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An experimental study to reduce the breakdown pressure of the unconventional carbonate rock by cyclic injection of thermochemical fluids
Journal of Petroleum Science and Engineering 187 (2020) 106859
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An experimental study to reduce the breakdown pressure of the unconventional carbonate rock by cyclic injection of thermochemical fluids Zeeshan Tariq a, Mohamed Mahmoud a, *, Abdulazeez Abdulraheem a, Ayman Al-Nakhli b, Mohammed BaTaweel b a b
Department of Petroleum Engineering, King Fahd University of Petroleum & Minerals, Dhahran, 31261, Saudi Arabia EXPECR ARC, Saudi Aramco, Dhahran, 31261, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords: Unconventional formation Breakdown pressure Thermochemical Cyclic fracturing Multiple fractures
Commercial production from unconventional reservoirs needs massive fracturing operations. Therefore, long horizontal wells are drilled, and multi-stage fractures are created to achieve the desirable production rates from these reservoirs. One common and major problem in most of the unconventional reservoirs is the high break down pressure due to the reservoir tightness. When fracturing these types of rocks, the hydraulic fracturing operation becomes much more challenging and difficult. In some scenarios reaches to the maximum pumping capacity without generating any fractures. In this study, a new approach to reduce the breakdown pressure of tight rocks is introduced and compared with the conventional method of fracturing. The new method in corporates the injection of thermochemical fluid in a series of cycles to create micro- and macro-fractures. The cyclic experiments performed in this study were concluded with the recording of breakdown pressure in each case. The post-treatment analysis using medical Computerized Tomography (CT) showed that multiple fractures were created due to the pressure pulse created in the thermochemical reaction. Results showed that the breakdown pressure of the rock decreases with the increasing number of cycles. The proposed method of cyclic thermochemical fracturing reduced the breakdown pressure by 33% in one-cycle, 41% in two-cycles, 53.5% in three-cycles, and 69% in four-cycles, when compared with conventional method of fracturing. An empirical relationship is also presented between the number of cycles of thermochemical fluid injection and the breakdown pressure of the rock.
1. Introduction Unconventional resources are playing a vital role in the global economy and has dramatically reduced the gap between world’s energy demand and supply. The economical production from unconventional resources needs the state-of-the-art stimulation techniques (Beugelsdijk et al., 2000; Etherington and McDonald, 2004; Glorioso and Rattia, 2012; Kim and Lee, 2018; Leimkuhler and Leveille, 2012; Pine and Batchelor, 1984; Polikar, 2009; Toscano et al., 2016; Xiao and Huang, 2018). Hydraulic fracturing is one of the most commonly used stimu lation technique adopted in the petroleum industry to produce from the unconventional resources (Beugelsdijk et al., 2000; Glorioso and Rattia, 2012; Zhang et al., 2016). Conventional hydraulic fracturing is imple mented by monotonically increasing the injection pressure of fracturing fluid until the rock fractures (Liu et al., 2018; Taleghani et al., 2018). Production from unconventional tight reservoirs requires massive
fracturing operations. Long horizontal wells with multi-stage fracturing can give the desirable production rates from these reservoirs. One common and major problem associated with most of the unconventional reservoirs is the high breakdown pressure due to the reservoir tightness. To address technical and operational difficulties associated with hori zontal well multistage fracturing, the alternative way to increase the stimulated reservoir volume (SRV) is by generating multiple fractures using pulse fracturing. In pulse fracturing, the basic principle is to raise the pressure up to several thousand psi in no time to create multiple fractures (Goodarzi et al., 2015; Malhotra et al., 2018; Safari et al., 2013; Yang and Risnes, 2000). Pulse fracturing can be achieved by many means such as explosive shooting, impact loading, and using thermo chemical fluids. Thermochemical fluids react to form exothermic reac tion, which release high heat and pressure to the surroundings. The detailed description of pulse fracturing with thermochemical fluids can be found in our previous publications (Al-Nakhli et al., 2019; Tariq et al.,
* Corresponding author. E-mail address: [email protected] (M. Mahmoud). https://doi.org/10.1016/j.petrol.2019.106859 Received 24 May 2019; Received in revised form 18 November 2019; Accepted 23 December 2019 Available online 27 December 2019 0920-4105/© 2019 Elsevier B.V. All rights reserved.
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2019b, 2019a). Cyclic fracturing is the technique to fracture the rocks by series of cycles to reach a fatigue failure (Lyu et al., 2016). The application of cyclic fracturing is broadly investigated in other fields of engineering such as civil, mechanical, and nuclear engineering (Patel et al., 2017a) and also widely studied in petroleum engineering applications (Agrawal and Sakhaee-Pour, 2017; Barreda et al., 2018; Hofmann et al., 2019; Patel et al., 2017b; Sakhaee-Pour and Agrawal, 2018; Salim et al., 2016; Zang et al., 2017). Hofmann et al. (2018) compared the performance of conventional hydraulic fracturing with cyclic fracturing at laboratory, mines, and field scales. They have found that cyclic fracturing can reduce the seismic events created during conventional hydraulic frac turing in addition to lowering the breakdown pressure of the rock. Erarslan and Williams (2013) carried out cyclic loading experiments on Brisbane Tuff and observed significant reduction in tensile strength of the rock using Brazilian disk test analysis. Mighani et al. (2016) observed increased in number of intergranular cracks in scanning elec tron microscopy (SEM) analysis after cyclic loading experiments. Patel et al. (2017a, 2017b) carried out cyclic fracturing on Tennessee sand stone and found the reduction in breakdown pressure by 16% compared to the breakdown pressure obtained in conventional hydraulic frac turing. Falser et al. (2016) reduced the breakdown pressure of the rock by cyclic pressure ramp-up and improving the near wellbore geometry. AlTammar and Sharma (2019) investigated the effect of constant in jection rate, cyclic injection, and constant bore-hole pressurization rate on the breakdown pressure of the rock. They have found that a constant bore-hole pressurization scheme resulted in lower breakdown pressure. Based on the literature survey, it was observed that previous re searchers studied only the effect of cyclic loading on the reduction of break down pressure. The main objective of this study is to develop a new fracturing technique to reduce the breakdown pressure of the un conventional rock as well as to increase the SRV by creating multiple fractures using thermochemical fluids. Laboratory experiments were carried out on the outcrop samples of the unconventional tight car bonates. Multiple experiments were performed with conventional and cyclic thermochemical fracturing techniques. A new method of cyclic fracturing with thermochemical is introduced for the first time in the present study. A relationship is also developed between the breakdown pressure and the number of cycles of thermochemical fluid injected. By creating microfractures and improving the injectivity, the required breakdown pressure can be reduced, and fractures can be enhanced. Therefore, the new method enables the fracturing of high strength rocks more economically and more efficiently.
