Feasibility study on application of volume acid fracturingtechnology to tight gas carbonate reservoir development*,**
Nianyin Li a, Jinxin Dai a, *, Chao Liu b, Pingli Liu a, Yanming Zhang b, Zhifeng Luo a,Liqiang Zhao a
a State Key Lab of Oil and Gas Reservoir Geology and Exploitation, South West Petroleum University, Chengdu, Sichuan 610500, PR Chinab National Engineering Laboratory of Low Permeability Oil and Gas Field Exploration and Development, Xian, Shanxi 710018, PR China
a r t i c l e i n f o
Article history:Received 24 April 2015Received in revised form2 June 2015Accepted 3 June 2015
Keywords:Tight oil and gas reservoirTight carbonate rockVolume acid fracturingFracture networkOrdos basinSRV
How to effectively develop tight-gas carbonate reservoir and achieve high recovery is always aproblem for the oil and gas industry. To solve this problem, domestic petroleum engineers use thecombination of the successful experiences of North American shale gas pools development bystimulated reservoir volume (SRV) fracturing with the research achievements of Chinese tight gasdevelopment by acid fracturing to propose volume acid fracturing technology for fractured tight-gas carbonate reservoir, which has achieved a good stimulation effect in the pilot tests. To deter-mine what reservoir conditions are suitable to carry out volume acid fracturing, this paper firstlyintroduces volume acid fracturing technology by giving the stimulation mechanism and technicalideas, and initially analyzes the feasibility by the comparison of reservoir characteristics of shale gaswith tight-gas carbonate. Then, this paper analyzes the validity and limitation of the volume acidfracturing technology via the analyses of control conditions for volume acid fracturing in reservoirfracturing performance, natural fracture, horizontal principal stress difference, orientation of in-situ stress and natural fracture, and gives the solution for the limitation. The study results showthat the volume acid fracturing process can be used to greatly improve the flow environment oftight-gas carbonate reservoir and increase production; the incremental or stimulation response isclosely related with reservoir fracturing performance, the degree of development of natural frac-ture, the small intersection angle between hydraulic fracture and natural fracture, the large hori-zontal principal stress difference is easy to form a narrow fracture zone, and it is disadvantageous tocreate fracture network, but the degradable fiber diversion technology may largely weaken thedisadvantage. The practices indicate that the application of volume acid fracturing process to thetight-gas carbonate reservoir development is feasible in the Ordovician Majiagou Formation oflower Paleozoic, which is of great significance and practical value for domestic tight-gas carbonatereservoir development and studies in the future.
chnology is a new technology which apply to stimulate tight reservoir, it is a combination of SRV fracturing andplex fracture network is created by SRV fracturing, fracture conductivity is created by multistage alternating
ience and Technology Major Project of China (2011ZX05044) and National Natural Science Foundation of China
troleum University.
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1. Introduction volume acid fracturing pilot tests for tight-gas carbonate reser-voir have been carried out in China, and it creates acid etched
The rapid development of unconventional natural gas hasdrawn the attention of theworld, and caused significant effect onthe global energy structure. As the leader of the exploration anddevelopment of unconventional natural gas, the United Statesmade a major breakthrough in the early 1980s. According to thestatistics of the U.S. Energy Information Administration (EIA), theproduction of unconventional gas has reached 1690 � 108 m3 in2011, which accounts for about 26% of the US natural gas output.Predictably, the proportion will continue to rise in the nextperiod of time.
Unconventional natural gas reserves of China are rich aswell, among which the amount of tight gas prospective re-sources reaches to (12e100) � 1012 m3 [1]. Consequently, do-mestic related work for the exploration of the tight gas hasbeen carried out and made substantial progress. The tight oiland gas resources have been discovered in Ordos basin,Sichuan basin, Junggar basin, Tarim basin, Songliao basin, andalmost all the petroliferous basins, mainly concluding threetypes of reservoirs, like lacustrine carbonate rocks, deep lakedelta sandstone and deep lake gravity flow sandstone, with amore than 20 � 104 km2 of total favorable exploration area andapproximately 106.7e111.5 � 108 t of total geological re-sources. There are great differences between the geologicalfeatures of the tight oil and gas reservoir and conventional oiland gas reservoir. Generally, the former one has low porosity(less than 10% generally), low permeability (less than 0.1 mDgenerally), various types of reservoir, complex lithology, highcalcium content (about 40% generally, in addition to the Yan-chang Group of Ordos Basin and the cretaceous system ofSongliao Basin), mostly proximal accumulation, low naturaldeliverability, developed natural fractures and essentiallycontrol the production of oil and gas reservoirs, and so on.
