By horizontal well multi-staged fracturing technology, shale rocks can be broken to form fracture networks via hydraulic force and increase the production rate of shale gas wells. Nonetheless, the fracturing stimulation effect may be offset by the water phase trapping damage caused by water retention. In this paper, a technique in transferring the negative factor of fracturing fluid retention into a positive factor of changing the gas existence state and facilitating shale cracking was discussed using the easy oxidation characteristics of organic matter, pyrite and other minerals in shale rocks. Furthermore, the prospect of this technique in tackling the challenges of large retention volume of hydraulic fracturing fluid in shale gas reservoirs, high reservoir damage risks, sharp production decline rate of gas wells and low gas recovery, was analyzed.
You Lijuna, Kang Yilia, Chen Qianga, Fang Chaoheb, Yang Pengfeia
aState Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China. bNational Energy Shale Gas R&D (Experiment) Center, Langfang, Hebei 065007, China.
Received 18 January 2017; accepted 25 May 2017
The organic matter and pyrite in shale rocks can produce a large number of dissolved pores and seams to improve the gas deliverability of the matrix pore throats to the fracture systems. Meanwhile, in the oxidation process, released heat and increased pore pressure will make shale rock burst, inducing expansion and extension of shale micro-fractures, increasing the drainage area and shortening the gas flowing path in matrix, and ultimately, removing reservoir damage and improving gas recovery. To sum up, the technique discussed in the paper can be used to “break” shale rocks via hydraulic force and to “burst” shale rocks via chemical oxidation by adding oxidizing fluid to the hydraulic fracturing fluid. It can thus be concluded that this method can be a favorable supplementation for the conventional hydraulic fracturing of shale gas reservoirs. It has a broad application future in terms of reducing costs and increasing profits, maintaining plateau shale gas production and improving shale gas recovery.
The key of shale gas recovery enhancement is to improve the methane gas desorption and diffusion capacity. Hydraulic fracturing is conducive to improving the seepage capacity of shale gas reservoirs, but it still cannot solve the problem of low desorption, diffusion and transmission capacity of methane gas in nanopores. As a result, the gas deliverability of shale matrix is much lower than the gas transmission capacity in the fractures. Thus, the production rate of gas wells in the initial period of exploitation is exponentially decreasing [1–3]. As the pore size decreases, the methane desorption, diffusion and transmission resistance increases and gas reservoir recovery decreases. For example, the recovery in shale gas reservoirs in the United States mostly ranges between 5% and 20%, but the recovery in Barnett shale gas reservoir is only 10%.
A large number of nano-scale pores are produced in organic matters. The spatial distribution of pyrite is closely related to organic matters. Organic matters and pyrite are closely related to the methane gas transmission path. They belong to the deposits under reductive environment , prone to oxidative dissolution. Therefore, due to the characteristics of fracturing fluid easy retention and difficult backflow, oxidizing fluid can be added into hydraulic fracturing fluid [5–12] to oxidize and dissolve the organic matter and pyrite, thus to produce a large number of dissolved pores and seams, and ultimately enhancing the conductivity of shale pores and seams.
In this paper, based on the analysis of the influence of engineering geological characteristics of shale gas reservoirs on the gas transmission capacity, the feasibility of the technique of oxidation-induced shale rock burst is demonstrated and the application prospect of oxidation in the fracturing stimulation of shale gas wells and gas recovery enhancement is discussed.
2. Engineering geological characteristics and gas transmission capacity of shale gas reservoirs
2.1. Multi-scale space of pores and seams determines shale gas transmission capacity
Shale gas mainly occurs in the pores of organic matters and the intergranular pores of clay minerals, which are mainly nano-scale [13,14]. According to their sizes, pores are divided into micropores (d < 2 nm), mesopores (2 nm < d < 50 nm) and macropores (d > 50 nm) [15–17]. The average diameter of shale pores is less than 100 nm and they are mainly mesopores and macropores [16–18]. Fractures are one of the controlling factors for shale gas transmission capacity [19–22], while shale fractures are mainly at the scale of macropores . The methane gas in shale micropores and mesopores is dominated by desorption–Knudsen diffusion and slip flows, while the methane gas in shale macropores is dominated by viscous flow and Knudsen diffusion/slip [19,23]. According to the characteristics of multi-scale pores and seams of shale rocks, Alharthy et al.  proposed a “triple” pore network model of shale gas transmission and “serial” and “parallel” transmission mechanisms. In the “serial” transmission, shale gas transmits towards the micropores, mesopores and macropores in sequence; in the “parallel” transmission, gas in the micropores and mesopores transmits towards the macropores simultaneously.