Table 1 Characteristics of unconventional carbonate rock. Tests
Values
Units
XRD Analysis
95.1 Calcite 4.1 Dolomite 0.6 Quartz 0.2 Augite 22.57 Carbon 41.37 Oxygen 5.27 Sodium 4.11 Chlorine 26.68 Calcium 12.20 0.08 2.72
% % % % % % % % % % mD g/cc
XRF Analysis
Porosity Permeability Bulk Density
enhance the productivity index by removing water blockage in tight sandstone rocks. Mahmoud (2019) applied thermochemical technology to remove the filter cake after drilling technology. 3. Materials and methods 3.1. Rock characterization In this study, outcrop samples from the unconventional carbonate formation were collected, analyzed, and tested. Routine core analysis (SCAL), X-ray fluorescence (XRF), and X-ray diffraction (XRD) analysis tests were carried out. The rock properties obtained from these char acterization tests are listed in Table 1. The outcrop samples had 95% calcite, 4.1% dolomite, 0.6% quartz, and some traces of augite mineral with 0.2% only. XRF analysis showed that major element in the studied rock sample is calcium with 26% weightage. The average porosity and permeability of the samples were 12% and 0.08 mD, respectively. The approximate density of the samples was 2.72 g/cc. Fig. 1 shows the SEM image at 500 micro-meter level. 3.2. Scratch test analysis Scratch test machine was used to measure the continuous scratch strength profile and compressional (P-) and shear (S-) waves velocities of the samples. Scratch strength is approximately equal to the unconfined compressive strength (UCS) of the rock (Mitaim et al., 2004). Scratch test has several advantages over conventional compressive strength measurement tests. It is a non-destructive test, it requires minimum
2. Application of thermochemical fluids in oil and gas industry Thermochemical fluids are those which can generate heat and pressure upon reaction. In oil and gas industry, the application of thermochemical fluids is widely reported. Amin et al. (2007) applied thermochemical technology to remove the organic scale deposited in side the tubing. Singh and Saidu Mohamed (2013) also carried out similar study to remove the organic scale formed inside the tubing of twenty-three wells and witnessed 100% increment in the production rate from these wells. Al-Nakhli and Abass (2014), Al-Nakhli (2015), and Al-Nakhli et al. (2013) carried out experimental work to investigate the increment in SRV due to the application of thermochemical fluids injection. They conducted experiments on different rock samples and cement blocks. Mustafa et al. (2018) conducted core-flooding experi ments and found that due to the application of thermochemical flooding the rock strength reduced by 30%. They carried out experiments on Berea and Scioto sandstones. Wang et al. (2018) used thermochemical fluids with surfactant to generate in situ heat foams in enhanced oil recovery (EOR) project. They conducted core-flooding experiments with thermochemical fluids on homogeneous and heterogeneous rocks and found 20% increase in oil recovery in homogeneous rock and 34% in non-homogeneous rocks. Hassan et al. (2019) used thermochemical to
Fig. 1. SEM image of carbonate rock sample. 2
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Fig. 2. Continuous log of scratch strength for tight carbonate sample obtained from the scratch test.
sample preparation, it is quicker, and has high repeatability (Richard et al., 1998; Schei et al., 2000a). The test is associated with the mea surement of normal and tangential forces acting on the sharp cutter while making a groove on the surface of the rock. The forces computed from the scratch tests are used to obtain the intrinsic specific energy (ε). ε is the amount of energy required to break a unit volume of the rock and is highly correlated with UCS of the rock (Schei et al., 2000b). The relationship between ε, cross sectional area ðAÞ, and the average horizontal force ðFt Þ is given by Eq. (1) Ft ¼ ε A
(1)
In this study, scratch strength for the entire length of the core sample was obtained by creating various grooves on the surface of the sample with different depth of cuts. The width of the PDC cutter was 10 mm. The inclination angle of the PDC cutter with respect to vertical position was set to 15� in all the experiments. Cutting velocity was kept constant to 30 mm/s in each experiment. Generally, in scratch test, there are two modes of failure: brittle mode of failure and ductile mode of failure.
Fig. 3. Continuous log of scratch strength on the 2-inch core plug drilled from the outcrop sample.
Fig. 4. Schematic illustration of the apparatus used in the fracturing experiment. 3
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Fig. 5. a) A view of the core plug with 6.5 mm drilled hole at the center, on the left side. b) Core plug with steel tubing inserted and fixed with HPHT epoxy. c) Idealized view of core sample used in this study. L ¼ 2-inch, D ¼ 2-inch, l ¼ 0.75-inch, and d ¼ 0.25-inch.
Brittle mode of failure is usually observed in deeper depth of cuts while ductile mode of failures is observed in shallow depth of cuts. The ε ob tained during ductile mode of failure is very well correlated with the UCS of the rock (Mitaim et al., 2004). Therefore, in this study, the ductile mode of failure was avoided by fixing the depth of cut in each groove to 0.5 mm. Fig. 2 shows the superimposed image of logs of continuous scratch strength obtained from two different grooves created on the tested specimen of the length 5-inch after performing scratch test. For the sake conciseness only groove 1 is shown in Fig. 2. The second groove is created at the 60� angle from the first groove. The average scratch strength of the two grooves was found to be 9000 psia (62 MPa). Fig. 3 shows the continuous scratch strength on the 2-inch core plug before performing any fracturing experiment. The average scratch strength was found to be 8700 psia (60 MPa). P and S-waves velocities were also measured on 2-inch core plug and found to be 4850 m/s and 2300 m/s, respectively. The dynamic Young’s modulus (Edyn ) was found to 38.99 GPa, and dynamic Poisson’s ratio ðυdyn ) was 0.37. The Edyn and υdyn were calculated using Eqs. (2) and (3). � 2 � 3VP 4V2S Edyn ¼ ρV2S (2) V2P V2S
υdyn ¼
V2P 2V2S � 2 V2P V2S
thermochemical and fracturing fluid from the piston accumulators to the core-holder. Multiple high pressure high temperature (HPHT) valves were installed at the inlet and outlet of the accumulators to control the fluid switching during the injection process. A high frequency pressure transducer was installed to record the continuous injection pressure and the results were sent through a data acquisition system to the computer operating software. Before performing any experiment, it was ensured that: 1. There is no trapped air in the system. 2. There is no leakage in the connections and tubing. 3. All tubing and connections were pressure-tested by initially pres surizing the system to 1500 psi and observing the decline in pressure. If any pressure drop happened due to the leak, then the faulty tubing or connections was fixed or replaced. 4. In all cyclic experiments the core holder was placed inside the oven for a period of 2 h to ensure uniform heating of the sample. Fig. 5a shows the front view of one of the core sample, Fig. 5b shows the core sample with steel tubing attached using HPHT epoxy, and Fig. 5c shows the idealized view of the cylindrical sample with proper dimensions. All the experiments were performed on a cylindrical sample of diameter and length 2-inches. A 6.5 mm synthetic bore-hole was drilled at the center of each sample which represents the wellbore. The hole was drilled to a depth of 19 mm. The bore-hole was created using heavy duty drill press. The drill bit used for this purpose was concrete drill bit. A steel tubing of 6.35 mm OD was inserted inside the drilled hole and cemented using JB Weld™ epoxy. An open hole section of ½ inches between the depth of the bore-hole and steel tubing were left for the fracturing fluid to make a contact with specimen. The samples were left for 24 h to ensure proper settling of epoxy. After the steel tubing was properly attached with the sample using epoxy, the sample was properly placed inside the core-holder and the top and bottom end caps of the core-holder were tightly fastened to avoid any leakage. Then core-holder was placed inside the electric oven and connected with the flow lines. The samples were left inside the oven for few hours in order to ensure that the sample temperature reached exactly the oven set temperature. The injection pressure was continuously logged until the specimen breakdown happens.
(3)
where VP and VS are the compressional and shear wave velocities in km/ s, and ρ is the bulk density in g/cc. 3.3. Experimental procedure Fig. 4 shows a process flow diagram (PFD) of the breakdown pressure experiment adopted in this study. Experimental setup consists of a core holder, high pressure syringe pump, two piston transfer cells, digital pressure gauge to monitor the confining pressure, and electronic pres sure gauge to record injection pressure. Each transfer cell has the ca pacity of 1 L which were used to accumulate the fracturing fluid and the thermochemical. Core-holder was used to accommodate the core sample of diameter of 2 inches and length up to 12 inches. In present study, all the samples were 2 inches long. The length was controlled by placing the spacers to ensure the contact with the upper and lower end cap of the core-holder. Core-holder was then placed inside an electric oven to heat the sample. An ISCO syringe pump was used to displace the 4
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Fig. 6. Time to reach the maximum reaction temperature as a function of reservoir temperature.
