Tight Oil In China Research Articles

The effect of engineering parameters on production capacity of a fractured horizontal well in tight oil reservoirs

Terrestrial tight oil in China has the characteristics of poor reservoir properties and strong heterogeneity. Large-scale volumetric fracturing of horizontal wells is one of the few solutions to achieve economic exploitation. Horizontal well productivity after fracturing is affected by many complex factors. Therefore, by using the numerical simulation method, the ellipsoidal fracture network containing the major fracture and the minor fracture is designed by the method of locally densifying grids to simulate the productivity of a single well of the tight reservoir of Changqing Oilfield and analyse the affecting factors on the productivity. The results show the horizontal well productivity increases with the length of the horizontal section, the number of fractured stages, the volume of SRV, and the complexity of the fracture network, but the increment gradually decreases. The orthogonal test method was used to rank the influencing degree of engineering parameters on productivity. It was found that the influencing degree of fractured horizontal well productivity in tight oil reservoirs was decreasing from SRV volume, fractured stages, the complexity of fracture network to the length of horizontal section.

Advances on enrichment law and key technologies of exploration and development of continental tight oil in China (2016–2018)

Continental tight oil is a promising unconventional resource found in recent years and an important supplement to the growth of domestic petroleum reserves and production in China. Guided by the research objectives set by the China National Science and Technology Major Project “Enrichment Law and Key Technologies of Exploration and Development of Tight Oil,” and based on laboratory experiments and field tests by eight PetroChina petroleum companies, three signs of progress in theoretical technology and practical exploration and development have been made to continental tight oil in China. (1) Forming conditions: It is found that laminated organic-rich shale is primary source rock. Furthermore, many types of favorable reservoirs, such as terrigenous clasts and internal clasts, are developed and accumulate into two kinds of large-scale and continuous distribution of in-source and near-source assemblages. (2) Enrichment Law: Favorable reservoir facies zones, being near the hydrocarbon source center, and having a supportive tectonic background are three common factors controlling oil enrichment and high production. The proven tight oil resources are 10.65 × 109 t, whereas 40 enrichment zones have been identified and a number of drillable “sweet sections” have been found. (3) Key technologies of exploration and development: Key technologies have been developed involving multi-scale tight reservoir characterization, resource classification evaluation, seismic response and engineering evaluation of “sweet sections,” as well as reservoir stimulation and effective development. The research results support the rapid continental tight oil in China. By the end of 2018, proven tight oil geological reserves were 437 × 106t, while proved, controlled and predicted reserves were 3.019 × 109t, and the new production capacity was 3.31 × 106t. It is suggested that follow-up studies should focus on three fields—clastic rocks, peperite-sedimentary tuff, and lacustrine carbonate rocks. The above should be done in five aspects – enrichment law, sweet area evaluation and prediction, key development technology, efficient reservoir stimulation, and practical application in key basins so as to support steady integration of geology and engineering for developing continental tight oil in China.

Classification evaluation criteria and exploration potential of tight oil resources in key basins of China

In general, the migration distance of continental tight oil in China is relatively short, and the formation and distribution are mainly controlled by the tectonic setting, source rock, reservoir, and source-reservoir matching relationship. It is characterized by continuous or quasi-continuous distribution and local enrichment in the development area of high-quality source rocks. The matching relationship between source rock and reservoir in vertical and horizontal directions controls the formation, distribution, and scale of tight oil reservoirs. Based on the source-reservoir matching relationship, tight oil resources are classified between in-source type and out-source type. These two types of tight oil are widely distributed in China's major petroliferous basins. The distribution and enrichment of in-source tight oil resources are controlled by the distribution of source rocks and favorable reservoirs. Out-source tight oil resources are controlled by the transport system and the distribution of favorable reservoirs, but the effectiveness of the transport system is controlled by the distribution of source rocks. The two important parameters of source rocks and reservoirs should be taken into account in the classification evaluation of tight oil resources. The classification evaluation of source rocks and reservoir conditions are both bases for the evaluation of the exploration potential of tight oil. The total amount of tight oil resources is 9.051 × 109 t in six major basins, (e.g., Songliao, Bohai Bay, Ordos, Sichuan, Santanghu, Qaidam, and Junggar basins), which are primary areas for tight oil exploration.

