The clean fracturing fluid, thickening water, is a new technology product, which promotes the advantages of clean fracturing fluid to the greatest extent and makes up for the deficiency of clean fracturing fluid. And it is a supplement to the low permeability reservoir in fracturing research. In this paper, the study on property evaluation for the new multicomponent and recoverable thickening fracturing fluid system (2.2% octadecyl methyl dihydroxyethyl ammonium bromide (OHDAB) +1.4% dodecyl sulfonate sodium +1.8% potassium chloride and 1.6% organic acids) and guar gum fracturing fluid system (hydroxypropyl guar gum (HGG)) was done in these experiments. The proppant concentration (sand/liquid ratio) at static suspended sand is up to 30% when the apparent viscosity of thickening water is 60 mPa·s, which is equivalent to the sand-carrying capacity of guar gum at 120 mPa·s. When the dynamic sand ratio is 40%, the fracturing fluid is not layered, and the gel breaking property is excellent. Continuous shear at room temperature for 60 min showed almost no change in viscosity. The thickening fracturing fluid system has good temperature resistance performance in medium and low temperature formations. The fracture conductivity of thickening water is between 50.6
Fracturing is one of the main measures for low-permeability reservoir reconstruction. The performance of fracturing fluid is an important factor affecting the increasing production of oil field after construction [
No damage or negative fracturing fluid represents the direction of fluid development in the fracturing industry. At present, clean fracturing fluid is a representative system of low-damage fracturing fluid [
In 2009, Xinquan et al. published a report on the clean fracturing fluid of viscoelastic surfactant, and pointed out that the viscoelastic surface active agent formed a colloidal vermicular structure in the presence of brine, thus changing the viscoelasticity of the solution [
But, application of many fields proves that all the current clean fracturing fluids have residues without exception. These residues are bound to cause serious clogging of the strata and filling layers to greatly reduce the permeability, and the cumulative damage can reach more than 90%, which greatly reduces the effect of fracturing and cannot achieve continuous mixing. This situation is especially prominent for low pressure and low permeability reservoirs, which often leads to fracturing failure.
In order to solve the above problems, a new type of multicomponent thickening water clean fracturing fluid system has been studied. The main component of the thickening fracturing fluid system (2.2% octadecyl methyl dihydroxyethyl ammonium bromide (OHDAB) +1.4% dodecyl sulfonate sodium +1.8% potassium chloride and 1.6% organic acids) can be quickly cross-linked to carry sand, when it encounters crude oil and formation water, it can break gel without adding a gel breaker. After breaking glue, no residue is found, and the viscosity of gel breaker is <5 mPa·s. In addition, the system can be continuously mixed to shorten the fracturing cycle and can carry acid (HCL and HF) of concentration 1%–9% and organic acid 1%–3%, to achieve combination of acidizing and sand fracturing. The flow-back liquid can be recovered directly, and the same thickener can be recycled and reused. It saves water consumption and is suitable for horizontal wells to improve efficiency, to meet the needs of saving energy and environmental protection and large-scale factory operation.
The clean fracturing fluid, new multicomponent and recoverable thickening water, is mainly composed of a variety of special surfactants. The application of the composite liquid mutually contains molecular force between material and attachment, and the complex becomes transparent liquid substances, namely thickener. When the thickener is exposed to aqueous solution, the surfactant molecules are released and diffused rapidly under the influence of polar substances and aggregated to form wormlike micelles. The equilibrium system and micelle charge of the special organic ions make the micelles grow. A salt of the variable length wormlike micelles entangled, thus forming a uniform mesh space-like floc-like in the system, so as to form a gel [
In the gel system, there are mainly physical interactions between the surfactant and the salt, which are different from guar gum in the polymer fracturing system connected with each other in chemistry. Therefore, when the surfactant fracturing fluid meets the appropriate amount of formation water and oil gas, the intermolecular interaction distance between the surfactant and salt will increase. Furthermore, the entanglement state of wormlike micelles is destroyed, even the wormlike micelles disintegrated into simple micelles, and these can make the gel system automatically break [
Schematic diagram of the mechanism of thickening water.
HAD-CQ2A double deflector long-term conductivity tester (Beijing Heng Aude instrument limited company), SNB-2 digital viscometer (Shanghai Jingke day U.S. Trade Co. Ltd.), mixer, 250 mm cylinder, and electric thermostatic water bath (DGSY Beijing) were used.
