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FORUM HULU PERTAMINA 2019
20–22 November 2019
A Gas-Condensate Well Performance in the Naturally Fractured Reservoir PHE ONWJ: A
Case Study
Wisnu Wijanarko, Jemi Jaenudin,1
Abstract
Naturally Fracture Reservoir is characterized by pore
systems that considerably affect the fluid flow properties
(contrast permeability between fracture and matrix system).
Matrix has capability of hydrocarbon storage meanwhile
fracture provides the flow path of the hydrocarbon. The
presence of the gas condensate in the reservoir thus escalate
the reservoir complexity considering the liquid
condensation of the hydrocarbon once the reservoir
pressure drops below the dew point pressure. The presence
of the fracture in PHE ONWJ is identified by the total loss
evidence of five out of thirteen wells experience total loss
during drilling and discontinuity feature in seismic has high
correlation with the depth of total loss circulation in the
offset wells.
Naturally Fractured Reservoir in PHE ONWJ (K-85) has
been producing since 2011 and plays an important role in
the development of PHE ONWJ in the upcoming years.
The observation is carefully taken to the existing producing
wells to identify the well productivity variation, water
movement, and GOR. Historically, the well experience a
significant skin addition indicating considerably well
productivity impairment during the production time is
probably due to condensate blockage. Moreover, Static
bottom-hole pressure, material balance analysis, and PVT
data are also evaluated to support the possibility of
condensate banking phenomenon. This paper also will
implement the analytical methodology to predict the
performance of NFR gas-condensate.
The result will be valuable to identify production
characteristics and forecast of condensate well
performance. In addition, the result could be an insight to
the development strategy of naturally fractured reservoir.
Introduction
Gas condensate fluids are mixtures of hydrocarbon
molecules that are initially present as a gaseous phase at
reservoir conditions. Reservoir temperature lies between
the critical temperature Tc and cricondenterm Tct of the
reservoir fluid. During the depletion of the reservoir fluid,
the gas condensate as the pressure of the reservoir reduces
below the hydrocarbon dew point pressure, which
introduces a liquid phase called retrograde condensate.
Figure 1 showed the typical phase diagram of a retrograde
system. Initial condition of the reservoir is represented by
point 1 where only single phase gas exist in the reservoir.
As reservoir pressure declines isothermally to the upper
dew-point pressure (point 2), the attraction between the
molecule of the light and heavy components causes them to
move further apart, thus liquid begin to condense. The
condensate accumulation continue until a maximum liquid
drop out reached (point 3). Retrograde condensate starts to
vaporize until the pressure reach lower dew point pressure.
Naturally fractured reservoirs (NFRs) are defined as
reservoirs where fractures created by geological processes
have a significant impact on the migration, storage, and
extraction of hydrocarbons
Naturally fracture gas condensate reservoir in Offshore
North West Java at Kuala Field was discovered by
exploration well K-1 in 1988. Many wells have been drilled
since then and those wells indicated good gas reading and
good oil shows. It has xx gas column and it suggested
intense natural fracture by the appearance of significant
fluid loss during drilling operation.
The ONWJ area is located on the north Coast of Java Island
in Indonesia. The ONWJ PSC has been operated by PT.
Pertamina Hulu Energi ONWJ since 2009. The PSC consist
of several fields and has been producing since 1971 with
more than 700 wells and 150 production platforms. The
Kencanaloka Field, one of the field within PSC, is located
approximately 59 miles northeast of Jakarta in the Ardjuna
Sub Basin Southern flank area. The structure has been
developed by three platforms (K-A, K-B and K-C) since
1986
Koala Field is a compartmentalized by fault and it may
have different compartmentalization each zone, depends on
its faults. K-85 Zone is a limestone reservoir and lies in
Koala Field at approximately -8500 TVDSS. It is divided
into two zone, East and West block separated by major
fault across North-South direction. This hydrocarbon
presence was proved from exploratory well K-1 in 1988 in
which its exploratory well primary target zone was Talang
Akar located deeper than K-85.
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Open Hole Drill Stem Test (DST) was conducted in east
block K-85 zone through K-1 well within 150 ft thickness
interval and showed significant hydrocarbon potential with
8.5 MMscfd, 545 BOPD @ 1770 psi. This hydrocarbon
has not been developed until a new infill well in 2011, K15, was drilled twinning to exploratory well. There are
three development well have been producing from K-85
zone, the well
In gas/condensate system, depletion upon fluid withdrawal
results in condensate dropout once the system pressure
blow dew point conditions. The fracture network and the
external edges of the matrix blocks will be the first to host
condensate in the system. If the matrix block are extremely
tight, the inner portion of the matrix blocks may not feel the
pressure change until a later depletion stage. Even if the
fracture condensate had considerable mobility, matrix
condensate is expected to be nearly immobile. Therefore,
appearance of condensate on the matrix-block faces further
constrains gas withdrawal from the inner parts.
