Expulsive force in the development of CO₂ sequestration: application of SC-CO₂ jet in oil and gas extraction

Abstract

With the rapid development of global economy, an increasing amount of attention has been paid to the emission of greenhouse gases, especially CO₂. In recent years, dominated by the governments around the world, several significant projects of CO₂ sequestration have been conducted. However, due to the huge investment and poor economic effects, the sustainability of those projects is not satisfactory. Supercritical CO₂ (SC-CO₂) has prominent advantages in well drilling, fracturing, displacement, storage, plug and scale removal within tubing and casing, which could bring considerable economic benefits along with CO₂ sequestration. In this paper, based on physicochemical properties of SC-CO₂ fluid, a detailed analysis of technical advantages of SC-CO₂ applied in oil and gas development is illustrated. Furthermore, the implementation processes of SC-CO₂ are also proposed. For the first time, a recycling process is presented in which oil and gas are extracted and the CO₂ generated could be restored underground, thus an integrated technology system is formed. Considering the recent interests in the development of enhancing hydrocarbon recoveries and CO₂ sequestration, this approach provides a promising technique that can achieve these two goals simultaneously.

Graphical abstract

Keywords

CO₂ sequestration / SC-CO₂ jet / well drilling / fracturing / oil and gas

1 Introduction

Carbon dioxide (CO₂), a kind of odorless, colorless and nonflammable gas, is a common component in air. It has been produced and utilized industrially for more than one hundred years. It is ubiquitous, filling every corner of the earth. The volume fraction of CO₂ in air is approximately 0.03%–0.04%. However, with the development of human life and production, CO₂ concentration in air is increasing year by year [1]. As one of the most important members of greenhouse gases, CO₂ could warm the earth, causing natural disasters such as glacial ablation, sea level rising, flooding, hurricane, and drought to rage around the world [2]. To combat with those natural disasters, a series of important documents including United Nations Framework Convention on Climate Change, Kyoto Protocol, Bonn Agreement, and Buenos Aires Plan of Action have been issued, aiming at reducing the emission of greenhouse gases and slowing down global warming. Meanwhile, scientists and researchers around the world are working actively to collect CO₂ from power plants and steel mills and inject the gas underground for permanent seal, such as Future Gen in America, the Sleipner project in Norway, the Schwarze Pumpe Power Station in Germany, and NZEC (China-EU Cooperation on Near Zero Emissions Coal) [3–7]. Although those projects have been successful from a technical perspective, plenty of manpower, material and financial supports are consumed in CO₂ capture, transport and storage, without bringing any economic benefit. Now, due to the huge financial cost, only the government can afford such projects, making the development process quite slow. Therefore, to fully arouse the enthusiasm of capital and attract more investment into CO₂ sequestration projects, it is essential to make CO₂ sequestration economically benefical, so that impetus is given to the development of CO₂ sequestration.

When pressurized to 7.38 MPa and heated to 31.1°C, CO₂ transforms from gaseous state to supercritical state, accompanied with sharp changes in its properties. For example, SC-CO₂ has almost the same viscosity and diffusivity as gas, but approximately the same density as liquid. Those properties give CO₂ great advantages in oil and gas extraction. SC-CO₂ causes no damage to formations [8]. Besides, it is easier to achieve underbalanced drilling, balanced drilling and overbalanced drilling with SC-CO₂ by controlling wellhead back pressure. Furthermore, compared with water jet, the SC-CO₂ jet has a lower rock breaking threshold pressure and higher rock breaking efficiency. A detailed analysis of SC-CO₂ drilling and some correlative experiments have been conducted to confirm the feasibility of SC-CO₂ drilling [9]. Shen, the academician of the Chinese Academy of Engineering, has proposed the method of utilizing SC-CO₂ in fracturing and stimulation in unconventional reservoirs [10]. Researchers have shown that SC-CO₂ is able to generate numerous and complicated fracture networks in formations. Better effect of pressure boosting within cavity can be obtained when SC-CO₂ is used in jet fracturing. Besides, SC-CO₂ has an easier adsorbability on rock than methane. This is important, because it is able to replace absorbed methane, enhancing methane recovery and sequestrating CO₂ permanently [10–12]. The possibility has been raised to remove scale in tubing and casing with SC-CO₂, which is superior to the mechanical and chemical descaling methods [13].

Therefore, it can be seen that CO₂ can be utilized in the whole process of oil and gas extraction to improve production and bring economic benefits. Meanwhile, CO₂ can be stored underground permanently. This paper gives a detailed analysis of physicochemical properties of SC-CO₂ and technical advantages of utilizing SC-CO₂ in drilling, fracturing, displacement stimulation, plug removal, and descaling, along with a detailed implementation process of those technologies, aiming to provide a route of sustainable development for CO₂ sequestration.

