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The evaluation on physical property and fracture conductivity of a new self-generating solid proppant
Journal of Petroleum Science and Engineering 177 (2019) 841–848
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The evaluation on physical property and fracture conductivity of a new selfgenerating solid proppant
T
Chengcheng Zhanga, Liqiang Zhaoa,∗, Donghe Yub, Guohua Liub, Yuxin Peic, Fushan Huanga, Biao Liud a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, PR China PetroChina Huabei Oilfield Company, Hebei, 062550, PR China c Research Institute of Petroleum Engineering, PetroChina Dagang Oilfield Company, Tianjin, 300280, PR China d Southwest Petroleum University, Chengdu, 610500, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Self-generating solid proppant Self-propping fracturing fluid Physical property Fracture conductivity
Most fracturing treatments today require injecting solid proppants to keep hydraulic fractures open. However, traditional proppants have disadvantages such as causing the reduced propped-fracture volume and abrasion to the pumping equipments and tubulars. To solve these problems, a new self-generating solid proppant has been developed through liquid-solid transition of self-propping fracturing fluid under reservoir conditions. To select the best self-generating solid proppant and compare it with conventional ones, physical property and fracture conductivity tests were done. Experimental results show that two self-generating solid proppants, including S101 and S505, are firstly selected because of their low density, high strength and excellent fracture conductivity. The apparent density is 1.09 g/cm3 for S101 and 1.10 g/cm3 for S505. The crushing rate of S101 is 1.47% while that of S505 is 1.29%. These data illustrate physical properties of self-generating solid proppant are obviously better than those of conventional proppants. Under the same conditions, the larger the spherical particle size of S101 and S505, the higher the fracture conductivity. Non-uniform distribution of proppant pack can increase fracture conductivity. Fracture conductivities of S101 and S505 are higher than that of quartz sand, and even higher than that of ceramsite at low closure stresses. Based on the experimental results, the pumping rate and injecting proportion of self-propping fracturing fluid can be optimized during the fracturing process.
1. Introduction As the major stimulation treatment, hydraulic fracturing technology is widely used in the low permeability reservoir, realizing the increase of oil and gas production (Tan et al., 2006; Gandossi and Von Estorff, 2015; He et al., 2017). Currently, hydraulic fracturing processes require pumping sand-carrying fluid mixed with solid proppant into the fractures, reaching the purpose of propping formation fractures (Rickards et al., 2006; Dmour, 2013; Chang et al., 2017). However, injecting solid proppants presents several challenges and disadvantages. Firstly, highdensity solid proppant requires highly viscous fracturing fluid, which could leave gel residue to reduce fracture permeability and conductivity (Barati and Liang, 2014). Secondly, the process of sand-carrying fracturing is complex. High displacement and pump pressure accompanied by high sand filling have a serious abrasion to wellhead, pumping equipments and tubulars (Yu et al., 2018). Moreover, it is uneasy for solid proppants to enter the complex fracture networks such as branch
∗
fractures and micro-fractures. This limits the transported distance and reduces fracture conductivity (Weijers et al., 2000). Lastly, pumping solid proppant easily causes sand plug, which has potential risks of field operation and personnel safety (Yuan et al., 2016). Proppants are the core of hydraulic fracturing technology, which play a key role in fracture conductivity (Liu et al., 2003). Conventional proppants can be broadly divided into three types including quartz sand, artificial ceramsite and resin-coated sand (Du et al., 2017). Several attempts have been made to generate solid proppant in the formation. Malone et al. (1968) described a method of fracturing and propping a subterranean formation using a liquid hardenable carrier composition. The liquid composition hardens to form a propping porous material capable of conducting fluids, whose permeability can reach 10–30 darcies or larger. Himes et al. (1994) presented a method of stimulating a subterranean formation using a foamed cement composition. When hardened, the cement composition has a permeability of at least about 0.3 darcies. Nguyen et al. (2004) improved methods and
Corresponding author. E-mail address: [email protected] (L. Zhao).
https://doi.org/10.1016/j.petrol.2019.03.013 Received 24 September 2018; Received in revised form 18 February 2019; Accepted 4 March 2019 Available online 07 March 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
Journal of Petroleum Science and Engineering 177 (2019) 841–848
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compositions for coating proppant particles with a hardenable resin composition. Suspending the coated proppant particles in a fracturing fluid and consolidating the proppant particles after being placed in fractures into permeable masses are provided. These methods and compositions are especially suitable for low temperature well, specifically those around the 15.6–121.1 °C. Chang et al. (2015) introduced a new idea which can generate proppants in-situ by converting injected fracturing fluid to discrete solid particles. This new fracturing fluid is solids free and contains chemical precursors that will set into spherical particle beads deep within the reservoir to serve as proppants which keep the flow channels open and allow oil and gas to be easily transported into the well. Liang et al. (2017) prepared a new kind composite particle which could be utilized as ultra-light weight proppant via suspension polymerization. While the heat-treated composite particles have a higher glass transition temperature of 146.1 °C than that of pure polystyrene particles. However, a proppant that can be generated in the formation and effectively used for field fracturing is seldom published. Chen (2017) described a new type of self-propping fracturing technology utilizing supramolecular self-assembly mechanism. The new self-generating solid proppant is transformed by self-propping fracturing fluid in the fracture under the formation temperature stimulation. With this technology, it is unnecessary to pump solid proppants into the reservoir, which will reduce abrasion to the pumping equipments and tubulars, and effectively increase the transported distance in areas such as branch fractures and micro-fractures. Compared with other proppants, self-generating solid proppant has obvious advantages. For example, because self-generating solid proppant is formed in the formation fracture, highly viscous fracturing fluid is not required. What's more, self-generating solid proppant is suitable for the reservoir temperature ranging from approximately 80 °C–150 °C, and the pressure-resistant proppant beads can be formed within 60 min. Therefore, this paper focuses on the evaluation of physical property and fracture conductivity of self-generating solid proppant based on experimental methods.
Fig. 1. The formation of self-generating solid proppant in the reservoir fracture.
1990), obtaining self-generating solid proppant in the formation fracture, as shown in Fig. 1. 2.2. Liquid-solid transition experiment To study the physical property and fracture conductivity of this new proppant, self-generating solid proppant was made by liquid-solid transition experiment in laboratory at the temperature of 80 °C, stirring speed of 110 r/min, self-propping fracturing fluid and channel fracturing fluid mixing ratio of 1:2, and different monomer ratios of selfpropping fracturing fluid. Recording the time and temperature of initial phase transition, total phase transition, pressure-resistant beads forming. After 60 min, the self-generating solid proppants were taken out and then cooled, washed, filtered and dried. Experimental results show that only 13 min are needed to form pressure-resistant self-generating solid proppant. Experimental phenomena are shown in Table 1. By only adjusting the monomer ratios of self-propping fracturing fluid, 9 kinds of self-generating solid proppants were obtained in two batches. The first batch proppants were numbered respectively as S901, S802, S703, S604 and S505, and the numbers of the second batch proppants were S101, S102, S103 and S104.