Fig. 7. Generation of pressure pulse due to thermochemical reaction, effect of reactants molar concentration.
3.4. Thermochemical preparation
thermochemical fluids and found that the enthalpy change ðΔHÞ during the thermochemical reaction is 369 kJ=mol, thermal conductivity ðλÞ of
Thermochemical fluid consists of ammonium chloride ðNH4 ClÞ and sodium nitrite ðNaNO2 Þ. A 1:1 M concentration of NH4 Cl and NaNO2 was used. The solution was mixed at ambient conditions, and the mixture was used as a fracturing fluid. NH4 Cl reacts with NaNO2 at the reservoir conditions and as a result very high pressure and temperature is generated. The reaction was triggered by the initial temperature of 80 � C. For all thermochemical reagents ðNH4 Cl and NaNO2 Þ, the chemical concentration of 1M was used. Al-Nakhli et al. (2016) used the same chemical and found that upon reaction, these thermochemical reagents can generate the pressure up to 3470 psia and raise the temperature up to 371 � C. Alade et al. (2019) analyzed the same concentration of the
thermochemical fluid lies between 0.1 and 0.6 W= and specific heat m:K capacity ðCÞ between 85 and 110 J= . The chemical reaction of mol:K thermochemical can be described by the following equation: NH4 Cl þ NaNO2 → NaCl þ 2H2 0 þ N2 ðgasÞ þ ΔHðheatÞ
(4)
where ΔH is the heat generated from the reaction. The reaction needs high temperature to trigger and the reaction time is a strong function of the initial temperature. This reaction at one to 1 M ratio generated an additional temperature of 90 � C and at two to 2 M ratio generated additional temperature of 115 � C. Different initial temperatures, 5
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Fig. 8. Pressure profile with time for conventional hydraulic fracturing experiment.
representing the downhole reservoir temperature, were used from 50 to 100 � C (Fig. 6). As a result of thermochemical reaction, the nitrogen gas was released, which creates the pressure pulse. Pressure pulse level was recorded in a small cell experiment having a volume of 20 cm3. Ther mochemical reaction was triggered by pre-heating the cell with an initial temperature of 80 � C. The reaction is described by Eq. (4). Fig. 7 illus trates the resulted pressure pulse as a function of reactants concentration (NH4 Cl and NaNO2 ). The cell was initially pressurized to 1000 psia and the generated pressure as a function of time was recorded. It may be noted that the pressure pulse is generated after a few seconds when the reaction starts (at 100 � C). For the case of 2:2 Molar concentrations of NH4 Cl and NaNO2 , the peak pressure recorded was 5550 psia which means that an additional 4550 psia pressure generated due to the re action. This peak pressure generated as a result of the release of nitrogen
gas. In the rock sample, this peak pressure pulse is responsible for creating micro-fracture and weakening of the rock strength. The peak pressure recorded for the case of 1:1 M concentration of the reactants was 3500 psia which is an additional 2500 psia due to the reaction. The same peak pressure is 2000 psia less than the 2:2 M concentrated re actants. This is due to the fact that less nitrogen gas generated when low concentration of reactants used. 4. Results and discussions 4.1. Conventional hydraulic fracturing experiment Conventional hydraulic fracturing experiment was carried out at room temperature (30 � C). Distilled water with a pH of 7 was used as the fracturing fluid. Distilled water was continuously injected with a flow
Fig. 9. A comparison of conventional hydraulic fracturing experiment with cyclic thermochemical fracturing experiments. 6
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Table 2 Breakdown pressure and number of cycles of tight carbonate samples under unconfined compression. Samples
Experiment Type
Fracturing Fluid
Number of Cycles
Breakdown Pressure, psia
Percentage Reduction in Breakdown Pressure
1 2 3 4 5
Conventional Cyclic Cyclic Cyclic Cyclic
Water Thermochemical Thermochemical Thermochemical Thermochemical
– 1 2 3 4
2169 1455 1280 1008 653.7
– 32.92% 40.99% 53.53% 69.86%
rate of 5 ml/min which resulted in a monotonic increase of pressure as shown in Fig. 8. Specimen failed within a short time at a breakdown pressure of 2169 psia. After the breakdown pressure the injection pressure dropped to zero because there was no confining pressure applied on the core sample. The breakdown pressure obtained from the conventional hydraulic fracturing experiment is referred to as a base pressure.
the reaction triggering temperature of 100 � C. An optimum temperature of 100 � C was selected after performing multiple experiments in a small cell experiments with changing initial temperature in each experiment as shown in Fig. 6. In cyclic thermochemical fracturing experiments, the thermochemical fluid was injected in series of cycles and the maximum pressure allowed to raise in each cycle was 500 psia. In every cycle the fracturing fluid was injected at a specified constant rate of 5 ml/min until the borehole pressure reached to a constant value of 500 psia, and then, fluid injection was stopped to allow fracturing fluid to penetrate inside the sample. In every cycle the flow rate and the peak pressure (500 psia) were kept constant. The shut-in time was 10 min which remained constant for every cycle. In one-cycle experiment, the thermochemical fluid was continuously injected with a constant flow rate of 5 ml/min until it reached to a pressure of 500 psia. After reaching to 500 psia, pumping was stopped for a period of 10 min to allow thermochemical to react inside the rock pores, create micro-fracture, increase the permeability, and saturate the core sample. At the end of first cycle (after 10 min of shut-in period), the pressure was dropped to 300 psia. It means that the reaction happened, and due to the reaction micro-fractures, and small channels were created which allowed fluid to penetrate deep inside the core sample. The injected chemicals partially penetrated the rock and the reaction took place inside the rock pores. The generated micro-fractures resulted in the drop of pressure at the end of the cycle. After 10 min of shut in time, thermochemical were injected with the same flow rate and pressure was monotonically increased until the breakdown of rock happened. The
4.2. Cyclic thermochemical fracturing experiments A total of four cyclic experiments were performed with different number of cycles in each experiment. Experiments were performed with one cycle, two cycles, three cycles, and four cycles of thermochemical fluid injection. All experiments were carried out on the core plugs drilled out from the large outcrop sample of the unconventional tight carbonate rocks. All plugs had identical mechanical and petrophysical properties. These properties are listed in Table 1. Fig. 9 shows the comparison of breakdown pressure profile recorded in the conventional hydraulic fracturing experiment with the breakdown pressure profiles recorded in the cyclic thermochemical fracturing experiments. Pressure profiles shown in Fig. 9 are logged till the rock specimen fails in each experi ment. Number of cycles in each experiment are plotted against the corresponding injection pressure of the fracturing fluids. All experi ments were performed with no confining pressure, which were initiated at an ambient pressure of 14.7 psia. Cyclic thermochemical fracturing experiments were carried out at
Fig. 10. A view of core samples treated with water and thermochemical after the experiment. 7
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Fig. 11. Medical CT scanned images of the fractured sample using cyclic experiment. (a) one cycle of thermochemical fluid, (b) two cycles of thermochemical fluid, (c) three cycles of thermochemical fluid, (d) four cycles of thermochemical fluid.