2 Physicochemical properties of SC-CO₂

According to the phase diagram of CO₂ , we can get that the triple point for CO₂ is at a temperature of –56.56 °C and a pressure of 0.52 MPa. The critical point is at 31.1 °C and 7.38 MPa. CO₂ reaches its supercritical state when its temperature and pressure reach the critical temperature and pressure simultaneously [1].

SC-CO₂ has its special physicochemical properties which are different from those of either gas or liquid. The density, viscosity and diffusivity of SC-CO₂, gas and liquid are compared in Table 1. The density of SC-CO₂ is close to that of liquid while its viscosity is close to that of gas. The diffusivity of SC-CO₂ is larger than that of liquid, presenting a higher mass transfer capacity. Besides, the surface tension of SC-CO₂ is almost zero, making it easier to enter any space that is larger than itself. Below critical temperature, the liquid phase condenses continuously with the compression of CO₂. However, when CO₂ is in a supercritical state, the compression can only lead to an increase in its density without any liquid coming out.

3 SC-CO₂ drilling

The special physical properties of SC-CO₂ give its incomparable technical advantages in drilling over others. Specifically, SC-CO₂ drilling can decrease rock breaking threshold pressure, and hence obtains a higher rate of penetration. Besides, SC-CO₂ has a better performance in wellbore cleaning and cuttings carrying. Furthermore, no pollution is generated during the whole process. Therefore, SC-CO₂ drilling becomes a promising new drilling method [14].

3.1 Advantages of SC-CO₂ drilling

3.1.1 Low rock breaking threshold pressure and fast rate of penetration (ROP)

An SC-CO₂ jet can break indurate marble, granite and shale easily with a much lower rock breaking threshold pressure compared with a water jet, making the ROP of SC-CO₂ drilling higher. Laboratory experiments have shown that the rock breaking threshold pressures of marble and shale with an SC-CO₂ jet are about 66% and 50% or less than those pressures with a water jet, respectively. Laboratory experiments of hydraulic assisted mechanical rock-breaking have shown that the ROP of SC-CO₂ drilling is 3.3 times faster than water jet drilling in Mancos Shale. The specific energy (The ratio of the sum of mechanical and hydraulic energy and volume of breaking rocks) of SC-CO₂ drilling is only 20% of water jet drilling [15]. Figure 1 is a comparison of rock breaking effects of a water jet and a SC-CO₂ jet in granite and Mancos shale. It can be seen that narrower grooves with clear profile are generated by using a water jet and small brokenrock volume is obtained. In contrast, larger tunnels without clear cutting edges are produced when an SC-CO₂ jet is applied, getting a larger fracturing rock volume and a larger area of caving. Wang et al. [9] have also confirmed this conclusion by experiments.

3.1.2 Effective protection of oil and gas reservoirs

When the conventional drilling fluids (such as water-based drilling fluids) are used to drill the oil and gas reservoir formations, solid particles in drilling fluids such as barite and clay can easily enter formations to plug the pore throats due to the pressure difference. Moreover, mud filtrate can also invade formations to cause clay minerals to swell, plugging the pore throats more severely. If the formations are water sensitive, water-blocking can happen, which increases oil and gas flowing resistance. In contrast, there are no solid particles or liquid in SC-CO₂ fluid. Therefore, the damages mentioned above can be avoided when SC-CO₂ is used as the drilling fluid. In fact, when CO₂ enters the formation, the permeability and the porosity of the formation are both improved and oil mobility is increased [16–18]. The reason for this is that the density of SC-CO₂ is large and has a high capacity of solvation. In well drilling, SC-CO₂ can dissolve heavy oil components and other organics near the wellbore, and hence reduces the near wellbore damage and decreases the skin factor. Consequently, the resistance of oil and gas flowing into the wellbore is lowered. Besides, in the presence of water, carbonic acid with weak acidity of SC-CO₂ is produced, which could inhibit the clay from swelling. Furthermore, SC-CO₂ is able to dehydrate the compact clayey sand and open sand pores, which increases the porosity and permeability of the formation and improves reservoir physical properties.

Thus, SC-CO₂ drilling can protect formations from damage effectively. It will definitely become a promising technology for tight and ultra-tight oil fields.