2. Self-generating solid proppant working mechanism 2.1. Self-propping fracturing technology and liquid-solid transition mechanism Self-propping fracturing technology is defined as injecting immiscible self-propping fracturing fluid and channel fracturing fluid to the formation fractures. Under the formation temperature stimulation, self-propping fracturing fluid transforms to the self-generating solid proppant in the fracture, realizing effective support to fracture surface. On the other hand, channel fracturing fluid plays a role of occupying flow channels without response to external stimuli. After fracturing, it flows back to surface and leaves high conductivity oil and gas flow channels. This technology completely avoids sand carrying issues of traditional hydraulic fracturing technology. During the whole fracturing process, only fluids will be injected and no solid proppants will be carried. Self-propping fracturing fluid is much easier to delivery in the fracture networks than solid proppant, so as to maximize the effective propping area of fractures and the volume of reservoir reconstruction. Self-propping fracturing fluid is a supramolecular fracturing fluid based on supramolecular self-assembly mechanism (Zhao et al., 2018). Lehn (1988) first proposed the concept of “supramolecular chemistry”, and supramolecular self-assembly mechanism works via non-covalent intermolecular interactions which mainly embody hydrogen bonds (Terech and Weiss, 1997; Paramonov et al., 2006), solvophobic forces (Meazza et al., 2013), donor acceptor (Wang and Hao, 2011), π-π stacking (Ahn et al., 2013), and metal-ligand coordination (Brinker and Scherer, 1990). Because of the dynamic non-covalent interaction between moleculars, self-propping fracturing fluid transition can be easily controlled by formation temperature stimulation (Rieck and Radford,
3. Physical property measurements of self-generating solid proppant The physical properties of self-generating solid proppant differ greatly from those of conventional proppants, influencing fracture conductivity and fracturing effect. To select self-generating solid proppant with excellent performance, the particle size distribution, roundness, sphericity, bulk density, apparent density, crushing rate were measured and compared among self-generating solid proppant, ceramsite and quartz sand with reference to SY/T standard (National Energy Board, 2015). The experimental results are shown in Table 2. As the representatives of low and high crushing rate, S505, S101 and S103 were observed before and after the crushing pressure 55 MPa by stereomicroscope. Their shape changes are shown in Fig. 2. Experimental results show the particle size distribution of the selfgenerating solid proppant is mainly concentrated at 20/40 mesh. The roundness and sphericity of them are 0.9 which are higher than SY/T standard (National Energy Board, 2015). The bulk densities range from 0.62 g/cm3 to 0.67 g/cm3 and apparent densities vary from 1.02 g/cm3 to 1.12 g/cm3 for all self-generating solid proppants. Both two densities are much lower than those of ceramsite and quartz sand, and even lower than those of ultralightweight proppant (Rickards et al., 2006). Except S103, the crushing rate of samples under the pressure of 55 MPa are lower than SY/T standard (National Energy Board, 2015). 842
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Table 1 Phase transition phenomena of self-propping fracturing fluid at 80 °C. Heating time, min
System temperature, °C
Experimental phenomena
2 5 6 13 60
38 60 71 88 80
Self-propping fracturing fluid was evenly distributed in channel fracturing fluid as spherical droplets. The viscosity of self-propping fracturing fluid increased, and partial spherical solid beads were formed. Self-propping fracturing fluid droplets were transformed to spherical solid beads competely and lost their fluidity. The hardness of solid beads increased, forming pressure-resistant self-generating solid proppant. The hardness of self-generating solid proppant increased continuously, and the sample were taken out.
(4) Experimental parameters: the closure stress, flow rate and the time to reach semisteady-state of each particle size under the closure stress as shown in Table 3.
According to the shape observations of S505, S101, S103, the surface of spherical beads is smooth. After 55 MPa, the shapes change a little, proving the material has a high strength. The self-generating solid proppant has the characteristics of large particle size, good sphericity, smooth surface, low density, high strength. These physical properties are obviously superior to most existing proppants. However, fracture conductivity is the key index to evaluate the performance of proppant, which plays a vital role in predicting fracturing effect and optimizing fracturing design. So next works have to test fracture conductivity of 9 kinds of self-generating solid proppants, purposing to select the high conductivity propping materials.
4.3. Experimental procedures (1) The self-generating solid proppant was weighed and filled into the flow diversion chamber. (2) The flow diversion chamber was installed on the conductivity measuring instrument, and the original width of proppant pack was measured. (3) The maximum value of pressure protection of the advection pump was set at 1.0 MPa. The target pressure values of conductivity measuring were set at 6.9, 13.8, 27.6, 41.4, 55.2, 69, 82.7 and 96.5 MPa. Usually, the initial value of the differential pressure transmitter was 3.90–4.00 mA. When the indication of differential pressure transmitter was at least 4.0 mA, the displacement could be performed normally. (4) The outlet flow rate, left and right widths of proppant pack, differential pressure transmitter readings under each testing pressure were recorded and fracture conductivity values were calculated.
4. Fracture conductivity experiments and data processing methods 4.1. Experimental assumptions and purposes Under the assumptions that there were no issues of particle migration and blockage in the flow diversion chamber, the formation temperature, rock hardness, proppant embedding, downhole fluid are not considered. Conductivity experiments of self-generating solid proppant were carried out according to SY/T standard (China National Petroleum Corporation, 2009) to prefer to select the high conductivity proppants, optimizing the field treatment parameters.
4.4. Data processing method of fracture conductivity testing According to Darcy's law, fracture conductivity of self-generating solid proppant was tested by using a flow diversion chamber of API standard. The width of chamber is 3.81 cm, and the length between two pressure measuring holes is 12.70 cm. So the area of proppant pack is 64.5 cm2. These above data are taken into Darcy formula to correct fracture conductivity of proppant pack, obtaining the corrected formula (1) as follows.
4.2. Experimental conditions (1) Experimental environment: At the temperature of 24 °C (Although this temperature differs from formation real temperature, the influence on the results of fracture conductivity is in the field permissible range), distilled water is used as the test liquid. (2) Experimental samples: self-generating solid proppants of S901, S802, S604, S703, S505, S101, S102, S103, S104, ceramsite used for fracturing and quartz sand used for fracturing. (3) Experimental apparatus: LD-1A conductivity measuring instrument (Fig. 3a), advection pump (Fig. 3b), differential pressure transmitter, electronic balance, stopwatch, vernier caliper, measuring vessel.
kWf =
5.555uQ Δp
(1)
Due to the pressure difference △p couldn't be tested directly, the mechanical signal of water pressure should be transmitted to the current signal to monitor by pressure transmitter. There is a linear relationship between current signal and current size of the pressure transmitter as shown in formula (2).
Table 2 Experimental results and comparisons of physical properties among self-generating solid proppants, ceramsite and quartz sand. Sample
Main particle size distribution, Mesh
Roundness
Sphericity
Bulk density, g/cm3
apparent density, g/cm3
Crushing rate under 55 MPa, %
Crushing ratio less than ceramsite, %
Crushing ratio less than quartz sand, %
S901 S802 S703 S604 S505 S101 S102 S103 S104 Ceramsite Quartz sand SY/T Standard
20/40 20/40 20/40 20/40 20/40 20/40 20/40 20/40 20/40 20/40 20/40 –
0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.7 ≥0.7
0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.7 ≥0.7
0.66 0.67 0.67 0.65 0.65 0.63 0.63 0.63 0.62 1.47 1.56 –
1.11 1.12 1.12 1.10 1.10 1.09 1.12 1.03 1.02 2.71 2.62 –
2.94 8.49 5.43 1.49 1.29 1.47 0.45 11.07 7.86 23.38 29.93 ≤9%
87.43 63.69 76.78 93.63 94.48 93.71 98.08 52.65 66.38 – – –
90.18 71.63 81.86 95.02 95.69 95.09 98.50 63.01 73.74 21.88 – –
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Fig. 2. Stereomicroscope observation of 20/40 mesh original samples-samples after the crushing pressure of 55 MPa-fragnents of S505, S101, S103.