Currently, to develop fractured tight-gas carbonate, conven-tional acid fracturing mode is generally used around the world.Although this mode can obtain a certain degree of originalstimulation, it results in rapid decline of production and diffi-culty in obtaining stable production [2,3]. Furthermore, thismode connects little of the natural fracture system and offersacid very small swept volume. Therefore, there is a need infurther exploration, study and field practice for this kind of gasreservoir. Until now, acid fracturing for high-calcium tight oil andgas reservoir hasn't been seen all around the world, but the
Table 1Comparative table of shale gas reservoir attributes and objective gas reservoir attribu
Parameter Shale-gas reservoir
Stress <2000 psia net lateral stressReservoir temperature >230 �FPressure >0.5 psi/ft
Mineralogy >40% quartz or carbonates<30% claysLow expandability
Fracture fabric and type Vertical vs. horizontal orientationOpen vs. filled with silica or calcite
Internal vertical heterogeneity Less is betterSeals Fracture barriers present top and baseGas type ThermogenicGas composition Low CO2, N2 and H2SThermal maturity Dry gas window > 1.4RoTotal organic content >2%Permeability >100 ndYoung's modulus >3.0 MMPSIAPoisson's ratio (static) <0.25
complex fracture network with the conductivity (HFM micro-seismic monitoring of adjoining wells have indicated that thevolume acid fracturing creates multiple branch non-planarcomplex fracture network). The results of practice have indi-cated that the effect of volume acid fracturing is very similar withSRV sand fracturing and the volume acid fracturing has a goodapplication prospect.
In this paper, tight-gas carbonate reservoir in the Ordos Basinis investigated by using many theoretical methods such as themechanics, the probability statistics, scanning electron micro-scope (SEM), X-ray diffraction (XRD) techniques, well logging,and using technical means of combination of laboratory exper-iment with field practice. The research results will promotethe theoretical development of volume acid fracturing in thefuture.
2. Stimulation mechanism and technical ideas
2.1. Stimulation mechanism
Horizontal well drilling technology increases reservoir con-tact area, slick water fracturing creates hydraulic fractures andreopens most natural fractures or makes part of them slippage,the hydraulic fractures communicate with the natural fracture,which creates initially fracture network; using the degradablefiber diversion agent to overcome the disadvantage of largehorizontal principal stress difference, increasing SRV, at thismoment complex fracture network is created initially; acid is animportant factor for creating complex fracture network, first,acid heterogeneously etches fracture walls and increases theirroughness, and so fracture obtains conductivity after fractureclose; second, natural fracture conductivity is created by acidleak-off, and simultaneously the formation of a small amount ofacidizing wormhole makes fracture network further complex; Amassive slick water is injected to reduce reservoir temperature,which makes aciderock reaction velocity slow down and acideffective time increase. Meanwhile the phenomenon of acidfingering in preflush also increases the distance of acid pene-tration, and acid is pushed to fractures in the remote by overflushfluid, which improves the stimulation rate of the fracture in theremote.
Horizontal orientation √Filled with authigenic carbonate √
Less √Fracture barriers present top and base √e e
Low N2 and some layers contain H2S, CO2 √Dry gas window is 1.6e4.5Ro √0.1e24 √<1 md √4.93 � 106 MMPSIA √0.22 √
N. Li et al. / Petroleum 1 (2015) 206e216208
2.2. Technical ideas
Tight carbonate reservoir is stimulated bymulti-cluster stagedhorizontal well slick water fracturing. ① Acid preflush is used toreduce hydraulic fracturing initiation pressure, and then slickwater is used to initiate the fracture and to reduce the tempera-ture of the fracturewalls;② The reduction friction acid is injectedto etch fracture surface and dissolve fillers in natural fractures,simultaneously propagate the fracture;③ Slickwater overflush isused to put the reduction friction acid into deep fracture;④Highviscosity fracturing fluid with degradable fiber temporary plug-ging agent is used for fracture temporary plugging, and then lowviscosity slick water is used to created hydraulic fractures indifferent directions, so SRV increases further; ⑤ The reductionfriction acid is used to etch new fracture surface and graduallydissolve fiber temporary plugging agent; ⑥ Steps 3e5 are con-ducted successively alternately until the end of frac job.