8
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Fig. 12. CT number profiles of one slice of CT scanned images obtained from one-cycle, two-cycle, three-cycle, and four-cycle thermochemical experiments.
breakdown pressure recorded for one-cycle experiment was 1455 psia. The same procedure was repeated for experiments with two, three, and four cycles of thermochemical fluid injections. The injection pres sures were recorded in each experiment till the point when the break down of the rock occurred. The breakdown pressure recorded for twocycle experiment was 1280 psia, for three-cycle experiment break down was 1008 psia, and for four-cycle experiment the breakdown pressure recorded was 653.7 psia. By comparing with reference break down pressure recorded in conventional hydraulic fracturing experi ment, one cycle of thermochemical fluid injection experiment resulted in the reduction of breakdown pressure by 32.92%, for two cycles the reduction was 40.99%, for three cycles the reduction was 53.5%, and for
four cycles the reduction was 69.86%. The summary of reduction in breakdown pressure is listed in Table 2. In the case of one-cycle exper iment, a single linear fracture was observed along the length of the core plug. For two- and three-cycle experiments, radial fractures were dominant along the bedding plane. In case of four cycles of thermo chemical fluid injection, multiple fractures (combination of linear and radial) were observed as shown in Fig. 10. In a conventional hydraulic fracturing experiment, water does not have enough time to penetrate into the rock matrix, while for the cases of cyclic injection with thermochemical fluid with large number of cy cles, thermochemical fluid had enough time to infiltrate deeper into the specimen along the directions of weaker zone or in the areas of
Fig. 13. A non-linear relationship between number of cycles and corresponding breakdown pressure in cyclic experiments. 9
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Table 3 Comparison of ultrasonic velocities, dynamic Poisson’s ratio, and dynamic Young’s Modulus before and after thermo-chemical treatment. Parameters
Before Thermochemical Treatment
After Thermochemical Treatment
Percentage Reduction
VP , m/s
4850
3200
34.02
2300
1850
19.56
υdyn
0.37
0.25
32.43
38.99
23.254
40.35
VS , m/s
Edyn , GPa
4.3. Scratch test analysis of post thermochemical treatment The scratch test is used to estimate the decrease in strength after the fracturing experiment. However, it was possible after core-flooding experiment (without breaking the rock sample) only because the sam ples in other experiments (1-cycle and up) were too fragmented to conduct this test. Fig. 14 shows the continuous scratch strength obtained before and after performing the core-flooding experiment with ther mochemical fluids. Table 3 shows the values of P-wave velocity, S-wave velocity, υdyn , and Edyn of the samples before and after thermochemical flooding. The post treatment results showed the decrease in P- and S- waves velocities after thermochemical treatment. The measured P and S-wave velocities were 4850 m/s and 3200 m/s respectively which were reduced to 2300 m/s and 1850 m/s after thermo-chemical treatment. The stiffness of the rock samples was decreased substantially after the treatment as exhibited by elastic parameters (dynamic Poisson’s and dynamic Young’s modulus). The remarkable variation has been observed in me chanical parameters in terms of decrease in Youngs’s modulus by 40.3% and Poisson’s ratio by 32.43%.
Fig. 14. Comparison of continuous strength profiles before and after per forming thermochemical fracturing.
microcracks. Because of which it became possible to see the specimen failed at multiple locations. In addition, nitrogen gas created high pressure pulse inside the rock pores and led to the formation of microfractures. All the core samples tested in this study were scanned using tomo graphic non-destructive micro computed tomography (CT) scanner (Toshiba Alexion TSX-032A). The purpose of conducting medical CT analysis is to better characterize the features of the samples by obtaining the descriptive images of density variations within an object. The scanned images also helped in visualizing if there is any fracture and wormhole already existed or generated by to the experiment. The scanner used in this study has a spatial resolution of 1-mm. Images ob tained from macro-CT scanner can be visualized in specialized software. The cores were scanned before and after carrying out the conventional and cyclic fracturing experiments. In every scan a total of 42 images were obtained. Top and bottom images of the samples were removed because of the unclarity caused by the noise. Medical-CT images exhibited changes in terms of micro cracks and pores generated in the samples after the cyclic fracturing experiments. Fig. 11 shows scanned images obtained after the one-cycle, two-cycle, three-cycle, and four-cycle thermochemical experiments. The figure contains slice by slice images from the top of the samples to the bottom. The scale at the bottom of images shows the distribution of CT number and the density in g/cc. CT number indirectly determines the density of the sample. As the density and CT number goes higher, the color in CT scan images goes lighter. The definition of CT number is given by Eq. (5) CT Number ¼
μ
μW x 1000 μW
5. Conclusions In this study hydraulic fracturing experiments were carried out in the laboratory on unconventional tight carbonate outcrop samples. Con ventional hydraulic fracturing and cyclic thermochemical fracturing schemes were applied and their performance in terms of breakdown pressure was compared. Based on the experimental evidence and the discussion presented in this study, the following conclusions can be drawn: 1. Cyclic experiment with four cycles of thermochemical fluids resulted in the reduction of breakdown pressure by 70% compared to the conventional hydraulic fracturing experiment with water. 2. Cyclic fracturing with four cycles of thermochemical fluids resulted in the creation of multiple fractures which are the combination of linear and radial fractures. 3. Thermochemical fracturing generates synthetic sweet-spots by creating multiple micro fractures around the region of wellbore. The generated micro-fractures resulted in the reduction of breakdown pressure. 4. The developed relationship between number of cycles and break down pressure can give a quick prediction of breakdown pressure. 5. Core flooding experiment shows that the stiffness of the rock samples was decreased substantially after the treatment with thermochem ical. Dynamic Young’s modulus decreased by 32.43% and dynamic Poisson’s ratio decreased by 40.35%.
(5)
where μ is the linear attenuation coefficient value of the specimen and μW is the linear attenuation coefficient of water. The CT number of air is
1000 and the CT number of water is 0. Fig. 12 shows the CT number profile of one slice of CT scanned im ages obtained after one-cycle, two-cycle, three-cycle, and four-cycle thermochemical experiments. The lower value of CT number indicates higher fracture conductivity. For case of 4-cycle of thermochemical the lowest value of CT number found to be was 650, which means the fractures created during 4-cylce of thermochemical was highly conductive. Also, medical CT images clearly illustrate the number and pattern of fractures generated from the experiments. An attempt was made to correlate the number of cycles in thermo chemical fracturing experiments with the corresponding breakdown pressures as shown in Fig. 13. A perfect polynomial equation with de gree of two was fitted on the experimental dataset. The trend yields a maximum value coefficient of determination (R2) of 1.
Author contributions 1. Zeeshan Tariq: initiated the paper write up and conducted the experimental work, 2. Mohamed Mahmoud: initiated the idea of the paper and supervise the work, 3. Abdulazeez Abdulraheem: Supervised the work and planned/analyzed the scratch test results, 4. Ayman Al10
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Nakhli: provided the reaction kinetics of thermochemicals and help design the fracture experiments., 5. Mohammed BaTaweel: contributed to the design of the cyclic fracturing experiments and the analysis part.
Acknowledgment The College of Petroleum and Geoscience, at King Fahd University of Petroleum & Minerals, and Saudi Aramco are acknowledged for the support and permission to publish this work. Saudi Aramco is also acknowledged for funding this research under project number CIPR 2316.
Declaration of competing interest We confirm that there is conflict of interest.
Nomenclature A C CT E Edyn F EDS EOR FEM HPHT ISRM NMR P PDC PFD PR S SEM SRV Vp Vs XRD XRF
Cross-sectional Area Specific Heat Capacity Computerized Tomography Young’s Modulus, GPa Dynamic Young’s Modulus, GPa Axial force Energy Dispersive X-ray spectroscopy Enhanced Oil Recovery Finite Element Method High Temperature High Pressure International Society of Rock Mechanics Nuclear Magnetic Resonance Compressional wave Poly Diamond Crystalline Process Flow Diagram Poisson’s Ratio Shear wave Scanning Electron Microscopy Stimulated Reservoir Volume Compressional wave velocity, m/s Shear wave velocity, m/s X-Ray Diffraction X-Ray Fluorescent
Greek Symbols Intrinsic specific energy Dynamic Poisson’s Ratio Bulk density, g/cc λ Thermal Conductivity μ Linear attenuation coefficient μW Linear attenuation coefficient of water
ε υdyn ρ
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.petrol.2019.106859.