The new type thickening water fracturing fluid is mainly composed of viscoelastic surfactant solution. The main agent of this experiment is the ionic surfactant with concentration of 2.2% octadecyl methyl dihydroxyethyl ammonium bromide (OHDAB). The role of the additives in thickening water fracturing fluid mainly depends on the cationic group adsorbed on the surfactant, which reduces the repulsive force between the cationic groups and increases the micelle growth. In this experiment, 1.4% dodecyl sulfonate sodium, 1.8% potassium chloride, and 1.6% organic acids are selected as the auxiliaries. The guar gum system used in this experiment is the most commonly used hydroxypropyl guar gum (HGG) in China.
The new type thickening water gel and guar gum fracturing fluid gel are packed in No. 1 and No. 2 cylinder, and then the proppant with different sand ratio (volume ratio) is added. The static sand-carrying capacity is measured by observing the proppant in two cylinders at the time of settlement, and the results are shown in Table
Comparison of static sand-carrying capacity of two kinds of fracturing fluids.
Sand/liquid ratio (%) | Settling velocity (mm·s−1) | |||||
---|---|---|---|---|---|---|
60 (mPa·s) | 90 (mPa·s) | 120 (mPa·s) | ||||
Thickening water | Guar gum | Thickening water | Guar gum | Thickening water | Guar gum | |
5 | 0.02973 | 0.04136 | 0.02461 | 0.03162 | 0.02016 | 0.02914 |
10 | 0.07823 | 0.09161 | 0.07012 | 0.08136 | 0.06812 | 0.07216 |
20 | 0.11986 | 0.14161 | 0.11014 | 0.13264 | 0.10011 | 0.12234 |
30 | 0.14832 | 0.17214 | 0.14036 | 0.16213 | 0.13613 | 0.15614 |
40 | 0.19613 | 0.21463 | 0.18214 | 0.20436 | 0.17614 | 0.19632 |
Comparison of static sand-carrying capacity of two kinds of fracturing fluids.
It can be seen from Table
The dynamic sand-carrying capacity of thickening water (120 mPa·s) and guar gum (120 mPa·s) at different sand ratios was determined at the stirring rate of 100 r/min and 70°C of temperature. The results are shown in Table
Comparison of dynamic sand-carrying capacity of two kinds of fracturing fluids.
Sand/liquid ratio (%) | Thickening water | Guar gum | Clean water |
---|---|---|---|
5 | No layered | No layered | Layered |
10 | No layered | No layered | Layered |
20 | No layered | No layered | Layered |
30 | No layered | A little precipitation | Layered |
40 | No layered | Layered | Layered |
It can be seen from Table
After thickening fracturing fluid is subjected to shear action, the network structure formed between micelles is destroyed, resulting in reduced number of micelles. And, the shear does not destroy the structure of the small molecule, but only reduces the degree of polymerization of the micelles; when the shear action stops, the micelles in the solution are rewound through the association to restore the reticular structure, and the viscosity is also restored. However, under the external shear effect, the network structure of gum fracturing fluid is destroyed, and the polymer molecular chain also shortens and degrades, and the viscosity of the system decreases. When the shear effect disappears, the molecule cannot recover the original viscosity through cross-linking. Therefore, compared with the polymer system, the carrying capacity of the thickened water fracturing fluid system is better.
The experiment is divided into two groups. The volume ratio of simulated oil to thickening water (120 mPa·s) and guar gum fracturing fluid (120 mPa·s) is 1 to 20. The viscosity in different temperature and shear time was measured by the rotating viscometer at the speed of 7.5 r/min, and the results are shown in Table
Comparison of gel breaking property of two kinds of fracturing fluids.
Time (min) | Apparent viscosity (mPa·s) | |||||
---|---|---|---|---|---|---|
50 (°C) | 60 (°C) | 70 (°C) | ||||
Thickening water | Guar gum | Thickening water | Guar gum | Thickening water | Guar gum | |
10 | 100.3 | 115.6 | 80.3 | 98.2 | 50.2 | 70.2 |
30 | 80.6 | 96.5 | 60.1 | 75.2 | 26.2 | 50.6 |
60 | 40.2 | 62.4 | 30.6 | 45.8 | 12.3 | 30.7 |
120 | 10.2 | 20.1 | 10.1 | 19.6 | 2.6 | 16.8 |
240 | 4.3 | 10.6 | 1.9 | 8.7 | 1.3 | 5.6 |
Comparison of gel breaking property of two kinds of fracturing fluids.
