The Research on Dynamic Changes of Fracture Width in

The Research on Dynamic Changes of Fracture Width in
Fractured/Caved Carbonate Reservoirs
LI Daqi, KANG Yili
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,
Southwest Petroleum University, P.R.China, 610500
[email protected]
Abstract: To predict the dynamic width of the fracture during drilling in fractured/caved carbonate
reservoirs, the physical model and finite element model of mud losses have been established, basing on
the condition of fractures or caves and the mechanisms of lost circulation. How fracture length, cave
diameter, overbalance pressure differential and leakoff time effectting fracture width changes are
investigated. The results show that the dynamics fracture width increases with the increase of
overbalance differential pressure, cave diameter, fracture length and leakoff time, among which the
overbalance pressure differential and cave diameter are the most important factors. Fracture has "flat,
straight and wide" features in high differential pressure, large diameter, short fracture length, and long
leakoff time. This increases the difficulty of lost circulation prevention and control and the difficulty of
reservoirs protection. Therefore, the lost circulation prevention should be put in the first place, and
plugging measures should be executed timely, quickly, and efficiently.
Keywords: fractured-caved reservoirs, carbonate rock, fracture width, formation damage, lost
circulation, numerical simulation
1 Introduction
Fractured/caved carbonate reservoirs are rich in the gas and oil resources, and its storage environment is
very complicated. The reservoir is usually buried deep, as well as pores, fractures and caves develop
remarkably, with the result that lost circulation accidents frequently happen while drilling, which have
brought about serious loss of human, material, financial resource and reservoir damage [1-3].Mud losses
control is always a worldwide problem [4-10], it is pivotal to forecast the fracture width reasonably and
know its dynamic change behavior in lost circulation prevention and plugging or formation damage
control. At present, there are lab simulated experiment, theoretical model, numerical simulation and
such methods for researching the change of fracture width [11-12].Lab simulated experiment can only
study the small-scale variation of fracture width. The theoretical model, however, predict the change of
fracture width of a single crack more often. The research on the change of fracture width of large-scale
cracks, mainly rely on numerical simulation to achieve, because it can reflect the complexity of its
geological conditions and has a unique technological advantage. With the finite element method, Lian
zhanghua, Kang yili, etc(2001,2003,2006) simulate the change behavior of fracture width of a single
horizontal fracture, a single vertically slit and two vertical slit next to hole wall. However, the
predecessors do not take the influence into account which the caves and lost circulation have on the
change of fracture width. Fractured/caved carbonate reservoirs often leak out owing to the crack and
cave system in drilling, therefore the study on the influence which the existence of caves and mud losses
have on the change of fracture width has importance meaning for targeted lost circulation prevention
and control measures.
2 Geology of Tahe Oilfield
Tahe oilfield is a typical fractured/caved carbonate reservoir in our country, and the demonstrated gas
and oil reserves are more than one billion tons of oil equivalent. The reservoir is usually buried
deep(generally deeper than 5300m), the pressure coefficient is 1.07 1.12 and the geothermal gradient
～
268
～ ℃
is 1.95 2.2 /100m. The poroperm characteristics of the reservoir matrix is poor, as the average
porosity is about 1% and the mean permeability is less than 0.1×10-3µm 2. The space of available reserve
and permeation is formed by fissure, vug and cave as a result of structure and karst. The vertical and
horizontal aeolotropy of the reservoir is very strong, and there is a big difference between scales of
fissure and limestone cave which distribute unevenly, with the result that leakage frequently happen in
drilling. Clay minerals such as goeschwitzite and interbed between goeschwitzite and montmorillonite
in the reservoir can easily lead to potential store damage, for example, speed, water and alkali
sensitivity.
It can be seen that the development of fractured hole in the reservoir and leakage during well drilling are
widespread according to well drilling data of several wells and statistics of core seam. Taking Tahe
oilfield Area 12 for example, we can find that there are 35 wells which leak out in the reservoirs out of
the 81 wells drilled. The rate of leakage is 43% and the leakage per leaker is up to 364.5m3. It can be
found that above 80% of lost circulation is due to natural fractures and cave development though the
analysis of the working condition of leakage (Figure 1).The main is fractured leakage (58%), followed
by cave leakage (35%). Among the above lost circulation less attributes to improper operation.
Analyzing 27 unloading wells (Figure 2), we can see the distribution of their length is between 0.2-23m,
most of them are less than 10m, and the average length is 5.07m. According to statistics of imagery
logging and coring, Cracks and micro-cracks developed in this area are at multiple cross cutting. Most
of cracks are high-angle fractures and part of them is vertical fracture.
