коефіцієнт газонасиченості) з урахуванням впливу термобаричних умов залягання. Виконано
аналіз варіантів усереднення позірних пористостей за ННК і за ГГК при отриманні істинної
пористості. Розроблено спосіб визначення коефіцієнта газонасиченості, який використовує
той же комплекс, що й при визначенні істинної пористості газових колекторів. Приведені
результати отримано для широкого інтервалу глибин (до 10 км). Застосування розвинутих
підходів до визначення петрофізичних параметрів газових колекторів продемонстровано на
прикладі обсадженої метановугільної свердловини.
Ключові слова: газові пласти-колектори, термобаричні умови залягання, комплекс
нейтрон-нейтронного і гамма-гамма каротажу, виділення газових пластів-колекторів, позір-
ні пористості, істинна пористість, коефіцієнт газонасиченості.
НАУЧНЫЕ СООБЩЕНИЯ
1997; Mao, 2012; Ijasan et al., 2013]. In particular,
neutron logging or density logging separately do
not allow determining the true porosity and other
parameters of the gas reservoirs. The solution of
this problem requires their combined use [Кulyk,
Bondarenko, 2013a, b; Bondarenko et al., 2014].
At the same time, the existing approaches of
combined use of neutron logging and density log-
ging have shortcomings and restrictions. Among
these are absence of substantiation of an optimal
method of averaging the neutron-apparent and
density-apparent porosities for determining true
porosity of gas reservoir, neglect of occurrence
depth, necessity to use the electric logging for
determining gas saturation (by means of water
saturation) and others.
The paper proposes a new approach of using
radioactive logging combination, which allows
determining set of the gas reservoir parameters
(nature of saturation, true porosity and gas satu-
ration) taking into account influence of pressure
and temperature conditions of gas reservoir oc-
currence. Results presented in the paper were
obtained for a wide depth interval (up to 10 km).
The application of developed approaches to de-
termine petrophysical parameters of gas reser-
voirs has been demonstrated by the example of
cased coal-bed methane well.
IDENTIFICATION OF GAS RESERVOIRS AND DETERMINATION OF THEIR PARAMETERS ...
Геофизический журнал № 2, Т. 38, 2016 107
2. Features of radioactive logging in both
open and cased holes. Investigation of reservoirs
in both open and cased wells has its own features.
2.1. Open hole. After drilling, the near-well-
bore region of gas reservoir can have properties,
different from a virgin bed.
When drilling on water-based mud in conven-
tional reservoirs the invaded zone is formed.
The problem of determining true porosity in
the presence of invaded zone under certain con-
ditions can be solved, as shown in [DasGupta,
1997], with the help of combination of density-
neutron loggings. However, this paper disre-
gards the influence of residual gas saturation,
which mostly is equal about 20 % [Hallenburg,
1998].
Residual gas essentially influences on the
readings of radioactive logging tools and, hence,
on porosities determined by density-neutron
loggings. In actual practice, when the invasion
depth exceeds the depth of investigation of den-
sity-neutron loggings, identification of gas reser-
voirs and determination of their true porosity on
the basis of the combination of radioactive log-
gings are possible, as well as a quantitative es-
timation of residual gas saturation [Кulyk et al.,
2014].
When drilling on oil-based mud the invaded
zone practically is not formed [Cholet, 2000].
Invasion is also a minimal when logging while
drilling. It is absent also in gas-saturated tight
sandstones and gas-saturated shales because of
low permeability (unconventional reservoirs). In
these cases, combination of radioactive loggings,
under certain conditions, allows to determine the
true petrophysical parameters of gas reservoir.
2.2. Cased hole. The investigation of conven-
tional gas reservoirs in cased holes has become
an increasingly important. This is due to follow-
ing problems: old wells reevaluation, control over
gas-saturation in the course of gas reservoir de-
velopment, monitoring of wells of underground
gas storages, necessity of immediate casing the
holes (for example, when drilling in the compli-
cated geological conditions). The latter takes
place in Ukraine, in particular, during both ex-
ploration of hydrocarbons at great depths (over 6
km) and investigation of coal-bed methane wells.
After drilling and casing the holes, the mud
filtrate dissipates during the time from a few
weeks [Serra, 1984] to a few months and even to a
few years, depending on the reservoir properties.
In immediately cased wells invaded zone mostly
has no time to form or this zone is negligible. The
invaded zone is absent in old wells.
Radioactive logging is practically without
alternative in the cased holes for identification
gas-bearing reservoirs, for determination of their
porosity, gas saturation and other parameters.
3. Pressure and temperature conditions. Es-
timation of PT-conditions influence in determin-
ing the parameters of gas reservoirs is the topical
task due to the facts that the depth of commercial
gas production increases and the reservoirs with
abnormally high formation pressure (AHFP) ex-
ist.
At the present time the gas presence of for-
mations at great depths (up to about 10 km) is
proved [Reeves et al., 1998; Thaddeus, Cook,
2001; Lukin, 2014b]. Total porosity as a whole
declines with the depth, whereas the gas density
and the amount of hydrogen per unit volume in-
crease. In other words, hydrogen index of a gas
increases with increasing formation pressure.