The viscosity of water at normal temperature is 1.3 mPa·s. It can be seen from Table
The gel breaking property of fracturing fluid directly affects the damage degree to formation. Gum fracturing fluid needs to add a certain amount of the gel breaker, so that fracturing fluid can break glue and it is difficult to control the degree of gel breaking. Unlike the gum fracturing fluid, it is not necessary to add the gel breaking agent in the thickened water clean fracturing fluid, but after the hydrocarbon compounds are encountered, the compound can be transformed into a spherical micelle by solubilizing to the micelles formed by the surfactant, and the mesh structure disintegrates to make it lose the viscoelasticity. Or, under the action of dilution of formation water, the content of surfactant is lower than the critical micelle concentration CMC, which makes the solution lose viscoelasticity and automatically breaks glue.
The main experimental instrument is the conductivity tester. The pressure difference and flow rate at each end of each fluid are measured quantitatively when the fracture is supported by simulation. And, when the liquid viscosity and fracture model size are replaced by Darcy’s formula, the flow conductivity tester can automatically calculate the diversion capacity value under different closed pressure conditions [
Comparison of three properties of two kinds of fracturing fluids.
Closing pressure (MPa) | Thickening water fracturing fluid system | Guar gum fracturing fluid system | ||||
---|---|---|---|---|---|---|
Fracture conductivity ( |
Diversion retention rate (%) | Diversion damage rate (%) | Fracture conductivity ( |
Diversion retention rate (%) | Diversion damage rate (%) | |
5 | 150.4 | 82.3 | 17.6 | 120.1 | 61.2 | 40.2 |
10 | 130.6 | 83.1 | 15.3 | 100.6 | 52.3 | 30.7 |
20 | 100.3 | 84.5 | 12.1 | 80.6 | 63.2 | 27.6 |
30 | 80.1 | 84.7 | 10.1 | 50.1 | 64.7 | 16.1 |
40 | 50.6 | 86.2 | 8.9 | 26.3 | 65.1 | 10.8 |
Comparison of fracture conductivity of two kinds of fracturing fluids.
Comparison of diversion retention rate and diversion damage rate of two kinds of fracturing fluids.
It can be seen from Table
Firstly, when the two fracturing fluids are used as prefluid, respectively, the viscosity loss of the thickening water is less than the gum fracturing fluid because of the shear recovery of the thickening water, which is more likely to achieve the fracturing effect. Secondly, when the two fracturing fluids are used as sand-carrying fluid, respectively, the carrying capacity of the thickening fracturing fluid is better than gum fracturing fluid. When the viscosity of the thickening water fracturing fluid is restored through the wellhead hole, it is beneficial to the transportation of the proppant, and the proppant can enter the depth of the crack. After the fracture is closed, the capacity of the support filling belt to the reservoir fluid is higher than the gum fracturing fluid. Finally, the thickening water fracturing fluid has good gel breaking performance in the formation, and the formation damage rate after controlled pressure reentry is small.
The viscosity changes of two kinds of fracturing fluids under continuous shear for 60 min at room temperature (25 r/min) were measured by a rotating viscometer at 750 rpm. The viscosity of two kinds of fracturing fluids at 30∼90°C under rotational speed 75 r/min with a rotational viscometer is shown in Figures
Comparison of viscosity changes with time: two kinds of fracturing fluids.
Comparison of viscosity changes with temperature: two kinds of fracturing fluids.
From Figure
The viscoelastic micelle system in the condensed water fracturing fluid has a strong self-healing property, and at the same shear rate, the system reaches a dynamic equilibrium of micelle destruction and repair. Therefore, the system can maintain the degree that will not be significantly decreased in long time under shear. In the guar gum fracturing fluid system, the polymer chains are sheared and the viscosity of the polymer decreases with the decrease of molecular weight.
From Figure
When the temperature rises, the solubility of the surfactant molecules in the water increases, thus, increasing the aggregation number of the micelles of the surfactant and speeding up the irregular movement of the micelles. This movement of micelles helps the intertwining between the rod-like micelles, and this movement maintains the viscosity of the gelatin. As the temperature continues to rise and when the temperature exceeds the critical micelle temperature, the micelle molecules dissociate and destroy the structure of some small micellar molecules. The viscosity of the thickened water system is not in a dynamic equilibrium state, so the viscosity of the gel decreases.
