Prospect of Shale Gas Recovery Enhancement by Oxidation-Induced Rock Burst

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Natural Gas Industry B xx (2017) 1e8 www.elsevier.com/locate/ngib Research Article Prospect of shale gas recovery enhancement by oxidation-induced rock burst

You Lijuna,*, Kang Yilia, Chen Qianga, Fang Chaoheb, Yang Pengfeia

a State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China b National Energy Shale Gas R&D (Experiment) Center, Langfang, Hebei 065007, China

Received 18 January 2017; accepted 25 May 2017

Abstract

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. 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. © 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Shale gas; Oxidization; Gas recovery; Rock burst; Organic matter; Pyrite; Conductivity

1. Introduction transmission resistance increases and gas reservoir recovery decreases. For example, the recovery in shale gas reservoirs in The key of shale gas recovery enhancement is to improve the United States mostly ranges between 5% and 20%, but the the methane gas desorption and diffusion capacity. Hydraulic recovery in Barnett shale gas reservoir is only 10%. fracturing is conducive to improving the seepage capacity of A large number of nano-scale pores are produced in organic shale gas reservoirs, but it still cannot solve the problem of low matters. The spatial distribution of pyrite is closely related to desorption, diffusion and transmission capacity of methane gas organic matters. Organic matters and pyrite are closely related in nanopores. As a result, the gas deliverability of shale matrix to the methane gas transmission path. They belong to the is much lower than the gas transmission capacity in the frac- deposits under reductive environment [4], prone to oxidative tures. Thus, the production rate of gas wells in the initial dissolution. Therefore, due to the characteristics of fracturing period of exploitation is exponentially decreasing [1e3].As fluid easy retention and difficult backflow, oxidizing fluid can the pore size decreases, the methane desorption, diffusion and be added into hydraulic fracturing fluid [5e12] to oxidize and dissolve the organic matter and pyrite, thus to produce a large number of dissolved pores and seams, and ultimately * Corresponding author. enhancing the conductivity of shale pores and seams. E-mail address: [email protected] (You LJ.). Peer review under responsibility of Sichuan Petroleum Administration. https://doi.org/10.1016/j.ngib.2017.05.014 2352-8540/© 2017 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: You LJ, et al., Prospect of shale gas recovery enhancement by oxidation-induced rock burst, Natural Gas Industry B (2017), https://doi.org/10.1016/j.ngib.2017.05.014 + MODEL 2 You LJ. et al. / Natural Gas Industry B xx (2017) 1e8