2.2. Methane occurrence state affects shale gas transmission capacity
Natural gas in shale gas reservoirs consists of three parts: free gas in fractures, free gas and adsorbed gas in matrix pores such as organic pores and clay mineral intergranular pores. The proportion of adsorbed gas in the North American shale ranges from 20% to 85%. Adsorbed methane mainly occurs in micropores and mesopores. At this scale, the methane gas transmission mechanism is desorption–diffusion and slip flows, and the production rate and yield of adsorbed methane are lower than that of free methane in a larger–scale space [24–27]. In addition, the permeability of methane gas is significantly smaller than that of nitrogen and helium, since the adsorption of methane on the pore wall makes the effective transmission path smaller and increases the methane transmission resistance . Methane adsorption affects the effective aperture of shale , which reduces the apparent permeability of shale nanopores [30,31].
2.3. Rich organic matter and pyrite is the prerequisite to enhancing shale gas transmission capacity via oxidation-induced rock burst
Organic matter is an important part of high-quality shale. Shale organic pores develop in organic matters, and organic pores are well developed and have a good connectivity in the high-over mature organic matters [32–35]. The organic carbon content of shale in the Lower Cambrian Niutitang Fm in the SE Chongqing region ranges from 2% to 10%, with an average of 7.0% . The organic carbon content of the Chang71 Member of the Upper Triassic Yanchang Fm in the continental shale of the Ordos Basin is generally 4–12% . The content of organic matter in the “sweet spots” of the Upper Ordovician Wufeng–Lower Silurian Longmaxi Fms in the Sichuan Basin is greater than 3.0%. Assuming that the density of organic matter is 1.2 g/cm3, the volume proportion of shale organic matter is 4–25%.
The organic carbon content of shale in the Junggar Basin is up to 79.44% . The occurrence state of shale organic matter is diverse [39–41]. According to the contact relation between organic matters and minerals, the occurrence state of organic matter is divided into four types : striped, interstitial, film-like, and fragmental. Loucks et al.  classified shale organic matters into dense continuous, sparse continuous and disperse organic matters. Nie et al.  argued that shale organic matters were mainly distributed along micro-bedding surfaces or sedimentary discontinuity surfaces. This mode of organic matter occurrence was prone to produce interconnected organic pore networks with generally good permeability. Kuila et al.  argued that shale organic matters existed in the form of dispersed particles and continuous layers, and that the intergranular pore connectivity between granular organic matters and clay minerals was good.
As the most important sulfide mineral of black shale, pyrite (FeS2) is a kind of diagnostic mineral rich in organic deposits [46–50]. Pyrite is common in shale gas reservoirs, with a content of 1–5%, and it is mainly in the shape of a strawberry or a mold ball and local enrichment blocks. The pyrite of idiomorphic crystal is few and the particle size ranges from several micrometers to tens of microns.
Organic matter and pyrite, both with active chemical properties, are probable to generate dissolved nano-scale pores and seams via oxidation. Anderson et al.  found that sodium hypochlorite solution and bromine water could efficiently remove organic matters in clay rocks. Under the oxidation of potassium permanganate (KMnO4) solution, organic matters in soil are easily oxidized and decomposed . For example, when the concentration of KMnO4 solution is 0.3 mol/L, the oxidative decomposition rate of organic matter is between 60% and 98% .
Zhang Mengyan et al.  pointed out that when the organic matter in soil contacts with the oxidizing solution, the carbonyl could be oxidized to form carboxylic acid, and the aromatic carbon might open ring to form saturated aliphatic carbon and water molecule organic acid. Kuila et al.  studied the removal efficiency of shale organic matter using NaOCl solution. It was found that immature organic matter was not easy to be oxidized and dissolved, high-over mature organic matter was easy to be oxidized and decomposed, and the nanometer pore diameter of shale was significantly increased after oxidation. Pyrite is a kind of indissolvable stabilized sulfide, which can be efficiently removed with a strong oxidizing agent . The S atom contained in the shale pyrite (FeS2) is at −1 valence with reducibility. When contacting with the oxidizing solution, pyrite can be oxidized and removed, while other inorganic mineral components will not be affected . Wu Xiyong et al.  believed that the water–rock interaction of black shale was mainly manifested in the oxidative decomposition of pyrite.