3. Comparison and analysis of reservoir characteristics
The volume acid fracturing is a new stimulation technology,in order to further study this topic, this paper compares tight-gascarbonate reservoir characteristics with shale gas reservoircharacteristics [4] and is presented in Table 1. It can be seen thattight-gas carbonate reservoir attributes are roughly similar withshale gas reservoir attributes, so this paper initially determinesthe volume acid fracturing technology is available for this car-bonate reservoir.
4. Interpretation of well logging data
The well logging data not only may optimize drilling andfracturing deployment which may provide effective guidanceparameters for horizontal well landing point and completion job,but also may provide Young's modulus, Poisson's ratio, tensilestrength, fracture toughness, the maximum and minimal hori-zontal principal stress by the theoretical formula, and these pa-rameters are very important for judging whether the volumeacid fracturing is available for tight-gas carbonate reservoir.
Young's modulus and Poisson's ratio can be obtained byAcoustic logging data and Equation (1) and (2) [5]and which arepresented as follows:
At present, horizontal principal stresses are obtained mainlyby three methods which are numerical simulation, laboratorycore test and well logging data [8,9]. In this paper well loggingdata is used to predict horizontal principal stresses bymulti-poromedium model [10] which is presented as follows:
sv ¼ gZDTV
0
rbðhÞdhþ O (5)
sh ¼ m
1� msv � m
1� mavertpp þ ahorpp þ E
1� m2xh þ mE
1� m2xH
(6)
sH ¼ m
1� msv � m
1� mavertpp þ ahorpp þ E
1� m2xH þ mE
1� m2xh
(7)
where sv is the total vertical stress, MPa; DTV is vertical depth, m;g is gravitational acceleration, m/s2; O is deviant, unitless; rb isbulk density, kg/m3; sH and sh are maximum and minimumhorizontal principal stress, MPa; sv is total vertical stress, MPa;avert is Biot coefficient at vertical direction, unitless; ahor is Biotcoefficient at horizontal direction, unitless; m is static poisson'sratio, unitless; a is effective stress coefficient, unitless; pp is porepressure, MPa; E is static Young's Modulus, MPa; xh is the strainat minimum horizontal principal stress direction, unitless; xH isthe strain at maximum horizontal principal stress direction,unitless.
Interpretation results of well logging data are presented inFig. 1.
5. Feasibility analysis on volume acid fracturing
SRV fracturing stimulation experiences of shale gas reservoirhave indicated that reservoir geological conditions such as rockbrittleness, natural fracture, horizontal principal stress differ-ence, intersection angle of between hydraulic fracture and nat-ural fracture etc. control whether complex fracture network iscreated. However, it is not reasonable to evaluate reservoirfracturing performance by rock brittleness because it does notindicate rock strength. For example, fracture barrier betweenupper and lower Barnett can be dolomitic limestone with higherbrittleness [11]. To avoid the shortage due to the single use ofbrittleness index for fracturing evaluation, linear elastic fracturetheory was adopted to create the fracturing performance indexthrough combination of brittleness index and fracture toughness[12e14]. Therefore, this paper analyzes the feasibility by fourways of fracturing performance, natural fracture, horizontalprincipal stress difference, as well as intersection angle betweenhydraulic fracture and natural fracture.
5.1. Fracturing performance evaluation
5.1.1. Rock brittlenessRock brittleness is one of themost important rockmechanical
properties for judging whether complex fracture network isformed during hydraulic fracturing [15e18], brittle rock isbeneficial to the development of natural fracture and createsfabric fracture. Therefore, brittleness has been used as adescriptor in screening hydraulic fracturing candidates [19e21].However, the absence of universally accepted definition andmeasurement of brittleness has led to various methods or
Fig. 1. Logging curve of tight carbonate reservoir in Ordos Basin.
