The development of shale gas in the United States has made a breakthrough, profoundly changed the pattern of oil and gas supply. It created the “shale gas revolution” on a global scale. Based on the practice of shale gas development in China and abroad, this paper aims to: (1) Summarize 5 basic characteristics, which are shale gas resource distribution, reservoir space, sweet spot area (section), hydrocarbon type and development mode. (2) Divide China shale development into three stages: scientific exploration, technological breakthrough and mature development. The United States is divided into three stages: scientific exploration, technological breakthrough and mature development. (3) Identify 4 challenges in the future development of China shale gas industry.
Caineng Zoua,b, Qun Zhaoa,b, Dazhong Donga,b, Zhi Yanga, Zhen Qiua,b,Nan Wanga,b, Yong Huangc, Anxiang Duana, Qin Zhanga, Feng Lianga,b, Zhiming Hua
aResearch Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China. bNational Energy Shale Gas R&D (Experimental) Center, Langfang 065007, China. cSouthwest Petroleum University, Chengdu 610600, China
Received 30 September 2017; revised 24 October 2017
Available online 28 November 2017
It includes non-marine shale gas potential, core technology and equipment for resource deep than 3500 m, complex surface “factory mode” production, human geography and other non-technical factors. (4) Process economic evaluation under the conditions of government financial subsidies. China’s shale gas project FIRR is about 8.0%–9.0%. Considering the global shale gas resources, consumer demand and other factors, it global shale gas production is expected to reach 1.1 × 1012m3 by 2040.
After the US made a breakthrough in the development of shale gas, with the aid of core technologies including horizontal well volume fracturing, micro-seismic surveillance and multi-well pad based exploitation to constantly promote the development of tight gas and rapid increase in the output of unconventional oil and gas, a worldwide “Shale Gas Revolution” has been brought about in energy sector.
Shale is fragile fine grained sediment layer  with shaly or lamellar bedding composed of clastic grains with grain size smaller than 0.0625 mm and clay together with organic substance. The shale in conventional oil and gas system has been considered as “an iron plate” without pore space and taken as source rock or cap-rock factor . Since a breakthrough made in shale gas development in Barnett in 1997, a number of shale gas has been developed in Haynesville, Marcellus and Utica in succession , it is known as a common knowledge that pore space of shale contains rich natural gas resource and that more and more attention has been paid to shale gas as a key natural gas resource. China is also abundant in shale gas resource , , having taken shape in production and size and become the largest shale gas production area in addition to those in North America after 10 years of development.
Discrepancy between shale gas and conventional natural gas in terms of forming condition and distribution characteristic has enriched traditional natural gas reservoir theory and improved level of technology for exploration and development to a certain extent, “shale gas revolution” plays a landmark role in promotion of development in oil and gas geology and update and upgrade of oil and gas industry. The paper summaries basic characteristic of shale gas and divides stages of development and recognition of shale gas field based on detailed analysis of typical shale gas fields in China and the US and determines challenge to development of shale gas in China inspired by “shale gas revolution” and draws a comparison of economical efficiency in shale gas development between China and the US as well as analyzes and predicts prospect for development of shale gas.
Basic characteristics of shale gas
Oil and gas in black shale bed series contain proximal tight oil, shale oil and gas as well as coal crevice gas as 3 types of unconventional oil and gas, the exploitation of tight oil and shale oil depends mainly on the output of dissociated oil and the exploitation of tight gas, shale gas and coal crevice gas depends mainly on continuous output of dissociated and adsorbed gas (Table 1).
Table 1. Main geologic characteristic of oil and gas in black shale bed series.
According to the division by United States Geological Survey , gas-bearing shale system is typical unconventional natural gas system, being continuous natural gas accumulation. Curtis  defined shale gas as biogenic gas, thermogenic gas or multi-genetic gas continuously distributed in rich organic shale system characterized by full of gas, continuous distribution in large area, multi-lithology trap and shorter migration distance. Zou  defines shale gas as natural gas contained in organic-rich shale in dissociated state and adsorbed state in most cases, being authigenic reservoir and continuous accumulation of natural gas in large area.
The paper defines shale gas as natural gas generated from and subjected to continuous accumulation in organic-rich black shale, with numerous and abundant nanoscale pores in shale reservoir serving as primary reservoir space. It is necessary to locate “sweet spot area”for development of shale gas field to achieve effective development by fracturing “sweet spot section ”to ensure “artificial permeability”.
Gas bearing shale beds are characterized by continuous distribution in large area, lamina and bedding in widespread development, abundant nanoscale organic pores, “sweet spot area (section)”enrichment, thermogenic free gas as decisive factor to determine output, high output to be achieved by control of natural fracture density and over pressure development degree and development of “artificial shale reservoir” as horizontal well fracturing network, which are considered as 7 basic characteristics.
Continuous distribution of shale gas in large area
Shale gas reservoir integrating source and storage is subject to continuous distribution in large area along slope and depression zone, being large in resource extent. The natural gas contained in reservoir after hydrocarbon generation and expulsion by organic substance in shale is to form shale gas accumulation, shale is not only source rock but also reservoir.