2.4. Large retention volume of fracturing fluid can be transformed into a favorable condition for enhancing shale gas transmission capacity via oxidation
Since the shale has a strong spontaneous imbibition effect and gas reservoirs have a strong aqueous phase trapping effect, fracturing fluid flowback rate is low. The water saturation of the gas-enriched shale is generally low [9,57,58]. The ultra-low water saturation increases the shale storage space and improves the gas phase permeability, but it accelerates the aqueous phase imbibition rate and strengthens the aqueous phase trapping effect during an engineering operation [10,11]. For shale gas wells, the horizontal well multi-staged fracturing technology is usually adopted, with single-well water consumption of tens of thousands of cubic meters. However, the volume of flowback water only accounts for 10–40% of the total injection volume [12,59], and most of it remains in shale gas reservoirs. As hydrophilic shale nanopores have high capillary pressure , the fracturing fluid will invade by spontaneous imbibition  when it is in contact with shale.
Shut-in of shale gas wells after fracturing will reduce the flowback rate of fracturing fluid, and the flowback rate will decrease with the increase of shut-in time . The flowback rate is usually less than 10% . Civan et al.  pointed out that the capillary pressure and relative permeability greatly affected the fracturing fluid flowback process . Makhanov et al.  argued that spontaneous imbibition effect might be the main cause for fracturing fluid intrusion into shale reservoirs and low flowback rates [64,65].
Gao Shusheng et al.  performed experiments on shale powder expansion and core water absorption and estimated the water absorption strength of shale gas wells after volume fracturing according to the principle of equivalent network seepage capacity. Zhang Lei  et al. used 3D digital core of shale to perform the lattice Boltzmann method (LBM), so as to simulate the shale fracturing fluid flowback rates at the pore scale. The fracturing fluid absorption during the soak time of shale gas wells is mainly related to the fracturing fluid properties, the surface area of the artificial fracture networks and the soak time .
Ruppert et al.  suggested that water could invade most of the shale pores with 10 nm–10 μm in size through a small angle neutron scattering experiment. Kuila et al.  found that shale porosity obtained by the water displacement method was very close to that obtained through He gas logging. In addition, they believed that water could invade almost all nano-scale pores of shale and that incompatible fluid entering shale could induce damage  and fracture expansion [72,73].
3. Technical idea of improving the transmission capacity of methane via oxidation-induced shale burst
Some technical challenges exist in the development of shale gas.
- How to improve the gas reservoir stimulation cycle? After fracturing stimulation of shale gas wells, the gas is produced through the desorption-diffusion-seepage process. However, the existing fracturing stimulation and cutting capacity of shale matrix is limited, resulting in insufficient gas deliverability and faster pressure and output decline.
- How to enhance the recovery of shale gas? How to enhance the recovery of shale adsorbed gas? Shale gas produced is mainly free gas. The adsorbed gas volume is large in shale gas with 20–80% of it in an adsorbed state. Therefore, the recovery of shale gas can be enhanced by improving the penetration capacity of the matrix, by increasing the density of fracture networks, and by improving the desorption rate of adsorbed gas.
Matrix of shale gas reservoirs has the nD-level permeability. Gas occurs in the matrix pore throats in a state of free gas and adsorbed gas. The bedding or natural fractures of shale can improve the rock permeability, only with limited effect. Accordingly, the volume of fractures must be expanded via volume fracturing.
In China, shale gas development aims to increase single well production and maintain a long-term production stability, and also to reduce costs and protect the environment. To this end, it is necessary to improve the fracturing stimulation scale and fracturing fluid flowback rate and reduce the consumption of fracturing water and treatment agent. Safe and environment-friendly exploitation of shale gas is a rigid requirement of scientific development, and also an inevitable requirement of industrialization . There is a need to open up a “new way” that can enable both the protection of the ecological environment and the economical and effective shale gas development.