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models for its quantification [22,23]. Jin et al. [13] has assembleda table of definitions of brittleness, as presented in Table A. Fromwhich it can be seen that brittleness is obtained mainly by rockmechanics tests and well logging data. Therefore, in order todetermine accurate brittleness of tight carbonate rock, this paperdetermines brittleness by combining rock mechanics tests andwell logging data.
From a physical viewpoint, mineralogical brittleness isconsidered more reliable [19,24]. However, originally, themineralogical brittleness accounted only for the weight fractionof quartz [19]. Afterward, it was observed that the presence ofdolomite tends to increase the brittleness of shale, so both thefractions of quartz and dolomite were included [25]. It is alsoobserved that silicate minerals such as feldspar and mica (thechemical expression of mica is X2Y4e6Z8O20(OH,F)4, if X ion iscalcium, it is considered as “brittle” mica) are more brittle thanclay in shale reservoirs. Besides the dolomite, other carbonateminerals, such as calcite in limestone, are also more brittle thanclay [26e28]. Therefore, a new expression of brittleness is pro-posed by Jin et al. [13] in the following:
B ¼ WQFM
WTotþWCarb
WTotz
WQFM þWcalcite þWdolomite
WTot(8)
where WQFM/WTot is the weight fraction of quartz, feldspar, andbrittle mica, which are silicate minerals;WCarb/WTot is theweightfraction of carbonate minerals consisting of dolomite, calcite, andother brittle carbonate (if there are WCARB, WDOL and WCLC,
Table 2XRD analysis results of carbonate samples.
Cores no. Depths (m) Mineral percentage of core (�10�2)
WCARB represents all other carbonate minerals except calcite,and all of them should be included in brittleness calculation; ifthere are only WCLC and WDOL, but no WCARB, include both ofthem in brittleness calculation).
XRD analysis results of carbonate samples are presented inTable 2, showing homogeneity in mineralogical compositions.The dominant minerals in the carbonate samples were dolomiteand calcite. Dolomite contents are between 0.00 and 99.57% withan average of 77.04%, whereas calcite contents range from 0.00 to97.31% with an average of 19.88%. In addition, the average ofquartz contents is very low and clay contents are zero. Themineralogical brittleness results are presented in Table 2 byEquation (8).
The stressestrain curves are obtained by the triaxial rockmechanics experiment, of which the curve shape reflects brittle,plastic, subdued, fractured and other all kinds of deformationpath during thematerial is acted by various outside forces, Heard[29] considered that the deformation is less than 3% before rockfailure as brittle failure. The stressestrain curves are presented inFig. 2 and it can be seen that all strains are between 0.5 and 1.03,while the average is about 0.8. Therefore, according to the theoryof Heard, carbonate samples are brittle rock.
In order to qualitatively evaluate carbonate sample brittle-ness, whole-diameter core sample is used to carry out spilt test,and rock crushing characteristic is obtained, as shown in Fig. 3. Itcan be seen that core sample is cracked extensively and showssignificant brittleness, so it is favorable to SRV fracturingstimulation.
Fig. 2. The stressestrain curve of three axis rock mechanics test.
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It is not feasible to evaluate brittleness for tight carbonateformation by laboratory measurements due to the limitation ofturnaround time and expense. Therefore, well logging dataestimation is more practical for the evaluation of brittleness intight carbonate reservoirs. In this paper, the brittleness evalua-tion method is used and proposed by Rickman et al. [20], whichhas been verified by Jin et al. [13] and Sun et al. [14]. Rickmanet al. [20] think that the concept of rock brittleness combinesboth Poisson's Ratio and Young's Modulus to reflect the rocksability to fail under stress (Poisson's Ratio) and maintain a frac-ture (Young's Modulus) once the rock fractures. In terms ofPoisson's Ratio, the lower the value, the more brittle the rock,and as values of Young's Modulus increase, the more brittle therock will be. Because the literature formula for this method is notclear, and after in-house verification, it is redefined as:
B ¼ �E þ s
��2 (9)
where E and s are normalized Young's modulus and Poisson'sratio, and are defined below:
E ¼ ððE � EminÞ=ðEmax � EminÞÞ � 100 (10)
s ¼ ððs� smaxÞ=ðsmin � smaxÞÞ � 100 (11)
where Emin and Emax are the minimum and maximum dynamicYoung's modulus for the investigated formation, MPa; smin andsmax are dynamicminimum andmaximum Poisson's ratio for theinvestigated formation, unitless; E and s are Young's modulusand Poisson's ratio along the depth.