Sedimentary environment and the controlling factors of organic-rich rocks in the Lucaogou Formation of the Jimusar Sag, Junggar Basin, NW China

The Lucaogou Formation in the Jimusar Sag of the eastern Junggar Basin is an important sedimentary stratum accumulating huge amounts of lacustrine tight oil in China, where organic-rich rocks are commonly observed. Focusing on the Lucaogou Formation, a precise analysis of the inorganic and organic petrology and the inorganic geochemistry characteristics was conducted. The paleoclimate and paleoenvironment during sedimentation of the Lucaogou Formation were established, and the key factors that were controlling the accumulation of organic matter during this time were identified. The results of this study suggest that during the sedimentation of the Lucaogou Formation, the paleoclimate periodically changed from a humid environment to an arid environment. As a result, the salinity of the water and the redox environment fluctuated. During the sedimentation period, the lake showed sufficient nutrient supplies and a high primary productivity. The interval studies in the Lucaogou Formation were divided into five sedimentary cycles, where the first, second, and fifth sedimentary cycles consisted of cyclical paleoclimate fluctuations varied from a humid environment to an arid environment and shifted back to a humid environment with levels of salinity from low to high and decreased again. The third and fourth cycles have cyclical fluctuations from a humid to an arid environment and corresponding salinity variation between low and high levels. During the period when organic-rich rocks in the Lucaogou Formation deposited in the Jimusar Sag, the paleoclimate and the water body were suitable for lower aquatic organisms to flourish. As a result, its paleoproductivity was high, especially during the early period of each cycle. A quiet deep water body is likely to form an anoxic environment at the bottom and is also good for accumulation and preservation of organisms. Fine-grained sediments were accumulated at a low deposition rate, with a low dilution of organic matter. Therefore, high paleoproductivity provided a sufficient volume of organisms in the studied area in a quiet deep water body with an anoxic environment and these were the key factors controlling formation of organic-rich rocks.

Characteristics and distribution of continental tight oil in China

Tight oil in China is mainly distributed in Mesozoic-Cenozoic continental petroliferous basins, either generally occurs in tight sandstone or tight carbonate reservoirs which are paragenetic to or in contact with lacustrine petroleum source rocks. By the end of 2016, the continental tight oil in China has established an annual production capacity of 1.55 million tons, and a cumulative oil production of 2.52 million tons. In 2016, the continental tight oil production in China was about 800,000 tons. By analyzing the characterization of the tight oil in Ordos Basin, Songliao Basin, Jungar Basin, the organic-rich shale in China were formed in multiple geological periods including the Permian, Triassic, Jurassic, Cretaceous, Neogene, and Paleogene, in freshwater, semi-saline to hyper-saline lacustrine depositional setting. They are generally tens of meters to several hundreds of meters thick, have total organic carbon (TOC) contents of 0.4–16%, and thermal maturity of 0.4–1.4%. Algal bloom as results of volcanism, low sedimentation rate, transgression, and stratification of water body are prominent factors controlling the formation of organic-rich shales. Tight oil reservoirs in China mainly include tight sandstone, tight carbonate rock, tight sedimentary tuff, etc., all of which are characterized by strong heterogeneity and poor petrophysical properties, with in situ porosity and permeability generally no more 12% and 1 × 10−3 μm2 respectively. They are also characterized by tight pore-throat systems of nano- to micro-scales, with pore throat diameters in the range of 80–1800 nm, and complex pore structures. The producing reservoirs have pressure coefficients between 0.7 and 1.8, covering under-pressure to over-pressure. Crude oil properties vary greatly with densities ranging from 0.75 to 0.92 g/cm3. It is believed that the abundance and effectiveness of hydrocarbon supply controls the formation of oil saturation, and tight oil “sweet spots” are generally in or near the areas with high hydrocarbon expulsion intensities. In addition, the quality of the source rock and the type of source reservoir control the distribution of the “sweet spots”, and storage space and movable fluid guarantee the formation of the “sweet spot” area of high oil saturation. “Sweet spots” are often developed in local mini-structural highs under a broad and gentle-sloped background. Continental tight oil in China has estimated geological resource of 14.66 × 109 tons with a technically recoverable resource of 1.45 × 109 tons. Tight oil is mainly distributed in Ordos Basin, Songliao Basin, Bohai Bay Basin, Junggar Basin, and Qaidam Basin, within the Mesozoic and Cenozoic formations. Overall China has a relatively large tight oil reserve and good exploration prospect.