Δp =
(I − Imin)pmax In
Table 3 The combinations with closure stress, flow rate and the time to reach semisteady-state of each particle size under the closure stress.
(2)
In order to measure fracture conductivity conveniently and accurately, the outflow liquid quality was weighed every 2 min. By bring formula (2) into formula (1), the simplified formulas for calculating the permeability and conductivity of the self-generating solid proppant pack are shown as formulas (3) and (4).
k=
6.4499μm ρ (I − Imin)Wf
(3)
6.4499μm ρ (I − Imin)
(4)
kWf =
Closure stress, MPa (psi)
Flow rate combination, cm3/ min
The time to reach semisteady-state under the closure stress, h
6.9 (1000) 13.8 (2000) 27.6 (4000) 41.4 (6000) 55.2 (8000) 69 (10000) 82.7 (12000) 96.5 (14000)
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
In the formula: k is proppant pack permeability, μm2; Wf is proppant pack width, cm; kWf is fracture conductivity, μm2·cm; ρ is test liquid
Fig. 3. The experimental apparatuses. 844
5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.5
10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
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density, g/cm3; I is real-time current value of ammeter, mA; Imin is the minimum output signal of ammeter, 4 mA; In is measure range of output signal of pressure transmitter, 16 mA; Pmax is calibration range of pressure transmitter, 6.89 kPa; μ is test liquid viscosity, mPa·s; m is the outflow liquid quality every 2 min, g/2min; Q is test liquid flow rate, cm3/min; △p is the pressure difference of liquid import and export, kPa.
However the large particle beads still have a fracture conductivity of around 30 μm2 cm. 5.1.3. Different propping concentrations Taking S101 and S505 as research objects, this group experiment was used to test their fracture conductivity with the same particle size distribution and different propping concentrations. Selecting the particle size of 6/20 mesh of S101, the propping concentrations were 0.33 kg/m2 (partial single-layer propping), 0.68 kg/m2 (single-layer propping), 1 kg/m2 (double-layer staggered propping) and 5 kg/m2 (multi-layer propping) respectively. Similarly, the beads size of S505 was used 20/40 mesh with the propping concentration of 0.23 kg/m2 (partial single-layer propping), 0.4 kg/m2 (single-layer propping), 1 kg/ m2 (double-layer staggered propping) and 5 kg/m2 (multi-layer propping) respectively. Experimental results of fracture conductivity of S101 and S505 with different propping concentrations are shown in Fig. 6. With the increase of propping concentration under the uniform propping conditions, the fracture conductivities of S101 and S505 become higher. However, fracture conductivity of partial single-layer propping concentration is obviously higher than that of single-layer propping concentration. Fig. 7 shows the single-layer propping morphology of the self-generating solid proppant, so the partial single-layer propping is distributed unevenly in the flow diversion chamber. Because there is a large gap between adjacent particles under partial single-layer propping condition, the flow channels of high permeability are formed. The higher the permeability is, the larger the fracture conductivity becomes. On the other hand, non-uniform distribution will reduce the effective propping to the fractures, which is determined by the ratio of selfpropping fracturing fluid and channel fracturing fluid. Hence, the injecting ratio of fracturing fluid can be optimized to form non-uniform proppant pack as well as ensure the propping quality to the fractures.
5. Results and analysis of fracture conductivity experiments 5.1. Experimental data analysis of fracture conductivity testing 5.1.1. The same particle size of different self-generating solid proppants This group experiment tested fracture conductivity of the two batches of self-generating solid proppants S901, S802, S703, S604, S505 and S101, S102, S103, S104. The particle size of all proppants was chosen 20/40 mesh and the propping concentration (Because the selfgenerating solid proppant discussed in this paper is different from conventional proppants, the concept of propping concentration is proposed. To distinguish from the filling concentration of conventional proppants, propping concentration is defined as the quality of selfgenerating solid proppant generated on unit area of fracture) was 5 kg/ m2. The experimental data of fracture conductivity under different closure stresses are fitted, obtaining the curves of fracture conductivity shown in Fig. 4. Under the same conditions, fracture conductivity of all self-generating solid proppants decrease with the increase of closure stress. The results show that fracture conductivity of S505 is obviously higher than that of the others in the first batch, and fracture conductivity of S101 is higher than that of the others in the second batch. At the closure stress of 55.2 MPa, fracture conductivity of S505 is even 35 μm2·cm. The self-generating solid proppants S505 and S101 are selected preliminarily.
5.1.4. Relative long-term fracture conductivity In this experiment, self-generating solid proppant S101 was selected with propping concentration of 5 kg/m2. The experiment was carried out under the closure stress of 55.2 MPa. As shown in Fig. 8, fracture conductivity of S101 decreased with the prolonging of time under 55.2 MPa. At the first 30 min, it reduced rapidly, and then cut down gradually. After 110 min, fracture conductivity of S101 still remained 46.03 μm2 cm with the crushing rate of 7.90%. Therefore, S101 has the advantages of low crushing rate, high strength, good relative long-term fracture conductivity.
5.1.2. Different particle sizes of the same proppant Under the propping concentration of 5 kg/m2, fracture conductivity of different particle sizes of the same proppant were compared, including S101 and S505. The particle sizes were chosen with 6/20, 20/ 40, 40/70 mesh. As shown in Fig. 5, the larger the particle size, the higher the fracture conductivity. The reason is large partical size in the fracture can increase the flow channels. However, the particle size of self-generating solid proppant is determined by the size of the droplet of selfpropping fracturing fluid. Additionally, droplet size is influenced by the pump rate. The faster the fracturing fluid injects, the smaller the particle size is. Therefore, the pump rate of fracturing fluid can be optimized based on the particle size, increasing fracture conductivity. Under high closure pressure, the self-generating solid proppants are crushed or deformed, and the gap between particles is gradually filled.
5.1.5. Different proppant types This group tested fracture conductivity of the same particle size of different proppant types. The proppants, including S101, S505, ceramsite (used for fracturing) and quartz sand (used for fracturing), were
Fig. 4. Fracture conductivity curves of different self-generating solid proppans at the same particle size of 20/40 mesh. 845
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Fig. 5. Fracture conductivity curves of different particle sizes of the self-generating solid proppants S101 and S505.
Fig. 6. Experimental results of fracture conductivity of S101 and S505 with different propping concentrations.
Fig. 7. The single-layer propping morphology of the self-generating solid proppant in the flow diversion chamber.
considered and improved next.
used in the experiment. The size of beads was 20/40 mesh, and the propping concentration was 5 kg/m2. Fig. 9 shows that fracture conductivities of self-generating solid proppants S101 and S505 are obviously higher than that of quartz sand, and even higher than that of ceramsite under the low closure stresses. Compared with the conventional proppants, the self-generating solid proppants still have high fracture conductivities.
