Based on resource, technology and actuality, the critical factors of the development of tight gas and shale gas are discussed. The development graph from quality and types of resource and foreground of “three-step development” are presented. The development of tight gas and shale gas will help to improve the quality of China’s energy structure and ensure the safety of energy demands.
The tight gas has already been the important field of the global unconventional resource exploration. The distribution of tight gas in China is very wide; for example, the scale development of Ordos Basin and Sichuan Basin have been achieved. The Rokey Mountain in the United States is the area where the tight gas is well developed. Comparative study on the generation condition and reservoir characters of American tight gas is the useful way to accelerate the development of tight gas and open up the thought of exploration. The result of comparative analysis of tight gas reservoir between China and the United States shows: tight gas of China and the United States havs many common characters, for example, most source rock being coal measure strata, tight reservoir, abnormal formation pressure, resource and reservoir intimate contract and gas reservoir gross distribution and so on; otherness is mainly reflected in tight gas source depositional environment,thermal evolution extent, reservoir heterogeneity, densifying factor, the longitudinal and plane distribution characteristics of gas reservoir. The main factors of are the character of depositional basin, sedimentary environment and deuterogene tectonic feature. In allusion to the particularity of Chinese tight gas, the key points of tight gas exploration and development are intensifying reservoir heterogeneity, high grade reservoir forecast, and the study of gas reservoir distribution and striving to make technological breakthrough to increase well cumulative production.
According to relevant estimation, the recoverable reserve of shale gas in China is up to 20 to 30 trillion m3. Although shale gas reserves evaluation is still in the early stage, it quite sure that China is one of the countries who are rich in shale gas resources. If developed subject to current gas annual production, it can be developed for more than 200 years. At present, many companies home and aboard focus on shale gas and compete it in China, and it is expected that shale gas will be the substitution of conventional energy resource in China. Thus the exploration and development of shale gas will bring great effect not only on China but also on the world. However, due to the tightness of shale gas reservoir and its permeability is as low as 10-9 Darcy scale, if especial stimulation treatments are not taken, it would be very difficult to achieve the goals of commercial development. For the moment, America is the only country in the world that is successful in commercial development of shale gas and its technology and commercial operating mode has been very mature too. CNOOC has invested more than 1 billion US dollar to buy 33.3 % interest of Eagle Ford, where its reserves reach about 600 billion m3, and where its development prospect is valuable. In order to get a comprehensive acknowledgement on the development technology, the author, as a leader, accompanied by a professional team has had a tour observation to Eagle Ford in America and has got a further understanding on key technologies in horizontal drilling and multi-stages fracturing etc. Meanwhile the author also has analyzed the relevant supporting policy framed by USA government, which will be important and valuable for shale gas exploration and development in China.
The unconventional hydrocarbon resources are very abundant within the registered blocks of SINOPEC (China Petroleum & Chemical Corporation). The potential exploration area for coalbed methane (CBM) reaches 101.9 thousand km2 and the resource with the buried depth smaller than 2 000 m is 11.02 trillion m3. The potential exploration area for shell gas is 175 thousand km2 and the volume is 15.9 trillion m3. The potential exploration area for shell oil reaches 117.6 thousand km2 and the resource is huge. Since 2004, SINOPEC has started to pay attention to unconventional hydrocarbon resources and launched research and exploration on CBM, shale gas and shale oil, etc. Recently, unconventional hydrocarbon resources exploration of SINOPEC has obtained breakthrough and progress. By the horizontal well fracturing section, Biye HF-1 obtains 23.6 m3/d commercial shell oil flow in Biyang Depression. By testing, several continental wells get high-yield commercial shell gas flow in Yuanba Block, and Jianye HF-1 obtains 12 300 m3/d commercial shell gas flow in Jiannan Block. The CBM proved reservoir in Yanchuan south block has reached 10 647 millon m3, and lots of wells get commercial CBM gas flow, which makes the pattern of “various resources type” and “multi-stage” exploration and development form on unconventional hydrocarbon resources in SINOPEC.
