CC BY
27
УДК 551.521.9
DOI: 10.18303/2618-981 X-2018-4-136-142
АНАЛИЗ ВЛИЯНИЯ ДИНАМИКИ ЗОНЫ ПРОНИКНОВЕНИЯ БУРОВОГО РАСТВОРА НА СИГНАЛЫ ГАММА- И ИМПУЛЬСНОГО НЕЙТРОННОГО КАРОТАЖА
Александр Викторович Серяков
Новосибирский технологический центр компании «Бейкер Хьюз», 630090, Россия, г. Новосибирск, ул. Кутателадзе, 4А, кандидат технических наук, научный сотрудник, тел. (383)332-94-43, e-mail: Alexander.Seryakov@bakerhughes.com
Баир Валерьевич Банзаров
Новосибирский технологический центр компании «Бейкер Хьюз», 630090, Россия, г. Новосибирск, ул. Кутателадзе, 4А, научный сотрудник, тел. (383)332-94-43, e-mail: Bair.Banzarov@bakerhughes.com
Александр Анатольевич Винокуров
Новосибирский технологический центр компании «Бейкер Хьюз», 630090, Россия, г. Новосибирск, ул. Кутателадзе, 4А, научный сотрудник, тел. (383)332-94-43, e-mail: Alexandr.Vinokurov@bakerhughes.com
Олег Борисович Бочаров
Новосибирский технологический центр компании «Бейкер Хьюз», 630090, Россия, г. Новосибирск, ул. Кутателадзе, 4А, кандидат физико-математических наук, научный сотрудник, тел. (383)332-94-43, e-mail: Oleg.Bocharov@bakerhughes.com
Александр Игоревич Макаров
Новосибирский технологический центр компании «Бейкер Хьюз», 630090, Россия,
г. Новосибирск, ул. Кутателадзе, 4А, кандидат технических наук, (383)332-94-43, e-mail: alexniler@gmail.com
В работе продемонстрировано влияние процесса эволюции зоны проникновения бурового раствора вокруг обсаженной скважины на интервале коллектора, вызванное гравитационной силой, на измерения естественного гамма- и импульсного нейтронного каротажа. Чувствительность ядерных методов к перераспределению флюидов показывается с помощью моделирования фильтрационного проникновения бурового раствора в пласт при проходке скважины, последующей гравитационной сегрегации флюидов в течение значительного времени после обсадки и расчета показаний приборов радиоактивного каротажа для различных стадий процесса.
Ключевые слова: зона проникновения, фильтрационное моделирование, гамма каротаж, импульсный нейтронный каротаж, радиоактивные изотопы, макроскопическое сечение захвата тепловых нейтронов.
ANALYSIS OF INVASION ZONE DYNAMIC IMPACT ON GAMMA RAY AND PULSED NEUTRON TOOL RESPONSES
Alexander V. Seryakov
Baker Hughes' Novosibirsk Technology Center, 4A, Kutateladze St., Novosibirsk, 630090, Russia, Ph. D., Researcher, phone: (383)332-94-43, e-mail: Alexander.Seryakov@bakerhughes.com
Bair V. Banzarov
Baker Hughes' Novosibirsk Technology Center, 4A, Kutateladze St., Novosibirsk, 630090, Russia, Researcher, phone: (383)332-94-43, e-mail: Bair.Banzarov@bakerhughes.com
Alexander A. Vinokurov
Baker Hughes' Novosibirsk Technology Center, 4A, Kutateladze St., Novosibirsk, 630090, Russia, Researcher, phone: (383)332-94-43, e-mail: Alexandr.Vinokurov@bakerhughes.com
Oleg B. Bocharov
Baker Hughes' Novosibirsk Technology Center, 4A, Kutateladze St., Novosibirsk, 630090, Russia, Ph. D., Researcher, phone: (383)332-94-43, e-mail: Oleg.Bocharov@bakerhughes.com
Alexander I. Makarov
Baker Hughes' Novosibirsk Technology Center, 4A, Kutateladze St., Novosibirsk, 630090, Russia, Ph. D., phone: (383)332-94-43, e-mail: alexniler@gmail.com
In this paper, we demonstrate the influence of gravity on invaded zone evolution and its effect on nuclear log data. We analyze nuclear tool response changes with fluid distribution changes in a near-wellbore zone. This study is based on a combination of invasion zone while-drilling simulation for computation of near-wellbore distributions of nuclear isotopes and macroscopic neutron capture cross-section and numerical modeling of nuclear tool responses. The nuclear logs sensitivity was shown using a synthetic example where the time-lapsed salinity profiles were obtained with the help of the Buckley-Leverett model.