In this paper, based on the analysis of the influence of 2.3. Rich organic matter and pyrite is the prerequisite to engineering geological characteristics of shale gas reservoirs enhancing shale gas transmission capacity via on the gas transmission capacity, the feasibility of the tech- oxidation-induced rock burst nique of oxidation-induced shale rock burst is demonstrated and the application prospect of oxidation in the fracturing Organic matter is an important part of high-quality shale. stimulation of shale gas wells and gas recovery enhancement Shale organic pores develop in organic matters, and organic is discussed. pores are well developed and have a good connectivity in the high-over mature organic matters [32e35]. The organic car- 2. Engineering geological characteristics and gas bon content of shale in the Lower Cambrian Niutitang Fm in transmission capacity of shale gas reservoirs the SE Chongqing region ranges from 2% to 10%, with an average of 7.0% [36]. The organic carbon content of the 2.1. Multi-scale space of pores and seams determines Chang71 Member of the Upper Triassic Yanchang Fm in the shale gas transmission capacity continental shale of the Ordos Basin is generally 4e12% [37]. The content of organic matter in the “sweet spots” of the Shale gas mainly occurs in the pores of organic matters Upper Ordovician WufengeLower Silurian Longmaxi Fms in and the intergranular pores of clay minerals, which are the Sichuan Basin is greater than 3.0%. Assuming that the mainly nano-scale [13,14]. According to their sizes, pores are density of organic matter is 1.2 g/cm3, the volume proportion divided into micropores (d < 2 nm), mesopores of shale organic matter is 4e25%. The organic carbon content (2 nm < d < 50 nm) and macropores (d > 50 nm) [15e17]. of shale in the Junggar Basin is up to 79.44% [38]. The The average diameter of shale pores is less than 100 nm and occurrence state of shale organic matter is diverse [39e41]. they are mainly mesopores and macropores [16e18].Frac- According to the contact relation between organic matters and tures are one of the controlling factors for shale gas trans- minerals, the occurrence state of organic matter is divided into mission capacity [19e22], while shale fractures are mainly at four types [42]: striped, interstitial, film-like, and fragmental. the scale of macropores [19]. The methane gas in shale mi- Loucks et al. [43] classified shale organic matters into dense cropores and mesopores is dominated by desorptioneKnud- continuous, sparse continuous and disperse organic matters. sen diffusion and slip flows, while the methane gas in shale Nie et al. [44] argued that shale organic matters were mainly macropores is dominated by viscous flow and Knudsen distributed along micro-bedding surfaces or sedimentary diffusion/slip [19,23]. According to the characteristics of discontinuity surfaces. This mode of organic matter occur- multi-scale pores and seams of shale rocks, Alharthy et al. rence was prone to produce interconnected organic pore net- [19] proposed a “triple” pore network model of shale gas works with generally good permeability. Kuila et al. [45] transmission and “serial” and “parallel” transmission mech- argued that shale organic matters existed in the form of anisms. In the “serial” transmission, shale gas transmits to- dispersed particles and continuous layers, and that the inter- wards the micropores, mesopores and macropores in granular pore connectivity between granular organic matters sequence; in the “parallel” transmission, gas in the micro- and clay minerals was good. pores and mesopores transmits towards the macropores As the most important sulfide mineral of black shale, pyrite simultaneously. (FeS2) is a kind of diagnostic mineral rich in organic deposits [46e50]. Pyrite is common in shale gas reservoirs, with a 2.2. Methane occurrence state affects shale gas content of 1e5%, and it is mainly in the shape of a strawberry transmission capacity or a mold ball and local enrichment blocks. The pyrite of idiomorphic crystal is few and the particle size ranges from Natural gas in shale gas reservoirs consists of three parts: several micrometers to tens of microns. free gas in fractures, free gas and adsorbed gas in matrix Organic matter and pyrite, both with active chemical pores such as organic pores and clay mineral intergranular properties, are probable to generate dissolved nano-scale pores pores. The proportion of adsorbed gas in the North American and seams via oxidation. Anderson et al. [51] found that so- shale ranges from 20% to 85%. Adsorbed methane mainly dium hypochlorite solution and bromine water could effi- occurs in micropores and mesopores. At this scale, the ciently remove organic matters in clay rocks. Under the methane gas transmission mechanism is desorp- oxidation of potassium permanganate (KMnO4) solution, tionediffusion and slip flows, and the production rate and organic matters in soil are easily oxidized and decomposed yield of adsorbed methane are lower than that of free [52]. For example, when the concentration of KMnO4 solution methane in a largerescale space [24e27]. In addition, the is 0.3 mol/L, the oxidative decomposition rate of organic permeability of methane gas is significantly smaller than that matter is between 60% and 98% [53]. Zhang Mengyan et al. of nitrogen and helium, since the adsorption of methane on [54] pointed out that when the organic matter in soil contacts the pore wall makes the effective transmission path smaller with the oxidizing solution, the carbonyl could be oxidized to and increases the methane transmission resistance [28]. form carboxylic acid, and the aromatic carbon might open ring Methane adsorption affects the effective aperture of shale to form saturated aliphatic carbon and water molecule organic [29], which reduces the apparent permeability of shale acid. Kuila et al. [45] studied the removal efficiency of shale nanopores [30,31]. organic matter using NaOCl solution. It was found that