According to Young' modulus and Poisson's ratio which areobtained by well logging data and the investigated reservoircharacteristics, brittleness coefficients are obtained by Equations(9)e(11), as presented in Fig. 4. It can be seen that brittleness
Fig. 3. Core sample crushing characteristic.
coefficients are mainly between 40 and 80. Rickman et al.considered the rock of which brittleness coefficients exceed 40 asbrittle rock. Therefore, a large proportion of rock is brittle.
Analysis results of mineralogical brittleness, stressestraincurve, rock crushing characteristic and Rickman method indicatethat reservoir rock investigated is mainly brittle rock, and somerocks are high brittleness, it is favorable to create fracturenetwork for volume acid fracturing.
5.1.2. Fracture toughnessFracture toughness represents the ability of rock to resist
fracture propagation from preexisting cracks. In order to improvestimulation effect and gain the greatest economic benefits, it'srequired to createmaximum SRV under the condition of a certainamount of fracturing fluid injection, that is to say hydraulicfracturing should have the high fracture-making ability in theinvestigated intervals. Under the condition of external factorssuch as pumping pressure are the same, fracture-making abilityof hydraulic fracturing completely depends on rock itself to resistfracture propagation ability. Rock fracture toughness representsfracture-making ability of hydraulic fracturing, and indicateshowdifficult reservoir is fractured. It has been proven that higherthe fracture toughness, higher is the breakdown pressure [30].
Fracture toughness is a material property, and can bemeasured with various methods, but fracture toughness mea-surement of rock is more difficult and complex than other rockmechanics tests. Therefore, to save time and expense, manycorrelations of fracture toughness are proposed by well loggingdata. In this paper, fracture toughness derived from the corre-lation proposed by Jin [7]. Correlation is presented in Equations(3) and (4), and fracture toughness in Fig. 1.
Fig. 4. A cross plot of Young's Modulus and Poisson's ratio showing the brittlenesspercentage increasing to the south west corner of the plot.
N. Li et al. / Petroleum 1 (2015) 206e216 211
5.1.3. Reservoir fracturing performanceThe objectives of massive hydraulic fracturing in tight car-
bonate reservoirs are: (1) creating and connecting complexfracture network; (2) maximizing stimulated reservoir volume(SRV). To create complex fracture network and connect them, thecandidate should have relatively higher brittleness. To maximizeSRV, the candidate should have relatively lower fracture tough-ness. According to the study results by Sun [14], formation withhigh brittleness may not have good fracturing performance, andformation with low fracture toughness may not have goodfracturing performance, while only formation with high brittle-ness and low fracture toughness have good fracturing perfor-mance. This paper uses fracturing performance index modelproposed by Sun Jianmeng et al. to evaluate reservoir fracturingperformance.
The mathematical model of fracability index in terms ofbrittleness and fracture toughness is defined as follows:
FI ¼ BInKIC_n (12)
where BIn and KIC_n are normalized brittleness (Eq. (12)) andnormalized fracture toughness as defined below:
BIn ¼ BI � BIminBImax � BImin
(13)
KIC_n ¼ KIC_max � KIC
KIC_max � KIC_min(14)
where BImax and BImin are the maximum and minimum brittle-ness index for the investigated formation, unitless; KIC_max andKIC_min are the maximum and minimum mode-I fracturetoughness for the investigated formation, MPa m1/2; BI is brit-tleness index at formation depth, unitless; KIC is fracturetoughness at formation depth, MPa m1/2.