The key to effective development and recovery enhancement of shale gas lies in the improvement of desorption and diffusion rates. As for the new stimulation technique – volume fracturing (or SRV), the main technical idea is to make natural fractures and bedding communicated and break up reservoir bodies to form fracture networks using staged multi-cluster perforation, the high-displacement, large-volume and low-viscosity liquid, and steering materials and technologies based on reservoir in-situ stress field, rock mechanics parameters, natural fractures and other factors [75–77]. However, this technique neglects the positive effect of the low flowback and long-term retention of fracturing fluid .
The “new way” of shale gas reservoir stimulation is to make full use of fracturing energy and the action of fracturing fluid. Specifically, after main fracture networks are generated, shale matrix in SRV is further cut or “broke up” by using the mechanical–chemical interactions between the retained fracturing fluid and shale, that is, the efficiency or density is modified, so as to further improve the gas transmission rate (Fig. 1).
Fig. 1. Schematic diagram of gas transmission capacity enhancement by oxidative dissolution of organic matters/pyrite and by the “fracture network” stimulating technique.
How can the retained fracturing fluid be used to break up rocks? Based on the fact that rock organic matter and pyrite in shale gas reservoirs are the products under a reducing environment and susceptible to oxidation and that the fracturing fluid is easy to be retained and difficult to flowback, due to the characteristics of easy oxidative dissolution of organic matter and pyrite, shale nanometer pore systems are modified in order to increase the connectivity of the methane gas transmission path, increase the fracture density or volume, shorten the desorption–diffusion path, and skip the gas diffusion phase, finally enhancing shale gas transmission capacity and recovery (Fig. 2). In this way, a long-term effect of once fracturing stimulation can be achieved. Moreover, oxidation can eliminate the clogging of the polymer in the fracturing fluid and eliminate the organic matter in the flowback fluid.
Fig. 2. Schematic diagram of gas transmission capacity enhancement via oxidative dissolution of shale organic matters (modified from Loucks et al. ).
The technical idea is as follows. Firstly, under oxidation, the organic matter and pyrite in shale are oxidized to generate dissolved pores and seams and then to connect the pores. Then, a lot of heat, gas and organic acids are produced in the oxidation. Heat and gas can make the dense shale pore pressure increase rapidly, which may cause shale burst and then increase the depth of stimulation. The organic acids can dissipate the carbonate minerals in natural fractures and reduce rock strength, thus to generate acid-etched fractures. Finally, the organic matter is removed, which significantly reduces the methane adsorption capacity of shale, while the heat generated by oxidation can promote shale gas desorption. Under the action of the capillary imbibition, oxidative dissolution, thermal cracking, and acid corrosion, etc., the continuous organic matter and dispersed organic matter can be oxidized to induce rock burst and then enhance shale gas transmission capacity (Fig. 2).
4. Application prospect of oxidation-induced rock burst of organic-rich shale
4.1. Producing synergistic effect with existing hydraulic fracturing to increase fracture density and enhance recovery
By adding oxidizing agent to the fracturing fluid, the fracturing fluid retained in the induced fractures are used to consume the organic matter, improve the matrix permeability, produce more microfractures on the wall of the fractures, increase the fracture network density and improve the interporosity flow coefficient between the matrix and fractures.
The oxidation of organic matter and pyrite can help to create new fractures in shale. The heat released during the oxidation can trigger the local temperature of the reservoir to rise greatly. The mineral heterogeneity of shale reservoirs is strong, and the expansion ability of different minerals is obviously distinct. In this case, a large thermal stress will be produced. Meanwhile, as the shale permeability is low, when the thermal stress concentrates to a certain extent in the local area, fractures will be generated in the matrix within the SRV scope, thereby increasing the fracture network density. After oxidative dissolution of the layered organic matter in the shale, dissolved pores and seams are formed , which further interconnect the matrix pore throats and fractures and increase the fracture density (Fig. 3).
Fig. 3. Schematic diagram of fracture network density increasing and recovery enhancement via organic-rich shale oxidation.