4%
2%
2%
2%
Total number N=27
Mean length=5.07m
6
13%
45%
5
Well number
2%
4
3
2
1
30%
0
Drilling
Flushing
Bit drop
Ream down
Overflow
Trip out
Trip in
Pumping
0~1 1~2 2~3 3~4 4~5 5~6 6~7 7~8 8~9 9~10 >10
Length of bit drop ,m
Fig.1 Condition of Lost circulation
Fig.2 Length distribution of bit drop
3 Mathematical Model
①
④
②
③
3.1 Basic assumptions
isotropic formation rock;
fracture surface is flat;
cavity without filling and it is spherical;
formation rock is elastic body; matrix permeability is zero.
⑤
269
F
cave
wellbore
fracture
E
cave
fracture
G
P
Fig.3 Physical model
Pd B
Pd
C
D
Fig.4 Finite element model
3.2 Mechanical model
According to crack and cave development status in Tahe oil field, we establish physical model of a
fracture-cave mix (Figure 1). According to elastic-plasticity FEM theory, this study belongs to plane
strain problem. As the model has the symmetry, we adopt one quarter to study and establish a finite
element mechanical model (Figure 2).
3.3 Boundary Conditions
DE section exerts the maximum effective horizontal principal stress P1, EF segment imposes the
minimum effective horizontal principal stress P2. GA arc is a bore hole exerted an effective pressure P.
AB section is a crack exerted a pressure gradient between P and Pd. BC is the cave section exerted an
effective pressure Pd. CD segment and FG segment are imposed the symmetry boundary constraints. In
accordance with the actual process of mud losses occurrence, the pressure on the crack surface and
inside the cave is a process of change and should be calculated separately.
3.4 Basic parameters
According to a large number of test statistics of some well reservoir sector of Tahe oil feild, the
mechanics parameters of formation rock are as follows: Elastic modulus is 3.06 × 104MPa; Poisson's
ratio is 0.32 in average; Biot’s factor is 0.8; Pore pressure P0 = 65MPa; Maximum horizontal stress
Shmax = 118.5MPa; Minimum horizontal stress Shmin = 92MPa; Well bores’ effective pressure P =
Pb-P0; Caves’ effective pressure Pd = Pc-P0. Using the mechanical model when the crack length is fixed,
we can analyze and predict dynamic width by adjusting well bores’ effective pressure P and caves’
effective pressure Pd.
4 Results
Assuming other parameters are fixed, changing the fracture length, the cavity diameter, the positive
differential pressure and the leakage time in turn, we analyze the effect which each parameter has on the
change of fracture width.
4.1 Fracture lengths impact on the change of fracture width
The positive differential pressure P is set to 3MPa, the effective pressure Pd in cave is equal to the
positive differential pressure P and the cavity diameter is 1000 mm. Crack lengths were taken 500mm,
1000mm, 2000mm, 5000mm respectively. At different crack lengths and under constant pressure, the
change of fracture width increment along the direction of fracture length was shown in Figure 5. Thus,
in the same conditions, the fracture width increases with the fracture length increasing. When fracture
270
length was doubled, the fracture width increment increases by about 20 percentage points, and the
increase speed continuously decreases. The fracture width can reach 1.5mm when the fracture length is
up to 5000mm.
4.2 Cavity diameters impact on the change of fracture width
The positive differential pressure P is set to 3MPa, the effective pressure Pd in cave is equal to the
positive differential pressure P and the fracture length is 1000 mm. the cavity diameter were taken 0mm
(the equivalent of a single crack), 10mm, 100mm, 1000mm, 4000mm, 10000mm respectively. At
different cavity diameters, the fracture width along the direction of fracture length was shown in Figure
6. Studies have shown that the presence of caves makes the fracture width easier to change. When cavity
diameter is 1m, the fracture width increment is about 20 times as large as that when no cave exists. With
the cavity diameter increasing, the fracture width increment increase sharply, and the increment reaches
unanimity along the direction of the fracture length.
Cave diameter 1000mm
1.2
1.0
0.8
500mm
0.6
1000mm
0.4
2000mm
5000mm
0.2
10mm
100mm
1000mm
4000mm
10000mm
0mm
Pressure drawdown 3MPa
6.0
1.4
Fracture width increment, mm
Fracture width increment, mm
1.6
5.0
4.0
3.0
0.5
2.0
1.0
0.0
0.0
0
1000
2000
3000
4000
Along fracture length, mm
5000
Fig.5 Fracture width increment in different
fracture length
0
200
400
600
Along fracture length, mm
800
1000
Fig.6 Fracture width increment in different
cave diameter
4.3 Positive differential pressures impact on the change of fracture width
The crack length is set to 1000 mm, cavity diameter is 1000 mm, the effective pressure Pd in cave is
equal to the positive differential pressure P and P was respectively 1MPa, 3MPa, 5MPa, 7.5MPa, 10MPa.