Consequently, condensate may never be formed in the
innermost portion of the matrix, and the hydrocarbon gas
located in the deeper portions of the matrix blocks may not
be easily recovered.
Recovery of gas stored can be largely driven by
concentration gradient. Rather than pressure gradient alone
as predicted by darcy’s law. Diffusion coefficient in the
hydrocarbons to be dependent on viscosity and molar
density of reservoir fluids. Diffusion coefficient tends to be
lower at higher pressure and higher at lower pressure
compared to constant diffusion
Condensate dropout triggers an intense property change in
the gas phase, which is being stripped of its heavy
components. This result in a sharp decrease of gas density
and viscosity, in turn translates into an enhanced diffusive
environment. Under this conditions, concentration
gradients in the reservoir will play an increasingly
important role in hydrocarbon recovery. Intensified
concentration gradient that are more quickly established at
higher depletion rate but also because of the larger
diffusion coefficient.
Highly fractured reservoirs where the effect of gravity
drainage is minimized. Higher matrix block sizes can
reduce the amount of trapped liquid, however, high critical
condensate saturation can dampen the effect to some
extent. Pore size uniformity is another important factor that
causes less condensate build up due to less capillary
pressure. Liquid drop out in the reservoir and becomes
wetting phase are trapped as immobile fluid by capillary
forces. Higher production rates result in earlier condensate
dropout peak. The peak is lower for higher rates, a
phenomenon attributed to velocity stripping. Furthermore,
the effect of gas and liquid diffusion is the most crucial in
tighter matrices and can significantly reduce the trapped
condensates.
Data and Method
Well Data
There are three producing wells from K-85 that will be
evaluated their performances. All of these wells are
completed in open hole completion with 3 ½” perforated
tubing enabling well intervention activities to reach in front
of reservoir target
PVT Analysis
Separator samples from K-85 zones was acquired not in the
initial condition, resulting in uncertainty of data analysis,
dew point from the fluid sampling showed dew point
higher than initial pressure, so it has problem with the
material balance.
Material Balance
Material balance analysis is a powerful method to validate
hydrocarbon in place, reservoir connectivity, drive
mechanism, and reservoir prediction based on production
and static pressure data. Furthermore, in naturally fracture
reservoir as K-85, in which apparently fracture has an
important role not only matrix to occupy great amount of
hydrocarbon, material balance analysis must be carefully
evaluated from data preparation to data analysis.
Well Deliverability
During retrograde gas production, when the initial reservoir
pressure is above the dew point pressure value, the oil
production of the well will be purely from the condensate
dropout of the gas phase. Therefore, it is believed that the
following relationship exist during the well production
history:
𝐴
𝐵 𝑙𝑛 𝐺
𝐶𝐺𝑅
It will observe a straight line exist on the semi log plot
starting at certain time point after reservoir experiences
two-phase plot
Result and Discussion
Material balance analyses were evaluated from Pressure
Build Up (PBU) and Static Bottom Hole Pressure (SBHP)
over Gas Cumulative Production. The quality of the
pressure data is favorable as the pressure is already stable at
interest depth over time. MBAL evaluation showed the
drive mechanism of K-85 zone is depletion drive as
indicated a straight line slope hence the result of the
evaluation is more than acceptable and could guide the
static volumetric.
Pressure data at the new drilled well in the west block in
2017 showed the pressure at K-85 is not original (P Res
current: 3300 psi, P Res Initial: 3730 psi). Moreover, when
we plot the latest pressure to the line generated from the
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reservoir pressure in the east block, it is obviously follow
the trend. It is arguably that between east and west block
are in communication
through 2nd packer and SSD in K-77. Due to the fact that
production of K-85 pass through several obstruction, it is
also suspected associated with high skin value.
First well to develop K-85, KLB-15, has experienced
significant production loss from nearly 900 bopd & 6
MMscfd @ 700 psi in 2017 to 350 bopd & 2 MMscfd in
2019. Within those period, K-15 production was relatively
stable as it is used as in situ gas lift for nearby oil wells.
Preliminary reservoir evaluation was conducted and it was
suggested to perform well surveillance by deploying
wireline unit to acquire static and flowing gradient survey
due to the fact that it is unlikely to have reservoir pressure
that low (Nodal Analysis indicated P Res should be 1700
psi).
Two new wells, KLB-18 and KLB-21 drilled in 2017, are
below dew point and has indicated GOR increasing over
time until it reach the critical condensate saturation,
followed by increasing CGR as condensate becomes
mobile. Even in immobile phase, velocity stripping of gas
phase in the fracture system when condensate saturation
has not exceeded critical could promote condensate in the
outermost matrix to produce up to the surface.