Accordingly, PT-conditions significantly affect
the radioactive logging tools readings, measured
neutron and density porosities and other param-
eters.
Formation pressure is the pressure of the flu-
ids (formation water, oil, gas) in the pore space of
the formation. It is generally accepted that nor-
mal formation pressure is equal to hydrostatic
pressure. The hydrostatic pressure is caused by
the water column which has a certain density and
is extending from the wellhead to the investigat-
ed formation. Conditional hydrostatic pressure
(CHP) is equal to the pressure of the column of
water with density of 1 g/cm3. Pressure gradient
which corresponds to conditional hydrostatic
pressure is equal to 10 MPa/km [Fertl et al., 1976;
Melik-Pashaev et al., 1983; Chilingar et al., 2002].
Manifestations of AHFP in gas reservoirs ly-
ing at different depths are recorded practically
in all regions of the world. For the reservoir,
which is located at a given depth, the lower limit
of AHFP is determined by the density of forma-
tion water at a maximal salinity ( 1.3 g/cm3). The
upper limit of AHFP corresponds to overburden
pressure which produces formation with an aver-
age bulk density 2.3 g/cm3. Thus, the pressure
gradient corresponding to AHFP lies in the range
about 13—23 MPa/km [Fertl et al., 1976; Melik-
Pashaev et al., 1983; Chilingar et al., 2002].
The average geothermal gradient in most of
oil and gas fields is equal to 30 °C/km [Petrenko
et al., 2004; William, Plisga, 2005], while pressure
gradient corresponds to CHP [Petrenko et al.,
2004].
Gas density and hydrogen index of gas with
consideration of PT-conditions were obtained
V. V. KULYK, M. S. BONDARENKO
108 Геофизический журнал № 2, Т. 38, 2016
below on a basis of real gas equation of state with
compressibility factor to account for non-ideal
behavior of real gas [Reid et al., 1977]. Most of
the parameter calculations were obtained for the
CHP and average geothermal gradient.
4. Petrophysical model. For investigation of
principal aspects of determination of both the
porosity and the gas saturation we accepted the
following simplest petrophysical model of reser-
voir.
By «gas reservoir» we shall mean rock, which
contains free gas in open, closed pores or in both.
Solid constituent of the rock (matrix) consists
of single mineral (quartz, calcite, dolomite), ex-
cluding shale. Pores are filled with fresh water
and gas (methane СН4) in various proportions.
The relative volume of gas in the pores of a rock
is the gas saturation. Gas saturation vary from 0
(water) to 1 (total gas saturation).
Matrix density and density of interstitial wa-
ter are accepted as constant in the depth interval
from 0 to 10 km; PT-conditions of gas reservoir
occurrence determine gas density.
5. Apparent porosities of gas reservoirs. 5.1.
Neutron porosity. Neutron porosity is closely re-
lated to the hydrogen content in the rock. It is
necessary to construct a calibration curve for de-
termination of neutron porosity ϕN. This calibra-
tion curve is a relation between readings of neu-
tron tool and porosity of water-filled pure rock
(for example, non-shale limestone) for the given
technical conditions of measurement.
Gas reservoir has a lower number of hydrogen
nuclei in unit volume than a water-filled forma-
tion of the same porosity. When gas-filled forma-
tion is logged, the neutron porosity ϕN, which
is estimated using the «water-filled» calibration
curve, will be apparent. This apparent porosity
will be lower than the true porosity.
The relative hydrogen content (hydrogen in-
dex, ω) of porous gas-water-filled non-shale for-
mation in terms of hydrogen indices of water ωw
and gas ωg is written as [Serra, 1984]:
(1 )w g g gS S , (1)
where ϕ is the porosity of gas reservoir; Sg is the
gas saturation; ωw is the hydrogen index of water
(hydrogen index of fresh water ωw=1); ωg is the
hydrogen index of gas; the hydrogen index of
methane is given by:
2,25g g w , (2)
where ρg is the gas density in the pores for the
given pressure-temperature conditions, ρw is the
water density.
As a first approximation the hydrogen index
of a water-filled reservoir is equal to neutron
porosity. The hydrogen index of a gas-filled for-
mation can be approximately considered as the
neutron-apparent porosity, i. e. ≈ϕN. Then, from
the Eq. (1) it follows that
N N , (3)
where
( )N w g gS . (4)
According to Eq. (3), neutron-apparent poros-
ity of gas reservoirs will be lower than the true
porosity on the value ϕN. In the case of a fully
water-saturated reservoir (Sg=0, ϕN=0), neutron-
apparent porosity becomes true: ϕN=ϕ. In the
case of a fully gas-saturated reservoir (Sg=1), neu-
tron-apparent porosity takes a minimum value:
ϕN=ωgϕ.
