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Quasi-dry CO2 Fracturing - A New Breakthrough of Liquid CO2 FracturingLiang Jin*2, Yan Zheng1, 1. Beijing AP Polymer Technologies Co. Ltd, Beijing, China. 2. BrightGold Consulting LLC, Houston, TX, USA

Copyright 2019, Unconventional Resources Technology Conference (URTeC) DOI 10.15530/urtec-2019-1049

This paper was prepared for presentation at the Unconventional Resources Technology Conference held in Denver, Colorado, USA, 22-24 July 2019.

The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper by anyone other than the author without the written consent of URTeC is prohibited.

Abstract

Liquid CO2 fracturing (Dry CO2 fracturing) is an important solution to reduce the damage to water sensitive or low-pressure formations by water based fracturing fluids. The application of dry CO2 fracturing is limited by the requirement of high cost pressurized blending systems. Fracture scope is also limited by the equipment capacity and the carrying capacity of proppant by liquid CO2.

A quasi-dry CO2 fracturing method has been developed enabling the blending of CO2 viscosifier and proppant at ambient conditions. The method has eliminated the fracture scope limitation caused by the capacity of pressurized blending equipment and simplified the operational complexity making wider application of liquid CO2 fracturing possible. The main characteristics of the quasi-dry CO2 technology will be described in this paper.

Field test was conducted recently on two wells in one of the unconventional blocks in Ordos basin in October 2018. The fracturing operations were successful indicating that the new method is operationally feasible. In comparison, two offset wells were fractured one of which was with energized crosslinked gel and another with slick water/crosslinked gel system. The two offset wells did not have gas production after flowing back. The quasi-dry CO2 fractured wells had significant gas production and 60-90% of the water was recovered.

The new procedure significantly simplifies the operation and reduces the cost of liquid CO2 fracturing. It may find more applications in unconventional reservoirs and EOR with liquid CO2.

Introduction

Hydraulic fracturing is an essential measure in developing tight and shale reservoirs. There are many reservoirs that are water sensitive. Using water based fracturing fluid may cause formation damage such as clay swelling and water blocking in Ordos basin and other areas in China. In some low-pressure reservoirs in this area, energized fluid such as CO2 or N2 are used to help fracturing fluid flow back in the past. In recent years, efforts have been put on the application of alternative fracturing fluids such as liquid CO2.

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Liquid CO2 has high rock breakdown capability and high flow capacity in the broken rock due to its low viscosity (Li, Yang et al. 2018). The advantage of liquid CO2 fracturing has been reported since the beginning of the application of the technology. Dry CO2 fracturing technology has been deployed in Changqing and Jilin oil fields in China in recent years (over 30 wells). Chen Chen (2018) had an extensive literature review in the area of supercritical CO2 fracturing technology development.

Compared to water based fracturing fluid, pure liquid CO2 fracturing (dry CO2 fracturing) has the following advantages:

Non-damaging: no water sensitive damage or water blocking

No residue: no damage to the formation and propped fractures.

Provides energy to assist in flow back after the well is fractured.

CO2 has very low viscosity (high flow capacity), easy to communicate with formation.

CO2 can reduce the viscosity of the crude making it easier to flow, assists in production.

CO2 can replace absorbed hydrocarbon from rock surface through a solvent effect, increases well production

A comparison of friction loss by liquid CO2 has been studied by various authors both in analytical and empirical forms (Li, 2018). Recent study of liquid CO2 on rock strength and breakdown pressure has been conducted (Li, 2015).

One way to resolve the poor proppant suspending problem is to reduce the density of proppant. An ultra-low-density proppant (density of 0.95 to 1.05) was used in liquid CO2 fracturing in Yanchang shale gas field in China in 2018 (Zhang Juntao, 2018). It was operationally successful. However, due to the limited volume constrained by the capacity of the pressurized blender, the desired results did not meet expectation. The high cost of low-density proppant is also a concern. A higher friction loss of liquid CO2 fracturing fluid compared to water based fracturing fluid was observed during the operation. Due to the low viscosity of liquid CO2, creating desired fracture geometry is another challenge.

Another route is to improve the proppant carrying capacity of liquid CO2 by increasing its viscosity. A detailed review of CO2 thickener development was conducted by Lee indicating that this has been a very challenging practice (Lee, et al. 2014).

Despite the above-mentioned efforts, pure liquid CO2 or viscosified liquid CO2 fracturing (Dry CO2 Fracturing) is facing the common challenges:

Low viscosity (The viscosity of liquid CO2 is 0.1 mPa.s), poor proppant carrying capacity, and high friction lead to low proppant concentration (usually less than 1 ppg) and less desired fracture geometry with poor proppant placement.