References
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An experimental study to reduce the breakdown pressure of the unconventional carbonate rock by cyclic injection of thermochemical fluids Zeeshan Tariq a, Mohamed Mahmoud a, *, Abdulazeez Abdulraheem a, Ayman Al-Nakhli b, Mohammed BaTaweel b a b
Department of Petroleum Engineering, King Fahd University of Petroleum & Minerals, Dhahran, 31261, Saudi Arabia EXPECR ARC, Saudi Aramco, Dhahran, 31261, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords: Unconventional formation Breakdown pressure Thermochemical Cyclic fracturing Multiple fractures
Commercial production from unconventional reservoirs needs massive fracturing operations. Therefore, long horizontal wells are drilled, and multi-stage fractures are created to achieve the desirable production rates from these reservoirs. One common and major problem in most of the unconventional reservoirs is the high break down pressure due to the reservoir tightness. When fracturing these types of rocks, the hydraulic fracturing operation becomes much more challenging and difficult. In some scenarios reaches to the maximum pumping capacity without generating any fractures. In this study, a new approach to reduce the breakdown pressure of tight rocks is introduced and compared with the conventional method of fracturing. The new method in corporates the injection of thermochemical fluid in a series of cycles to create micro- and macro-fractures. The cyclic experiments performed in this study were concluded with the recording of breakdown pressure in each case. The post-treatment analysis using medical Computerized Tomography (CT) showed that multiple fractures were created due to the pressure pulse created in the thermochemical reaction. Results showed that the breakdown pressure of the rock decreases with the increasing number of cycles. The proposed method of cyclic thermochemical fracturing reduced the breakdown pressure by 33% in one-cycle, 41% in two-cycles, 53.5% in three-cycles, and 69% in four-cycles, when compared with conventional method of fracturing. An empirical relationship is also presented between the number of cycles of thermochemical fluid injection and the breakdown pressure of the rock.
1. Introduction Unconventional resources are playing a vital role in the global economy and has dramatically reduced the gap between world’s energy demand and supply. The economical production from unconventional resources needs the state-of-the-art stimulation techniques (Beugelsdijk et al., 2000; Etherington and McDonald, 2004; Glorioso and Rattia, 2012; Kim and Lee, 2018; Leimkuhler and Leveille, 2012; Pine and Batchelor, 1984; Polikar, 2009; Toscano et al., 2016; Xiao and Huang, 2018). Hydraulic fracturing is one of the most commonly used stimu lation technique adopted in the petroleum industry to produce from the unconventional resources (Beugelsdijk et al., 2000; Glorioso and Rattia, 2012; Zhang et al., 2016). Conventional hydraulic fracturing is imple mented by monotonically increasing the injection pressure of fracturing fluid until the rock fractures (Liu et al., 2018; Taleghani et al., 2018). Production from unconventional tight reservoirs requires massive
fracturing operations. Long horizontal wells with multi-stage fracturing can give the desirable production rates from these reservoirs. One common and major problem associated with most of the unconventional reservoirs is the high breakdown pressure due to the reservoir tightness. To address technical and operational difficulties associated with hori zontal well multistage fracturing, the alternative way to increase the stimulated reservoir volume (SRV) is by generating multiple fractures using pulse fracturing. In pulse fracturing, the basic principle is to raise the pressure up to several thousand psi in no time to create multiple fractures (Goodarzi et al., 2015; Malhotra et al., 2018; Safari et al., 2013; Yang and Risnes, 2000). Pulse fracturing can be achieved by many means such as explosive shooting, impact loading, and using thermo chemical fluids. Thermochemical fluids react to form exothermic reac tion, which release high heat and pressure to the surroundings. The detailed description of pulse fracturing with thermochemical fluids can be found in our previous publications (Al-Nakhli et al., 2019; Tariq et al.,
* Corresponding author. E-mail address: [email protected] (M. Mahmoud). https://doi.org/10.1016/j.petrol.2019.106859 Received 24 May 2019; Received in revised form 18 November 2019; Accepted 23 December 2019 Available online 27 December 2019 0920-4105/© 2019 Elsevier B.V. All rights reserved.
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2019b, 2019a). Cyclic fracturing is the technique to fracture the rocks by series of cycles to reach a fatigue failure (Lyu et al., 2016). The application of cyclic fracturing is broadly investigated in other fields of engineering such as civil, mechanical, and nuclear engineering (Patel et al., 2017a) and also widely studied in petroleum engineering applications (Agrawal and Sakhaee-Pour, 2017; Barreda et al., 2018; Hofmann et al., 2019; Patel et al., 2017b; Sakhaee-Pour and Agrawal, 2018; Salim et al., 2016; Zang et al., 2017). Hofmann et al. (2018) compared the performance of conventional hydraulic fracturing with cyclic fracturing at laboratory, mines, and field scales. They have found that cyclic fracturing can reduce the seismic events created during conventional hydraulic frac turing in addition to lowering the breakdown pressure of the rock. Erarslan and Williams (2013) carried out cyclic loading experiments on Brisbane Tuff and observed significant reduction in tensile strength of the rock using Brazilian disk test analysis. Mighani et al. (2016) observed increased in number of intergranular cracks in scanning elec tron microscopy (SEM) analysis after cyclic loading experiments. Patel et al. (2017a, 2017b) carried out cyclic fracturing on Tennessee sand stone and found the reduction in breakdown pressure by 16% compared to the breakdown pressure obtained in conventional hydraulic frac turing. Falser et al. (2016) reduced the breakdown pressure of the rock by cyclic pressure ramp-up and improving the near wellbore geometry. AlTammar and Sharma (2019) investigated the effect of constant in jection rate, cyclic injection, and constant bore-hole pressurization rate on the breakdown pressure of the rock. They have found that a constant bore-hole pressurization scheme resulted in lower breakdown pressure. Based on the literature survey, it was observed that previous re searchers studied only the effect of cyclic loading on the reduction of break down pressure. The main objective of this study is to develop a new fracturing technique to reduce the breakdown pressure of the un conventional rock as well as to increase the SRV by creating multiple fractures using thermochemical fluids. Laboratory experiments were carried out on the outcrop samples of the unconventional tight car bonates. Multiple experiments were performed with conventional and cyclic thermochemical fracturing techniques. A new method of cyclic fracturing with thermochemical is introduced for the first time in the present study. A relationship is also developed between the breakdown pressure and the number of cycles of thermochemical fluid injected. By creating microfractures and improving the injectivity, the required breakdown pressure can be reduced, and fractures can be enhanced. Therefore, the new method enables the fracturing of high strength rocks more economically and more efficiently.
