High-Efficiency Proppant Placement Technology and Its Application

Abstract

Aiming at solving the problem of sand production and limited sand carrying distance during slick water fracturing of unconventional oil and gas reservoirs, an efficient proppant placement technology based on fiber structure stabilizer is innovatively developed: During the sand fracturing construction of oil and gas wells, the fiber and fiber structure stabilizer is pumped into the reservoir with proppant in sand-adding stage, and the components including fiber, stabilizer, proppant and drag reducing agent form a stable net-like structure to improve the proppant placement distance and stacking height, thus increasing the effective support volume. With this technology, the seepage mode of artificial fracture is changed and its conductivity of supporting fracture body is greatly improved; the critical flow rate of proppant flowback is increased up to tens of folds compared in the same operation condition. It has been applied in more than 30 wells of tight gas and shale oil and gas in China. The sand production rate after fracturing in tight gas was 79% lower than conventional fracturing without additives of fiber and fiber structure stabilizer. The effect of proppant flowback control and hydrocarbon stimulation is remarkable.

1. Introduction

Large-scale volume fracturing with a complex fracture network mainly composed of low-viscosity slick water is an effective measure for unconventional oil and gas reservoirs to achieve production [1]. In order to achieve a high production capacity for fracturing wells, it is necessary to carry the proppant to the far end of the fracture and support the artificial fracture for a long time without displacement of the proppant [2]. How to improve the proppant delivery efficiency, how to maintain the stable placement of the proppant, and how to obtain better stacking height and porosity in the fracture have been challenging for researchers [3, 4, 5, 6, 7]. In the past few decades, researchers have carried out a large amount of research to determine the key factors affecting proppant transportation in order to effectively improve its efficiency.

Among them, fluid viscosity has a key impact on proppant transport efficiency. Research shows that the higher the viscosity of fracturing fluid, the longer the proppant transport distance. However, there will be residue after fracturing fluid gel breaking [8, 9]. The higher the concentration of thickener, the greater the damage of residue. For unconventional reservoirs, it is difficult for high-viscosity fracturing fluid to form a complex fracture network; meanwhile, it aggravates reservoir pollution. Nevertheless, the low-viscosity fracturing fluid has a limited ability to carry proppant, which is prone to accumulate near the fracture opening and difficult to penetrate the deep part of the fracture [10]. Therefore, improving the transport efficiency of proppant in clean water or low-viscosity fracturing fluid has become an important research direction.

In order to improve the sand transportation efficiency of clean water or low-viscosity fracturing fluid, the petroleum industry has developed a large number of new products and processes, including self-suspension proppant and low-density proppant. They can significantly improve the efficiency of support transportation, but at present, the manufacturing of self-suspension proppant and low-density proppant is relatively complex and costly, and it cannot solve the problems of proppant backflow [11, 12]. In order to prevent proppant backflow, a film-coated proppant was introduced, which not only has higher mechanical properties but also effectively prevents the backflow of proppant in the blasthole. Nonetheless, it cannot prevent the flow of the distal proppant to the blasthole. Research showed that this kind of proppant is greatly affected by temperature. When the temperature reaches the critical value, it may reduce the conductivity of the proppant [13].

Therefore, researchers began to focus on a simple physical method—combining fiber with proppant to effectively prevent proppant backflow. In high-viscosity fracturing fluid, fiber and proppant form a fiber-proppant cluster to improve the delivery efficiency of proppant and increase the conductivity of proppant. The network structure formed by the fiber can maintain the stability of the proppant and prevent the backflow of the proppant [14, 15, 16]. However, this technology still has certain limitations, and the fracturing fluid still needs to be highly viscous [17]. In low-viscosity fracturing fluid, the synergy of fiber-proppant is poor; it is difficult to form a fiber-proppant cluster, and the sand-carrying performance is poor. In the process of sedimentation, the fiber settles slowly and is prone to escape from the proppant (the escape rate is about 50%). The fiber cannot form a stable network structure inside proppants and cannot effectively prevent its backflow.

In order to solve the above problems, a new technology, proppant efficient placement technology is developed. It can carry proppant farther, form a sand body with higher conductivity, and greatly reduce proppant backflow.