Key words: invaded zone, invasion simulation, gamma ray logging, pulsed neutron logging, radioactive isotopes, and macroscopic neutron capture cross section.
Usually, drilling mud penetrates into the reservoir while drilling and forms an "invasion zone". We consider the invasion zone evolution as a complex process that changes the fluids pattern in the near-wellbore zone and affects the tool measurements. The first stage of this process includes the mud-oil-salt components redistribution in permeable intervals produced by the mud pressure gradient into the formation. The drilling regimes, mud properties, technologic stoppages, formation filtration characteristic, initial brine distribution and reservoir pressure play major roles in the invasion zone structure development. The after-drilling fluid behavior, when the gravity effect impacts the fluids redistribution during relative long times, can be referred to as the second stage of invasion zone evolution. It has been established in [1] that the differences in mud filtrate and reservoir fluids densities lead to the fluids segregation and invasion zone restructuring. This activity can be registered by the induction logging tools. The influence continues when the casing is set, complicating the electromagnetic logging efforts. As a rule, this problem is overcome by application of nuclear logging techniques. In this paper, we perform a feasibility study of nuclear tools sensitivity responses to fluid distribution changes in a near-wellbore zone at reservoir intervals in the presence of casing.
To perform numerical modeling of an invasion zone around a vertical well, a 2D axisymmetric Buckley-Leverett model [2, 3] was applied. The main computational equations of the model are represented by relation for the first phase ("water") saturation distribution s, equation (1) and equation (2) for pore pressure p:
MwJ-p^Vp-ftg)'
dt y ^
0.
(1)
dm(p)[(pt -P2)* + P2]
- div
k
dt
k1( * ) k2 ( * ) p1--+p2
^2
Vp - k
P2k1 ( * ) , P2k2 ( * )
^2
g
= 0,
(2)
where p, are fluid densities; k is formation permeability; k, ^, I = 1,2 are phase
permeabilities and viscosities of fluids; m(p) = m0 (1 + ep), m0 is initial porosity;
e is formation compressibility. The mud cake dynamics equation [3] is solved together with (1), (2)
dh/ dt = aq - t
(3)
where h(t) is mud cake thickness, a is mud cake growth parameter, which is a function of solid particles concentration in the mud and cake porosity, q is filtration rate through the borehole wall, x is mud cake washout parameter. The filtration software based on system (1)-(3) can simulate the drilling, technologic stoppage, casing, and compute the long time interval to reveal the gravity effect. To clearly demonstrate the invasion zone fluids redistribution and its influence on the nuclear measurements, a homogeneous highly-permeable reservoir of 15 m thickness was considered. We considered an incompressible reservoir that was placed between impermeable shale bands of infinite thickness and had filtration properties presented in Table 1.
Table 1
Input parameters to model invaded zone formation
Group Parameter Value
Reservoir properties Absolute permeability 600 mD
Porosity 25 %
Initial water saturation 10 %
Water phase viscosity 0,3 cP
Fluid properties Formation oil viscosity 3,0 cP
Formation oil density 0,9 g/cm3
Irreducible oil saturation 10 %
Formation water density 1,041 g/cm3
Drilling mud filtrate density 1,05 g/cm3
Mudcake permeability 0,001 mD
The vertical borehole was drilled with an overbalanced pressure dP equal to 40 bar on bit. After the productive layer was pierced, the pressure in the wellbore was maintained with dP equal to 20 bar for 1 day and then the casing completely isolate the reservoir. The process of further invasion zone fluids segregation was computed for a 30-day timeframe. We consider the mud containing KCl potassium salt that affects nuclear measurements and the reservoir fossil water that usually contains the NaCl solution. After computation of the mud filtrate and brine saturation, the results were used to calculate radioactive element (40K) and neutron capture cross-section (Da) distributions.
Figure 1 displays the contour maps for the mud filtrate saturation and potassium salt concentration parameters at 12 and 30 days after reservoir penetration. During these periods the gravity force influence becomes noticeable and clearly seen on the mud filtrate saturation profile that shifts down and forms the lengthy water layer at the reservoir's bottom. The KCl distribution qualitatively reflects the parameter's behavior because salt concentration is computed as the function of saturation following the instantaneous mixing assumption. Therefore, at the distance of 0,2 to 0,7 m the evidence of the gravity segregation must be observed with the help of nuclear methods.