Oxidation of organic matter can reduce methane adsorption capacity of shale and stimulate the adsorbed gas desorption of shale. Organic matter produces carbon dioxide and heat during oxidation. Shale has a stronger adsorption capacity for carbon dioxide than for methane. The generated carbon dioxide can replace methane. The high temperature during oxidation can reduce the methane adsorption capacity of shale. Therefore, this idea can improve the recovery of adsorbed gas, increase the fracture density to increase the adsorbed gas and free gas production, and finally contribute to the enhancement of shale gas production and shale gas recovery.
4.2. Extending the validity period of reservoir stimulation to achieve high and stable production of shale gas wells
For the existing hydraulic fracturing technology, a gas reservoir is mainly stimulated under the hydraulic action to break the rocks to produce fractures. However, since shale is tight, gas in the matrix nanometer pores is still difficult to enter the fractures and wellbores. In this case, the reservoir stimulation of gas wells has a short validity and the output declines rapidly. If the oxidative modification of the existing fracturing fluid is conducted and spontaneous imbibition of fracturing fluid through the shale bedding, microfractures and nanopores are generated, the oxidizing fracturing fluid can still continue to oxidize and dissolve the shale in a wide range after the hydraulic fracturing. In this case, the dissolved pores and seams caused by the oxidative dissolution will induce oxidative liquid to gradually enter the deeper area of the matrix. Then the reservoir stimulation range will increase with time, so that the purpose of the long-term reservoir stimulation through a fracturing can be achieved and shale gas wells can maintain a high and stable production for a long time.
4.3. Repeatedly stimulating shale gas reservoirs to economically and effectively enhance gas recovery
As the drilling and completion costs of shale oil and gas wells account for 50% of the upstream costs, the cost of production stabilizing or recovery enhancement through drilling new wells is high. E, especially in the current context of low oil prices, this method may generate low economic efficiency. In this case, repeated stimulation of shale gas reservoirs is a recommended. The injection of oxidizing liquid into the low-yield or unproductive shale gas reservoirs will eliminate the polymer or shale powder clogging clogged in the original fracture networks. Fracturing stimulation of the existing fracture networks through soaking and other measures can increase the fracture network density, restore or improve the production of shale gas wells, and finally economically and effectively improve gas recovery.
4.4. Mitigating reservoir damage by retained fracturing fluid and environmental pollution by flowback fluid
The retained conventional fracturing fluid forms a zone with high water saturation in the microfractures or the matrix pore throats near the fractures, which affects the gas transmission and damages the gas deliverability of the gas reservoirs. According to the stimulation via oxidation proposed in this paper, it is necessary to reasonably extend the shut-in time after the fracturing stimulation of shale gas wells, make full use of the chemical action between water (oxidized modified fracturing fluid) and rock (shale) during the “soak”, so that organic matter and pyrite can be consumed to weaken the strength of shale, induce microfracture initiation, expansion and extension, and improve the fracture density. Moreover, the gas generated during the oxidation reaction of shale desorbs the gas adsorbed on the original organic matter, which can increase the pore pressure. The oxidative dissolution of pores increases the pressure transmission rate and increases the flowback force of the fracturing fluid, thus weakening the damage of retained fracturing liquid trap to reservoirs.
After the fracturing stimulation of shale gas wells, since the fracturing fluid has a long-term contact with the organic-rich shale, the flowback fluid has a high content of organic matter. For example, in some Marcellus shale gas wells, the organic carbon content in the flowback fluid is up to 509 mg/L . The flowback fluid may cause serious pollution to the environment , for which the treatment measures are relatively complex. The oxidizing agent added into the fracturing fluid can oxidize the organic matter in shale formations, and also oxidize the organic matter in flowback fluid, thereby alleviating the environmental pollution of the flowback fluid or reducing the processing costs of the flowback fluid.
- The key of shale gas recovery enhancement is to improve methane gas desorption and diffusion capacity. Hydraulic fracturing is conducive to improving the seepage capacity of shale gas reservoirs, but it still cannot solve the problem of low desorption, diffusion and transmission capacity of methane gas in nanopores.
- In terms of shale gas transmission capacity enhancement via oxidative dissolution, large content of organic matter and pyrite is a favorable geological condition and the large retention volume of fracturing fluid in shale gas wells is an available engineering condition.
- The organic matter and pyrite in shale are oxidated to consume organic matter, release heat and produce organic acids and carbon dioxide, which can increase the hydraulic fracturing network density and improve shale gas recovery.