The accumulation in authigenic reservoir makes shale reservoir in continuous distribution become one of the main control factors for distribution of shale gas fields, it is common that shale gas is in continuous distribution in large area along slope and depression zone. Barnett shale gas is taken as an example: it is observed that shale reservoir is in wide distribution along slope zone in Ouachita mountainous region , with transverse continuous distribution up to more than 2000 km2, gas-bearing area of about 1.55 × 104 km2, and technical recoverable reserves up to 7362 × 108 m3.
Widespread development of lamina and bedding
Bedding development has significantly improved the horizontal seepage capability of shale reservoir and enhanced output of shale gas well. Shale as pulveryte is under the combined action of physical, chemical and biological processes in sedimentation period to form sedimentation of lamina different in component, including brittle mineral (calcite and dolomite etc.), clay mineral (illite and illit/smectite interstratifications) and organic substance (algae etc.), with bedding (seam) developed between laminas in continuous or intermittent distribution and combination of similar laminas to form lamina group and further form single layer with horizontal bedding (seam) in development and continuous distribution in most cases .
It is observed from research that lamina and bedding (seam) are able to communicate inorganic mineral pores and nanoscale organic pores in shale reservoir to form high speed channel ,  for horizontal migration of oil and gas. Bedding is in widespread distribution in gas bearing shale in Wufeng Formation-Longmaxi Formation in Sichuan Basin, with its horizontal permeability higher than 0.01 × 10−3μm2 (average value is 1.33 × 10−3μm2), being much higher than its vertical permeability in similar depth (lower than 0.001 × 10−3μm2, average value is 0.0032 × 10−3μm2), the difference between which is more than three orders of magnitude .
Gas bearing shale is compacted along vertical upward direction, with vertical permeability on the low side to prevent shale gas from rapid dissipation and make for preservation, however, horizontal bedding (seam) development has greatly improved horizontal seepage capability of shale reservoir  and is able to make horizontal well more likely to form complex fracture network ,  after transformation by hydraulic fracturing to enhance output of shale gas.
Abundant nanoscale organic pores
Shale reservoir is compacted, with development of nanoscale pore throat volume and organic pores in most cases, and the nanoscale pores are the main volume for gas accumulation. The shale reservoir is super compacted, with pore diameter from 50 mm to 400 mm and permeability from 10−9 to 10−6μm2 (Table 2). A number of analyses and test data show that nanoscale pore and pore-throat volume are in development in organic system in shale, accounting for more than 90% of total pores in shale, having offered a large number of effective spaces  for shale gas reservoir. Basing on molecular dynamics simulation, pore throat radius which is movable for methane is 2.4–7.8 nm, with 5.0 nm for average, considering water film.
Table 2. Comparison of parameters in main shale reservoirs both here and abroad.
Loucks , based on analysis of Barnett shale, presented that nanoscale pore is the main space in shale gas reservoir. Curtis  expressed that pore diameter in shale is from 4 nm to 200 nm. It is observed from argon ion polishing and scanning electron microscopy that pore diameter in organic-rich shale in Wufeng Formation-Longmaxi Formation in the south of Sichuan Basin is from 150 nm to 400 nm (Fig. 1), with pore throat radius from 10 nm to 30 nm, shale porosity from 2% to 12%, permeability from 20 × 10−9μm2 to 1.73 × 10−3μm2.
Fig. 1. Distribution of nanoscale pore in shale samples of Well Wei 201.
“Sweet spot area” and “sweet spot section” enrichment
There is a certain change of indicators including shale reservoir TOC, Ro and brittle mineral along horizontal and upward direction, there is “sweet spot area” in region and “sweet spot section” along longitudinal direction for shale gas; since many shale gas fields both here and abroad came into being long time ago, the evaluation of preservation condition is key factor taken into consideration to determine whether they are able to subject to accumulation on a large scale. Barnett shale gas is taken as an example, the gas field is about 1.55 × 104 km2, in which the sweet spot area for shale gas development is 5000 km2, non-sweet spot area is 1.05 × 104 km2.
Mineral Wells fault zone in the middle of core area for Barnett shale gas cuts through shale reservoir along near north east direction, with shale gas wells near to the fault zone generating a large amount of water and a small amount of gas, therefore, it is necessary to avoid the fault zone  in drilling shale gas well. Two sets of organic rich shale developed in Qiongzhusi Formation in Cambrian system, Wufeng Formation in Ordovician system-Longmaxi Formation in Silurian system in middle and upper Yangtze Region in China.
Cambrian shale under the action of tectonic stress develops calcite lamellae in reservoir, with poor gas-bearing performance and without breakthrough in exploration. Shale in Wufeng Formation and Longmaxi Formation is from 100 to 200 m in total thickness, in which the reservoir from 10 m to 40 m in thickness with development of graptolite fossil and TOC >3% is “sweet spot section” for shale gas development, there are three “sweet spot ”areas established for shale gas development in Jiaoshiba, Changning-Zhaotong and Weiyuan.