Reservoir Pressure and Flowing Bottom Hole Pressure
from K-15 was 3100 psi and 1500 psi, respectively,
indicating reservoir pressure remained high, only depleting
approximately 600 psi to produce nearly 20 Bscf. However,
a high drawdown pressure of 1600 psi was observed from
this well which is probably due to two reasons; firstly, the
obstruction in the wellbore and formation damage as the
tool could not go deeper to observe accurately the main
issue contributing to loss production of K-15 well.
Secondly, During the hydrocarbon production in the gascondensate reservoir and reservoir pressure falls below
dew-point pressure, condensate will accumulate in the near
wellbore and eventually when it exceed critical saturation
the condensate in the reservoir will be produced along with
the gas. In the naturally fracture reservoir with high fracture
intensity, matrix porosity ~ 6% having Pc > 800 psi in
which it will have abundant gas condensation in the
outermost of the matrix. Pressure depletion is introduced
initially and foremost in the fracture and subsequently in
the matrix was felt in the later phase. Once the pressure
drop below the dew-point felt in the fracture, it is
instantaneously mobile due to negligible capillary pressure.
Meanwhile, condensate will drop out and adhere on the
outermost of the matrix in immobile phase to hinder gas
flowing out from the matrix to the fracture up to the
surface. Condensate in the outermost matrix immediately
will flow when reach the critical saturation. This
phenomenon could be observed in K-15 well production,
firstly the well flowing with constant CGR and following
pressure drop below dew point it increases considerably to
150 STB/SCF. Meanwhile, the gas production decreases as
condensate hinders gas flow from inside matrix to the
fracture system.
KLB-15 well is completed by open hole completion and it
was originally produced from two zones, K-77 and K-85
which separated by SSD and currently K-85 is producing
From two new wells, fracture intensity seismic in the well
influence how long it will take the condensate become
mobile. KLB-21 penetrates more fractures than KLB-18 as
its penetration point is near major fault which is indicated
from total loss rate and interval during drilling operation
(KLB-18: 200 bph/74 ft, KLB-21: 2000 bph/101 ft). The
Pressure at East Block
Figure 1 P/Z Analysis of K-85 Reservoir
This figure shows the P/Z Analysis of K-85 reservoir. East
and West Block separated by fault indicated from the
Material Balance analysis those two blocks have the same
pressure trend.
Figure 2: KLB-15 Production Production
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Figure 3 SBHP and FGS result from KLB-15
Data Acqusition by wireline stopped at 8500 md.ft due
to obstruction occurred, hence the pressure was
extrapolated to in front of reservoir. It is also observed
fluid level at 5300 TVD.
Figure 5 Production Profile KLB-21
This figure shows the well performance of two new infill in
2017, KLB-18 and KLB-21. Cumulative Gas, Gp vs CGR
Cum plot indicates when condensate becomes mobile. As
KLB-18 has fewer fracture intensity, it takes almost 3 Bscf
production while KLB-21 only takes 1 Bscf. This
deflection point (A and B) CGR will increase over time and
could be applied to forecast CGR production in the future
well
Figure 6 Depth Structure Map overlay with fracture
This figure shows the depth structure map overlay with
fracture intensity in K-85. East and West Block are
separated by major fault N-S direction and it is indicated
from the Material Balance analysis those two block has the
same pressure trend
Figure 4. Production Profile KLB-18
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Conclusions

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PVT Sampling in gas-condensate reservoir at not
original pressure could add more uncertainty to data
analysis
Material Balance evaluation shows both East and
West Block are connected
Condensate Banking phenomenon could be
observed from KLB Wells
Condensate Gas Ratio (CGR) trend could be
generated from two existing wells to predict CGR in
other K-85 reservoir wells
Wells having more fracture intensity could impact
the critical condensate saturation
References
Badley, E, M., (1985) Practical Seismic Interpretation.
International
Human
Resources
Development
Corporation. Boston.
Pandito, R.., (2016) Passive Seismic for Hydrocarbon
Indicator : between expectation and reality, 41st HAGI
Annual Meeting.
Syaraf, E., Simo, J. A., Carrol, A, R., dan Shields, M.,
(2005) Stratigraphic Evolution of Oligomiocene abd
Siliciclastics, East Java Basin, Indonesia, AAPG.
Numerical Analysis of Condensate-Flow
Parametric Study of Condensate Build up (SPE 133435)
Acknowledgements
The authors wish to thank the respective management of
Pertamina Hulu Energi and PHE ONWJ for permission to
publish this work. Special acknowledgment is given to
West Reservoir Management and KL Team of their support
and encouragement during the publication of this paper and
their contributions to the outcome.