Natural gas resource of low porosity and permeability reservoir is the main body of natural gas exploration and development of current and future period in China, with the characteristics of largescale hydrocarbon accumulation. This paper focuses on the large-scale gas accumulation characteristics and distribution law of this type gas resource, and it is recognized that reservoir-forming factors have the characters of development and change in a large scale, which is the foundation of gas accumulation in a large scale. The main types of gas accumulation in a large scale are the “pancake type” outside and “cluster type” inside;the scale“energy storage” in burial period and scale“exhausting gas” in uplift stage are the important gas source;the volume flow and diffusion flow filling are the main migration and accumulation modes. The distribution of low porosity and permeability gas reservoir is close to source , and reservoir combination mainly contains two types,which are large area and large scope types, and the stratigraphic and lithologic traps are the main type. The synclinal area of marine cratonic basin, the broad slope zone of intracontinental DownWarped basin and slope slow wing area of foreland basin are the main areas of large scale accumulation of this kind of natural gas resources. The recognization of large scale gas accumulation of low porosity and permeability gas resources has been put forward, which prompts the discovery potential of natural gas resource, expands the range of exploration, provides the theoretical basis for objective evaluation of the total amount and distribution of natural gas resources,and promotes the large gas field effective discovery in China.
Through abundant data of seism, drilling, logging, mineral, geochemistry and physical properties, it is concluded that the shale of Paleogene Es1, Lower Es3 and Upper Es4 in Bonan sub-sag, Jiyang Depression is characterized by wide distribution and high abundance, better types (mainly with type I and II) and wide maturity range of organic matter, with a material basis for the formation of shale oil and gas in a large scale. Meanwhile, the low content of clay mineral and high content of brittle mineral (mainly with carbonate) in the 3 sets of shale are favorable for the exploitation of shale oil and gas. In addition, the high pressure coefficient and the development of different micropores and microcracks in the shale are also favorable for the preservation of shale oil and gas, because the reticular storage system is easy to form under this condition. According to the oil and water saturation data, the upper of No.12 layer to upper of No.13 layer in the Lower Es3 formation are the favorable layers for shale oil and gas exploration in Bonan sub-sag because of their high oil and gas content in unit rock. Especially, as the most favorable rock type, the lamiated argillaceous limestone could be effectively explored .
Shale gas in America mostly comes from thermogenic marine shales, and gas shales mainly distribute in Devonian and Carboniferous of Foreland and Craton basins, while the buried depth is 1 500~3 500 m. Shales in China include 3 types: marine, transitional and lacustrine. Marine shales basically distribute in the lower Paleozoic of Aulacogen and Craton basins;organic-rich concentrated parts are steady-going,and thermal maturity phase and gas content are high,so the explorational prospect is the best. Transitional shales mostly distribute in Carboniferous-Jurassic of Foreland and Craton basins;there is no obvious organic-rich concentrated part, and gas content changes greatly, so the explorational potential needs to be proved. Lacustrine shales typically distribute in Mesozoic and Cenozoic of rift and Craton basins;organic-rich concentrated parts are thick and stable, and explorational potential is probably great in center of depression which reaches generating gas stage.It is forecast that shale gas exploration will take a long time and output will be 50 billion~60 billion m3 in the future.
The tight sand gas is becoming one of the most important domains in unconventional natural gas exploration & development in the world. Especially the large-scale exploration and utilization of American tight sandstone gas promotes the exploration & development in other countries.Based on a brief overview of global tight sandstone gas exploration & development，the paper carries out a systematic study on the exploration history, the exploration effect as well as the development tendency and proposals of China's tight sandstone gas. The result shows that the development process of China's tight gas development can be divided into three stages and is entering the fast-developing stage recently.In recent years, several large tight sandstone gas reservoirs are discovered in upper Palaeozoic of Ordos Basin, Xujiahe Formation of Sichuan Basin and deep formations of Kuqa, Tarim Basin. The reserve and production of tight sand gas have both experienced rapid growth. The production accounts for 1/5 of China's total nature gas production in 2011, and it is predicted to reach 1/3 in 2020, and at that time the tight sandstone gas would become the key role in natural gas industry.
A lot of outcrop geological survey works, basic research and pilot of exploration and development of shale gas were undertaken in China. Especially in the marine black shale of the South, more than 40 wells were drilled and more than 10 wells had got shale gas. The initial production of some wells has reached more than 1×104 m3/d. It was showed that the prospect of shale gas was better in China. Because of the obvious particularity in geologic settings, many geological and development problems have to be solved . With the breakthrough of the theories and key technologies in exploration and development of shale gas, improvement of economic conditions, and the support of national policy, the exploration prospect of shale gas is better in China.