3.1.3 Effective improvement for well production and recovery

In SC-CO₂ drilling, the rock breaking threshold pressure is lower and the ROP is higher, which decreases the pressure needed in jet drilling. Moreover, the utilization of coiled tubing can significantly extend its service life and lower the pressure requirement for surface equipment and down-hole tools. When a down-hole motor is used to perform underbalanced drilling, gases such as air, nitrogen and natural gas cannot provide enough torque for the motor due to their low densities. The foam is able to increase its density by decreasing gas liquid ratio (GLR) to supply sufficient torque for the motor. However, it is hard for the foam to guarantee an underbalanced state of the wellbore [3]. Fortunately, SC-CO₂ is able to provide sufficient power for the down-hole motor working and keep the wellbore in an underbalanced state due to its special property of density. And the viscosity of SC-CO₂ is much lower than that of other conventional drilling fluids, making the frictional head loss much lower. Thus, it can supply the drilling bit with enough hydraulic power while decreasing the size of the coiled tubing as much as possible. Furthermore, the low viscosity of SC-CO₂ can make the fluids flow in the annulus in a turbulent state, which is beneficial for the cuttings carrying [19].

As stated above, coiled tubing drilling with SC-CO₂ as the drilling fluids is suitable to drill the slim hole, the micro-hole, the ultra-short radius horizontal well and the complex well. It can provide a low-cost drilling service for low and ultralow permeability formations, such as coalbed methane and shale gas, which is not economically recoverable. It can also offer an effective technical guarantee for the fault blocking, confined aquifer, naturally fractured and depleted reservoirs.

3.2 Process of SC-CO₂ drilling

Figure 2 illustrates the process of drilling a radial horizontal well by using of the coiled tubing and the drilling fluid of SC-CO₂. Liquid CO₂ is stored in a high-pressure tank where the temperature is kept between –15 and 5 °C and the pressure between 4 and 8 MPa, ensuring that CO₂ is in a liquid state before entering the high-pressure pump. To keep the temperature in the tank, a chiller is installed outside and the outer wall of the tank coated with a heat insulation layer.

Liquid CO₂ is pumped by the high-pressure pump to the bottom hole through the coiled tubing. At the wellhead, the CO₂ in the coiled tubing is in a low temperature and high pressure liquid state. With the flowing of liquid CO₂ to a deeper depth, its temperature and pressure increase with the underground environment. When its temperature and pressure exceed the critical ones, the CO₂ transfers into a supercritical state. Under conventional gradients of temperature and pressure, the CO₂ is in a supercritical state when the depth is over 750 m. When the SC-CO₂ fluids pass through the bit, an SC-CO₂ jet is generated to fracture the rocks.

The cuttings at the bottom hole are performed by the SC-CO₂ fluids to the wellhead through the annulus. Due to the invasion of a small amount of water and hydrocarbon into the drilling fluids, solid cuttings must be separated first to prevent the pipe valve from erosion by the high speed mixture. Then the mixture enters the gas-liquid separator and the gas purifier to get the purified CO₂. The purified CO₂ is delivered into the tank for recycling [20].

For horizontal drilling in old wells, if the well pocket is deep enough, the cuttings can be left in the pocket. When the formation is opened, the CO₂ delivered to the well site can be directly injected into the formation to improve formation energy and recovery.

4 Supercritical CO₂ fracturing

4.1 Advantages of SC-CO₂in fracturing

SC-CO₂ is a type of non-aqueous fracturing fluid which does not need the addition of other chemical agents. Therefore, no pollution will be brought into the reservoir as well as the environment, achieving much better stimulation result.

First, SC-CO₂ is environmentally friendly, causing no harm to human body. Next, as SC-CO₂ is a type of non-aqueous fracturing fluid, the weakness of clay expansion is solved essentially, thus the reservoir will be protected effectively. In addition, with a low viscosity and a high diffusion coefficient, the fracture network is generated in the reservoir and the conductivity capacity of the reservoir is further improved. Based on experiments, it is found that the fracture surface caused by SC-CO₂ is rougher than that by water and N₂, and the rough surface is beneficial to improving the fracture conductivity [21]. More importantly, when SC-CO₂ is applied to fracturethe shale gas or coal gas reservoir, the adsorbed methane will be replaced. The reason for this is that the adsorption capacity of SC-CO₂ is higher than that of methane, so that the recovery rate of unconventional gas will increase significantly. Therefore, the supercritical CO₂ jet fracturing is expected to become a new type of environmentally friendly, efficient and safe fracturing technique.