2. Experimental preparation

2.1 Materials and instruments

The experimental materials include: drag reducing agent (Polyacrylamide(PAM) lotion), gel breaker, polyester fiber (6 mm in length), fiber structure stabilizer, and distilled water, and the materials used are all from Chengdu Laoenpusi Technology Co., Ltd. (LEPS); Quartz sand for fracturing site (70/140 mesh).

Experimental instrument: self-made visual proppant settling device (device specification: 200 × 300 × 5 mm), from LEPS; American Petroleum Institute (API) conductivity tester, from Southwest Petroleum University; self-made simulated proppant conveying device, from LEPS; ZEISS EV0 MA15 Scan electron microscope (SEM), from Zeiss, Germany.

2.1.1 Microstructure observation of high-efficiency placement agent

The procedure for sample preparation is as follows: mix 15 g of quartz sand, 0.45 g of fiber (0.3 wt%), and 0.1 mL of drag reducer in 100 mL of water; stir until completely dispersed; add 0.3 mL of fiber structure stabilizer into the solution, stirring for 2 min. SEM was used to characterize the mixture of fiber, proppant, fiber structure stabilizer, and drag reducer, as well as a blank control of the mixture without fiber and fiber structure stabilizer.

2.1.2 Evaluation of proppant placement height

The proppant placement height: The height of the opaque part accumulated in the visual sinking device.

The visual proppant settling device (Figure 1) was used to observe the proppant placement height after adding different fiber structure stabilizers at 10%, 20%, and 30% of sand ratio, respectively. The experimental steps are as follows: mix appropriate amounts of water, quartz sand, drag reducer, fiber, and fiber structure stabilizer in a 1000 mL beaker; set the stirrer speed at 600 r/min and stir for 2 min until the liquid is completely uniform. Pour the configured supporting agent solution into the visual device, and use the ruler of the device as the reference to observe and record the layout height of the support agent after 1 hour of settlement time.

Figure 1.

Visual proppant settling device.

2.1.3 Conductivity test

An API conductivity tester was used to test the conductivity of proppant only and proppant with fiber and fiber structure stabilizer.

Preparation for proppant high-efficiency placement samples: add a mixture of 0.6 g of fiber, 0.4 ml of resistance reducer, 2 ml of fiber structure stabilizer, 0.4 ml of gel breaker, and 120 g of proppant in a beaker containing 400 ml of clean water; use a stirrer to stir (600 r/min) for 2 minutes; then take out the sample and heat the sample in a 70°C water bath for 2 h to break the gel completely and complete the preparation of quartz sand+ fiber+ fiber structure stabilizer system sample. Preparation of a blank sample: The procedure is similar to the one described above with the difference of no fiber and fiber structure stabilizer added.

The conductivity tests of different samples were carried out according to the China Petroleum Industry Standard of SY/T 6302–2009—Test method for conductivity of fracturing Proppant.

Test steps of diversion capacity: Assemble the diversion chamber, weigh 48 g sample evenly into the diversion chamber, put the upper piston into the diversion chamber, connect the hydraulic press and pipeline, set the flow rate of constant flow pump 2 ml/min, discharge the air in the diversion chamber, start and set the hydraulic press to the required pressure, improve the displacement flow, observe and record parameters, and calculate the diversion capacity.

$${\mathit{kW}}_f=\frac{5.55\mathit{\mu Q}}{\mathrm{\Delta }P}$$E1

where: ${\mathit{kW}}_f$— Diversion capacity, $\mu$— Fluid viscosity, $Q$— Displacement flow, and $\mathrm{\Delta }P$— Pressure difference.

2.1.4 Critical flow rate of proppant flowback test

Critical flow rate of proppant flowback: The lowest flow rate at the beginning of sand production.

An API conductivity tester was used to test the sand control performance of proppant. Samples were prepared following the procedures described in 2.2.2. The critical flow rates of proppant flowback under different closure stresses were tested using different fiber dosages.

Test steps of a critical flow rate of proppant flowback: Assemble the diversion chamber, weigh 72 g samples and pour them evenly into the diversion chamber, put the upper piston into the diversion chamber, connect the hydraulic press and pipeline, start the hydraulic press to press to the set pressure, increase the displacement flow rate, observe the outlet, and record the displacement flow rate during sand production at the outlet.