According to Sun's [14] method, a plot of fracturing perfor-mance index is derived and presented in Fig. 5. In the case of Sun'spaper [14] the strata fracturing which performance index is morethan 0.24 is considered as fracturing strata, from Fig. 5 we can seemost of fracturing performance indexes are more than 0.24.Therefore, the investigated reservoir has many fracturing strata.
Through above analysis, it is concluded that: (1) the investi-gated reservoir rockwidespreadly characterizes high brittleness, itis favorable to create complex fracture network and connect them;(2) the investigated reservoir rockwidespreadly characterizes lowfracture toughness, and it is favorable to maximize SRV; (3) the
Fig. 5. Fracturing performance index variation of tight-gas carbonate reservoir.
investigated reservoir rock widespreadly characterizes high frac-turing performance index, and it is favorable to create acid etchedcomplex fracture network for volume acid fracturing.
5.2. Development status of natural fracture
The development degree of natural fracture is a prerequisitefor the volume acid fracturing, because hydraulic fracture needsto maximally connect natural fracture system, and complexfracture network could be created. Formation microresistivityscanning imaging logging (FMI) technology is an effectivemethod for investigating formation fracture at present. Accord-ing to fracture origins and performance characteristics, forma-tion fracture is divided into natural fracture and drilling inducedfracture [31]. Meanwhile, this paper investigates the develop-ment degree of natural fracture from microcosmic point of viewby scanning electron microscope (SEM) and microcosmic shapeanalysis on rock slices.
The analysis results of FMI and SEM are presented in Fig. 6,from which it can be seen that natural fracture is developed inthe investigated reservoir, but some natural fractures are packedby carbonate rock. The analysis results of microcosmic shapeanalysis on rock slices are presented in Table 3, fromwhich it canalso be seen that natural fracture is developed but some naturalfractures are packed by carbonate rock. Therefore, GMI-MohrFracs software is used to simulate and analyze the open-ing degree of cracks in investigated reservoir.
Fig. 7 shows that most of natural fracture may open whenbottomhole pressure is up to 1.80 g/cm3. Therefore, hydraulicfracturingmaymakemost of natural fracture open, it is favorableto create complex fracture network.
5.3. The horizontal principal stress difference
The size and azimuth of the in-situ stress determine the az-imuth and morphology of artificial fracture during hydraulicfracturing. According to elasticity mechanics theory and rockfailure criterion, fracture initiation and propagation generallyfollow the orientation of maximum horizontal principal stress,therefore, when the horizontal principal stress difference islarge, bi-wing fracture geometry is easily formed; when thehorizontal principal stress difference is small, natural fracturemay largely influence on the orientation of fracture initiation,and hydraulic fracture may propagate at various direction alongrandom natural fracture, so fracture network is created [32,33].
In this paper, well logging data are used to predict themaximum and minimum horizontal principal stress throughporous elastic horizontal strain model. As shown in Fig. 1, hori-zontal principal stress difference is large and the average is about20MPa, so it is disadvantage for creating fracture network duringvolume acid fracturing.
To reduce or remove this disadvantage and obtain maximumSRV, the degradable fiber diversion technology is used and hasgot a good application in acid fracturing and hydraulic fracturingnowadays [34e36].
The degradable fiber diversion technology means that thedegradable fiber agglomerates are injected into bullet hole or theformation fracture during construction, following the principleof fluid flow in the direction of least resistance. Fiber flows intothe natural fractures or artificial fractures, and forms temporaryplugging on the fracture end. This makes the following fracturingfluid can't flow into the natural fractures or artificial fractures. Atthis time, bottomhole pressure will rise. Under the condition ofthe differential stress at horizontal two directions, the secondarybreakdown will appear and then the fracture orientation will be
Fig. 6. Fracture developmental status.
N. Li et al. / Petroleum 1 (2015) 206e216212
changed, then the new fracture is formed. The schematic dia-gram is presented in Fig. 8.
The study results of Zhao [37] indicate that the degradablefiber can effectively improve SRV, and give construction sug-gestions, through improving pump injection displacement andusing low concentration degradable fiber to achieve diversion infracture; through reducing pump injection displacement andusing high concentration degradable fiber to achieve diversion inlayer-to-layer.