Fracture scope is limited by the capacity of the pressurized mixing system.

High cost.

A new procedure has been developed by us aiming at taking the advantages of liquid CO2 fracturing, increasing proppant carrying capacity, and mixing at ambient conditions to eliminate fracture scope limitations. This procedure uses water (10 to 30% of total fluid volume) and liquid CO2 to meet our purposes. Since CO2 takes most or a large portion of the fracturing fluid volume (unlike energized fracturing fluids), we call this procedure quasi-dry CO2 fracturing.

Quasi-Dry CO2 Fracturing Method

In the development of the new procedure, we are trying to answer three questions:

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1. Can we avoid the use of a pressurized mixing system?

2. Can we pump at high proppant concentration to improve proppant placement?

3. Can the volume of liquid CO2 be reduced to lower the total cost?

The key element of the new procedure is the joint thickening effect of a self-crosslinking water viscosifier (Chemical A) and a special liquid CO2 viscosifier (Chemical B) mixed in water. The water viscosifier is dissolved in water and the liquid CO2 viscosifier is dispersed in the viscosified water system. The viscosified water system can make high concentration proppant slurry with moderate viscosity so that the high concentration proppant slurry can be pumped to the wellhead. Liquid CO2 is then mixed with the proppant laden water system to form a multi-phase proppant slurry. Figure 1 is the viscosity versus temperature profile of a 30% water, 70% liquid CO2 mixture viscosified by the two chemicals.

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Figure 1. Viscosity – Temperature/Pressure Profile of a 70% CO2 - 30% Water System

Lab tests indicate that the two chemicals dissolve and disperse very rapidly in water to reach desired rheology making it possible to mix proppant at high concentration to form a high concentration proppant slurry. With such a high concentration proppant slurry (Note: It contains dispersed liquid CO2 viscosifier), it is possible to pump liquid CO2 separately and combine the two fluid systems to form a quasi-dry fracturing fluid. Such a fluid system would have the desired rheology and high concentration of proppant. See-through cell tests indicate that proppant can be suspended very well at high temperature in the slurry and the fluid viscosity measured by falling ball viscometer was about 40 mPa.s. Figure 2 shows a perfect proppant (high density 30/50 ceramic proppant) suspension at 89 °C and 20 MPa. No proppant settling was observed for quite a long time (30 minutes without proppant settling). As can be seen from the viscosity versus temperature/pressure data presented in Figure 1, at low temperature (surface), the suspending capacity would be even better since the viscosity is almost doubled. Such a good suspending capability was enabled by the high elasticity of the fluid. Rheological properties will be discussed in our future papers.

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Figure 2. See-through Cell Showing a Perfect Proppant Suspension (30 minutes without proppant settling).

Having the main roadblocks resolved, a quasi-dry CO2 fracturing technology was proposed. The main characteristics of the quasi-dry CO2 fracturing technology include:

Joint thickening effect realized by a water based viscosifier and a special liquid CO2 thickener making the liquid CO2 a good proppant carrying fluid with low friction (70% friction reduction).

Mix proppant with small amount of water (10 to 30% of the total fluid volume) viscosified by above-mentioned chemicals to form a highly viscoelastic high concentration (9.5-15.9 ppg) proppant slurry. The mixing is at ambient condition.

Pump liquid CO2 at high pumping rate and mix it with the high concentration proppant slurry at the wellhead. At turbulent flow condition, the liquid CO2 immediately increase its viscosity and form a multi-phase proppant slurry. The viscoelastic behavior of the combined slurry has high proppant carrying capacity. The combined effective proppant (high density ceramic proppant) concentration can reach 4.8 to 6.3 ppg.

Field Application

The first field test was conducted recently in one of the unconventional blocks in Ordos basin in October 2018 in a vertical well. The depth of the well is 3300 m. Reservoir temperature is 110 °C. Reservoir permeability is 0.02 mD with porosity of 11%. Clay content is 23% and the formation is very water sensitive. An operator had accepted the quasi-dry CO2 fracturing concept and provide this well for field application.

As mentioned above, the equipment set up is separated into two parts: chemical and proppant mixing with water and pumping to the wellhead; and pure liquid CO2 pumping directly to the wellhead. The two-fluid system are mixed at the wellhead and through the tubing in turbulent flow. Figure 3 is the layout of equipment on the wellsite.

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Figure 3. Quasi-Dry CO2 Fracture Operation Well Site Layout

Figure 4 is the treatment plot. Since this is the first field trial, every step of change was very cautious. In general, as can be seen in the plot, the operation went smoothly as designed without any issues.

