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A study of influencing factors on fracture
initiation pressure of cemented sliding sleeve fracturing
Article in Journal of Natural Gas Science and Engineering · May 2014
DOI: 10.1016/j.jng.2014.03.002
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A study of influencing factors on fracture initiation pressure of cemented sliding sleeve fracturing
Huanqiang Yang*,Ruihe Wang,Weidong Zhou,Luopeng Li,Fei Chen
College of Petroleum Engineering,China University of Petroleum,Dongying257000,China
描写清明节的诗a r t i c l e i n f o
Article history:
Received11December2013 Received in revid form
28February2014
Accepted3March2014
Available online
Keywords:
Cemented sliding sleeve fracturing Fracture initiation pressure Interfacial parameters
Valve parameters
Ports orientation angle a b s t r a c t
As a new well completion technology of multiple-layer fracturing,the main concept of cemented sliding sleeve fracturing(CSSF)is to place sliding sleeve valves in the casing string and complete the well with normal cementing operations.The potential high fracture initiation pressure is a key issue to overcome with this technique.This paper mainly prents the theoretical and experimental studies on the influ-encing factors of CSSF on fracture initiation pressure,hoop stress distribution in the cement and for-mation near the wellbore was predicted withfinite element analysis(FEA):1.the linear spring element was ud to simulate the cementing strength of thefirst interface between casing and cement;2.the finite element model of CSSF was established under the condition of in-situ stress;3.laboratory tests of the CSSF was conducted with the full-scale fracturing simulation experimental system.The results indicate that by improving the cementation strength of thefirst interface and reducing the ports orientation angle can reduce the fracture initiation pressure.Addition
ally,the length and the number of ports are the main influencing factors of the fracture initiation pressure.Incread the length and the number of ports can achieve the aim of reducing the fracture initiation pressure.The valve with slots turns out to be a better choice compared to the valve withfins.
Ó2014Elvier B.V.All rights rerved.
1.Introduction
Unconventional oil and gas resources,such as shale gas,tight gas,coal bed methane,sandstone oil and gas et al.have properties of low porosity,low permeability,strong heterogeneity,low re-rves abundance,thin producing formation and multiple layers (Holditch,2013;McGlade et al.,2013;Jia et al.,2012;Xie et al., 2012).It is generally accepted that fracturing is the most common method to stimulate the unconventional oil and gas rervoirs.The industry has been arching and experimenting with a variety of methods to optimize the unconventional oil and gas production. Ajao et al.(2013)has revealed the distinguishing features of the fracture designs of two wells,each at the opposite extremes of industry application:a higher permeability(17md)gas rervoir in Siberia and a very low permeability(0.0001md)shale formation in the United States through the u of Unified Fracture Design(UFD). According to the fracturing horizontal wells of unconventional gas rervoirs,
Soliman et al.(2012)investigated the effect of various fracturing scenarios on the stress distribution around the wellbore and optimized the number of fractures using bothfluidflow and geo-mechanics aspects.Wu et al.(2012)studied the connotation, design optimization and implementation of volume fracturing technique on unconventional rervoirs.
小米6什么时候上市的To address the increasing need for completion efficiency,a new well completion technology of cemented sliding sleeve fracturing is being propod.Schlumberger called this technology Treat And Produce(TAP)completions.The main concept of TAP is to place sliding sleeve valves in the casing string and complete the well with normal cementing operations.The sliding sleeves would then be opened one at a time to fracture each production interval inde-pendently without perforating.There is a C-ring in the main TAP valve,which reduces in size when squeezed by a piston.The C-ring originally has the same size inner diameter(ID)as the valve,but when reduced in size,the C-ring becomes a at on which the next ball or dart can al.Fig.1(a)shows the cross ction of the main TAP valve and the C-ring.When the dart lands on the C-ring at,it creates a al and pump pressure pushes the sleeve open,then the ports can be activated(reference Fig.1(b)).
Schlumberger has applied this technology to stimulate many wells and the results are quite favorable.After the operations of TAP,the well produced20%more gas than an offt well(Rytlewski a
nd Schlumberger,2008).
*Corresponding author.Tel.:þ8613605464005.
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dx.doi/10.1016/j.jng.2014.03.002
1875-5100/Ó2014Elvier B.V.All rights rerved.
Journal of Natural Gas Science and Engineering18(2014)219e226
Xiao and Tang (2012)and Wang et al.(2012)pointed out that the cemented sliding sleeve fracturing technique requires high quality of cementation.According to the study of Nelson and Guillot (2006),the quality of the first interface cementation is affected by cement slurry properties,displacement ef ficiency and volume shrinkage of cement paste,which may lead to the first interface imperfect.So,it is important to consider the interface effect of the first interface in the rearch of CSSF.Meanwhile,according to the theory of hydraulic fracture,in-situ stress is one of the most important characteristics in fluencing on the fracture initiation pressure (Lian et al.,2011;Crosby et al.,2002).So,the effect of ports orientation angle at which the ports of valves oriented from the maximum horizontal stress (MHS)on the fracture initiation pres-sure is worth studying.