There are mainly 3 sets of volcanic rocks in sedimentary basins of China: the strata of Carboniferous-Permian, the strata of Jurassic-Cretaceous and the strata of Palaeogene-Neogene. Volcanic rocks distributed widely and developed associated with hydrocarbon source rocks form good combination of source-reservoir-cap assemblage. In the eastern basins of China, intermediate-acidic volcanic rocks and primary type reservoir are mainly developed. In the western basins of China, mid-mafic volcanic rocks and weathering crust type reservoir are often developed. Almost all of the lithology and lithofacies of volcanic rocks can form effective reservoir. And the reservoir characteristic of weathering crust type is better than that of primary type. Lithological reservoir and fault-lithological reservoir are often developed in the eastern basins of China. Stratum reservoir, structure-stratum reservoir and structure-lithological-stratum combined reservoir are often developed in the western basins of China. In Songliao Basin, deep continued settlement faults control the distribution of gas regional; hydrocarbon generation faults trough controls the gas distribution in the fault depression; fault structure regions near the hydrocarbon generation faults trough are the gas enriched accumulation region, and high-quality volcanic reservoir controls gas accumulation. In the north part of Xinjiang area, the Carboniferous residual hydrocarbon-generation sags control area distribution of oil and gas; weathering crust scale controls the hydrocarbon accumulating scale and degree; the type of weathering crust-stratum traps could control reservoir forming effectively; the positive tectonic background controls oil and gas migration and accumulation. This paper points out that the area of volcanic rocks for exploration is 3.6×105 km2 in China, and the predicted equivalent reserve in volcanic rocks is above 6×109 t.
Based on the reservoir forming condition research of tight sandstone gas in Ordos Basin, Sichuan Basin and Tarim Basin, the authors give two types of tight sandstone gas reservoirs,sloped lithologic type and structural type which are the main of tight sandstone gas reservoir. The favorable gas region is predicted with multi-factor overlay method. And based on the existed tight sandstone gas reservoir analysis, the authors get the main parameters such as reservoir abundance, gas-bearing area coefficient which are necessary for gas resource calculation of the large basin through analogism method. According to the result of this resource evaluation, the tight sandstone gas geological recoverable resource in China is 9.2×1012～13.4×1012 m3 which is widely distributed over coal measure basins. The tight sandstone gas is the most realistic source in present. Due to the support of government policies and the motivation of technological progress, it might become one of the pillar sources in natural gas development in 10 to 20 years.
Many factors affect the size of the shale gas development. First and foremost factor is how to establish the geological evaluation standards and select the core area of shale gas. In the early stage of exploration the optimized shale gas core area makes it possible to identify the target and to select the most favorable region for exploration. The main purpose is to find shale gas and achieve economic development. Based on the analysis of shale gas reservoir characteristics and the enrichment factors we discuss the organic-rich shale maturity, mineral content, gas content and conservation parameters in China. Learn from the U.S. shale gas exploration and development and research, combined with the new progress of China's shale gas exploration and development and research, we establish china's first shale gas core area parameter system and value standards. Finally, we optimize some favorable areas and core areas of the marine shale in southern China and point out Southern Sichuan Basin is the most realistic area for shale gas development.
There are extensively three sets of marine regional organic-rich shale stones in southern China as lower Cambrian, lower Silurian and upper Permian and also continental organicrich shale stones in Sichuan Basin, Jianghan Basin and Subei Basin, possessing geological conditions to creating shale gas. Meanwhile, comparing with those in U.S., the shale gas in southern China is featured by multiple superimposition and reforms by structural movements, high Ro, complicated preserving conditions while the TOC and porosity of continental shale stones are relatively lower. Therefore, many challenges are faced for shale gas explorations in southern China and blind optimism should be avoided. The authors consider that the marine and continental shale stones are the realistic fields to realize shale gas exploration and development with large scales and the continental shale stones in Jianghan Basin, Subei Basin can be taken as the mothball field for shale gas exploration in southern China.
Shale reservoir fracturing is the critical and center technology for shale oil and gas efficient exploration and development. Different from conventional low permeability oil and gas reservoir whose goal is to improve fracturing effectively by single fracture, the main goal of low porosity and low permeability shale is to form effective conductivity network fracture, ensuring fracturing volume large enough and economical and efficient. Proposed the concept of network effective fracturing stimulation volume of shale ( ESRV ), in reference to North American shale gas fracturing experience and the domestic shale gas fracturing based on practice, in view of Chinese specific characteristics of shale reservoir, it has been doing exploratory research in some aspects, including prefrac evaluation method, the optimization of perforating parameters, such as compound test fracturing, network fracturing methods, flowback technology and other aspects. The initial shale network fracturing technique is obtained, and it has good effect in field experiment, solving the problem of brittle shale sand of easily being screened out and low success rate.