Along the direction of fracture length, the change of fracture width along with P was shown in Figure 5.
The results showed that the fracture width increment increases with P increasing, and the nearer cracks
are away from wall along the direction of fracture length, the greater their width change.
4.4 Leakage times impact on the change of fracture width
The positive differential pressure P of well bore is set to 5MPa, crack length is 1000 mm and cavity
diameter is 1000 mm. The increasing effective pressure in cave Pd was taken 0MPa, 1MPa, 3MPa, 5MPa
respectively, which simulates the process of the change of fracture width accompanying drilling fluid
leaks into the cave constantly. At different pressures, the crack width increment along the direction of
the crack length was shown in Figure 6. Thus it can be seen that the crack width increment increases
gradually as leakoff time increases (namely Pd increased). The nearer cracks are away from wall along
the direction of fracture length, the greater their width changes, and this difference decrease gradually
along with the reduction of the leakage time. The crack length increment reaches unanimity along the
direction of fracture length after a long time mud losses.
271
Cave diameter 1000mm
1MPa
3MPa
2.5
5MPa
2.0
7.5MPa
10MPa
1.5
1.0
0.5
Cave diameter 1000mm
1.4
Fracture width increment, mm
Fracture width increment, mm
3.0
0.0
1.2
1
Pd=0MPa
Pd=1MPa
Pd=3MPa
Pd=5MPa
0.8
0.6
0.4
0.2
0
0
200
400
600
800
Along fracture length, mm
1000
0
200
400
600
800
Along fracture length,mm
1000
Fig.7 Fracture width increment in different presurre Fig.8 Fracture width increment in different leakoff time
5 Discussion
5.1 Dynamic fracture width
The natural fractures and solution caves are highly developed in fractured/caved carbonate reservoirs,
near the wellbore, the true fracture width is the sum of original width and width increment under
positive differential pressure. To prevent the outgoing of poisonous gas such as H2S etc, we generally
use positive differential pressure drilling, so the fracture of borehole wall broadening, with the original
width, dynamic fracture width becomes bigger. Besides, it’ll make a dynamic variation of fracture width,
because the deeper the borehole is, the greater the pressure fluctuation it exerts when making a trip etc.
The formation protection capacity of drilling fluids has been weaken by the bigger dynamic fracture
width and the width variation with the downhole effective pressure variation, So, loss circulation is more
likely to happen, and it is rather difficult to plugging. For example, when the cave diameter is 10m,the
fracture length is 1m,the width increment will approximately be 5mm under the smaller positive
differential pressures (3MPa) ,adding the original width ,the total is more than 5mm.Once drilling these
bigger fracture, drilling fluid will loss, and the normal plugging measures hardly to work ,we must take
specialized plugging technology.
～
5.2 Analysis of fracture morphology
With reference to Fig.5 Fig.8, we can see that in the direction of fracture length, the width increment
gradually become a constant with the decrease of fracture length and the increase of cave diameter,
positive differential pressure and leakoff time. If the nature fracture surfaces are flat, high differential
pressure make the plate fracture more likely to happen relative to wedge fracture. Especially when the
cave diameter is very large, the fracture has "flat, straight and wide" features which make it difficult to
plug and bridge the fracture.
5.3 Analysis of parameter sensitivity
The dynamics fracture width increases with the increase of positive differential pressure, cave diameter,
fracture length and leakoff time in Figure 9.The width is sensitive to the variation of positive differential
pressure, and the width increment linearly increases with the increase of positive differential pressure.
When keeping the same positive differential pressure, along with the drilling fluid going into the
fractured/caved system, the mean pressure in fracture is on the increase, and the width will increase as
well , finishing at a constant width. When keeping the same differential pressure and fracture length, the
fracture width increases rapidly with the increase of solution cave diameter, the fracture open easier
under the existing of solution caves. when keeping the same differential pressure and cave diameter, the
width variation is most obvious if the fracture length is smaller, and it will leveled off with the increase
of fracture length. It shows that stress sensitivity is stronger in fractured/caved carbonate reservoirs, the
272
leakage is more prone to happen in well completion, we should to the greatest extent keep balance
pressure to drill and avoid pressure fluctuation while making a trip quickly.