The area of A block in YC low permeability oil field is 2.03 km2, and the underground pore volume is 196.3 × 104 m3. The geological reserve of the target layer is 132.1 × 104 t, the average single well shooting sandstone thickness is 14.32 m, the effective thickness is 10.23 m, and the average effective permeability is 12.3 × 10−3
The new multicomponent thickened water clean fracturing fluid is a viscoelastic surfactant-based fracturing fluid system. The main component, liquid thickener, can be quickly cross-linked and carries sand when mixed with water. Without adding new equipment, continuous mixing operation can be realized, and the glue can be broken when oil and formation water are encountered. The fracturing fluid can carry different types and concentrations of acid solution. It has the technical features of no residue and low damage, and can be put into operation without swabbing and draining. The new type thickening water can carry acid (HCL and HF) concentration of 1%–9% and organic acid 1%–3% to achieve acidification combined with sand fracturing. The temperature resistance and shear resistance: temperature of new type thickening water is not higher than 75°C and 40°C fracturing fluid continuous shear viscosity of >100 mPa·s, no change of viscosity. The new type of gel breaking thickening water without adding breaking agent, gel breaking in crude oil and formation water, no residue after gel breaking, and gel breaking liquid viscosity of <5 mPa·s. The sand-carrying capacity: 40 centigrade and 35% sand ratio, the static state can be suspended more than 60 min. The core damage rate: the total is less than 20%. The antiswelling rate of clay is >89%.
Guar gum fracturing fluid ensures the success of high-temperature deep fracturing. However, when guar gum fracturing fluid enters the pipeline at high speed and enters the pipeline at high speed during construction, it will cause severe shearing degradation and produce permanent viscosity loss. Moreover, the compound chain of guar gum fracturing fluid is connected with the cross-linked chain between the chains, resulting in the body shape. The result is that the gel breaking is incomplete, so that the residue left behind after breaking the gel will remain in the crack, which will seriously reduce the permeability of proppant filling layer and damage the production layer, resulting in poor fracturing effect. In addition, the cross-linking fracturing fluid is not completely broken, causing sand fracturing well to scour, prolonging operation time, and increasing operating cost. At present, the guar gum fracturing fluid system has high content of water insoluble substance (the national standard requires less than 12% of water insoluble substance) and repeating damage to the formation. The construction process and operation cycle of the two kinds of fracturing fluid are shown in Tables
Comparison of construction process.
Process | Thickening water | Guar gum | Remark |
---|---|---|---|
Before fracturing construction | Preparation and thickener | Preparing and cleaning tank, fracturing fluid material to field distribution | Guar gum fracturing fluid is easy to deteriorate by long time of placement |
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Fracturing construction | Continuous mixing, how much design is required, and how much it can be matched | Construction according to the design requirements | The amount of thickening water can be determined according to the actual amount of field construction, and there is no problem of liquid waste |
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After fracturing construction | Sand washing, well washing, production | Injection, sand washing, well washing, swabbing, and production | |
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Overall evaluation | Thickening water clean fracturing fluid system is more practical than guar gum fracturing fluid system |
Comparison of operation cycle.
Time | Guar gum | Thickening water | Remarks |
---|---|---|---|
Preparation tank | 1-2 days | 1-2 days | Truckage |
Tank cleaning | 3-4 h | 0 h | |
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Liquid distribution | 10 h | 0 h | There is no need for advance solution for thickening water |
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Fracture | 1–1.5 h | 1–1.5 h | The continuous mixing of the thickened water mixture is used in the field, and the amount can be determined according to the actual construction amount of the site |
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Liquid discharge | 2–5 days | 1 day | The thickening water can not be discharged and put into production directly |
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Total | 4.5–8 days | 1.5–3.5 days | Save 3-4 days |
The development effect of the new multicomponent thickened water fracturing hydraulic fracturing 3 wells in A area is shown in Table
The effect of 3 wells fractured by thickening water.