(1) Fracture conductivity of self-generating solid proppant decreases rapidly with the increase of the closure stress. When self-generating solid proppant is taken out after fracture conductivity testing, partial deformation of proppant is discovered especially at the high closure stresses. Fortunately, phase transition curing agent and modifier have been developed successfully now which can greatly short the transition time and increase the strength of self-generating solid proppant. (2) If the assumptions such as particle migration and proppant embedding are considered, partial single-layer propping concentration may not be so suitable. The less the amount of self-generating solid proppant, the more the closure stress on each bead. As a result, the propping concentration should be chosen according to the multiple
6. Results and discussions Based on fracture conductivity experiments of particle sizes, propping concentrations, long-term pressure resistence of the proppant pack and different proppant types, S101 and S505 of self-generating solid proppants exhibit excellent fracture conductivities, which predict very encouraging fracturing effects. However, some problems remain to be 846
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smooth surface, high sphericity, low density and high strength, which are obviously superior to conventional proppants. (2) According to the analysis on the key parameter of fracture conductivity, fracture conductivities of S101 and S505 are higher than that of other self-generating solid proppants, quartz sand and ceramsite (at the low closure stresses). The larger the particle size, the higher the fracture conductivity. Non-uniform distribution of proppant pack can increase fracture conductivity. Based on the experimental results, the pump rate and injection proportion of selfpropping fracturing fluid can be optimized during the fracturing process. (3) The innovation of this paper is that self-generating solid proppant is formed by liquid-solid transition of self-propping fracturing fluid in the formation fracture. This new proppant has excellent performance and high fracture conductivity, providing a new idea for fracturing and acidizing direction in oil and gas industry. Acknowledgments Fig. 8. Relative long-term fracture conductivity of S101 under the closure stress of 55.2 MPa.
This work was supported by the joint scientific research project from PetroChina Huabei Oilfield Company and Southwest Petroleum University (China) (Grant HBYT-CYY-2016-JS-144). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.03.013. References Ahn, J., Park, S., Lee, J.H., et al., 2013. Fluorescent hydrogels formed by CH-π and π-π interactions as the main driving forces: an approach toward understanding the relationship between fluorescence and structure. Chem. Commun. 49 (21), 2109–2111. Barati, R., Liang, J.T., 2014. A review of fracturing fluid systems used for hydraulic fracturing of oil and gas wells. J. Appl. Polym. Sci. 131 (16), 318–323. Brinker, C.J., Scherer, G.W., 1990. Sol-gel Science : the Physics and Chemistry of Sol-Gel Processing. Academic Press. Chang, F., Berger, P.D., Lee, C., 2015. In-situ formation of proppant and highly permeable blocks for hydraulic fracturing. In: SPE Hydraulic Fracturing Technology Conference. Chang, O., Dilmore, R., Wang, J.Y., 2017. Model development of proppant transport through hydraulic fracture network and parametric study. J. Pet. Sci. Eng. 150, 224–237. Chen, Y.X., 2017. Experimental Study on a New Type of Self-Propping Fracturing Technology. Southwest Petroleum University. China National Petroleum Corporation, 2009. Recommended Practices for Evaluating Short Term Proppant Pack Conductivity: SY/T 6302-2009. Petroleum Industry Press, Beijing. Dmour, H.N., 2013. Productivity index enhancement of stimulated gas wells through hydraulic fracturing. Liq. Fuel. Technol. 31 (3), 225–236. Du, H.L., Zhang, W., M, F., et al., 2017. Research progress of hydraulic fracturing proppants. Bull. Chin. Ceram. Soc. 36 (08), 2625–2630. Gandossi, L., Von Estorff, U., 2015. An overview of hydraulic fracturing and other formation stimulation technologies for shale gas production. EUR 26347. https://doi. org/10.2790/379646. He, Y.W., Cheng, S.Q., Li, S., et al., 2017. A semianalytical methodology to diagnose the locations of underperforming hydraulic fractures through pressure-transient analysis in tight gas reservoir. SPE J. 22 (03), 924–939. Himes, R.E., Dalrymple, E.D., Dahl, J.A., et al., 1994. Injecting to Stimulate Subterranean Formation; Includes a Proppant Material; Hardening. US. US 5358047 A. Lehn, J.M., 1988. Supramolecular chemistry — scope and perspectives: molecules — supermolecules — molecular devices. J. Inclusion Phenom. 6 (4), 351–396. Liang, T., Yan, C., Zhou, S., et al., 2017. Silica fume reinforced polystyrene-based composite particles used as ultra-light weight proppants in hydraulic fracturing. Mater. Res. Express 4 (11). Liu, R.J., Zhang, J.T., Yin, C.B., et al., 2003. The Current Situation and Development Trend of Hydraulic Fracturing Proppants. Drilling & Production Technology. Malone, W.T., Williams, J.R., Derby, J.A., 1968. Method of Fracturing and Propping a Subterranean Formation. US. US3366178. Meazza, L., Foster, J.A., Fucke, K., et al., 2013. Halogen-bonding-triggered supramolecular gel formation. Nat. Chem. 5 (1), 42–47. National Energy Board, 2015. Measurement of Properties of Proppants Used in Hydraulic Fracturing and Gravel-Packing Operations: SY/T 5108-2014. Petroleum Industry Press, Beijing. Nguyen, P.D., Weaver, J.D., Mccabe, M.A., et al., 2004. Methods and Compositions for Consolidating Proppant in Subterranean Fractures. US. US6729404. Paramonov, S.E., Jun, H.W., Hartgerink, J.D., 2006. Self-assembly of peptide-amphiphile
Fig. 9. Fracture conductivity of different proppant types.
factors of reservoir. (3) On the one hand, the storage conditions of self-propping fracturing fluid are relatively strict, which must maintain liquid state on the surface. Hence, self-propping fracturing fluid can not be exposed in the extremely hot climate for a long time. On the other hand, the cost of self-propping fracturing fluid is a little high currently. Once the large-scale fracturing treatment is formed, the cost can effectively controlled. 7. Conclusions By the evaluation on physical property and fracture conductivity, the self-generating solid proppants, including S101 and S505, are selected firstly because of their excellent performance. The conclusions are as follows. (1) Based on the physical property measurements, the particle size of S101 and S505 are mainly 20/40 mesh. The roundness and sphericity are both 0.9. The bulk density is 0.63 g/cm3 for S101 and 0.65 g/cm3 for S505. The apparent density of S101 is 1.09 g/cm3 while that of S505 is 1.10%. Under the closure stress of 55.2 MPa, the crushing rate is 1.47% for S101 and 1.29% for S505. Hence, S101 and S505 have the characteristics of large particle size, 847
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nanofibers: the roles of hydrogen bonding and amphiphilic packing. J. Am. Chem. Soc. 128 (22), 7291–7298. Rickards, A.R., Brannon, H.D., Wood, W.D., et al., 2006. High strength, ultralightweight proppant lends new dimensions to hydraulic fracturing applications. SPE Prod. Oper. 21 (2), 212–221. Rieck, I., Radford, W., 1990. The benefit and use of phase change thermal storage in buildings in the antarctic environment. In: The First Pacific/Asia Offshore Mechanics Symposium, pp. 24–28 June, Seoul, Korea. Tan, M., Zhang, S., Zhang, C., et al., 2006. Application study of hydraulic fracturing technology in tight sandstone reservoir of suining formation with ultra-low permeability in luodai gas field. Nat. Gas. Ind. 26 (4), 89–91. Terech, P., Weiss, R.G., 1997. Chem. Rev. 97, 3133–3160.