Table 1 Characteristics of unconventional carbonate rock. Tests
Values
Units
XRD Analysis
95.1 Calcite 4.1 Dolomite 0.6 Quartz 0.2 Augite 22.57 Carbon 41.37 Oxygen 5.27 Sodium 4.11 Chlorine 26.68 Calcium 12.20 0.08 2.72
% % % % % % % % % % mD g/cc
XRF Analysis
Porosity Permeability Bulk Density
enhance the productivity index by removing water blockage in tight sandstone rocks. Mahmoud (2019) applied thermochemical technology to remove the filter cake after drilling technology. 3. Materials and methods 3.1. Rock characterization In this study, outcrop samples from the unconventional carbonate formation were collected, analyzed, and tested. Routine core analysis (SCAL), X-ray fluorescence (XRF), and X-ray diffraction (XRD) analysis tests were carried out. The rock properties obtained from these char acterization tests are listed in Table 1. The outcrop samples had 95% calcite, 4.1% dolomite, 0.6% quartz, and some traces of augite mineral with 0.2% only. XRF analysis showed that major element in the studied rock sample is calcium with 26% weightage. The average porosity and permeability of the samples were 12% and 0.08 mD, respectively. The approximate density of the samples was 2.72 g/cc. Fig. 1 shows the SEM image at 500 micro-meter level. 3.2. Scratch test analysis Scratch test machine was used to measure the continuous scratch strength profile and compressional (P-) and shear (S-) waves velocities of the samples. Scratch strength is approximately equal to the unconfined compressive strength (UCS) of the rock (Mitaim et al., 2004). Scratch test has several advantages over conventional compressive strength measurement tests. It is a non-destructive test, it requires minimum
2. Application of thermochemical fluids in oil and gas industry Thermochemical fluids are those which can generate heat and pressure upon reaction. In oil and gas industry, the application of thermochemical fluids is widely reported. Amin et al. (2007) applied thermochemical technology to remove the organic scale deposited in side the tubing. Singh and Saidu Mohamed (2013) also carried out similar study to remove the organic scale formed inside the tubing of twenty-three wells and witnessed 100% increment in the production rate from these wells. Al-Nakhli and Abass (2014), Al-Nakhli (2015), and Al-Nakhli et al. (2013) carried out experimental work to investigate the increment in SRV due to the application of thermochemical fluids injection. They conducted experiments on different rock samples and cement blocks. Mustafa et al. (2018) conducted core-flooding experi ments and found that due to the application of thermochemical flooding the rock strength reduced by 30%. They carried out experiments on Berea and Scioto sandstones. Wang et al. (2018) used thermochemical fluids with surfactant to generate in situ heat foams in enhanced oil recovery (EOR) project. They conducted core-flooding experiments with thermochemical fluids on homogeneous and heterogeneous rocks and found 20% increase in oil recovery in homogeneous rock and 34% in non-homogeneous rocks. Hassan et al. (2019) used thermochemical to
Fig. 1. SEM image of carbonate rock sample. 2
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Fig. 2. Continuous log of scratch strength for tight carbonate sample obtained from the scratch test.
sample preparation, it is quicker, and has high repeatability (Richard et al., 1998; Schei et al., 2000a). The test is associated with the mea surement of normal and tangential forces acting on the sharp cutter while making a groove on the surface of the rock. The forces computed from the scratch tests are used to obtain the intrinsic specific energy (ε). ε is the amount of energy required to break a unit volume of the rock and is highly correlated with UCS of the rock (Schei et al., 2000b). The relationship between ε, cross sectional area ðAÞ, and the average horizontal force ðFt Þ is given by Eq. (1) Ft ¼ ε A
(1)
In this study, scratch strength for the entire length of the core sample was obtained by creating various grooves on the surface of the sample with different depth of cuts. The width of the PDC cutter was 10 mm. The inclination angle of the PDC cutter with respect to vertical position was set to 15� in all the experiments. Cutting velocity was kept constant to 30 mm/s in each experiment. Generally, in scratch test, there are two modes of failure: brittle mode of failure and ductile mode of failure.
Fig. 3. Continuous log of scratch strength on the 2-inch core plug drilled from the outcrop sample.
Fig. 4. Schematic illustration of the apparatus used in the fracturing experiment. 3
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Fig. 5. a) A view of the core plug with 6.5 mm drilled hole at the center, on the left side. b) Core plug with steel tubing inserted and fixed with HPHT epoxy. c) Idealized view of core sample used in this study. L ¼ 2-inch, D ¼ 2-inch, l ¼ 0.75-inch, and d ¼ 0.25-inch.
Brittle mode of failure is usually observed in deeper depth of cuts while ductile mode of failures is observed in shallow depth of cuts. The ε ob tained during ductile mode of failure is very well correlated with the UCS of the rock (Mitaim et al., 2004). Therefore, in this study, the ductile mode of failure was avoided by fixing the depth of cut in each groove to 0.5 mm. Fig. 2 shows the superimposed image of logs of continuous scratch strength obtained from two different grooves created on the tested specimen of the length 5-inch after performing scratch test. For the sake conciseness only groove 1 is shown in Fig. 2. The second groove is created at the 60� angle from the first groove. The average scratch strength of the two grooves was found to be 9000 psia (62 MPa). Fig. 3 shows the continuous scratch strength on the 2-inch core plug before performing any fracturing experiment. The average scratch strength was found to be 8700 psia (60 MPa). P and S-waves velocities were also measured on 2-inch core plug and found to be 4850 m/s and 2300 m/s, respectively. The dynamic Young’s modulus (Edyn ) was found to 38.99 GPa, and dynamic Poisson’s ratio ðυdyn ) was 0.37. The Edyn and υdyn were calculated using Eqs. (2) and (3). � 2 � 3VP 4V2S Edyn ¼ ρV2S (2) V2P V2S
υdyn ¼
V2P 2V2S � 2 V2P V2S
thermochemical and fracturing fluid from the piston accumulators to the core-holder. Multiple high pressure high temperature (HPHT) valves were installed at the inlet and outlet of the accumulators to control the fluid switching during the injection process. A high frequency pressure transducer was installed to record the continuous injection pressure and the results were sent through a data acquisition system to the computer operating software. Before performing any experiment, it was ensured that: 1. There is no trapped air in the system. 2. There is no leakage in the connections and tubing. 3. All tubing and connections were pressure-tested by initially pres surizing the system to 1500 psi and observing the decline in pressure. If any pressure drop happened due to the leak, then the faulty tubing or connections was fixed or replaced. 4. In all cyclic experiments the core holder was placed inside the oven for a period of 2 h to ensure uniform heating of the sample. Fig. 5a shows the front view of one of the core sample, Fig. 5b shows the core sample with steel tubing attached using HPHT epoxy, and Fig. 5c shows the idealized view of the cylindrical sample with proper dimensions. All the experiments were performed on a cylindrical sample of diameter and length 2-inches. A 6.5 mm synthetic bore-hole was drilled at the center of each sample which represents the wellbore. The hole was drilled to a depth of 19 mm. The bore-hole was created using heavy duty drill press. The drill bit used for this purpose was concrete drill bit. A steel tubing of 6.35 mm OD was inserted inside the drilled hole and cemented using JB Weld™ epoxy. An open hole section of ½ inches between the depth of the bore-hole and steel tubing were left for the fracturing fluid to make a contact with specimen. The samples were left for 24 h to ensure proper settling of epoxy. After the steel tubing was properly attached with the sample using epoxy, the sample was properly placed inside the core-holder and the top and bottom end caps of the core-holder were tightly fastened to avoid any leakage. Then core-holder was placed inside the electric oven and connected with the flow lines. The samples were left inside the oven for few hours in order to ensure that the sample temperature reached exactly the oven set temperature. The injection pressure was continuously logged until the specimen breakdown happens.
(3)
where VP and VS are the compressional and shear wave velocities in km/ s, and ρ is the bulk density in g/cc. 3.3. Experimental procedure Fig. 4 shows a process flow diagram (PFD) of the breakdown pressure experiment adopted in this study. Experimental setup consists of a core holder, high pressure syringe pump, two piston transfer cells, digital pressure gauge to monitor the confining pressure, and electronic pres sure gauge to record injection pressure. Each transfer cell has the ca pacity of 1 L which were used to accumulate the fracturing fluid and the thermochemical. Core-holder was used to accommodate the core sample of diameter of 2 inches and length up to 12 inches. In present study, all the samples were 2 inches long. The length was controlled by placing the spacers to ensure the contact with the upper and lower end cap of the core-holder. Core-holder was then placed inside an electric oven to heat the sample. An ISCO syringe pump was used to displace the 4
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Fig. 6. Time to reach the maximum reaction temperature as a function of reservoir temperature.