The ports of sliding sleeve valves are channels connecting fracturing fluid with cement sheath.The effect of the length and the number of ports on fracture initiation pressure has been discusd by Rytlewski and Cook (2006).All sliding sleeve valves studied by Rytlewski have 5.5-in outer diameter (OD).In this paper,according to the field tests conducted in the He-8Block in Changqing Oil field,sliding sleeve valves of 6.625-in OD were ud to investigate.So,the sliding sleeve valves studied in this paper all had 6.625-in OD.In addition,to ensure each valve get the same hydraulic ene
rgy under the same pressure condition,the valves were tested with the same port area.The valves with ports lying along in different di-rections,noted as the angle of ports,were also discusd in this paper.Meanwhile,fracture shapes of different fractured specimens were analyzed.
To address the issues,FEA is ud to investigate the in fluence of cementation quality of the first interface cementation quality and the ports orientation angle on hoop stress distribution near the wellbore and how to improve the hoop stress.Then,the in fluence of valve parameters on fracture initiation pressure was analyzed by using experimental and theoretical study.2.Finite element analysis of CSSF
To reduce the fracture initiation pressure of He-8block in Changqing oil field,this paper tested the material properties of cement mortar with 1:0.5w 1:3cement-sand ratio samples in laboratory and compared to the properties of rervoir formation in He-8block.Then,a suitable ratio which was similar to the prop-erties of rervoir formation in He-8block was chon (reference Table 1).
According to the rearch of Hossain et al.(2000),initial fracture will appear on the wellbore when the maximum hoop stress rea-ches the tensile strength of the rock.It is assumed that by raising
the hoop stress of cement sheath in an attempt to reduce the fracture initiation pressure is a feasible
way to optimize the treat-ment.The hoop stress distribution around the cement sheath and formation near the ports is simulated using FEA.2.1.Finite element analysis with no in-situ stress
Some rearchers have ud many models to describe the interface effect,such as Wang et al.(2009)and Benveniste and Miloh (2001).The linear spring model can identify some experi-mental phenomena and has a wide range of applications,such as Zhong et al.(2009),Chen et al.(2009),Hashin (1991),Klarbring and Movchan (1998)and Antipov et al.(2001).Zhong et al.(2009)established the 2D calculation model of imperfect bi-material interface using the linear spring model and calculated the interfa-cial stress.Then the stress intensity factor was calculated using this interfacial stress.The in fluences of the imperfection of the interface on the stress intensity factors for a bi-material compod of aluminum and steel were prented.
In this paper,3D finite element models with no in-situ stress were ud to identify the hoop stress distribution.All the models were constructed with a 6.625-in OD sliding sleeve in a 9.45-in wellbore.According to the rearch of Hossain,the effect of stress and strain which is caud by fracturing fluid pressure can be neglected when the diameter of area is 5times bigger than the diameter of wellbore.So,taking 6.56-ft as the diameter of the for-mation is suitable in the study of FEA.
The linear spring model was ud to identify the cementing strength of the first interface in this paper.Similarly to the elastic coef ficient of spring,interface parameters b s and b z are introduced to describe the ability of sliding and compression or tension be-tween casing and cement in the linear spring model.When the b s and b z tend to be N ,the interface between casing and cement is perfect.Under this condition,the phenomenon of sliding and compression or tension will not occur;on the contrary,the inter-face is called parate interface when b s and b z are equal to 0and the sliding and compression or tension will occur more easily.Following this approach,we can u l ¼b s /E c and x ¼b z /E c to measure the value of b s and b z ,in which E c reprents the young ’s modulus of cement.When the fracturing fluid pressure about 5.8Â102psi was applied on the cement sheath near the ports and casing in wall,the hoop stress distributions of cement sheath near the ports with different values of interface parameters were shown as Fig.2.
From the results of Fig.2,it could be concluded that the maximum hoop stress in the cement sheath near the ports was 730psi when the first interface was perfect,which was 28%higher than the applied pressure of 580psi.The maximum hoop stress in the cement sheath near the ports was 217psi as l and x equals to 0,which is similar to pressurizing an openhole.The values of the maximum hoop stress were 684psi and 378psi respectively when l ,x ¼20and l ,x ¼0.2(reference Fig.2(b)and (c)).Ry
tlewski and Cook (2006)calculated hoop stress in the cement only in the cas of the cement bonded and not bonded to the formation with simple 2D finite element models.Hoop stress in the cement was 25%higher than the applied pressure when the cement was bonded to the formation.As expected,the hoop stress in the
西岩
unbonded
Fig.1.Working principle of TAP (a)TAP Valve with C-ring Clod,(b)The dart landing on the C-ring at and the sleeve opening.