2.1.5 Proppant delivery capacity test

Compared with conventional hydraulic fracturing technology, the key to high-efficiency proppant placement technology is the development of fiber structure stabilizers. The use of fiber and fiber structure stabilizers to improve the sand placement efficiency and placement height of proppant in fractures, reduce the return of proppant after fracturing, and maintain the conductivity of artificial fractures formed by fracturing is crucial to improve the reconstruction effect and recovery efficiency.

In order to fully understand the migration state and distribution law of proppant in the fracture, the simulated proppant conveying device was used to study the sand carrying capacity of the high-efficiency proppant placement technology and the placement profile of proppant in the simulated fracture (Figure 2). The migration states of quartz sand only and quartz sand added with fiber and fiber structure stabilizer in the simulated fracture of flat plate were tested, respectively.

Figure 2.

Flow chart of proppant transportation experiment (device specification: 300 × 30 × 0.5 cm)where: Mixer—Mixing preparation of sand carrying liquid; dispensing tank—Store carrying fluid; screw pump—Pump the sand carrying liquid to the settling device; self-made visual proppant settling device—Observe the migration characteristics of sand carrying fluid; and camera and data processing system—Collect and analyze experimental data.

The experimental procedure is as follows:

1. Check the sealing of the experimental device;

2. Preparation of sand-carrying fluid system;

3. Open the liquid inlet valve and pump the liquid into the device;

4. Use a high-definition camera to record the migration state of fiber and proppant after entering the plate;

5. Complete the experiment.

3. Action mechanism of high-efficiency proppant placement technology

3.1 Action mechanism of fiber structure stabilizer

The interaction diagram of the fiber structure stabilizer, fiber, and quartz sand is shown in Figure 3 as a special bridging molecular structure; the fiber structure stabilizer can behave like a stable bridging structure between the fiber surface and the polyacrylamide(the polymer) and increase the adsorption between the fiber and quartz sand, resulting a uniform and stable three-dimensional network structure containing the fiber and the proppant in the slick water. Consequently, it effectively reduces the escape of fiber from proppant in slick water and maintains a height of formed stable structure in the fracture for a long time.

Figure 3.

Schematic diagram of interaction among fiber structure stabilizer, fiber, and quartz sand.

Figure 4 shows the microstructures visualized by SEM: No fiber or structural stabilizer was added to the blank sample, the net-like PAM is adsorbed on the surface of quartz sand, and the quartz sand is tightly packed; with fiber and fiber structure stabilizer, PAM is not only adsorbed on the surface of quartz sand but also on the surface of fiber. The distance between quartz sand is 2–4 times greater than that of blank samples, providing more fluid flow channels. At the same time, the bridging structure of the stabilizer is formed between the fiber structure stabilizer and the polyacrylamide, fiber, and quartz sand, which greatly reduces the escape of fiber from quartz sand.

Figure 4.

Microstructure of proppant (left: no fiber and fiber structure stabilizer and right: With fiber and fiber structure stabilizer).

3.2 Technical mechanism of high-efficiency proppant placement

The effect of fiber and fiber structure stabilizer on proppant during fracturing is functionally similar to that of putting a “glider” on proppant as illustrated in Figure 5. Conventional proppant is mainly subject to the resistance of gravity, buoyancy, and drag force of slick water in the process of settlement; with fiber and fiber structure stabilizer, the force distribution of the proppant is changed with additional traction by the fiber.

Figure 5.

Proppant settlement mechanism under a static settlement.

Eqs. (2) and (3) are the drag and buoyancy equations, respectively. From Eq. (3), it can be seen that due to the small volume of the proppant, its buoyancy is small, therefore it is easier to settle in the process of flow. From Eq. (2), it demonstrates that in the fluid, the sectional area of the individual proppant is small, resulting in a small drag force. Hence the proppant is not easy to be carried by the fluid; after adding fiber and fiber structure stabilizer, the fiber, the fiber stabilizer, and the proppant form a composite structure. The relative cross-sectional area of the fiber is larger than that of the single-particle proppant, and the buoyancy and drag forces are increased, making the proppant more easily carried by the fluid into the fracture depth.

$$F_{\text{drag}}=\frac{1}{2}\rho v^2{\mathit{AC}}_d$$E2

where: $F_{\mathit{\text{drag}}}$—drag force, $C_d$—drag coefficient, $\rho$—Fluid density, $v$—flow velocity of the object relative to the fluid, and $A$- cross-sectional area of the object.

$$F_{\text{buoyancy}}={\rho }_{\text{liquid}}\mathrm{g}V$$E3

where: $F_{\mathit{\text{buoyancy}}}$—Liquid buoyancy, $\rho$—Fluid density, $g$—acceleration of gravity, and $V$—volume of object.