Figure 4. Treatment Plot

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Recently (May 2019), a second field test was conducted in the same area. The operation was completed without any issue. Flowback is on going but has shown a good water recovery and good gas production similar to the first well.

Results of Field Test

The fracturing operations were successful indicting that the new method is operationally feasible. In comparison, two offset wells were fractured one of which was with energized crosslinked gel and another with slick water/crosslinked gel system. The two offset wells did not have gas production after flowing back. The quasi-dry CO2 fractured wells had significant gas production and 60-90% of the pumped water was recovered. Flowed back fluid indicates a complete viscosifier breaking (flowed back fluid viscosity is around 1 to 2 mPa.s).

Discussions

Formation Damage - In order to make sure that the fluid system does not damage the reservoir rock, returned permeability tests were conducted. Results indicate that the fracturing fluid system has low to non-damage to the formation. Combined with clay stabilizers, the damage can well be controlled. Like dry CO2 fracturing, it prevents water sensitivity and water blocking effect. Table 1 shows the returned permeability test results on some of the formation cores.

Table 1. Returned Permeability of Cores with Quasi-dry Fluid Flown Through

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Table 2. Comparison of CO2 Fracturing Technologies

Comparison of CO2 Fracturing Technologies

Table 2 summarizes the main characteristics of CO2 related fracturing technologies. It shows that the new quasi-dry CO2 fracturing technology has its advantages over other techniques.

Figure 5 illustrates the evolution of the quasi-dry CO2 fracturing technology as part of an integrated fracturing fluid process. The two main viscosifiers (Chemical A and Chemical B) play different roles at different fracturing conditions. They work jointly to meet the engineering design based on subsurface requirement. The workflow shows that the whole hydraulic fracturing fluid system from 100% water-based fluid (slick water/cross-linked gel) to 100% liquid CO2 (viscosified dry CO2) becomes a fracturing fluid solution with the combined use of the two main chemicals. This would greatly simplify the selection of fracturing fluid and providing field services.

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Figure 5. The Evolution of Quasi-Dry CO2 Fracturing Technology

Conclusions

Based on theoretical and laboratory development and successful field test (in 2 wells at the time of writing this paper), the new procedure – quasi-dry CO2 fracturing technology has the following advantages compared to dry CO2 fracturing.

The method does not require pressurized blending equipment making operation simple. It can be used in liquid CO2 frac, energized frac, and foam frac.

The high viscosity and high pumpability of the quasi-dry CO2 slurry with high proppant concentration enables the creation of fractures with desired geometry and proppant placement.

Compared to traditional dry CO2 fracturing, the new method significantly reduces the complexity and cost of liquid CO2 fracturing.

Since the new method broke the limitation on dry CO2 fracturing, we see its immediate application in many unconventional and tight reservoirs with water sensitivity or low pressure. It provides a possible effective method for CO2 EOR and re-fracturing with liquid CO2.

We are continuously collecting more data and conducting more field test through the cooperation with operators and service providers. More work needs to be done to improve the procedure and reduce the risk in various operation conditions. In the meantime, effort needs to be paid to deeper understanding of the complex system so that the procedure can be improved and optimized. It is believed that with more field application, the methods will become more mature.

References

Chen, Chen. et al. 2018. Progress of Research on Carbon Dioxide Fracturing Technology. Exploration Engineering (Rock & Soil Drilling and Tunneling). Oct. 2018. Vol. 45 No. 10, 21-18.

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Lee, J.J. et al. 2014. Development of Small Molecule CO2 Thickeners for EOR and Fracturing. SPE- 169039-MS presented at the SPE Improved Oil Recovery Symposium held in Tulsa, Oklahoma, USA, April 12-16, 2014.

Li, Xiaojiang. et al. 2018. Estimating and Analysis of Carbon Dioxide Friction Loss in Wellbore During Liquid/Supercritical Carbon Dioxide Fracturing. SPE-191142-PA.

Li, Xiang. Feng, J. et al. 2015. Hydraulic Fracturing in Shale with H2O, CO2 and N2. AMRA 15-786.

Li Yang et al. 2018. Research and application of CO2 Dry Fracturing Technology. IFEDC-20182869. Presented at 2018 International Field Exploration and Development Conference in Xi’an, China, 18-20 September 2018.

Zhang Juntao et al. 2018. New Technology in Dry CO2 Fracturing in Shale Reservoirs. Unconventional Oil & Gas. Vol. 5 No. 5. Oct. 2018