Fig. 7. Generation of pressure pulse due to thermochemical reaction, effect of reactants molar concentration.
3.4. Thermochemical preparation
thermochemical fluids and found that the enthalpy change ðΔHÞ during the thermochemical reaction is 369 kJ=mol, thermal conductivity ðλÞ of
Thermochemical fluid consists of ammonium chloride ðNH4 ClÞ and sodium nitrite ðNaNO2 Þ. A 1:1 M concentration of NH4 Cl and NaNO2 was used. The solution was mixed at ambient conditions, and the mixture was used as a fracturing fluid. NH4 Cl reacts with NaNO2 at the reservoir conditions and as a result very high pressure and temperature is generated. The reaction was triggered by the initial temperature of 80 � C. For all thermochemical reagents ðNH4 Cl and NaNO2 Þ, the chemical concentration of 1M was used. Al-Nakhli et al. (2016) used the same chemical and found that upon reaction, these thermochemical reagents can generate the pressure up to 3470 psia and raise the temperature up to 371 � C. Alade et al. (2019) analyzed the same concentration of the
thermochemical fluid lies between 0.1 and 0.6 W= and specific heat m:K capacity ðCÞ between 85 and 110 J= . The chemical reaction of mol:K thermochemical can be described by the following equation: NH4 Cl þ NaNO2 → NaCl þ 2H2 0 þ N2 ðgasÞ þ ΔHðheatÞ
(4)
where ΔH is the heat generated from the reaction. The reaction needs high temperature to trigger and the reaction time is a strong function of the initial temperature. This reaction at one to 1 M ratio generated an additional temperature of 90 � C and at two to 2 M ratio generated additional temperature of 115 � C. Different initial temperatures, 5
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Fig. 8. Pressure profile with time for conventional hydraulic fracturing experiment.
representing the downhole reservoir temperature, were used from 50 to 100 � C (Fig. 6). As a result of thermochemical reaction, the nitrogen gas was released, which creates the pressure pulse. Pressure pulse level was recorded in a small cell experiment having a volume of 20 cm3. Ther mochemical reaction was triggered by pre-heating the cell with an initial temperature of 80 � C. The reaction is described by Eq. (4). Fig. 7 illus trates the resulted pressure pulse as a function of reactants concentration (NH4 Cl and NaNO2 ). The cell was initially pressurized to 1000 psia and the generated pressure as a function of time was recorded. It may be noted that the pressure pulse is generated after a few seconds when the reaction starts (at 100 � C). For the case of 2:2 Molar concentrations of NH4 Cl and NaNO2 , the peak pressure recorded was 5550 psia which means that an additional 4550 psia pressure generated due to the re action. This peak pressure generated as a result of the release of nitrogen
gas. In the rock sample, this peak pressure pulse is responsible for creating micro-fracture and weakening of the rock strength. The peak pressure recorded for the case of 1:1 M concentration of the reactants was 3500 psia which is an additional 2500 psia due to the reaction. The same peak pressure is 2000 psia less than the 2:2 M concentrated re actants. This is due to the fact that less nitrogen gas generated when low concentration of reactants used. 4. Results and discussions 4.1. Conventional hydraulic fracturing experiment Conventional hydraulic fracturing experiment was carried out at room temperature (30 � C). Distilled water with a pH of 7 was used as the fracturing fluid. Distilled water was continuously injected with a flow
Fig. 9. A comparison of conventional hydraulic fracturing experiment with cyclic thermochemical fracturing experiments. 6
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Table 2 Breakdown pressure and number of cycles of tight carbonate samples under unconfined compression. Samples
Experiment Type
Fracturing Fluid
Number of Cycles
Breakdown Pressure, psia
Percentage Reduction in Breakdown Pressure
1 2 3 4 5
Conventional Cyclic Cyclic Cyclic Cyclic
Water Thermochemical Thermochemical Thermochemical Thermochemical
– 1 2 3 4
2169 1455 1280 1008 653.7
– 32.92% 40.99% 53.53% 69.86%
rate of 5 ml/min which resulted in a monotonic increase of pressure as shown in Fig. 8. Specimen failed within a short time at a breakdown pressure of 2169 psia. After the breakdown pressure the injection pressure dropped to zero because there was no confining pressure applied on the core sample. The breakdown pressure obtained from the conventional hydraulic fracturing experiment is referred to as a base pressure.
the reaction triggering temperature of 100 � C. An optimum temperature of 100 � C was selected after performing multiple experiments in a small cell experiments with changing initial temperature in each experiment as shown in Fig. 6. In cyclic thermochemical fracturing experiments, the thermochemical fluid was injected in series of cycles and the maximum pressure allowed to raise in each cycle was 500 psia. In every cycle the fracturing fluid was injected at a specified constant rate of 5 ml/min until the borehole pressure reached to a constant value of 500 psia, and then, fluid injection was stopped to allow fracturing fluid to penetrate inside the sample. In every cycle the flow rate and the peak pressure (500 psia) were kept constant. The shut-in time was 10 min which remained constant for every cycle. In one-cycle experiment, the thermochemical fluid was continuously injected with a constant flow rate of 5 ml/min until it reached to a pressure of 500 psia. After reaching to 500 psia, pumping was stopped for a period of 10 min to allow thermochemical to react inside the rock pores, create micro-fracture, increase the permeability, and saturate the core sample. At the end of first cycle (after 10 min of shut-in period), the pressure was dropped to 300 psia. It means that the reaction happened, and due to the reaction micro-fractures, and small channels were created which allowed fluid to penetrate deep inside the core sample. The injected chemicals partially penetrated the rock and the reaction took place inside the rock pores. The generated micro-fractures resulted in the drop of pressure at the end of the cycle. After 10 min of shut in time, thermochemical were injected with the same flow rate and pressure was monotonically increased until the breakdown of rock happened. The
4.2. Cyclic thermochemical fracturing experiments A total of four cyclic experiments were performed with different number of cycles in each experiment. Experiments were performed with one cycle, two cycles, three cycles, and four cycles of thermochemical fluid injection. All experiments were carried out on the core plugs drilled out from the large outcrop sample of the unconventional tight carbonate rocks. All plugs had identical mechanical and petrophysical properties. These properties are listed in Table 1. Fig. 9 shows the comparison of breakdown pressure profile recorded in the conventional hydraulic fracturing experiment with the breakdown pressure profiles recorded in the cyclic thermochemical fracturing experiments. Pressure profiles shown in Fig. 9 are logged till the rock specimen fails in each experi ment. Number of cycles in each experiment are plotted against the corresponding injection pressure of the fracturing fluids. All experi ments were performed with no confining pressure, which were initiated at an ambient pressure of 14.7 psia. Cyclic thermochemical fracturing experiments were carried out at
Fig. 10. A view of core samples treated with water and thermochemical after the experiment. 7
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Fig. 11. Medical CT scanned images of the fractured sample using cyclic experiment. (a) one cycle of thermochemical fluid, (b) two cycles of thermochemical fluid, (c) three cycles of thermochemical fluid, (d) four cycles of thermochemical fluid.