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The evaluation on physical property and fracture conductivity of a new selfgenerating solid proppant
T
Chengcheng Zhanga, Liqiang Zhaoa,∗, Donghe Yub, Guohua Liub, Yuxin Peic, Fushan Huanga, Biao Liud a
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, PR China PetroChina Huabei Oilfield Company, Hebei, 062550, PR China c Research Institute of Petroleum Engineering, PetroChina Dagang Oilfield Company, Tianjin, 300280, PR China d Southwest Petroleum University, Chengdu, 610500, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Self-generating solid proppant Self-propping fracturing fluid Physical property Fracture conductivity
Most fracturing treatments today require injecting solid proppants to keep hydraulic fractures open. However, traditional proppants have disadvantages such as causing the reduced propped-fracture volume and abrasion to the pumping equipments and tubulars. To solve these problems, a new self-generating solid proppant has been developed through liquid-solid transition of self-propping fracturing fluid under reservoir conditions. To select the best self-generating solid proppant and compare it with conventional ones, physical property and fracture conductivity tests were done. Experimental results show that two self-generating solid proppants, including S101 and S505, are firstly selected because of their low density, high strength and excellent fracture conductivity. The apparent density is 1.09 g/cm3 for S101 and 1.10 g/cm3 for S505. The crushing rate of S101 is 1.47% while that of S505 is 1.29%. These data illustrate physical properties of self-generating solid proppant are obviously better than those of conventional proppants. Under the same conditions, the larger the spherical particle size of S101 and S505, the higher the fracture conductivity. Non-uniform distribution of proppant pack can increase fracture conductivity. Fracture conductivities of S101 and S505 are higher than that of quartz sand, and even higher than that of ceramsite at low closure stresses. Based on the experimental results, the pumping rate and injecting proportion of self-propping fracturing fluid can be optimized during the fracturing process.
1. Introduction As the major stimulation treatment, hydraulic fracturing technology is widely used in the low permeability reservoir, realizing the increase of oil and gas production (Tan et al., 2006; Gandossi and Von Estorff, 2015; He et al., 2017). Currently, hydraulic fracturing processes require pumping sand-carrying fluid mixed with solid proppant into the fractures, reaching the purpose of propping formation fractures (Rickards et al., 2006; Dmour, 2013; Chang et al., 2017). However, injecting solid proppants presents several challenges and disadvantages. Firstly, highdensity solid proppant requires highly viscous fracturing fluid, which could leave gel residue to reduce fracture permeability and conductivity (Barati and Liang, 2014). Secondly, the process of sand-carrying fracturing is complex. High displacement and pump pressure accompanied by high sand filling have a serious abrasion to wellhead, pumping equipments and tubulars (Yu et al., 2018). Moreover, it is uneasy for solid proppants to enter the complex fracture networks such as branch
∗
fractures and micro-fractures. This limits the transported distance and reduces fracture conductivity (Weijers et al., 2000). Lastly, pumping solid proppant easily causes sand plug, which has potential risks of field operation and personnel safety (Yuan et al., 2016). Proppants are the core of hydraulic fracturing technology, which play a key role in fracture conductivity (Liu et al., 2003). Conventional proppants can be broadly divided into three types including quartz sand, artificial ceramsite and resin-coated sand (Du et al., 2017). Several attempts have been made to generate solid proppant in the formation. Malone et al. (1968) described a method of fracturing and propping a subterranean formation using a liquid hardenable carrier composition. The liquid composition hardens to form a propping porous material capable of conducting fluids, whose permeability can reach 10–30 darcies or larger. Himes et al. (1994) presented a method of stimulating a subterranean formation using a foamed cement composition. When hardened, the cement composition has a permeability of at least about 0.3 darcies. Nguyen et al. (2004) improved methods and
Corresponding author. E-mail address: [email protected] (L. Zhao).
https://doi.org/10.1016/j.petrol.2019.03.013 Received 24 September 2018; Received in revised form 18 February 2019; Accepted 4 March 2019 Available online 07 March 2019 0920-4105/ © 2019 Elsevier B.V. All rights reserved.
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compositions for coating proppant particles with a hardenable resin composition. Suspending the coated proppant particles in a fracturing fluid and consolidating the proppant particles after being placed in fractures into permeable masses are provided. These methods and compositions are especially suitable for low temperature well, specifically those around the 15.6–121.1 °C. Chang et al. (2015) introduced a new idea which can generate proppants in-situ by converting injected fracturing fluid to discrete solid particles. This new fracturing fluid is solids free and contains chemical precursors that will set into spherical particle beads deep within the reservoir to serve as proppants which keep the flow channels open and allow oil and gas to be easily transported into the well. Liang et al. (2017) prepared a new kind composite particle which could be utilized as ultra-light weight proppant via suspension polymerization. While the heat-treated composite particles have a higher glass transition temperature of 146.1 °C than that of pure polystyrene particles. However, a proppant that can be generated in the formation and effectively used for field fracturing is seldom published. Chen (2017) described a new type of self-propping fracturing technology utilizing supramolecular self-assembly mechanism. The new self-generating solid proppant is transformed by self-propping fracturing fluid in the fracture under the formation temperature stimulation. With this technology, it is unnecessary to pump solid proppants into the reservoir, which will reduce abrasion to the pumping equipments and tubulars, and effectively increase the transported distance in areas such as branch fractures and micro-fractures. Compared with other proppants, self-generating solid proppant has obvious advantages. For example, because self-generating solid proppant is formed in the formation fracture, highly viscous fracturing fluid is not required. What's more, self-generating solid proppant is suitable for the reservoir temperature ranging from approximately 80 °C–150 °C, and the pressure-resistant proppant beads can be formed within 60 min. Therefore, this paper focuses on the evaluation of physical property and fracture conductivity of self-generating solid proppant based on experimental methods.
Fig. 1. The formation of self-generating solid proppant in the reservoir fracture.
1990), obtaining self-generating solid proppant in the formation fracture, as shown in Fig. 1. 2.2. Liquid-solid transition experiment To study the physical property and fracture conductivity of this new proppant, self-generating solid proppant was made by liquid-solid transition experiment in laboratory at the temperature of 80 °C, stirring speed of 110 r/min, self-propping fracturing fluid and channel fracturing fluid mixing ratio of 1:2, and different monomer ratios of selfpropping fracturing fluid. Recording the time and temperature of initial phase transition, total phase transition, pressure-resistant beads forming. After 60 min, the self-generating solid proppants were taken out and then cooled, washed, filtered and dried. Experimental results show that only 13 min are needed to form pressure-resistant self-generating solid proppant. Experimental phenomena are shown in Table 1. By only adjusting the monomer ratios of self-propping fracturing fluid, 9 kinds of self-generating solid proppants were obtained in two batches. The first batch proppants were numbered respectively as S901, S802, S703, S604 and S505, and the numbers of the second batch proppants were S101, S102, S103 and S104.