Initial output dependent on thermogenic free gas content
Thermogenic gas is prevailing in shale gas field, with two modes of occurrence including dissociated state and adsorbed state and initial output of gas well dependent on the free gas content. At the present time, there are more than 15 shale gas fields developed in the world, in which 13 fields are for thermogenic gas, 1 field is for biogenetic gas and 1 field is for multi-genetic gas . Shale gas in reservoir occurs in dissociated state and adsorbed state . The shale bed, after volume transformation, shall generate channel system composed of a number of highly permeable fracture networks to enlarge the contact area  between shale matrix and highly permeable fracture network channel to a greater extent.
Compared with free gas, the adsorbed gas shall be desorbed from desorbed state into dissociated state before migration to highly permeable fracture network channel within matrix, so the more free gas the higher intical production. Using the Physical Simulation Experiment Device of Coupled Seepage, a shale gas simulation test has been carried out with hermetic coring sample for 1764 days. There was 5970 ml shale gas and the ratio of adsorbed gas to free gas was 1:2. Almost all produced gas was free gas with the proportion of 98% till the 1.1 years. At the end of the third year, 83% produced gas was free gas, and the recovery of free gas was 47%.
Free gas shall be the first to come into play in shale gas well production, higher free gas content shall bring higher initial output of gas well. For example, free gas content in Barnett, Fayetteville and Woodford is 60%, 80% and 40%, with initial output of single well being 5.3 × 104 m3/d, 7 × 104 m3/d and 5.5 × 104 m3/d , ,  respectively; free gas content in Haynesville, Muskwa and Eagle Ford is 80%, 80% and 75%, with initial output of single well being 28 × 104 m3/d, 14 × 104 m3/d and 23 × 104 m3/d , ,  respectively.
High output to be achieved by control of natural fracture density and over pressure development degree
Fracturing process in shale reservoir is opening process of natural fracture in most cases, with development degree of natural fracture exerting an influence on effect of volume transformation for shale so as to control output of shale gas well. It is advisable to simulate fracture distribution characteristic in situ stress condition by physical model test for massive hydraulic fracturing and shale sample of 1 m × 1 m × 1 m in size to evaluate the mechanism to form fracture in volume transformation of shale.
It is observed from test result that most fractures in fracturing process are opened and propagate along natural gas fractures, with mutual restriction and mutual opening between natural gas fracture and bedding to form complex volume fracture network in shale reservoir. There is development of natural gas fracture ,  in organic-rich shale reservoir at the bottom of Wufeng Formation-Longmaxi Formation in Jiaoshiba Block, with shale gas well output on the high side, initial output of single well in excess of 104 m3/d and EUR value of single well being more than 1.5 × 108 m3.
Formation overpressure is an important factor to control gas content in shale and single well output of shale gas well. Drilling verifies that there is gas breakthrough in shale far and wide in Sichuan Basin and Longmaxi Formation in the surrounding; there is abnormal high pressure in basin slope and syncline area, with pressure coefficient from 1.4 to 2.2, overpressure area in excess of 25000 km2.
The average gas content in Longmaxi Formation in Changning Block in abnormal high pressure area is 4.1 m3/t, the average gas content in Longmaxi Formation in Fuling Area is 4.6 m3/t, with tested output of single horizontal well higher than 5 × 104 m3/d in most cases; the basin margin is under normal pressure, with gas content from 2.3 to 2.92 m3/t, tested output of single horizontal well being about 2 × 104 m3/d , .
Effective development based on “artificial gas reservoir” by horizontal well fracturing
“Artificial gas reservoir” is to achieve effective development of natural gas from shale formation on a large scale by manual intervention including taking black gas-bearing shale formation as “sweet spot area” unit, in which the well groups are deployed in a scientific and rational way and shale formation is subjected to transformation by massive hydraulic fracturing to form dense fracture network and build “artificial highly permeable area and rebuild seepage field” to greatly change subsurface fluid seepage environment and supplement formation energy.
Years of exploration shows that multi-stage fracturing technique for horizontal well is able to make “artificial permeability” in shale reservoir, being an effective technique to establish “artificial shale gas reservoir”. PurpleHayes Well 1H in Utica Shale Gas Field is taken as an example; horizontal section is 5652 m in length and subjected to fracturing in 124 stages, with fracturing fluid of 11.3 × 104 m3 injected to achieve that single well is 14.1 × 104 m3/d in gas output and 159.6 t/d  in oil output.
Free gas is prior product in production process of gas well, therefore, single well is higher in initial output; with the decrease of formation pressure, adsorbed gas is subjected to desorption in organic substance and flows into highly permeable fracture network channel, but the highly permeable fracture network channel shows shortage of gas supply, therefore, its later output is on the low side in long-period gas production.
Horizontal well in Haynesville Shale Gas is taken as an example, the single well is 28 × 104 m3/d in initial output, with output for the first year up to 30% of recoverable reserves and output for the first three years up to 50% ,  of recoverable reserves. On the premise of taking no account of economical efficiency in gas well development, production period shall be up to more than 30 years , .