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Fig. 12. CT number profiles of one slice of CT scanned images obtained from one-cycle, two-cycle, three-cycle, and four-cycle thermochemical experiments.
breakdown pressure recorded for one-cycle experiment was 1455 psia. The same procedure was repeated for experiments with two, three, and four cycles of thermochemical fluid injections. The injection pres sures were recorded in each experiment till the point when the break down of the rock occurred. The breakdown pressure recorded for twocycle experiment was 1280 psia, for three-cycle experiment break down was 1008 psia, and for four-cycle experiment the breakdown pressure recorded was 653.7 psia. By comparing with reference break down pressure recorded in conventional hydraulic fracturing experi ment, one cycle of thermochemical fluid injection experiment resulted in the reduction of breakdown pressure by 32.92%, for two cycles the reduction was 40.99%, for three cycles the reduction was 53.5%, and for
four cycles the reduction was 69.86%. The summary of reduction in breakdown pressure is listed in Table 2. In the case of one-cycle exper iment, a single linear fracture was observed along the length of the core plug. For two- and three-cycle experiments, radial fractures were dominant along the bedding plane. In case of four cycles of thermo chemical fluid injection, multiple fractures (combination of linear and radial) were observed as shown in Fig. 10. In a conventional hydraulic fracturing experiment, water does not have enough time to penetrate into the rock matrix, while for the cases of cyclic injection with thermochemical fluid with large number of cy cles, thermochemical fluid had enough time to infiltrate deeper into the specimen along the directions of weaker zone or in the areas of
Fig. 13. A non-linear relationship between number of cycles and corresponding breakdown pressure in cyclic experiments. 9
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Table 3 Comparison of ultrasonic velocities, dynamic Poisson’s ratio, and dynamic Young’s Modulus before and after thermo-chemical treatment. Parameters
Before Thermochemical Treatment
After Thermochemical Treatment
Percentage Reduction
VP , m/s
4850
3200
34.02
2300
1850
19.56
υdyn
0.37
0.25
32.43
38.99
23.254
40.35
VS , m/s
Edyn , GPa
4.3. Scratch test analysis of post thermochemical treatment The scratch test is used to estimate the decrease in strength after the fracturing experiment. However, it was possible after core-flooding experiment (without breaking the rock sample) only because the sam ples in other experiments (1-cycle and up) were too fragmented to conduct this test. Fig. 14 shows the continuous scratch strength obtained before and after performing the core-flooding experiment with ther mochemical fluids. Table 3 shows the values of P-wave velocity, S-wave velocity, υdyn , and Edyn of the samples before and after thermochemical flooding. The post treatment results showed the decrease in P- and S- waves velocities after thermochemical treatment. The measured P and S-wave velocities were 4850 m/s and 3200 m/s respectively which were reduced to 2300 m/s and 1850 m/s after thermo-chemical treatment. The stiffness of the rock samples was decreased substantially after the treatment as exhibited by elastic parameters (dynamic Poisson’s and dynamic Young’s modulus). The remarkable variation has been observed in me chanical parameters in terms of decrease in Youngs’s modulus by 40.3% and Poisson’s ratio by 32.43%.
Fig. 14. Comparison of continuous strength profiles before and after per forming thermochemical fracturing.
microcracks. Because of which it became possible to see the specimen failed at multiple locations. In addition, nitrogen gas created high pressure pulse inside the rock pores and led to the formation of microfractures. All the core samples tested in this study were scanned using tomo graphic non-destructive micro computed tomography (CT) scanner (Toshiba Alexion TSX-032A). The purpose of conducting medical CT analysis is to better characterize the features of the samples by obtaining the descriptive images of density variations within an object. The scanned images also helped in visualizing if there is any fracture and wormhole already existed or generated by to the experiment. The scanner used in this study has a spatial resolution of 1-mm. Images ob tained from macro-CT scanner can be visualized in specialized software. The cores were scanned before and after carrying out the conventional and cyclic fracturing experiments. In every scan a total of 42 images were obtained. Top and bottom images of the samples were removed because of the unclarity caused by the noise. Medical-CT images exhibited changes in terms of micro cracks and pores generated in the samples after the cyclic fracturing experiments. Fig. 11 shows scanned images obtained after the one-cycle, two-cycle, three-cycle, and four-cycle thermochemical experiments. The figure contains slice by slice images from the top of the samples to the bottom. The scale at the bottom of images shows the distribution of CT number and the density in g/cc. CT number indirectly determines the density of the sample. As the density and CT number goes higher, the color in CT scan images goes lighter. The definition of CT number is given by Eq. (5) CT Number ¼
μ
μW x 1000 μW
5. Conclusions In this study hydraulic fracturing experiments were carried out in the laboratory on unconventional tight carbonate outcrop samples. Con ventional hydraulic fracturing and cyclic thermochemical fracturing schemes were applied and their performance in terms of breakdown pressure was compared. Based on the experimental evidence and the discussion presented in this study, the following conclusions can be drawn: 1. Cyclic experiment with four cycles of thermochemical fluids resulted in the reduction of breakdown pressure by 70% compared to the conventional hydraulic fracturing experiment with water. 2. Cyclic fracturing with four cycles of thermochemical fluids resulted in the creation of multiple fractures which are the combination of linear and radial fractures. 3. Thermochemical fracturing generates synthetic sweet-spots by creating multiple micro fractures around the region of wellbore. The generated micro-fractures resulted in the reduction of breakdown pressure. 4. The developed relationship between number of cycles and break down pressure can give a quick prediction of breakdown pressure. 5. Core flooding experiment shows that the stiffness of the rock samples was decreased substantially after the treatment with thermochem ical. Dynamic Young’s modulus decreased by 32.43% and dynamic Poisson’s ratio decreased by 40.35%.
(5)
where μ is the linear attenuation coefficient value of the specimen and μW is the linear attenuation coefficient of water. The CT number of air is
1000 and the CT number of water is 0. Fig. 12 shows the CT number profile of one slice of CT scanned im ages obtained after one-cycle, two-cycle, three-cycle, and four-cycle thermochemical experiments. The lower value of CT number indicates higher fracture conductivity. For case of 4-cycle of thermochemical the lowest value of CT number found to be was 650, which means the fractures created during 4-cylce of thermochemical was highly conductive. Also, medical CT images clearly illustrate the number and pattern of fractures generated from the experiments. An attempt was made to correlate the number of cycles in thermo chemical fracturing experiments with the corresponding breakdown pressures as shown in Fig. 13. A perfect polynomial equation with de gree of two was fitted on the experimental dataset. The trend yields a maximum value coefficient of determination (R2) of 1.
Author contributions 1. Zeeshan Tariq: initiated the paper write up and conducted the experimental work, 2. Mohamed Mahmoud: initiated the idea of the paper and supervise the work, 3. Abdulazeez Abdulraheem: Supervised the work and planned/analyzed the scratch test results, 4. Ayman Al10
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Nakhli: provided the reaction kinetics of thermochemicals and help design the fracture experiments., 5. Mohammed BaTaweel: contributed to the design of the cyclic fracturing experiments and the analysis part.
Acknowledgment The College of Petroleum and Geoscience, at King Fahd University of Petroleum & Minerals, and Saudi Aramco are acknowledged for the support and permission to publish this work. Saudi Aramco is also acknowledged for funding this research under project number CIPR 2316.
Declaration of competing interest We confirm that there is conflict of interest.
Nomenclature A C CT E Edyn F EDS EOR FEM HPHT ISRM NMR P PDC PFD PR S SEM SRV Vp Vs XRD XRF
Cross-sectional Area Specific Heat Capacity Computerized Tomography Young’s Modulus, GPa Dynamic Young’s Modulus, GPa Axial force Energy Dispersive X-ray spectroscopy Enhanced Oil Recovery Finite Element Method High Temperature High Pressure International Society of Rock Mechanics Nuclear Magnetic Resonance Compressional wave Poly Diamond Crystalline Process Flow Diagram Poisson’s Ratio Shear wave Scanning Electron Microscopy Stimulated Reservoir Volume Compressional wave velocity, m/s Shear wave velocity, m/s X-Ray Diffraction X-Ray Fluorescent
Greek Symbols Intrinsic specific energy Dynamic Poisson’s Ratio Bulk density, g/cc λ Thermal Conductivity μ Linear attenuation coefficient μW Linear attenuation coefficient of water
ε υdyn ρ
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.petrol.2019.106859.
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