Cave diameter
Fracture width increment, mm
6
Sensitivity analysis
Fracture length
5
Pressure drawdown
Mean fracture pressure
4
3
2
1
0
0
2
4
6
8
10
12
Fracture length, m / Pressure drawdown, MPa
Fig.9 Parameter sensitivity analysis
6 Conclusion
，
(1) The stress sensitivity for fractured/caved reservoirs is stronge dynamic fracture width increase
rapidly with the increase of positive differential pressure, cave diameter, fracture length and leakoff time,
The existing of caves make the fracture open easier in positive differential pressures, and make the mud
losses more prone to happen in well drilling and completion.
(2) The fracture is characteristic of level , straight and wide, which generated in such circumstances as
high differential pressure big cave diameter short fracture length long leakage time. It will increase the
difficulty of prevention and control the lost circulation as well as reservoir protection.
(3) The numerical simulation results and drilling practice have shown that the forecast of fracture width
is of vital importance in fractured/caved reservoirs, the leakage controlling what should be mainly
carried out is prevention, and the plugging measures should be executed timely, quickly, and efficiently.
、
、
、
Acknowledgments:
The work is supported by 973 Plan Project (2010CB226705) and National Science and Technology
Major Projects (2008ZX05049-003-03, 2008ZX05005-006-08HZ). The authors wish to thank Professor
Kang for permission to publish and present this paper, and also express appreciation to Associate
Professor You and other colleagues in laboratory team for their help.
References
[1]. Kang Yili, Luo Pingya, Pu Xiaolin, et al. Key Problems and Advances in Formation Damage
Control of Deep High Sufur Content Carbonate Gas Reservoirs [C]. Professional Committee of
China Petroleum Natural Gas Society 2006 Annual Conference, Kunming, Yunnan, 2006, 11 (in
Chinese)
[2]. Li Peilian, Zhang Ximing, Chen Zhihai. Tahe Oilfild Ordovician Fractured carbonate Reservoir
development [M]. Beijing: Petroleum industry press, 2003(in Chinese)
[3]. Xu Tongtai, Liu Yujie, Shen Wei. Lost Circulation Prevention and Control Technique in Drilling
Engineering[M].Beijing: Petroleum industry press,1997:1-4 (in Chinese)
[4]. Wang Pingquan, Luo Pingya, Nie Xunyong, et al. The Complex Problems Solutions of Circulation
Lost and Well blow out Coexisting in Shuangmiao Well 1 [J].Natural Gas Industry, 2007,
273
27(1):60-63. (in Chinese)
[5]. Lu Kaihe,Xue Zhengsong, Wei Huiming. Study on techniques of auto-adapting lost circulation
resistance and control for drilling fluid[J]. Acta Petrolei Sinica,2008, 29(5):757-760,765(in
Chinese)
[6]. Majidi R., Miska S. Z., Yu M., et al. Modeling of Drilling Fluid Losses in Naturally Fractured
Formation[C]. SPE 114630, 2008
[7]. Majidi R., Miska S. Z., Yu M., et al. Modeling of Drilling Fluid Losses in Naturally Fractured
Formation[C]. SPE 114630, 2008
[8]. Wang H., Sweatman R., Engelman B., et al. Best Practice in Understanding and Managing Lost
Circulation Challenges[J]. SPE Drilling & Completion, 2008, 23(2):168-175.
[9]. Wang H., Soliman M. Y., Towler B. F. Investigation of Facttors for Strengthening a Wellbore by
Propping Fractures[J]. SPE Drilling & Completion, 2009, 24(3):441-451.
[10]. Oort E. V., Friedheim J., Pierce T., et al. Avoiding Losses in Depleted and Weak Zones by
Constantly Strengthening Wellbores[C]. SPE 125093, 2009
[11]. Wang Renfu.Advanced Well Completion Engineering [M]. Third Edition. Beijing, Petroleum
industry press, 2000:222
[12]. Economides, M. J., Nolte, K. G.. Reservoir Stimulation [M]. Third Edition. Translated by Zhang
Bao ping, Jiang Tian, Liu Liyun,et al. Beijing, Petroleum industry press,2002:182-209 (in Chinese)
[13]. Lian Zhanghua, Kang Yili, Xu Jin, et al. Predicting fracture width by finte element numerical
simulation[J] .Natural Gas Industry, 2001, 21 (3) :47-50(in Chinese)
[14]. Lian Zhanghua, Kang Yili, Tang Bo, et al. rediction of Vertical Fracture Widths near Borehole Face
of the Wall[J]. Natural Gas Industry, 2003, 23 (3) :44-46(in Chinese)
[15]. Li Xiangchen Kang Yili Zhang Hao et al. Computer Modeling of the Changes in Width of Two
Vertical Fractures in Tight Sand Connected to Bore hole[J]. Drilling Fluid & Completion Fluid,
2007, 24(4) :55-59(in Chinese)
274