2. Self-generating solid proppant working mechanism 2.1. Self-propping fracturing technology and liquid-solid transition mechanism Self-propping fracturing technology is defined as injecting immiscible self-propping fracturing fluid and channel fracturing fluid to the formation fractures. Under the formation temperature stimulation, self-propping fracturing fluid transforms to the self-generating solid proppant in the fracture, realizing effective support to fracture surface. On the other hand, channel fracturing fluid plays a role of occupying flow channels without response to external stimuli. After fracturing, it flows back to surface and leaves high conductivity oil and gas flow channels. This technology completely avoids sand carrying issues of traditional hydraulic fracturing technology. During the whole fracturing process, only fluids will be injected and no solid proppants will be carried. Self-propping fracturing fluid is much easier to delivery in the fracture networks than solid proppant, so as to maximize the effective propping area of fractures and the volume of reservoir reconstruction. Self-propping fracturing fluid is a supramolecular fracturing fluid based on supramolecular self-assembly mechanism (Zhao et al., 2018). Lehn (1988) first proposed the concept of “supramolecular chemistry”, and supramolecular self-assembly mechanism works via non-covalent intermolecular interactions which mainly embody hydrogen bonds (Terech and Weiss, 1997; Paramonov et al., 2006), solvophobic forces (Meazza et al., 2013), donor acceptor (Wang and Hao, 2011), π-π stacking (Ahn et al., 2013), and metal-ligand coordination (Brinker and Scherer, 1990). Because of the dynamic non-covalent interaction between moleculars, self-propping fracturing fluid transition can be easily controlled by formation temperature stimulation (Rieck and Radford,
3. Physical property measurements of self-generating solid proppant The physical properties of self-generating solid proppant differ greatly from those of conventional proppants, influencing fracture conductivity and fracturing effect. To select self-generating solid proppant with excellent performance, the particle size distribution, roundness, sphericity, bulk density, apparent density, crushing rate were measured and compared among self-generating solid proppant, ceramsite and quartz sand with reference to SY/T standard (National Energy Board, 2015). The experimental results are shown in Table 2. As the representatives of low and high crushing rate, S505, S101 and S103 were observed before and after the crushing pressure 55 MPa by stereomicroscope. Their shape changes are shown in Fig. 2. Experimental results show the particle size distribution of the selfgenerating solid proppant is mainly concentrated at 20/40 mesh. The roundness and sphericity of them are 0.9 which are higher than SY/T standard (National Energy Board, 2015). The bulk densities range from 0.62 g/cm3 to 0.67 g/cm3 and apparent densities vary from 1.02 g/cm3 to 1.12 g/cm3 for all self-generating solid proppants. Both two densities are much lower than those of ceramsite and quartz sand, and even lower than those of ultralightweight proppant (Rickards et al., 2006). Except S103, the crushing rate of samples under the pressure of 55 MPa are lower than SY/T standard (National Energy Board, 2015). 842
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Table 1 Phase transition phenomena of self-propping fracturing fluid at 80 °C. Heating time, min
System temperature, °C
Experimental phenomena
2 5 6 13 60
38 60 71 88 80
Self-propping fracturing fluid was evenly distributed in channel fracturing fluid as spherical droplets. The viscosity of self-propping fracturing fluid increased, and partial spherical solid beads were formed. Self-propping fracturing fluid droplets were transformed to spherical solid beads competely and lost their fluidity. The hardness of solid beads increased, forming pressure-resistant self-generating solid proppant. The hardness of self-generating solid proppant increased continuously, and the sample were taken out.
(4) Experimental parameters: the closure stress, flow rate and the time to reach semisteady-state of each particle size under the closure stress as shown in Table 3.
According to the shape observations of S505, S101, S103, the surface of spherical beads is smooth. After 55 MPa, the shapes change a little, proving the material has a high strength. The self-generating solid proppant has the characteristics of large particle size, good sphericity, smooth surface, low density, high strength. These physical properties are obviously superior to most existing proppants. However, fracture conductivity is the key index to evaluate the performance of proppant, which plays a vital role in predicting fracturing effect and optimizing fracturing design. So next works have to test fracture conductivity of 9 kinds of self-generating solid proppants, purposing to select the high conductivity propping materials.
4.3. Experimental procedures (1) The self-generating solid proppant was weighed and filled into the flow diversion chamber. (2) The flow diversion chamber was installed on the conductivity measuring instrument, and the original width of proppant pack was measured. (3) The maximum value of pressure protection of the advection pump was set at 1.0 MPa. The target pressure values of conductivity measuring were set at 6.9, 13.8, 27.6, 41.4, 55.2, 69, 82.7 and 96.5 MPa. Usually, the initial value of the differential pressure transmitter was 3.90–4.00 mA. When the indication of differential pressure transmitter was at least 4.0 mA, the displacement could be performed normally. (4) The outlet flow rate, left and right widths of proppant pack, differential pressure transmitter readings under each testing pressure were recorded and fracture conductivity values were calculated.
4. Fracture conductivity experiments and data processing methods 4.1. Experimental assumptions and purposes Under the assumptions that there were no issues of particle migration and blockage in the flow diversion chamber, the formation temperature, rock hardness, proppant embedding, downhole fluid are not considered. Conductivity experiments of self-generating solid proppant were carried out according to SY/T standard (China National Petroleum Corporation, 2009) to prefer to select the high conductivity proppants, optimizing the field treatment parameters.
4.4. Data processing method of fracture conductivity testing According to Darcy's law, fracture conductivity of self-generating solid proppant was tested by using a flow diversion chamber of API standard. The width of chamber is 3.81 cm, and the length between two pressure measuring holes is 12.70 cm. So the area of proppant pack is 64.5 cm2. These above data are taken into Darcy formula to correct fracture conductivity of proppant pack, obtaining the corrected formula (1) as follows.
4.2. Experimental conditions (1) Experimental environment: At the temperature of 24 °C (Although this temperature differs from formation real temperature, the influence on the results of fracture conductivity is in the field permissible range), distilled water is used as the test liquid. (2) Experimental samples: self-generating solid proppants of S901, S802, S604, S703, S505, S101, S102, S103, S104, ceramsite used for fracturing and quartz sand used for fracturing. (3) Experimental apparatus: LD-1A conductivity measuring instrument (Fig. 3a), advection pump (Fig. 3b), differential pressure transmitter, electronic balance, stopwatch, vernier caliper, measuring vessel.
kWf =
5.555uQ Δp
(1)
Due to the pressure difference △p couldn't be tested directly, the mechanical signal of water pressure should be transmitted to the current signal to monitor by pressure transmitter. There is a linear relationship between current signal and current size of the pressure transmitter as shown in formula (2).