With advantages of multilayer fracturing by a single trip, there is no compaction effect caused by the bullet perforation and there is no need to employ the mechanical packer, the coiled-tubing multistage hydraulic jet fracturing technique is an effective approach to stimulating the reservoir. However, severe challenges are faced with this technique. For example, for the case of small inner diameter of the coiled tubing, the flowing friction is great, resulting in a deficiency of the downhole hydraulic energy. Furthermore, the pressure bearing capacity of coiled tubing is limited, which could restrict the operation pressure [22]. Coiled-tubing jet fracturing by SC-CO₂ can strengthen the merits of coiled tubing fracturing effectively. First, the flow friction of SC-CO₂ in the coiled tubing is smaller, which ensures that the SC-CO₂ jet fracturing obtains sufficient energy. Secondly, for the low rock breaking threshold pressure of SC-CO₂, the jet fracturing by SC-CO₂ can be performed at a low operation pressure. Most importantly, SC-CO₂ is a type of clean fracturing fluids which requires no discharge after operation, which shortens the operation period and reduces operational costs [23–28].

Considering the superiority of SC-CO₂ fracturing, GE in the United States announced that it would invest 10×10¹⁰ dollars to develop the technology in 2014. Now, several projects are being studied by GE, cooperating with Statoil ASA and the Departmentof Energy of the United States.

4.2 Procedure of SC-CO₂ jet fracturing

Figure 3 is a schematic diagram of SC-CO₂ jet fracturing. As is shown, liquid CO₂ is stored in a tank. First, sand blast perforation is operated. Liquid CO₂ is then pumped into the blending equipment, mixing with abrasive sands that are 60 to 80 meshes in diameter. The mixture is pumped downhole through coiled tubing. When it flows into the nozzle of the jet fracturing tool, an abrasive SC-CO₂ jet is formed. Then, the perforating operation starts, lasting for about 5–10 minutes. After that, pure liquid CO₂ is pumped downhole, carrying the remaining abrasive sands out of well preventing the pipe from being stuck by sands. Then, fracturing operation is performed. A large amount of pure CO₂ is pumped into the well in liquid form continuously. When the bottom hole pressure exceeds the formation fracturing pressure, the CO₂ mixed with proppants is pumped downhole through coiled tubing, or though the annulus between coiled tubing and casing simultaneously to reduce the abrasion with coiled tubing. After that, pure liquid CO₂ is pumped into the bottom hole to carry the remaining proppants in the wellbore and the bottom out to prevent the pipe from being stuck by sands. If it is necessary to fracture the next layer, the coiled tubing and jet fracturing tool is pulled up to the target layer. Then, the second stage of fracturing is operated. In a similar way, multi-stage fracturing is operated. After fracturing operation, it is better to shut the well for 5 to 10 days, and then open the well to produce without open flow. As is described in Section 3.1, after SC-CO₂ flows into the reservoir, it will bring several benefits. The recovery rate is further improved. If the production task is urgent, it is feasible to release the downhole pressure slowly and begin production directly [29,30].

5 SC-CO₂ flooding and storage

Due to its special physicochemical properties, SC-CO₂ has very good performance in oil and gas displacement. Because the surface tension of SC-CO₂ is almost zero, it can enter any space that is bigger than the diameter of the CO₂ molecule. Moreover, the diffusion coefficient of SC-CO₂ is high, so that it diffuses easily in hydrocarbon reservoir, and expands the sweep area effectively. Furthermore, the dissolution of CO₂ makes crude oil expand, the viscosity decrease, the oil-water mobility ratio decrease, the flowing energy of crude oil increase, and oil-water interfacial tension decrease dramatically. Therefore, the residual oil saturation drops, and the recovery is enhanced. In addition, after CO₂ is mixed with crude oil, the light hydrocarbon in crude oil is extracted and gasified, and the oil band consisting of mixture of light hydrocarbon and CO₂ is formed. The move of oil band can displace oil, thus enhancing oil recovery dramatically. With large amounts of CO₂ dissolving in crude oil, the solution gas drive effect occurs, and with the pressure decreasing, CO₂ escapes from crude oil, which generates gas-driven power and enhances displacement effects [31–33].

Apart from displacing oil efficiently, SC-CO₂ can also displace adsorbed methane in shale and coal gas reservoirs, and with the content of free methane increasing, production is enhanced. Busch et al. [34] have studied Germanic coal with a vitrinite reflectance (Ro) between 0.25% and 1.68% and found that the adsorption capacity of CO₂ to coal is 2.7–3.16 times as much as that of methane. Mastalerz et al. [35] have studied Indiana coal in the United States with a Ro between 0.48% and 0.62% and found that the adsorption capacity CO₂ to coal is 3.5–5.3 times as much as that of methane.