Please cite this article in press as: You LJ, et al., Prospect of shale gas recovery enhancement by oxidation-induced rock burst, Natural Gas Industry B (2017), https://doi.org/10.1016/j.ngib.2017.05.014 + MODEL You LJ. et al. / Natural Gas Industry B xx (2017) 1e8 3 immature organic matter was not easy to be oxidized and was very close to that obtained through He gas logging. In dissolved, high-over mature organic matter was easy to be addition, they believed that water could invade almost all oxidized and decomposed, and the nanometer pore diameter of nano-scale pores of shale and that incompatible fluid entering shale was significantly increased after oxidation. Pyrite is a shale could induce damage [71] and fracture expansion kind of indissolvable stabilized sulfide, which can be effi- [72,73]. ciently removed with a strong oxidizing agent [55]. The S atom contained in the shale pyrite (FeS2)isatÀ1 valence with 3. Technical idea of improving the transmission capacity reducibility. When contacting with the oxidizing solution, of methane via oxidation-induced shale burst pyrite can be oxidized and removed, while other inorganic mineral components will not be affected [45]. Wu Xiyong Some technical challenges exist in the development of et al. [56] believed that the watererock interaction of black shale gas. shale was mainly manifested in the oxidative decomposition of pyrite. 1) How to improve the gas reservoir stimulation cycle? After fracturing stimulation of shale gas wells, the gas is 2.4. Large retention volume of fracturing fluid can be produced through the desorption-diffusion-seepage pro- transformed into a favorable condition for enhancing cess. However, the existing fracturing stimulation and shale gas transmission capacity via oxidation cutting capacity of shale matrix is limited, resulting in insufficient gas deliverability and faster pressure and Since the shale has a strong spontaneous imbibition effect output decline. and gas reservoirs have a strong aqueous phase trapping effect, 2) How to enhance the recovery of shale gas? How to fracturing fluid flowback rate is low. The water saturation of enhance the recovery of shale adsorbed gas? Shale gas the gas-enriched shale is generally low [9,57,58]. The ultra- produced is mainly free gas. The adsorbed gas volume is low water saturation increases the shale storage space and large in shale gas with 20e80% of it in an adsorbed improves the gas phase permeability, but it accelerates the state. Therefore, the recovery of shale gas can be aqueous phase imbibition rate and strengthens the aqueous enhanced by improving the penetration capacity of the phase trapping effect during an engineering operation [10,11]. matrix, by increasing the density of fracture networks, For shale gas wells, the horizontal well multi-staged fracturing and by improving the desorption rate of adsorbed gas. technology is usually adopted, with single-well water consumption of tens of thousands of cubic meters. However, the Matrix of shale gas reservoirs has the nD-level perme- volume of flowback water only accounts for 10e40% of the ability. Gas occurs in the matrix pore throats in a state of free total injection volume [12,59], and most of it remains in shale gas and adsorbed gas. The bedding or natural fractures of shale gas reservoirs. As hydrophilic shale nanopores have high can improve the rock permeability, only with limited effect. capillary pressure [60], the fracturing fluid will invade by Accordingly, the volume of fractures must be expanded via spontaneous imbibition [5] when it is in contact with shale. volume fracturing. Shut-in of shale gas wells after fracturing will reduce the In China, shale gas development aims to increase single well flowback rate of fracturing fluid, and the flowback rate will production and maintain a long-term production stability, and decrease with the increase of shut-in time [61]. The flowback also to reduce costs and protect the environment. To this end, it rate is usually less than 10% [62]. Civan et al. [60] pointed out is necessary to improve the fracturing stimulation scale and that the capillary pressure and relative permeability greatly fracturing fluid flowback rate and reduce the consumption of affected the fracturing fluid flowback process [63]. Makhanov fracturing water and treatment agent. Safe and environment- et al. [6] argued that spontaneous imbibition effect might be friendly exploitation of shale gas is a rigid requirement of the main cause for fracturing fluid intrusion into shale reser- scientific development, and also an inevitable requirement of voirs and low flowback rates [64,65]. Gao Shusheng et al. [66] industrialization [74]. There is a need to open up a “new way” performed experiments on shale powder expansion and core that can enable both the protection of the ecological environ- water absorption and estimated the water absorption strength ment and the economical and effective shale gas development. of shale gas wells after volume fracturing according to the The key to effective development and recovery enhance- principle of equivalent network seepage capacity. Zhang Lei ment of shale gas lies in the improvement of desorption and [67] et al. used 3D digital core of shale to perform the lattice diffusion rates. As for the new stimulation technique e volume Boltzmann method (LBM), so as to simulate the shale frac- fracturing (or SRV), the main technical idea is to make natural turing fluid flowback rates at the pore scale. The fracturing fractures and bedding communicated and break up reservoir fluid absorption during the soak time of shale gas wells is bodies to form fracture networks using staged multi-cluster mainly related to the fracturing fluid properties, the surface perforation, the high-displacement, large-volume and low- area of the artificial fracture networks and the soak time [68]. viscosity liquid, and steering materials and technologies Ruppert et al. [69] suggested that water could invade most of based on reservoir in-situ stress field, rock mechanics pa- the shale pores with 10 nme10 mm in size through a small rameters, natural fractures and other factors [75e77].How- angle neutron scattering experiment. Kuila et al. [70] found ever, this technique neglects the positive effect of the low that shale porosity obtained by the water displacement method flowback and long-term retention of fracturing fluid [78].