Fig. 1. Water saturation and KCl salt concentration in the near-wellbore zone of a reservoir at 12 and 30 days since reservoir penetration
To analyze the changes that occurred in the invaded zone due to gravity segregation, two methods of geophysical wellbore surveying were selected: gamma ray (GR) and pulsed neutron (PNL) logging in sigma mode [4]. The nuclear measurements were modeled using the Geant4 package [5]. The key idea is that both logging methods are sensitive to KCl. GR tools respond to natural gamma field, and its time-lapse measurements enable estimation of the changes in potassium distribution based on the changing number of registered gamma-quanta.
On the other hand, the changes in chlorine distribution can be detected by PNL measurements. PNL tools are equipped with sources generating 14-MeV neutrons and photon detectors. The sources emit neutrons that are absorbed by surrounding materials, resulting in photon emission. The detectors register the photons. This data
is then used to measure the lifetime of the neutrons. The measured signal is presented as a time spectrum. A range between 420 and 1 000 ^s of such spectra can be used for derivation of neutron capture cross-section Da. Different materials (see Table 2) have different intrinsic values of Da.
Table 2
Neutron capture cross sections for different materials
Material Sa (c.u.)
Water + NaCl, 60 kppm 43,9
Water + KCl, 80 kppm 45,3
Oil 25,9
Quartz 4,3
Steel 360
The measured Da values are the integrated responses of materials presented in borehole, casing and formation. Figure 2 demonstrates distributions of neutron capture macroscopic cross-section at different time moments after drilling. Invasion of KCl brine into formation raises the neutron capture cross-section in near-wellbore zone. Therefore, it is possible to evaluate its evolution using PNL methods.
Fig. 2. Evolution of distribution of neutron capture macroscopic cross-section
(sigma, c.u.) in the formation
Figure 3 demonstrates the borehole logs obtained by modeling the GR and PNL responses 12 and 30 days after drilling. The difference in the responses along a borehole axis enabled us to estimate mud filtrate propagation in the vertical and radial directions. The increased GR count rate meant higher potassium content in the invasion zone and indicated the high borehole fluid concentration. As for the PNL responses ("sigma' on the figure), the increased neutron capture cross-section was an indicator of growing chlorine concentration. Because chlorine is mainly concentrated in a borehole fluid, its increase was a sign of its high concentration in the invaded zone.
In our model, the highest responses characterized invasion zone expansion that can be observed at the reservoir bottom. On the other hand, the lowest responses related to the invasion zone disbanding commonly observed at the reservoir top.
For considered conditions, both logging methods provide logs that correlate to each other that can be used for additional control of measurements. On the other hand, the logs correlate with the saturation and salinity profiles that can be used for interpretation of measurements.
Fig. 3. GR and Sigma measurements performed after different time periods
after borehole drilling
The analysis verified the feasibility of nuclear logging for near-wellbore monitoring after drilling in presence of casing that enables selection of an optimal place and time for reservoir perforation based on GR and PNL responses. The nuclear logging monitoring enables determination of the true formation fluid (oil) location and reduces operational expenditures for establishment of production practices.
REFERENCES
1. Bocharov O. B., Makarov A. I., Seryakov A. V. Drilling Mud Gravity Segregation in Wellbore Zone and Its Effect on Induction and Electric Logging Measurements // SPE Annual Caspian Conference and Exhibition. - Astana, 2016. - SPE-182571-MS.
2. Aziz K., Settary A. Mathematical modeling of reservoir systems. - London and New York: Elsevier Applied Science Publishers, 1975. - 407 p.
3. Kashevarov A. A., Eltsov I. N., Epov M. I. Hydrodynamic model of invaded zone evolution while borehole drilling // AMTP. - Novosibirsk : SB RAS, 2003. - Vol. 44, N 6. - P. 148-157.
4. Ellis D. V., Singer J. M. Well Logging for Earth Scientists Dordrecht. - The Netherlands : Springer. - 2007. - 697 p.
5. Geant4 a simulation toolkit. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment / S. Agostinelli et al. -2003. - Vol. 506, Issue 3. - P. 250-303.
© А. В. Серяков, Б. В. Банзаров, А. А. Винокуров, О. Б. Бочаров, А. И. Макаров, 2018