4. Experimental results and discussions

4.1 Evaluation of proppant placement height

As shown in Figure 6, from left to right, are quartz sand only, quartz sand+0.3 wt% fiber, quartz sand+0.3 wt% fiber+0.1% fiber structure stabilizer, quartz sand+0.3 wt% fiber+0.3% fiber structure stabilizer, quartz sand+0.3 wt% fiber+0.5% fiber structure stabilizer, quartz sand+0.3 wt% fiber+0.7% fiber structure stabilizer, respectively. In low-viscosity slick water, only fiber is added, and the placement height of the proppant is not increased. After adding a fiber structure stabilizer further, the placement height increases apparently. The placement height increases with the increase of fiber structure stabilizer concentration. When the stabilizer dosage reaches 0.5%, the proppant placement height increases by 66.7% compared with the blank sample. Afterward, the proppant placement height shows little increase when the stabilizer dosage continues to increase.

Figure 6.

Effect of different dosages of fiber structure stabilizer on placement height with a 10% sand ratio.

The pure quartz sand settled evenly in the slippery water and was laid dense; in the process of settlement Fiber + quartz sand, there is a phenomenon of non-uniform settlement because the density of quartz sand is large, the settlement speed is fast, the fiber density is small, the settlement speed is slow, the fiber is concentrated in the upper part of the quartz sand, the influence of proppant placement height is small; In the sedimentation process of fiber + quartz sand + fiber structure stabilizer, under the action of fiber structure stabilizer, the three components form a composite proppant cluster, which can maintain its original structure after settlement, improve the placement volume of proppant, and increase the conductivity, as shown in Figure 7.

Figure 7.

Schematic diagram of proppant settlement.

4.2 Proppant delivery capacity test

For the control sample of quartz sand only in low viscosity and slick water, it is prone to accumulate at the entrance, and the migration distance was short as shown in Figure 8. The cross-section of such conventional quartz sand placement was relatively dense with conductivity attributed to the gap of quartz sand accumulation only.

Figure 8.

Quartz sand filling profile: With quartz sand only.

Figure 9 shows the filling profile of 0.2% fiber+0.5% fiber structure stabilizer+ quartz sand. Enhanced by fiber and fiber structure stabilizer, the quartz sand placement profile moved to the right as a whole, indicating its increased migration ability. The proppant stacking height increased with a relatively fluffy filling profile, resulting in a greatly improved oil/gas flow channel.

Figure 9.

Quartz sand filling profile: With 0.2% fiber+0.5% fiber structure stabilizer+ quartz sand.

When the amount of fiber increased to 0.4% and the other components were kept constant, the amount of quartz sand settling in the flat plane device was small (Figure 10), and a large portion of the quartz sand was carried out of the outlet end, indicating that the sand carrying capacity was further enhanced. Meanwhile, the filling profile was fluffier, which was more conducive to oil and gas seepage.

Figure 10.

Quartz sand filling profile: With 0.4% fiber+0.5% fiber structure stabilizer+ quartz sand.

4.3 Conductivity test

Closure stress refers to the force acting on the fracture wall to make the fracture appear closed after the hydraulic fracturing operation stops pumping. In this paper, the closure stress refers to the pressure exerted on the test sample by the hydraulic press.

As shown in Figure 11, the conductivity of the proppant decreased as closure stress increased. However, under the same closure stress, the conductivity of quartz sand applied with high-efficiency proppant placement technology was always higher than that of quartz sand only. When the closure stress was less than 30 MPa, the conductivity of the quartz sand+ fiber+ fiber structure stabilizer system was more than 40% higher than that of quartz sand only.

Figure 11.

Effect of fiber structure stabilizer on proppant conductivity.

After adding a fiber structure stabilizer to the fiber and quartz sand, the fiber and quartz sand form clusters, and the quartz sand distribution system changes from uniform to nonuniform. Under higher closure pressure, the local conductivity increases, making the test value of conductivity higher than that of quartz sand only.