The experiments conducted by Sun et al. [36] have shown that the adsorption capacity CO₂ to shale is higher than that of methane. Therefore, SC-CO₂ can displace adsorbed methane and increase the content of free methane in pores.

Figure 4 is the theoretical diagram of the CO₂ displacing adsorbed methane. As is shown, before displacement, the majority of CH₄ molecules are adsorbed on the surface of pore, while after displacement, the majority of adsorbed methane molecules are replaced by CO₂ molecules, which can increase free methane contents, along with CO₂ sequestration. Therefore, when SC-CO₂ is applied in shale and coal gas reservoirs to improve well production and enhance recovery, CO₂ can also be sequestrated permanently [37–44].

6 SC-CO₂ plug removal and descaling

6.1 SC-CO₂ jet sand-flushing

Problems such as sand production, fracturing sand remaining, and permeability damage near wellbore have attracted much attention of global operators. Currently, the majority of well cleaning up is accomplished by water, with some additives added if necessary. For a pressure-depleted reservoir, under balanced well cleaning up with N₂, CO₂ or air foam is adopted. These cleaning up methods, to some extent, slow down the rate of production decline caused by plug, but they cannot solve wellbore plug radically. For example, when water is injected into a water sensitive reservoir, it will damage the formation substantially. Even though the foam flushing method could effectively reduce bottom hole pressure, it is difficult to control foam quality, which has a high possibility to cause bottom hole pressure surge, thus damaging the reservoir. It is hard for the water jet to break the plug formed by macromolecule organics, such as asphalt, etc., mixed with sands and clay, which has a strong viscoelasticity. However, these cleaning up operations by SC-CO₂ can solve these problems efficiently [45,46].

First, because the rock breaking threshold pressure of the SC-CO₂ jet is relatively low and has a strong solvability, the SC-CO₂ jet can break and dissolve macromolecule organics at a low injection pressure and carry them out of the wellbore easily. Secondly, with low viscosity, almost zero surface tension, and large diffusion coefficient, it is easy for SC-CO₂ to enter tiny pores and fractures to dissolve macromolecule organics and other impurities, and to clean more thoroughly. Besides, with the wide range of adjustable density, at wellbore pressure and temperature, the bottom hole pressure can be controlled by adjusting wellhead pressure; then underbalanced, balanced and overbalanced sand-flushing operations can be realized. The principle of the SC-CO₂ jet flushing well is shown in Fig. 5. The top of the coiled tubing is connected with the SC-CO₂ rotating jet nozzle, which is driven by the SC-CO₂ jet. When the sand-flushing operation is over, the CO₂ carried to the well site can be injected into the formation, which will further enhance oil and gas recovery.

6.2 SC-CO₂ scale removal within tubing and casing

After producing for a long time, it is easy to generate scale on the wellbore surface, because the salinity of formation water is high. If the thickness of scale is large enough, the production performance will be lowered. The traditional descaling methods include mechanical descaling, chemical descaling, water jet descaling, and so on. For mechanical descaling, it is possible to damage casing and tubing. For chemical descaling, tubing and casing may get eroded. For water jet descaling, even though it does not damage tubing and casing, a relatively high pump pressure is needed. If hard scale cannot be cleaned, abrasive jet is supposed to be applied, which may pierce tubing and casing when pressure is not controlled well.

Because of low rock breaking threshold pressure and high speed of breaking rock, the SC-CO₂ jet needs a relatively low pump pressure. Thus, it is able to clean the scale at high speed without causing damage to tubing and casing. Figure 6 demonstrates the process of SC-CO₂ jet removing scales. The rotating jet nozzle could spin 360 degrees, achieving a thorough cleaning [47].

7 Conclusions and recommendations

SC-CO₂ is a type of fluid different from gas and liquid. The special physicochemical properties make it superior for being used in various aspects of petroleum engineering.

If CO₂ is merely sequestrated underground, it cannot bring economic benefits and the government investment is unsustainable. SC-CO₂ has prominent technical advantages in well drilling, fracturing, displacement, storage, plug removal and scale removal within tubing and casing. These applications could bring considerable economic benefits along with CO₂ sequestration. These applications will attract numerous social investments and are important approaches to making CO₂ sequestration realize its sustainable development.

It is recommended that theoretical and practical studies on SC-CO₂ drilling and completion technology be conducted, and an integrated system be formed concerning drilling, fracturing, displacement, plug removal, descaling and CO₂ sequestration in 20 years. The objective is to enhance oil and gas recovery along with CO₂ sequestration, and relieve the pressure from growing carbon emissions to the environment, and finally, realize the target of green development and utilization of fossil fuel resources.

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