Table 1
The material properties.Material
Young ’s modulus (psi)Poisson ratio Tensile strength (psi)Permeability (md)
Porosity (%)Casing 2.99Â1070.3
/
/
咸宁景点/Cement 1.60Â1060.258.7Â1022Â10À213Formation 5.08Â106
0.21
5.8Â102
1.51
9.5
H.Yang et al./Journal of Natural Gas Science and Engineering 18(2014)219e 226
220
model with micro annulus was the same as pressurizing an open-hole.The conclusion was similar to this paper.
When varying the pressure of fracturing fluid,the maximum hoop stress with different interface parameters are shown in Table 2.
The results indicated that under the conditions of different fracturing fluid pressure,the maximum hoop stress in the cement sheath near the ports would increa as the increa of interface parameter b s and b z ,in other words,we could improve cementing quality to decrea the fracture initiation pressure.2.2.Finite element analysis with in-situ stress
The in-situ stress consists of vertical principal stress s V ,maximum horizontal stress s H and minimu
m horizontal stress s h .Taking He-8block in Changqing oil field as an example,the in-situ stress data of this block is:s V ¼1.16Â104psi,s H ¼8.2Â103psi and s H ¼7.3Â103psi,the pore pressure is 4.1Â103psi.When the
number of ports is 6,the range of ports orientation angle a is 0 e 30 (reference Fig.3).
When 7.25Â103psi of the fracturing fluid pressure was applied,the hoop stress distributions under different ports orientation an-gles a were shown as Fig.4.
From Fig.4,it could be noted that the ports orientation angle a had great in fluence to hoop stress.The maximum hoop stress in the cement sheath near the ports reached 1.65Â103psi when a ¼0 .Additionally,the maximum hoop stress was 91.5psi when a ¼30 .It could also be concluded that the hoop stress distribution was very centralized when a ¼0 and relatively disperd when a ¼30 .When varying the pressure of fracturing fluid,the maximum hoop stress measured with different ports orientation angles were listed in Table 3.
Above all,the maximum hoop stress dropped as increasing of the ports orientation angle,that is,the fracture initiation pressure can be incread with the increa of the ports orientation angle.So,increasing the number of ports,which can decrea the ports orientation angle,is an effectiv
e method to reduce the fracture initiation pressure.
3.Experimental of the cemented sliding sleeve fracturing 3.1.Experimental method
In order to study the in fluence of various ports parameters on the sliding sleeve valves,such as the angle,the length,the number of ports and the valve with slots or fins on the fracture
initiation
Fig.2.Effect of interface parameters on hoop stress distribution.(a)l ,x /N ,(b)l ,x ¼20,(c)l ,x ¼0.2(d)l ,x ¼0.
Table 2
Effect of interface parameters on maximum hoop stress with different fracturing fluid pressures.Fracturing fluid pressure (psi)Maximum hoop stress (psi)
l 、x /N
l 、x ¼20
l 、x ¼0.2
l 、x ¼0
68086780744325978099492550829788011211043573336980
1251
1164
639
374
H.Yang et al./Journal of Natural Gas Science and Engineering 18(2014)219e 226221
pressure,the experiment was conducted by using full-scale frac-turing simulation experimental system.11kinds of reprentative valves were designed,and one type of them is t as a reference valve for comparison.The valves have the same area to ensure each valve get the same hydraulic energy under the same pressure condition.The types and parameters of the 11kinds of valves are shown in Table 4.
The valves of 6slots Â6.3-in Â30 and 6fins Â6.3-in Â0 were shown in Fig.5,and some of the sliding sleeve valves were shown in Fig.6.
The main goal of the tests was to optimize the valve shape to reduce fracture initiation pressure and analyze the fracture
shapes.
Fig.4.Effect of ports orientation angle on hoop stress阶梯式退休
distribution.
Fig.3.Schematic of ports orientation angle.
Table 3
Effect of ports orientation angle on maximum hoop stress with different fracturing fluid pressures.Fracturing fluid pressure (Â103psi)Maximum hoop stress (Â103psi)
a ¼0
a ¼10
a ¼20
a ¼30
7.54 2.24 1.99 1.290.227.83 2.89 2.65 1.940.888.12 3.54 3.29 2.59 1.528.41
4.18
3.94
七星花
3.16
2.18
H.Yang et al./Journal of Natural Gas Science and Engineering 18(2014)219e 226
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