4.4 Critical flow rate of proppant flowback test

Critical flow rate of proppant flowback reflects the proppant’s ability to resist fluid erosion. The higher the critical flow rate of proppant flowback, the stronger the anti-erosion ability, the harder the proppant returns, and the higher the long-term diversion ability, which is conducive to improving the long-term production capacity of the well.

Figure 12 illustrates the effect of fiber concentrations on the critical flow rate of proppant flowback, demonstrating that the closure pressure and the critical flow rate of proppant flowback increased as the dosage of fiber increased. The critical flow rate of proppant flowback was 5 mL/min without fiber. With 0.4 wt% of fiber, it increased about 40 times under the closure stress of 5 MPa; furthermore, the critical flow rate of proppant flowback increased with closure stress. When the closure stress reached 10 MPa, the critical flow rate of proppant flowback increased by 104 times.

Figure 12.

Effect of fiber concentration on a critical flow rate of proppant flowback of 70/140 mesh quartz sand.

Figure 13 shows the morphology comparison of with 0 wt% fiber and with 0.4 wt% fiber after the tests of critical flow rate of proppant flowback under 5 MPa closure stress. Without fiber, the proppant was carried out of the conductivity chamber by the backflow fluid in a large amount. With additive of 0.4 wt% fiber, the proppant placement structure remained as a whole, the network structure of which maintained its stability and reduced its backflow greatly.

Figure 13.

Proppant morphology after the completion of critical flow rate test of proppant flowback (top: With 0 wt% fiber, bottom: With 0.4 wt% fiber).

5. Field application

High-efficiency proppant placement technology has been applied in China’s tight oil/gas and shale oil/gas for more than 30 wells. Production data from Shengli oil Production Plant, Zhongjiang Gas Production Plant, Qiulin Gas Production Plant, and Changning Gas Production Plant for production and sand production comparison.

1. application in tight gas

   There are 23 wells for tight gas application, of which 10 are located in Zhongjiang Gas Field in West Sichuan Depression of Sichuan Basin, respectively, J343HF, J346HF, J220-2HF, J240-1HF, J220-5HF, J327-3HF, J330-3HF, J318-3HF, L89HF, and J347HF; Among them, 13 wells are located in Qiulin Gas Field in West Sichuan Depression of Sichuan Basin, including wells Q511-1, Q511-2, Q5113, Q511-4, Q516-1, Q507, Q822-2, Q206-6, Q206-9, Q202, Q212, Q220-2, and Q201. The 20 comparison wells are still from the above gas fields, including well numbers J318-2HF, J318-1HF, J220HF, J220-1HF, J220-3HF, J220-4HF, J33-46HF, J240HF, J33-58HF, Q220-1, Q220-3, Q516-2, Q209, Q13, Q822-1, Q822-2, Q211, Q823, Q503, Q817.

   The proppant efficient placement technology has been applied to 23 wells in tight gas. Compared with 20 wells without application of this technology based on the production data and sand production (as of January 30, 2023), the results show that the average daily gas production per kilometer increased by 32% in the initial stage as well as by 40% according to pressure drop per unit (Figure 14); the sand production rate was 79% lower than that of conventional process wells. For wells with similar geological conditions and construction parameters, the length of the supporting half fracture increased by 35.5%, and the volume of supporting fracture increased by 33.8%, respectively, based on the fitting of construction curves after hydrofracturing for the high-efficiency proppant placement technology when compared with the traditional technology. The high-efficiency proppant placement technology achieved similar supporting half fracture length and supporting fracture volume with 25% less sand-adding compared with those of the traditional process wells using quartz sand only in the whole process in similar geological conditions.

2. application in shale oil

   Shale oil application wells are located in Dongying Depression of Jiyang Depression, including FY1HF, FY2HF, FY3HF, FY101HF, and LY1HF.

   This technology has been used in five wells of shale oil with average daily oil production per kilometer increasing by 22% compared with those in an earlier stage. Take FY1HF well as an example: the horizontal section of the well is 1811 m long with mainly Class I+ II reservoirs. There are 236 clusters of staged fracturing in 28 sections, 25 sections of which adopt high-efficiency proppant placement technology, and the other 3 sections adopt conventional impulse sand fracturing with fiber. The construction was successfully completed according to the design, with an average sanding strength of 3.35m³/m and a liquid strength of 41m³/m. The chemical tracer was used to monitor the transformation effect and postfracturing production capacity.