5.2. Density porosity. The results of den-
sity logging are due to electron density of rock,
which, in turn, is closely connected with bulk
density. The total porosity of water-saturated res-
ervoirs obtained from the density logging is ex-
pressed as:
s
D
s w
, (5)
where ρs is the bulk density of solid component,
ρw is the water density, ρ is gamma-gamma log
density [Serra, 1984; Interpretation…, 1988; Ellis,
Singer, 2008].
The bulk density of gas-water-saturated re-
servoirs ρ is determined using the following
equation:
(1 ) (1 )s w g g gS S . (6)
Taking into account that ρ ≈ρ, and substitut-
ing Eq. (6) in Eq. (5), we get expression for the
density-apparent porosity of gas-water-saturated
reservoirs:
D D , (7)
where
( 1)D g gS . (8)
Here Δg is dimensionless density parameter
(Δg>1), which at given ρs=const and ρw=const is
determined by the gas density ρg under reservoir
conditions:
s g
g
s w
. (9)
According to Eq. (7), density-apparent poros-
ity of the gas reservoirs is higher than the true
IDENTIFICATION OF GAS RESERVOIRS AND DETERMINATION OF THEIR PARAMETERS ...
Геофизический журнал № 2, Т. 38, 2016 109
porosity on the value of ϕD. In the case of a fully
water-saturated reservoir, density-apparent po-
rosity becomes true: ϕD=ϕ. In the case of a fully
gas-saturated reservoir, density-apparent poros-
ity takes a maximum value: ϕD = gϕ.
5.3. Relation between apparent porosities
and true porosity. Three different porosities
(neutron-apparent porosity, density-apparent
porosity and true porosity) are used in the inves-
tigation of gas reservoirs with the aid of radioac-
tive logging combination.
As an example of gas-saturated sandstones
at 20 %-porosity and 10 %-porosity, the results
of estimating the neutron-apparent porosity and
density-apparent porosity on the basis of Eqs. (3),
(4) and Eqs. (7)—(9), respectively, for various gas
saturations depending on the occurrence depth
are shown in Fig. 1.
Fig. 1 shows that depth-dependences of the
apparent porosities ϕN and ϕD are nonlinear, in so
doing the nonlinearity increases with increasing
Sg. For reservoirs deeper than 4 km, porosities
N and D relatively depend only weakly on the
PT-conditions for all the values of Sg.
Thus, the differences between density-appar-
ent porosity and true porosity, between neutron-
apparent porosity and true porosity, between
density-apparent porosity and neutron-apparent
porosity are increasing as the gas saturation in-
creases. The same pattern is characteristic for gas
reservoir with increasing true porosity. The effect
of gas on both density porosity and neutron po-
rosity decreases with increasing depth at the ex-
pense of an increase in gas density and hydrogen
index of gas. In so doing effect of gas saturation
on the neutron logging is stronger than on the
density logging over the whole depth interval
and it doesn’t depend on both gas saturation and
true porosity.
We consider ratios of density-apparent poros-
ity and neutron-apparent porosity to true poros-
ity (RN =ϕN/ϕ and RD=ϕD/ϕ), respectively, as:
1 ( ) N w g gR S , (10)
1 ( 1) D g gR S , (11)
together with the ratio of neutron-apparent po-
rosity to density-apparent porosity, RN/D=ϕN/ϕD,
as
/
1 ( )
1 ( 1)
w g g
N D
g g
S
R
S
. (12)
Fig. 2 demonstrates depth dependences of the
ratios RN, RD and RN/D for various gas saturations
Sg.
Analysis of Eq. (10)—(12) along with Fig. 2
leads to the following conclusions:
to accepted approximation the ratios RN, RD
and RN/D are independent of porosity of gas res-
ervoir, i. e., Eq. (10)—(12) and Fig. 2 are valid for
any porosity;
dependences of the ratios RN, RD and RN/D
on the occurrence depth of beds h are nonlinear
for any values of Sg; in so doing for small values
Fig. 1. Apparent porosities ϕN and D of gas reservoirs vs depth for sandstone with ϕ=0.2 (a) and ϕ=0.1 (b)
at different gas saturation.
V. V. KULYK, M. S. BONDARENKO
110 Геофизический журнал № 2, Т. 38, 2016
Sg(<0.2) this nonlinearity is weak (R≈const(h)),
whereas for large values Sg (>0.8) nonlinearity is
relatively strong to a depth of 4 km; at the big-
ger depths parameters R depend relatively only
weakly on PT-conditions for all values of Sg;
if Sg=0 (fully water-saturated pores) it is ob-
vious that RN=RD=RN/D=1; at Sg=1 (fully gas satu-
ration) we may write that RN g(<1), RD= g(>1),
RN/D g g(<1);
for all the values of Sg and h the ranges of ra-
tios R are as follows: 0<RN 1; 1 RD g (maximum
value g 1.6 corresponds to shallow depth and to
maximum gas saturation); 0<RN/D 1.