5.4. The intersection angle between hydraulic fracture andnatural fracture
The angle between the hydraulic and natural fractures affectsthe formation of the complex fracture network. If the hydraulicfracture appears through natural fractures, the hydraulic fracturekeep the plane shape, but if the meeting of the hydraulic andnatural fractures makes natural fractures enlarge or along itspropagation, it may form a complex fracture network, as shownin Fig. 9.
The orientation of the hydraulic induced fracture and naturalfracture can be obtained from FMI image and is presented in
Table 3Analysis results of rock-pore casting slice.
Well name Core no. Depth (m) Pore type
Primaryintergranular pore
Secondaryintergranular po
G2-9 2-67/127 3236 Few 5G5-7 1-18/133 3224 1
1-74/106 3121 31-93/106 3124 2
G26-11 1-99/133 3386 21-110/133 3388 Few 43-48/66 3427 2
G37-10 1-34/91 3616 2 3G40-9 1-103/210 3577 Few 5
1-113/210 3578 Few 5G42-8 1-142/173 3674 Few 3
1-156/173 3676 1 4
Fig. 10, from which it can be seen that the major orientation ofthe hydraulic induced fracture is about southeast 120�, the majororientation of natural fracture is about southeast 140�, and it ismainly high angle crack with 50e90�.
The results of numerical simulation [39] indicated that thenatural fracture would open and the hydraulic fracture wouldchange extending path if intersection angle between the hy-draulic fracture and natural fracture is low (0� < intersectionangle < 30�) when hydraulic fracturing for the hard fracturedreservoir; intersection angle is 30�e60�, if horizontal principalstress difference is low, natural fracture will open. But if hori-zontal principal stress difference is large, natural fracturewill notopen, the hydraulic induced fracture will pass through naturalfracture; intersection angle is more than 60�, no matter whathorizontal principal stress difference, natural fracture would notopen. Many research results show that intersection angle be-tween the hydraulic fracture and natural fracture, horizontalprincipal stress difference control extending path of the hy-draulic induced fracture [40e42]. Fig. 10 shows that intersectionangle between the hydraulic fracture and natural fracture is20�e30�. So it is advantageous to create complex fracturenetwork.
Fracture type Facialporosity (%)
Filling
resMoldicpore
Primaryfracture
Distensibledissolved fracture
Dolomite
7 5 13 Y2 Several Several 4 Y5 3 82 4 5 Y3 2 6 Y4 1 8 Y
8 55 5 2 13 Y4 10 11 Y2 5 88 10 5 14 Y4 2 8 Y
Fig. 7. Diagram of opening degree of natural fracture under bottomhole pressure is 1.80 g/cm.3.
Fig. 8. The schematic diagram of the degradable fiber diversion technology.
Fig. 9. Breakdown of interaction process between hydraulic fracture (HF) andnatural fracture (NF) [38].
N. Li et al. / Petroleum 1 (2015) 206e216 213
6. Stimulation effect
The pilot tests of volume acid fracturing technology haveachieved success in tight-gas carbonate reservoir of Ordos Basin,whose reservoir type is Ordovician weathering crust in theLower Paleozoic, and themain gas layer is in the first, second andfifth member of Majiagou Formation. Stimulation effect ofexperimental wells are presented in Table 4, which indicates thatthe effect of increasing production is obvious, and the average ofstimulation ratio is about 10. In addition, stimulation effect ofvolume acid fracturing technology also proves that it is feasiblefor stimulation of tight-gas carbonate reservoir through practicalapplication.
7. Conclusions
(1) The investigated reservoir rock characterizes high brittle-ness widely, low fracture toughness and high fracturing
Fig. 10. Orientation diagram of natural fracture and induced fracture.
Table 4Stimulation effect of volume acid fracturing.
No. Well Operation parameter Open flow potentialbefore stimulation, 104 m3/d
1 A 4.8e6.5 46e59 437 6 2.5 36.22 B 4.9e6.1 45.8e68 670 8 5.5 58.693 C 4.1e6.8 57e71 706 7 4 524 D 4.8e6.3 52.5e62.8 579 5 3.4 38
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performance index, so it is favorable to create complexfracture network during fracturing.