Block | Well | Physical parameters | Oil test | Put into operation | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Shale content (%) | Porosity (%) | Permeability (md) | Oil saturation (%) | Daily oil (t) | Daily water (t) | Daily liquid (m³) | Daily oil (t) | Water cut (%) | ||
A | A1 | 10.3 | 11.5 | 9.2 | 62.2 | 55.3 | 1.2 | 11.9 | 10.8 | 9.2 |
A2 | 10.4 | 10.0 | 8.7 | 63.8 | 42.6 | 1.0 | 8.6 | 7.8 | 9.4 | |
A3 | 15.1 | 9.3 | 6.9 | 65.0 | 52.1 | 0.8 | 9.5 | 8.6 | 9.3 |
Production curve of A1 well.
Production curve of A2 well.
Production curve of A3 well.
At present, the average daily production of 3 wells is 10.3 m3, with an average daily oil production of 9.27 t. And, 3 horizontal wells were selected as contrast wells in the same block, and the average daily production of 3 wells was 8.2 m3, with an average daily oil production of 7.28 t. By comparison, the daily production of the test wells increased by 2.1 m3 compared to the selected wells in the block, and the daily production increased by 1.89 t. It indicates that the acidic clean fracturing fluid of thickening water has a good adaptability to A block, which will pave the way for further promotion Table
Comparison of effect between thickening water fracturing wells and conventional fracturing wells.
Well types | Well | Physical parameters | Oil test | Put into operation | ||||||
---|---|---|---|---|---|---|---|---|---|---|
Shale content (%) | Porosity (%) | Permeability (md) | Oil saturation (%) | Daily oil (t) | Daily water (t) | Daily liquid (m³) | Daily oil (t) | Water cut (%) | ||
Thickening water fracturing well | 3 | 13.2 | 8.3 | 10.3 | 68.3 | 50.2 | 1.0 | 10.3 | 9.27 | 10.0 |
Guar gum fracturing well | 3 | 14.2 | 9.4 | 11.5 | 60.1 | 45.2 | 1.2 | 8.2 | 7.38 | 10.0 |
As a new type of fracturing fluid, clean fracturing fluid have the characteristics of cleaning, low damage, thoroughly gel breaking, little residues, and high flow-back rate. So, it will have good application prospect in low permeability reservoirs. In particular, the reprocessing liquid can be directly recovered, and the same thickener can be added to recycle, which reduces the reservoir damage and saves the water consumption. It is very suitable for improving the efficiency and saving energy for the horizontal well, as well as protecting environment and meeting the demand of large-scale factory operation.
When the shear action stops in new type thickening water, the micelles in the solution are rewound through the association to restore the reticular structure, and the viscosity is also restored. The sand/liquid ratio at static suspended sand is up to 30% when the apparent viscosity of the new system is 60 mPa·s, which is equivalent to the sand-carrying capacity of guar gum at 120 mPa·s. When the viscosity of the thickening water is restored through the wellhead hole, it is beneficial to the transportation of the proppant, and the fracture conductivity is between 50.6 The new system does not need to be added to the adhesive; after the hydrocarbon compounds are encountered, the compound can be transformed into a spherical micelle by solubilizing to the micelles formed by the surfactant and the reticular structure disintegrates to make it lose the viscoelasticity. Under the action of dilution of formation water, the content of surfactant is lower than the critical micelle concentration CMC, which makes the solution lose viscoelasticity and automatically break the gel. The viscoelastic micelle system in the multicomponent thickening water has a strong self repair. Under the same shear rate, the system achieves a dynamic balance of the micellar damage and repair. At room temperature, the shear rate is 60 min, and the viscosity is almost unchanged. As the temperature continues to rise and when it exceeds the critical micelle temperature, the micelle molecules dissociate and destroy the structure of some small micellar molecules. The viscosity system is not in a dynamic equilibrium state, so the viscosity of the gel decreases. It shows that the new system has good temperature resistance in middle and low temperature formation. The actual production of A block shows that the damage of thickening fracturing fluid is low, and the long retention in reservoir will not cause great damage to the reservoir. Therefore, it cannot be pumped directly into production and continuous mixing, shortening the overall testing oil cycle. The flow-back liquid can be recovered directly, and the same thickener can be recycled and reused.
The data used to support the findings of this study are available in the supplementary materials, or from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work is financially supported by the National Natural Science Foundation of China under Grant no. 51504069. The foundation provides the author with many financial help, such as the cost of experimental materials, the layout of the articles, and so on.
The supplementary material includes the data of experiments and field application mentioned in the article, and the tables (from Tables 1–8) and figures (from Figures 1–10) are drawn from these data.