Thus, parameters RN and RD provide quanti-
tative estimating for the ratio of both apparent
porosities to true gas reservoir porosity as a func-
tions of the occurrence depth at a given value of
the gas saturation; whereas the parameter RN/D
gives a quantitative relationship between appar-
ent porosities depending on the same factors.
6. Identification of gas reservoirs. Estimation
of nature of saturation (gas, water) is impossible
using neutron logging or density logging sepa-
rately, whereas their combined use solves this
problem. One known way of solving the problem
of gas reservoirs identification is to compare of
neutron-apparent porosity and density-apparent
porosity as borehole logs [Ellis, Singer, 2008].
Disagreement between density porosity and
neutron porosity along geological section of
borehole can serve as convenient quantitative
criterion of gas-filled rocks. This disagreement is
determined by difference of two porosities
D N . (13)
Difference ϕ can be considered as a parameter
of gas reservoir identification. For water-saturated
rocks, difference is zero ( ϕ=0), whereas for gas-
bearing interval, wherein both density porosity
and neutron porosity are apparent, this parameter
is a positive value, and ϕ increases, as the poros-
ity and gas volume in pores are increased.
For reliable identification of gas reservoir, dif-
ference ϕ shall exceed the total error of porosity
determination with the help of density logging
and neutron logging. In our estimation the abso-
lute total error of difference ϕ in practice is less
than about ±3 %.
In Fig. 3 the calculated depth dependences of
the parameter ϕ at various gas saturations are
shown for the sandstone with porosity 20 % (it
corresponds to a conventional, good-quality res-
ervoir which occurs at a moderate depth) and 5 %
(it corresponds to an unconventional reservoir or
to a deep-seated conventional reservoir [Lukin,
2014a]). Dashed line indicates the accepted ab-
solute error (3 %) of parameter ϕ for fully water-
saturated pores ( ϕ=0); that is, identification of
gas reservoir in the range of parameter ϕ from
0 to 0.03 is impracticable through errors of mea-
surements.
As may be seen from Fig. 3, a, at relatively
high true porosity the neutron-density loggings
allow to identify gas reservoirs practically over
Fig. 2. Ratios RD, RN (a) and RN/RD (b) vs depth for sandstone at different gas saturation.
IDENTIFICATION OF GAS RESERVOIRS AND DETERMINATION OF THEIR PARAMETERS ...
Геофизический журнал № 2, Т. 38, 2016 111
the whole gas saturation range (at Sg>0.2), in par-
ticular also in the presence of residual gas satura-
tion in an invaded zone. From Fig. 3, b it follows
that the neutron-density loggings allow to iden-
tify low-porosity gas reservoirs with confidence
only at high gas saturation (at Sg>0.5—0.6). At
low porosity the shallowness (less than 2 km) of
gas reservoir is favorable factor for its identifica-
tion by neutron-density loggings.
Specific procedure of gas reservoirs identifi-
cation consists in the following (see Fig. 4). Ob-
tained neutron porosity log and density porosity
log should be plotted together (at the same scale
and use of the same porosity units). In gas reser-
voirs, the two porosity curves will cross over each
other: the density porosity log will show higher
porosity, while the neutron porosity log will show
a lower porosity (ϕD>ϕN).
The magnitude of the crossover (the amount
of separation between the curves) increases with
increasing both true porosity and gas saturation.
In water-saturation (non-shale) formations, the
porosity curves obtained from neutron log and
density log (ϕN and ϕD, respectively) are practi-
cally agreed (within the limits of errors).
Fig. 4 gives logging data obtained in imme-
diately cased coal-bed methane well (Donbas,
terrigenous geologic section). Gas reservoirs
were identified by the use of difference ϕ in the
following intervals (where the density porosity
curve and neutron porosity curve are separated):
Х15—Х20 m, Х23—Х30 m, Х35—Х40 m, Х41—
Х50 m, Х59—Х65 m, Х72—Х75 m, Х84—Х90 m.
Petrophysical parameters of gas reservoirs were
also determined. In so doing, methods of deter-
minations of true porosity ϕ and gas saturation Sg
Fig. 3. Identification parameter ∆ϕ vs depth for sandstone with ϕ=0.2 (a) and ϕ=0,05 (b) at different gas saturation. Dashed line
shows error of identification parameter.
Ta b l e 1. Example of determination of gas reservoir petrophysical parameters in the cased borehole
Number Interval, m ϕD, % ϕN, % ϕ, % ϕ, % Sg, %
1 X15—X20 17.5 11.4 6.1 15 26
2 X23—X26 19.8 13.4 6.4 18 24
3 X26—X30 22.2 3.1 19.1 16 80
4 X35—X40 19.1 13.3 5.8 17 22
5 X41—X47 21.8 14.8 7.0 19 24
6 X47—X50 15.5 9.6 5.9 13 29
7 X59—X62 19.6 13.0 6.6 17 25
8 X62—X65 23.0 5.4 17.6 17 68
9 X72—X75 17.9 8.1 9.8 14 44
10 X84—X90 17.3 10.2 7.1 15 31
V. V. KULYK, M. S. BONDARENKO
112 Геофизический журнал № 2, Т. 38, 2016
will be considered in the following sections (see
sections 7 and 8).