(2) The study results of FMI, SEM and microcosmic shapeanalysis on rock slices indicate that natural fracture isdevelopmental and some are filled with authigenic car-bonate rock in the investigated reservoir, and simulationresults of GMI-MohrFracs software indicate that most ofclosed natural fracture may reopen when bottomholepressure is up to 1.80 g/cm3; the analysis result of fractureazimuth indicates that intersection angle between thehydraulic fracture and natural fracture is less than 30�; theoccurrence of natural fracture is suitable for volume acidfracturing.
(3) The study results of horizontal principal stress differenceindicate that it is large in the investigated reservoir, it isdisadvantageous for volume acid fracturing. But study
Table AExpressions summary of existing brittleness index [13,14].
Principle classification Formula Variable declaration
Based on the hardnessor firmness
B1 ¼ (Hm � H)/K H and Hm are macro and micK is bulk modulus
B12 ¼ H/KIC H is hardness, KIC is fracture
B16 ¼ H � E=K2IC H is hardness, E is Young's m
KIC is fracture toughness
B2 ¼ qsc q is percent of debris (<0.6 msc is compressive strength
B13 ¼ c/d c is crack length, d is indentindents at a specified load;empirically related to H/KIC
B15 ¼ Fmax/P Fmax is maximum applied forP is the corresponding penet
B14 ¼ Pinc/Pdec Pinc and Pdec are average incrdecrement of forces
demonstrates that the degradable fiber diversion tech-nology can reduce or remove this disadvantage, as well asachieving maximum SRV, and microseismic monitoringhas been verified.
(4) Stimulation of carbonate rock reservoir may carry out SRVfracturing, but it should meet the following conditions:high fracturing performance index, developmental naturalfracture, as well as less intersection angle between thehydraulic fracture and natural fracture.
(5) How acid system, acidizing fluid volume, injection condi-tion and so on influence the final SRV and acid etchedcomplex fracture conductivity needs further studies.
m diameter); Proto impact test Protodyakonov (1962)
size for Vickers Indentation test Sehgal et al. (1995)
ce on specimen,ration.
Yagiz (2009)
ement and Copur et al. (2003)
Table A (continued )
Principle classification Formula Variable declaration Test method Reference
Based on the strengthratio
B8 ¼ sc/st sc and st are compressive and tensile strength Uniaxial compressivestrength and Braziliantest
Hucka and Das (1974)B9 ¼ (sc � st)/(st þ sc)B10 ¼ (scst)/2 Altindag (2003)B11 ¼ (scst)0.5/2
Based on thestressestrainfeatures
B3 ¼ 3ux � 100% 3ux is unrecoverable axial strain Stressestrain test Andreev (1995)B4 ¼ ( 3p � 3r)/ 3p 3p is peak of strain, 3r is residual strain Hajiabdolmajid and
Kaiser (2003)B5 ¼ tp � tr/tp tp and tr are peak and residual of shear strengths Bishop (1967)B6 ¼ 3p/ 3t 3p and 3t are recoverable and total strains Hucka and Das (1974)B7 ¼ Wr/Wt Wr and Wt are recoverable and total strain energiesB17 ¼ 45� þ 4/2 4 is internal friction angle Mohr circle or
logging dataB18 ¼ Sin 4
Based on the elasticitymechanics parameters
B19 ¼ (En þ vn)/2 En and vn are normalized dynamic Young's modulusand dynamic Poisson's ratio defined in Eq. (3) and (4)
Density and soniclogging data
Modified fromRickman et al. (2008)
Based on the rockmineralogicalcomposition
B20 ¼ (Wqtz)/WTot Wqtz is the weight of quartz, WTot is total mineral weight Mineralogical loggingor XRD in the laboratory
Jarvie et al. (2007)B21 ¼ (Wqtz þ Wdol)/WTot Wqtz and Wdol are weights of quartz and dolomite,
WTot is total mineral weightWang and Gale (2009)
B22 ¼ (WQFM þ Wcarb)/WTot WQFM is weight of quartz, feldspar, and mica;WCarb is weight of carbonateminerals consisting of dolomite, calcite, and othercarbonate components. WTot is total mineral weight.
Jin et al. (2015)
N. Li et al. / Petroleum 1 (2015) 206e216 215
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