The “new way” of shale gas reservoir stimulation is to Then, a lot of heat, gas and organic acids are produced in the make full use of fracturing energy and the action of fracturing oxidation. Heat and gas can make the dense shale pore pres- fluid. Specifically, after main fracture networks are generated, sure increase rapidly, which may cause shale burst and then shale matrix in SRV is further cut or “broke up” by using the increase the depth of stimulation. The organic acids can mechanicalechemical interactions between the retained frac- dissipate the carbonate minerals in natural fractures and turing fluid and shale, that is, the efficiency or density is reduce rock strength, thus to generate acid-etched fractures. modified, so as to further improve the gas transmission rate Finally, the organic matter is removed, which significantly (Fig. 1). reduces the methane adsorption capacity of shale, while the How can the retained fracturing fluid be used to break up heat generated by oxidation can promote shale gas desorption. rocks? Based on the fact that rock organic matter and pyrite in Under the action of the capillary imbibition, oxidative disso- shale gas reservoirs are the products under a reducing envi- lution, thermal cracking, and acid corrosion, etc., the contin- ronment and susceptible to oxidation and that the fracturing uous organic matter and dispersed organic matter can be fluid is easy to be retained and difficult to flowback, due to the oxidized to induce rock burst and then enhance shale gas characteristics of easy oxidative dissolution of organic matter transmission capacity (Fig. 2). and pyrite, shale nanometer pore systems are modified in order to increase the connectivity of the methane gas transmission 4. Application prospect of oxidation-induced rock burst of path, increase the fracture density or volume, shorten the organic-rich shale desorptionediffusion path, and skip the gas diffusion phase, finally enhancing shale gas transmission capacity and recovery 4.1. Producing synergistic effect with existing hydraulic (Fig. 2). In this way, a long-term effect of once fracturing fracturing to increase fracture density and enhance stimulation can be achieved. Moreover, oxidation can elimi- recovery nate the clogging of the polymer in the fracturing fluid and eliminate the organic matter in the flowback fluid. By adding oxidizing agent to the fracturing fluid, the The technical idea is as follows. Firstly, under oxidation, fracturing fluid retained in the induced fractures are used to the organic matter and pyrite in shale are oxidized to generate consume the organic matter, improve the matrix permeability, dissolved pores and seams and then to connect the pores. produce more microfractures on the wall of the fractures,

Fig. 1. Schematic diagram of gas transmission capacity enhancement by oxidative dissolution of organic matters/pyrite and by the “fracture network” stimulating technique.

Fig. 2. Schematic diagram of gas transmission capacity enhancement via oxidative dissolution of shale organic matters (modiﬁed from Loucks et al. [43]).

For the existing hydraulic fracturing technology, a gas The retained conventional fracturing fluid forms a zone reservoir is mainly stimulated under the hydraulic action to with high water saturation in the microfractures or the matrix break the rocks to produce fractures. However, since shale is pore throats near the fractures, which affects the gas trans- tight, gas in the matrix nanometer pores is still difficult to mission and damages the gas deliverability of the gas reser- enter the fractures and wellbores. In this case, the reservoir voirs. According to the stimulation via oxidation proposed in stimulation of gas wells has a short validity and the output this paper, it is necessary to reasonably extend the shut-in time declines rapidly. If the oxidative modification of the existing after the fracturing stimulation of shale gas wells, make full fracturing fluid is conducted and spontaneous imbibition of use of the chemical action between water (oxidized modified

Fig. 3. Schematic diagram of fracture network density increasing and recovery enhancement via organic-rich shale oxidation.

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