   The monitoring of chemical tracer shows that the average oil production of a single section using the high-efficiency proppant placement technology is 37% higher than that using the conventional impulse sand fracturing with fiber.

   As shown in Figure 15, the peak daily oil production of FY1HF is about 269.58m³ with daily gas production of about 41276m³ at the initial stage of production. When compared with an adjacent well of FY1-1HF, which has a peak daily oil production of 262.8m³ at the initial stage of production, the length of FY1HF horizontal section is 209 m shorter and the total hydrocarbon is 8% lower; however, the production after fracturing is 15% higher than that from FY1-1HF, far beyond the expected geological target.

3. application in shale gas

   Well NX4, an application well of shale gas, is located in the south wing of Ordovician top structure in the Changning anticline structure. There are also wells NX3 and NX5, the production wells of the platform at the same time.

   As shown in Figure 16, NX-4, NX-3, and NX-5 wells from the NX platform have similar horizontal section lengths, reservoir physical properties, and gas-bearing properties. The platform adopted processes such as multi-cluster in a section, large-displacement construction, and high-strength sanding process with a sanding strength of 2m³/m. After hydrofracturing, the same drainage measure was adopted with proppant flowback of about 15 and 20 m³ from NX-3 and NX-5, respectively. By comparison, only 0.2 m³ of proppant was back flowed from NX-4 after using the high-efficiency proppant placement technology, the amount of which was about 1% of that of adjacent wells. Moreover, the production of NX-4 well after fracturing was 1.2 and 1.35 times than that of the adjacent wells NX-3 and NX-5, respectively.

Figure 14.

Production comparison of tight gas applications (high-efficiency proppant placement technology wells: J343HF、J346HF、J220-2HF、J240-1HF、J220-5HF、J327-3HF、J330-3HF、J318-3HF、L89HF、J347HF、Q511-1、Q511-2、Q511-3、Q511-4、Q516-1、Q507、Q822-2、Q206-6、Q206-9、Q202、Q212、Q220-2, and Q201; conventional technology wells: J318-2HF、J318-1HF、J220HF、J220-1HF、J220-3HF、J220-4HF、J33-46HF、J240HF、J33-58HF、Q220-1、Q220-3、Q516-2、Q209、Q13、Q822-1、Q822-2、Q211、Q823、Q503, and Q817).

Figure 15.

Shale oil application yield comparison (high-efficiency proppant placement technology well: FY1HF; conventional technology well: FY1-1HF).

Figure 16.

Shale gas application yield comparison (high-efficiency proppant placement technology well: NX-4; conventional technology wells: NX-3 and NX-5).

6. Conclusions and suggestions

1. High-efficiency proppant placement technology can effectively reduce the fiber escape from the proppant in low-viscosity slick water and improve the proppant stacking height. When the sand ratio is 10% with additives of a fiber dosage of 0.3 wt% and a fiber structural stabilizer dosage of 0.5 wt%, the proppant stacking height is increased by 66.7%; it does not change noticeably when only fiber is added.

2. High-efficiency proppant placement technology has changed the seepage mode of artificially supporting fracture body, and the distance between proppants is increased by 2–4 times, thus greatly improving the conductivity of artificially supporting fracture body.

3. High-efficiency proppant placement technology has better sand carrying capacity. With additives of 0.3 wt% fiber and 0.5 wt% fiber structure stabilizer, it can increase the proppant placement distance by more than 30%.

4. The construction curve fitting shows that high-efficiency proppant placement technology can effectively enhance the proppant placement length by 35.5% and the effective support volume by 33.8% at the same scale; equivalent placement distance and supporting volume can be achieved with application of such technology with 25% less of sanding scale, resulting of more than 20% higher of oil and gas production. It demonstrates that the technology can reduce costs and increase efficiency.

5. High-efficiency proppant placement technology can effectively improve the sand control effect and reduce the sand production rate by 79% compared with those without measures.

6. The field test shows that the technology has good adaptability, can meet the sand control requirements of tight gas and shale oil/gas, improve the production effect after fracturing, and has broad application prospects.