The Tab. 1 gives, as an example, the results of
determination of gas reservoir petrophysical pa-
rameters in the same borehole. Namely the follow-
ing parameters are presented: apparent porosities
ϕD and ϕN, parameter of gas reservoir identifica-
tion ϕ, true porosity ϕ, gas saturation Sg.
Fig. 4. Diagrams of radioactive logging (1 — gamma-ray log, 2 — density log, 3 — neutron log; c. u. — conventional unit) and pet-
rophysical parameters of formations (4 — neutron porosity, 5 — density porosity, 6 — true porosity, 7 — gas saturation) obtained
in coalbed methane well (Donbas, terrigenous geologic section).
IDENTIFICATION OF GAS RESERVOIRS AND DETERMINATION OF THEIR PARAMETERS ...
Геофизический журнал № 2, Т. 38, 2016 113
Another known method exists for identifica-
tion of gas reservoirs, namely, the comparison of
neutron porosity and density porosity in the form
of crossplot [Hunt, Pursell, 1997; Ellis, Singer,
2008].
Fig. 5 demonstrates the comparison of neu-
tron porosity and density porosity for formations
identified in aforementioned borehole. Diagonal
line of crossplot represents the water-filled po-
rosity, whereas water-saturated formations and
low gas saturation formations fall within a dashed
interval. Gas-saturated formations fall outside of
dashed interval (the density porosity log shows
higher porosity, while the neutron porosity log
shows a lower porosity).
As may be seen from the crossplot, gas reser-
voirs are shifted up and to the left about the di-
agonal of this crossplot. This shift increases with
increasing both true porosity and gas saturation.
A comparison between results of gas reser-
voirs identification by the use of neutron-density
porosity logs plotted together (Fig. 4) and by the
use of density-neutron crossplot method (Tab. 1
and Fig. 5), verified the efficiency of both. In
practice it is expedient to use both approaches
together.
7. True porosity of gas reservoirs from densi-
ty-neutron loggings. 7.1. Variants of averaging
apparent porosities. In gas reservoirs, the neu-
tron porosity is lower than the true porosity due
to both low hydrogen content in the pore fluid
and, to some extent, decrease of density, whereas
the density porosity is higher than the true value
through decrease in bulk density. Therefore in
gas reservoirs the neutron porosity and the den-
sity porosity are apparent. Hence the determina-
tion of true porosity of gas reservoir using neu-
tron logging or density logging separately is im-
possible. It is necessary to use the combination of
these methods for determination of true porosity.
Historically true porosity in gas-bearing for-
mations is estimated by applying the root-means-
square equation [Gaymard, Poupon, 1968]:
2 2
2
D N . (14)
There are other approaches to determining
the true porosity of gas reservoirs with the help
of density-neutron loggings, namely as the fol-
lowing approaches:
( ) 2D N , (15)
0.55 0.45D N , (16)
0.70 0.02 0.30 0.02( ) ( )D N . (17)
0.63 0.37D N . (18)
The arithmetic mean of values ϕD and ϕN (see
Eq. (15)) is used, predominantly, for oil reservoirs,
but it can be applied in estimating the true poros-
ity as well as gas reservoirs [Hunt, Pursell, 1997].
The averaging equation (16) obtained according
to the experimental data of measurements [Alger,
Dewal, 1969]. The numerical factors on ϕD and ϕN
in Eq. (17) were obtained when estimating the true
porosity of the gas reservoirs (with mud-filtrate-
invaded zone) by a procedure of fitting using least-
squares method [DasGupta, 1997]. Equation (18)
was obtained by empirical way by the example of
near-surface aeration zone [Кulyk, Bondarenko,
2014]. We used Eq. (18) for determination of the
true porosity of gas reservoirs occurring at shallow
depths (up to about 1.0 km).
Thus the Eq. (14) and the Eq. (15)—(18) differ
in the structure. As well numerical factors on D
and ϕN in Eqs. (15)—(18) substantially differ from
each other in all variants.
The use of weighted arithmetic mean of the
neutron apparent-porosity and density apparent-
porosity with corresponding weight factors is a
generalization of Eq. (15)—(18) [Кulyk et al.,
2014]:
1 2D N . (19)
Weight factors i ( =1, 2) are, by definition, the
real non-negative numbers which are less than 1.
Fig. 5. Field example of neutron-density porosity crossplot:
1 — gas reservoir, 2 — water reservoir; 1—10 — number of gas
reservoirs in Tab. 1.
V. V. KULYK, M. S. BONDARENKO
114 Геофизический журнал № 2, Т. 38, 2016
Weight factors i are normalized such that their
sum is equal to 1:
1 2 1 . (20)
7.2. Weight factors of weighted arithmetic
mean of ϕD and ϕN. In the context of accepted
approximations it is possible to obtain the follow-
ing values of the weight factors αi:
1 (1 ) ( )g g ,
2 ( 1) ( )g . (21)
Hence, weight factors (20)—(21) used for ob-
taining the true porosity of gas reservoirs by Eq.