Table 2 Experimental results and comparisons of physical properties among self-generating solid proppants, ceramsite and quartz sand. Sample
Main particle size distribution, Mesh
Roundness
Sphericity
Bulk density, g/cm3
apparent density, g/cm3
Crushing rate under 55 MPa, %
Crushing ratio less than ceramsite, %
Crushing ratio less than quartz sand, %
S901 S802 S703 S604 S505 S101 S102 S103 S104 Ceramsite Quartz sand SY/T Standard
20/40 20/40 20/40 20/40 20/40 20/40 20/40 20/40 20/40 20/40 20/40 –
0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.7 ≥0.7
0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.8 0.7 ≥0.7
0.66 0.67 0.67 0.65 0.65 0.63 0.63 0.63 0.62 1.47 1.56 –
1.11 1.12 1.12 1.10 1.10 1.09 1.12 1.03 1.02 2.71 2.62 –
2.94 8.49 5.43 1.49 1.29 1.47 0.45 11.07 7.86 23.38 29.93 ≤9%
87.43 63.69 76.78 93.63 94.48 93.71 98.08 52.65 66.38 – – –
90.18 71.63 81.86 95.02 95.69 95.09 98.50 63.01 73.74 21.88 – –
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Fig. 2. Stereomicroscope observation of 20/40 mesh original samples-samples after the crushing pressure of 55 MPa-fragnents of S505, S101, S103.
Δp =
(I − Imin)pmax In
Table 3 The combinations with closure stress, flow rate and the time to reach semisteady-state of each particle size under the closure stress.
(2)
In order to measure fracture conductivity conveniently and accurately, the outflow liquid quality was weighed every 2 min. By bring formula (2) into formula (1), the simplified formulas for calculating the permeability and conductivity of the self-generating solid proppant pack are shown as formulas (3) and (4).
k=
6.4499μm ρ (I − Imin)Wf
(3)
6.4499μm ρ (I − Imin)
(4)
kWf =
Closure stress, MPa (psi)
Flow rate combination, cm3/ min
The time to reach semisteady-state under the closure stress, h
6.9 (1000) 13.8 (2000) 27.6 (4000) 41.4 (6000) 55.2 (8000) 69 (10000) 82.7 (12000) 96.5 (14000)
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25
In the formula: k is proppant pack permeability, μm2; Wf is proppant pack width, cm; kWf is fracture conductivity, μm2·cm; ρ is test liquid
Fig. 3. The experimental apparatuses. 844
5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.5
10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0
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density, g/cm3; I is real-time current value of ammeter, mA; Imin is the minimum output signal of ammeter, 4 mA; In is measure range of output signal of pressure transmitter, 16 mA; Pmax is calibration range of pressure transmitter, 6.89 kPa; μ is test liquid viscosity, mPa·s; m is the outflow liquid quality every 2 min, g/2min; Q is test liquid flow rate, cm3/min; △p is the pressure difference of liquid import and export, kPa.
However the large particle beads still have a fracture conductivity of around 30 μm2 cm. 5.1.3. Different propping concentrations Taking S101 and S505 as research objects, this group experiment was used to test their fracture conductivity with the same particle size distribution and different propping concentrations. Selecting the particle size of 6/20 mesh of S101, the propping concentrations were 0.33 kg/m2 (partial single-layer propping), 0.68 kg/m2 (single-layer propping), 1 kg/m2 (double-layer staggered propping) and 5 kg/m2 (multi-layer propping) respectively. Similarly, the beads size of S505 was used 20/40 mesh with the propping concentration of 0.23 kg/m2 (partial single-layer propping), 0.4 kg/m2 (single-layer propping), 1 kg/ m2 (double-layer staggered propping) and 5 kg/m2 (multi-layer propping) respectively. Experimental results of fracture conductivity of S101 and S505 with different propping concentrations are shown in Fig. 6. With the increase of propping concentration under the uniform propping conditions, the fracture conductivities of S101 and S505 become higher. However, fracture conductivity of partial single-layer propping concentration is obviously higher than that of single-layer propping concentration. Fig. 7 shows the single-layer propping morphology of the self-generating solid proppant, so the partial single-layer propping is distributed unevenly in the flow diversion chamber. Because there is a large gap between adjacent particles under partial single-layer propping condition, the flow channels of high permeability are formed. The higher the permeability is, the larger the fracture conductivity becomes. On the other hand, non-uniform distribution will reduce the effective propping to the fractures, which is determined by the ratio of selfpropping fracturing fluid and channel fracturing fluid. Hence, the injecting ratio of fracturing fluid can be optimized to form non-uniform proppant pack as well as ensure the propping quality to the fractures.
5. Results and analysis of fracture conductivity experiments 5.1. Experimental data analysis of fracture conductivity testing 5.1.1. The same particle size of different self-generating solid proppants This group experiment tested fracture conductivity of the two batches of self-generating solid proppants S901, S802, S703, S604, S505 and S101, S102, S103, S104. The particle size of all proppants was chosen 20/40 mesh and the propping concentration (Because the selfgenerating solid proppant discussed in this paper is different from conventional proppants, the concept of propping concentration is proposed. To distinguish from the filling concentration of conventional proppants, propping concentration is defined as the quality of selfgenerating solid proppant generated on unit area of fracture) was 5 kg/ m2. The experimental data of fracture conductivity under different closure stresses are fitted, obtaining the curves of fracture conductivity shown in Fig. 4. Under the same conditions, fracture conductivity of all self-generating solid proppants decrease with the increase of closure stress. The results show that fracture conductivity of S505 is obviously higher than that of the others in the first batch, and fracture conductivity of S101 is higher than that of the others in the second batch. At the closure stress of 55.2 MPa, fracture conductivity of S505 is even 35 μm2·cm. The self-generating solid proppants S505 and S101 are selected preliminarily.
5.1.4. Relative long-term fracture conductivity In this experiment, self-generating solid proppant S101 was selected with propping concentration of 5 kg/m2. The experiment was carried out under the closure stress of 55.2 MPa. As shown in Fig. 8, fracture conductivity of S101 decreased with the prolonging of time under 55.2 MPa. At the first 30 min, it reduced rapidly, and then cut down gradually. After 110 min, fracture conductivity of S101 still remained 46.03 μm2 cm with the crushing rate of 7.90%. Therefore, S101 has the advantages of low crushing rate, high strength, good relative long-term fracture conductivity.
5.1.2. Different particle sizes of the same proppant Under the propping concentration of 5 kg/m2, fracture conductivity of different particle sizes of the same proppant were compared, including S101 and S505. The particle sizes were chosen with 6/20, 20/ 40, 40/70 mesh. As shown in Fig. 5, the larger the particle size, the higher the fracture conductivity. The reason is large partical size in the fracture can increase the flow channels. However, the particle size of self-generating solid proppant is determined by the size of the droplet of selfpropping fracturing fluid. Additionally, droplet size is influenced by the pump rate. The faster the fracturing fluid injects, the smaller the particle size is. Therefore, the pump rate of fracturing fluid can be optimized based on the particle size, increasing fracture conductivity. Under high closure pressure, the self-generating solid proppants are crushed or deformed, and the gap between particles is gradually filled.
5.1.5. Different proppant types This group tested fracture conductivity of the same particle size of different proppant types. The proppants, including S101, S505, ceramsite (used for fracturing) and quartz sand (used for fracturing), were
Fig. 4. Fracture conductivity curves of different self-generating solid proppans at the same particle size of 20/40 mesh. 845
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Fig. 5. Fracture conductivity curves of different particle sizes of the self-generating solid proppants S101 and S505.
Fig. 6. Experimental results of fracture conductivity of S101 and S505 with different propping concentrations.
Fig. 7. The single-layer propping morphology of the self-generating solid proppant in the flow diversion chamber.
considered and improved next.
used in the experiment. The size of beads was 20/40 mesh, and the propping concentration was 5 kg/m2. Fig. 9 shows that fracture conductivities of self-generating solid proppants S101 and S505 are obviously higher than that of quartz sand, and even higher than that of ceramsite under the low closure stresses. Compared with the conventional proppants, the self-generating solid proppants still have high fracture conductivities.
(1) Fracture conductivity of self-generating solid proppant decreases rapidly with the increase of the closure stress. When self-generating solid proppant is taken out after fracture conductivity testing, partial deformation of proppant is discovered especially at the high closure stresses. Fortunately, phase transition curing agent and modifier have been developed successfully now which can greatly short the transition time and increase the strength of self-generating solid proppant. (2) If the assumptions such as particle migration and proppant embedding are considered, partial single-layer propping concentration may not be so suitable. The less the amount of self-generating solid proppant, the more the closure stress on each bead. As a result, the propping concentration should be chosen according to the multiple
6. Results and discussions Based on fracture conductivity experiments of particle sizes, propping concentrations, long-term pressure resistence of the proppant pack and different proppant types, S101 and S505 of self-generating solid proppants exhibit excellent fracture conductivities, which predict very encouraging fracturing effects. However, some problems remain to be 846
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smooth surface, high sphericity, low density and high strength, which are obviously superior to conventional proppants. (2) According to the analysis on the key parameter of fracture conductivity, fracture conductivities of S101 and S505 are higher than that of other self-generating solid proppants, quartz sand and ceramsite (at the low closure stresses). The larger the particle size, the higher the fracture conductivity. Non-uniform distribution of proppant pack can increase fracture conductivity. Based on the experimental results, the pump rate and injection proportion of selfpropping fracturing fluid can be optimized during the fracturing process. (3) The innovation of this paper is that self-generating solid proppant is formed by liquid-solid transition of self-propping fracturing fluid in the formation fracture. This new proppant has excellent performance and high fracture conductivity, providing a new idea for fracturing and acidizing direction in oil and gas industry. Acknowledgments Fig. 8. Relative long-term fracture conductivity of S101 under the closure stress of 55.2 MPa.
This work was supported by the joint scientific research project from PetroChina Huabei Oilfield Company and Southwest Petroleum University (China) (Grant HBYT-CYY-2016-JS-144). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.petrol.2019.03.013. References Ahn, J., Park, S., Lee, J.H., et al., 2013. Fluorescent hydrogels formed by CH-π and π-π interactions as the main driving forces: an approach toward understanding the relationship between fluorescence and structure. Chem. Commun. 49 (21), 2109–2111. Barati, R., Liang, J.T., 2014. A review of fracturing fluid systems used for hydraulic fracturing of oil and gas wells. J. Appl. Polym. Sci. 131 (16), 318–323. Brinker, C.J., Scherer, G.W., 1990. Sol-gel Science : the Physics and Chemistry of Sol-Gel Processing. Academic Press. Chang, F., Berger, P.D., Lee, C., 2015. In-situ formation of proppant and highly permeable blocks for hydraulic fracturing. In: SPE Hydraulic Fracturing Technology Conference. Chang, O., Dilmore, R., Wang, J.Y., 2017. Model development of proppant transport through hydraulic fracture network and parametric study. J. Pet. Sci. Eng. 150, 224–237. Chen, Y.X., 2017. Experimental Study on a New Type of Self-Propping Fracturing Technology. Southwest Petroleum University. China National Petroleum Corporation, 2009. Recommended Practices for Evaluating Short Term Proppant Pack Conductivity: SY/T 6302-2009. Petroleum Industry Press, Beijing. Dmour, H.N., 2013. Productivity index enhancement of stimulated gas wells through hydraulic fracturing. Liq. Fuel. Technol. 31 (3), 225–236. Du, H.L., Zhang, W., M, F., et al., 2017. Research progress of hydraulic fracturing proppants. Bull. Chin. Ceram. Soc. 36 (08), 2625–2630. Gandossi, L., Von Estorff, U., 2015. An overview of hydraulic fracturing and other formation stimulation technologies for shale gas production. EUR 26347. https://doi. org/10.2790/379646. He, Y.W., Cheng, S.Q., Li, S., et al., 2017. A semianalytical methodology to diagnose the locations of underperforming hydraulic fractures through pressure-transient analysis in tight gas reservoir. SPE J. 22 (03), 924–939. Himes, R.E., Dalrymple, E.D., Dahl, J.A., et al., 1994. Injecting to Stimulate Subterranean Formation; Includes a Proppant Material; Hardening. US. US 5358047 A. Lehn, J.M., 1988. Supramolecular chemistry — scope and perspectives: molecules — supermolecules — molecular devices. J. Inclusion Phenom. 6 (4), 351–396. Liang, T., Yan, C., Zhou, S., et al., 2017. Silica fume reinforced polystyrene-based composite particles used as ultra-light weight proppants in hydraulic fracturing. Mater. Res. Express 4 (11). Liu, R.J., Zhang, J.T., Yin, C.B., et al., 2003. The Current Situation and Development Trend of Hydraulic Fracturing Proppants. Drilling & Production Technology. Malone, W.T., Williams, J.R., Derby, J.A., 1968. Method of Fracturing and Propping a Subterranean Formation. US. US3366178. Meazza, L., Foster, J.A., Fucke, K., et al., 2013. Halogen-bonding-triggered supramolecular gel formation. Nat. Chem. 5 (1), 42–47. National Energy Board, 2015. Measurement of Properties of Proppants Used in Hydraulic Fracturing and Gravel-Packing Operations: SY/T 5108-2014. Petroleum Industry Press, Beijing. Nguyen, P.D., Weaver, J.D., Mccabe, M.A., et al., 2004. Methods and Compositions for Consolidating Proppant in Subterranean Fractures. US. US6729404. Paramonov, S.E., Jun, H.W., Hartgerink, J.D., 2006. Self-assembly of peptide-amphiphile
Fig. 9. Fracture conductivity of different proppant types.
factors of reservoir. (3) On the one hand, the storage conditions of self-propping fracturing fluid are relatively strict, which must maintain liquid state on the surface. Hence, self-propping fracturing fluid can not be exposed in the extremely hot climate for a long time. On the other hand, the cost of self-propping fracturing fluid is a little high currently. Once the large-scale fracturing treatment is formed, the cost can effectively controlled. 7. Conclusions By the evaluation on physical property and fracture conductivity, the self-generating solid proppants, including S101 and S505, are selected firstly because of their excellent performance. The conclusions are as follows. (1) Based on the physical property measurements, the particle size of S101 and S505 are mainly 20/40 mesh. The roundness and sphericity are both 0.9. The bulk density is 0.63 g/cm3 for S101 and 0.65 g/cm3 for S505. The apparent density of S101 is 1.09 g/cm3 while that of S505 is 1.10%. Under the closure stress of 55.2 MPa, the crushing rate is 1.47% for S101 and 1.29% for S505. Hence, S101 and S505 have the characteristics of large particle size, 847
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