(19) depend on the following factors: the matrix
density, which is due to reservoir lithology; den-
sities and hydrogen indices of pore water and
pore gas; as well as PT-conditions of occurrence,
which are the governing factors for gas density
and hydrogen index of gas.
In Fig. 6 the depth dependences of weight
factors αi for main reservoir lithologies with con-
ditional hydrostatic pressure are shown. Lithol-
ogy distinctions bound up with various matrix
density. As may be seen from Fig. 6, parameters
αi are substantially varying to a depth of about
4 km, while deeper the PT-conditions depen-
dence is essentially weaker. In addition, the
value of 1 is greater than 2 ( 1> 2) for depths
under consideration with conditional hydrostatic
pressure (CHP). Under CHP the weight factors αi
are approaching to 0.5 with increasing of depth
(see Fig. 6).
Fig. 7 compares the influence of CHP with
the influence of AHFP on the weight factor α1 for
sandstone (dashed curves correspond to lower
and upper limit of pressure gradient for AHFP).
Calculation of parameter α1 for examples of
AHFP in gas fields of both Dnieper-Donets De-
pression (DDD) and other regions of the world
(points) is also presented. As evident from the
Fig. 7, the increase of pressure gradient causes
the range extension of weight factors αi. In so
doing, both case of α1≥α2 and case of α1 α2 are
possible.
Thus as follow from Eq. (21) and Figs. 6, 7,
weight factors αi in the considered approxima-
tion possess the following properties:
– i substantially depends on PT-conditions
(in so doing they depend primarily on reservoir
pressure) through changes of both gas density ρg
and hydrogen index of the gas g;
– abnormally high PT-conditions strongly ef-
fect on the values of i;
– i depends on lithology (through density of
solid constituent ρs);
– i is independent of both porosity and gas
saturation;
– i is independent of both specification and
metrological characteristics of particular neu-
tron tools and density tools, as well as it is inde-
pendent of borehole effects (hole diameter, cas-
ing etc.).
Fig. 6. Weight factors αi vs depth occurrence of reservoir: 1 —
dolomite, 2 — limestone, 3 — sandstone.
Fig. 7. Weight factor α1 for sandstone. Pressure gradient: 1
— 10 MPa/km (CHP), 2 — 13 MPa/km (lower limit of AHFP),
3 — 23 MPa/km (upper limit of AHFP). Examples of AHFP:
4 — DDD [Melik-Pashaev et al., 1983], 5 — DDD [Lukin,
2014b]; 6 — other regions of the world [Petrenko et al., 2004].
IDENTIFICATION OF GAS RESERVOIRS AND DETERMINATION OF THEIR PARAMETERS ...
Геофизический журнал № 2, Т. 38, 2016 115
7.3. True porosity and variants of averag-
ing. In determining the true porosity of gas re-
servoirs using the different variants of averaging
of apparent porosities ϕD and ϕN the problem of
analysis of both accuracy and the area of applica-
tion of each Eq. (14)—(19) depending on the PT-
conditions, as well as lithology and other charac-
teristics of formations arises.
Analytical expressions (21) for the weight
factors αi make it possible to calculate their va-
lues depending on both gas reservoir depth and
lithology as well as to compare the calculated
weight factors with numerical factors on ϕD and
ϕN in Eq. (15)—(18). The results of calculations
are presented in Tab. 2.
From Tab. 2 it follows that variants of averag-
ing (15)—(18) do not fully agree with the theo-
retically substantiated variant (19)—(21).
This is more visually illustrated by Fig. 8. In
Fig. 8 the results of calculation of true porosity
for gas-bearing sandstone with porosity 20 and
10 % on a basis of methods (14)—(21) depending
on occurrence depth are shown. Thickened line
indicates the true porosity, calculated according
to Eq. (19)—(21).
Comparison of the results of porosity determi-
nation of gas reservoirs using the above-described
variants of averaging shows the following.
● The porosity obtained by using root-mean-
square equation (14) is overstated over the whole
depth interval and is weakly dependent on the
depth; systematic relative error of this averaging
variant is equal to about +10 % for any porosities.
● The porosity resulting from simple averag-
ing of neutron and density porosity (see Eq. (15))
is understated over the whole depth interval; this
porosity highly depends on the depth to a depth
of 4 km and in this interval is actually unusable;
deeper then 4 km systematic relative error is
equal to about — 5 % for any porosities.
● Averaging (16) gives porosity, which is near-
est to true porosity for actual depth; deeper then
1.5 km relative error is about ±3 %.
● Averaging (17) gives overstated porosity for
any depth with systematic relative error of about
+15 %.
● Averaging (18) is effective for shallow depth
(less than about 1 km); at these depths error is
less than about +5 %.
● Weighted arithmetic mean of porosity (19)—
(21) within the accepted approach allows obtain-
ing exact value of true porosity of gas reservoirs.
The field example of determining the true po-
rosity of gas-saturated reservoir with the help of
radioaktive logging by Eq. (19)—(21) is shown in
Fig. 4 and Tab. 1.
8. Gas saturation from density-neutron log-
gings. Gas saturation, Sg, is determined as a ratio
of volume of gas-filled pores to total volume of
pores. For two-phase (gas—water) reservoirs the
relation between gas saturation and water satura-
tion is given by: Sg+Sw=1.
In an open borehole the water saturation Sw of
conventional reservoirs is determined by electric
logging, using the Archie’s equation [Serra, 1984;
Interpretation…, 1988; Ellis, Singer, 2008; Alimo-
radi et al., 2011]. However, in high-resistivity re-
servoirs, in unconventional low-permeability re-
servoirs, as well as in cased wells, this approach
to determining the gas saturation (Sg=1–Sw) is
non-working.
We have proposed a method for determining
Sg [Кulyk et al., 2014], which allows determining
gas saturation from the combination of radioac-
Ta b l e 2 . Weight factors for true porosity determination of gas reservoirs by means of averaging the apparent
neutron and density porosities
D
ep
th
, k
m
Eq. (15) Eq. (16) Eq. (17) Eq. (18) Eq. (19)—(21)
Limestone,
sandstone,
dolomite
Limestone
Limestone,
sandstone,
dolomite
Sandstone Limestone Sandstone Dolomite
α1 α1 α1 α2 α1 α2 α1 α2 α1 α2 α1 α2 α1 α2
0.0
0.
50
0.
50
0.
55
0.
45
0.
70
±0
.0
2
0.
30
±0
.0
2
0.63 0.37 0.63 0.37 0.62 0.38 0.65 0.35
1.0 0.63 0.37 0.61 0.39 0.60 0.40 0.63 0.37
2.0 — — 0.58 0.42 0.57 0.43 0.61 0.39
4.0 — — 0.56 0.44 0.55 0.45 0.58 0.42
6.0 — — 0.54 0.46 0.53 0.47 0.56 0.44
8.0 — — 0.53 0.47 0.52 0.48 0.56 0.44
V. V. KULYK, M. S. BONDARENKO
116 Геофизический журнал № 2, Т. 38, 2016
tive loggings practically in all cases of the ab-
sence of mud-filtrate invasion into reservoir. In
case of presence of mud-filtrate invasion in open
boreholes, proposed method allows determining
residual gas saturation.
According to this method, the parameter Sg
can be obtained as a value, which is proportional
to the ratio of difference between density-appar-
ent porosity and neutron-apparent porosity to
true porosity:
gS , (22)
where is the proportionality factor, whereas ϕ
and are determined by Eq. (13) and (19), re-
spectively.
Within the approach, which is adopted in this
paper, the proportionality factor takes the form:
1
g g
, (23)
where g is determined by Eq. (9).
Thus, dimensionless proportionality factor
depends on matrix density, water density and gas
density as well depend on hydrogen index of a gas.
Since the parameters of the gas (density and
hydrogen index) depend on PT-conditions, the
factor β varies with the depth. Depth dependence
of the factor at pressure gradient, which cor-
responds to conditional hydrostatic pressure,
and at an average geothermal gradient is shown
in Fig. 9 for sandstone, limestone and dolomite.
There are sufficiently strong depth dependence
and the relatively weak lithology dependence of
the factor , as indicated in Fig. 9.
Fig. 10 gives calculation data of the factor
(points) for sandstone with abnormally high for-
mation pressure (by the example of gas fields
of both Dnieper-Donets Depression and other
regions of the world, occurring at the various
depths). Dashed curves correspond to the lower
and to the upper limits of pressure gradient for
Fig. 9. Proportionality factor β vs depth occurrence of reser-
voir: 1 — dolomite, 2 — limestone, 3 — sandstone.
Fig. 8. Porosity of gas reservoir from density-neutron log porosity vs depth by: 1 — root-means-square equation (14); 2 —
α1=0.50, α2=0.50 (Eq. (15)); 3 — α1=0.55, α2=0.45 (Eq. (16)); 4 — α1=0.70, α2=0.30 (Eq. (17)); 5 — α1=0.63, α2=0.37 (Eq. (18)); 6 — αi
from Eqs. (19)—(21). True porosity: a — ϕ=0.20, b — ϕ=0.10.
IDENTIFICATION OF GAS RESERVOIRS AND DETERMINATION OF THEIR PARAMETERS ...
Геофизический журнал № 2, Т. 38, 2016 117
abnormally high formation pressure (AHFP). For
comparison the depth dependence of factor for
typical PT-conditions is also given (curve 1). It is
seen that the dependence of factor on PT-con-
ditions increases sharply for the AHFP.
The example of determination of gas satura-
tion on the basis of combination of radioactive
loggings in cased well according to Eq. (22)—
(23) is presented in Fig. 4 and Tab. 1.
9. Conclusions. The problem of identifica-
tion of gas reservoirs and determination of their
petrophysical parameters in principle can not be
solved by separate (individual) logging methods
and requires a combined approach. One such ap-
proach is the use of combination of radioactive
logging methods (density logging, neutron log-
ging and gamma ray logging), which works in
both open and cased wells.
The paper is focused on the development of
theoretical and applied aspects of the gas reser-
voirs investigation with the help of combination
of radioactive loggings on the basis of simple
models. However, these simple models take into
account the basic properties of the subject under
investigation and conditions of logging, as well
as allow obtaining results in an explicit form. We
used the simplest petrophysical model of reser-
voir and simplified relation between measured
and petrophysical parameters. Identification of
gas reservoirs, choice of optimal methods of ob-
taining their true porosity, quantitative estima-
tion of porosity and gas saturation taking into ac-
count PT-conditions of reservoir occurrence are
consecutively considered.
Combination of radioactive loggings allows
identifying gas reservoirs by the parameter of
disagreement between the density and the neu-
tron porosity, which are presented in the form of
logs along the investigated borehole section or a
part thereof. In gas reservoirs a parameter of dis-
agreement is positive and increases with increas-
ing volume of gas in the rock. Together with this
approach, it is advantageous to use the method
of density-neutron porosity crossplot for identifi-
cation of gas reservoirs.
It has been proposed that in general case, the
true porosity of gas reservoirs is determined by the
combination of radioactive loggings as a weighted
arithmetic mean value of the measured both den-
sity-apparent porosity and the neutron-apparent
porosity with weight factors, which are derived
theoretically. The weight factors depend on PT-
conditions (primarily they depend on the reservoir
pressure) due to variation in both gas density and
hydrogen index of gas, as well as depend on reser-
voir lithology. The estimation of values of weight
factors for hydrostatic pressure and for abnormally
high formation pressure has been carried out.
Another important parameter is the gas satu-
ration, which can be obtained on data of radio-
active logging as quantity, which is proportion-
al to the ratio of difference between measured
density-apparent porosity and neutron-apparent
porosity to the determined true porosity. Propor-
tionality factor is estimated by calculated way
taking into account PT-conditions similarly to
the weight factors.
The proposed method allows determining
the gas saturation practically in all the cases of
absence of mud-filtrate invasion into reservoir
(cased wells with dissipated or unformed invaded
zone, open wells when drilling on oil-based mud,
unconventional reservoirs with low permeabi-
lity). In case of presence of mud-filtrate invasion
in open boreholes, proposed method allows de-
termining residual gas saturation.
In the considered approach, the cited both
the weight factors and the proportionality fac-
tor are independent of the following: particular
values of both porosity and gas saturation; both
specification and metrological characteristics of
neutron tools and density tools; borehole effects.
It is shown, that the investigated parameters
are substantially dependent on the hydrostatic
pressure and the overburden pressure; estimation
of affecting of abnormally high formation pres-
sures for gas fields of the Dnieper-Donets Depres-
sion and other regions of the world is presented.
Efficiency of the developed approaches is
Fig. 10. Proportionality factor β for sandstone. Legend see in
Fig. 7.
V. V. KULYK, M. S. BONDARENKO
118 Геофизический журнал № 2, Т. 38, 2016
demonstrated for the cased coal-bed methane
well (without mud filtrate invasion zone).
In general, the investigation of conventional
and unconventional gas reservoirs with the help
of radioactive logging shows practical univer-
sality and high informativity of combination ap-
proach for determination of set of petrophysical
parameters. Developed approaches allow the
generalization towards account of more realistic
petrophysical and calculated models. Combina-
tion of radioactive loggings and other logging
methods (electric logging, acoustic logging) al-
lows extending both capabilities of logging and
a set of determined parameters.
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Alimoradi A., Moradzadeh A., Bahtiari M. R., 2011.
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Identification of gas reservoirs and determination of their
parameters by combination of radioactive logging methods
© V. V. Kulyk, M. S. Bondarenko, 2016
The paper proposes a new approach for using a combination of radioactivity logging (neutron
and density logging), which allows determining a set of gas reservoir parameters (nature of satura-
tion, true porosity, gas saturation) taking into account influence of the pressure and temperature
conditions of occurrence. Analysis of the ways of averaging the neutron-apparent porosity and the
density-apparent porosity for obtaining the true porosity was made. Method of determination of gas
saturation, which uses the same combination of radioactivity logging as in determining the true porosity
of gas reservoirs, was developed. The results presented in the paper, were obtained over a wide interval
of depth (up to 10 km). Application of developed approaches for determination of petrophysical pa-
rameters of gas reservoirs is demonstrated by the example of cased coalbed methane well.
Key words: gas reservoirs, pressure-temperature conditions of occurrence, combination of neu-
tron logging and density logging, identification of gas reservoirs, apparent porosities, true porosity,
gas saturation.
IDENTIFICATION OF GAS RESERVOIRS AND DETERMINATION OF THEIR PARAMETERS ...
Геофизический журнал № 2, Т. 38, 2016 119
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