Exploration and development of continental tight oil in China

Based on the investigation of tight oil exploration and development in North America, the successful cases of tight oil exploration and development in North America are summarized. The geological differences between continental tight oil in China and marine tight oil in North America is analyzed to explore the technical strategies for the industrial development of continental tight oil in China. The experiences of large-scale exploration and profitable development of tight oil in North America can be taken as references from the following 6 perspectives, namely exploring new profitable strata in mature exploration areas, strengthening the economic evaluation of sweet spots and focusing on the investment for high-profitability sweet spots, optimizing the producing of tight oil reserves by means of repetitive fracturing and 3D fracturing, optimizing drilling and completion technologies to reduce the cost, adopting commodity hedging to ensure the sustainable profit, and strengthening other resources exploration to improve the profit of whole project. In light of the high abundance of tight oil in China, we can draw on successful experience from North America, four suggestions are proposed in sight of the geological setting of China's lacustrine tight oil: (1) Evaluating the potential of tight oil resources and optimizing the strategic area for tight oil exploration; (2) selecting “sweet spot zone” and “sweet spot interval” accurately for precise and high efficient development; (3) adopting advanced tight oil fracturing technology to realize economic development; (4) innovating management system to promote the large-scale profitable development of tight oil.

Distribution and characteristics of lacustrine tight oil reservoirs in China

Lacustrine tight oil resources in China are significant exploration targets, similar to the marine tight oil resources in North America, while there are clear differences in terms of the forming environment, as well as reservoir properties of these two systems. In this study, the geological characteristics of lacustrine tight rocks are discussed, including the distribution, petrology, storage space, and movable fluid saturation. The tight rocks are consisted of tight sandstone, tight carbonate rock, tight peperite, and tight sedimentary tuff, in which intergranular pores and dissolved pores with the dimensions of 30–900 nm are mainly dominated by the storage space. The oil saturation typically exceeds 50% and the oil mobility is basically controlled by pore-throats with diameter less than 1 μm. These four types of reservoirs show different potentials for tight oil, in which tight sandstone plays in the Ordos Basin and the Songliao Basin are taken as the preferred targets into account for exploration and development. Although the distribution area, physical properties, and mobility of lacustrine tight oil in China are not as good as those of tight oil in North America, the lacustrine reservoirs are often stacked in multiple layers, resulting in a great thickness, and possess relatively high oil saturation, making them producible and commercially viable. Fluid properties are critical to effectively recovery of tight oil, and more comprehensive geological evaluation on fluid mobility should be undertaken.

Assessment on tight oil resources in major basins in China

Large-scale commercial development of shale gas and tight oil resources in North American and continuous discoveries of tight oil in China lead to the recognition of the importance of assessment on tight oil resources. Based on the characteristics of China's tight oil reservoirs and exploration status, this paper discusses a workflow focusing on in-place resource evaluation. In the workflow, a Resource-Richness Analogy Approach (RR-AA) is proposed for assessment of tight oil resources in less explored areas, a Genesis-Statistics Assessment Approach (GS-AA) for lacustrine tight oil resources with strong heterogeneity and regions where more delicate assessment is required, and an EUR Analogy Approach (EUR-AA) for assessment on recoverable tight oil by fully using available EUR data of production wells. After analyzing the geological characteristics of the formation of tight oil in the Ordos, Bohai Bay, Songliao, Junggar, Sichuan and other major Basins, the distributions of essential parameters such as reservoir thickness, area and porosity have been defined, and the parameter evaluation criteria have been established. Finally, the tight oil resource potentials in the major basins in China have been evaluated using one of the three methods and the prospective areas of high resource abundance are outlined in each of basins. The results from this study show that China's tight oil resources have a large potential and good exploration future. The estimated in-place tight oil resources are 12.580 × 109 tons, mainly distributed in the Ordos, Songliao, Bohai Bay and Junggar Basins. 15 most favorable zones are proposed having in-place resources of 5.296 × 109 tons.

Evaluation criteria, major types, characteristics and resource prospects of tight oil in China

AbstractTight oil refers to a petroleum play that occurs in a free or adsorbed state in source rocks or tight reservoir rocks (e.g., sandstone and carbonate rock) interbedded with or close to source rocks. Tight oil has generally not experienced large-scale, long-distance migration. According to such a definition and its characteristics, 10 key indices are proposed for tight oil resource evaluation in China. Tight oil reservoirs are divided into three groups in terms of porosity and permeability. Tight oil can be classified into three types according to the contact relationship between the tight oil reservoirs and source rocks, i.e., tight lacustrine carbonate oil, tight deep-lake gravity flow sandstones oil, and tight deep-lake deltaic sandstones oil. In China, tight oil resources are widely distributed and significant exploration discoveries have been achieved in the sixth member and seventh member of the Triassic Yanchang Formation in the Ordos Basin, the Permian Lucaogou Formation in the Junggar Basin, the Middle-Lower Jurassic strata of the Sichuan Basin, and the Cretaceous Qingshankou and Quantou Formations in the Songliao Basin. The total geological resources of tight oil in China assessed by using the “analog” method are estimated to be (10.67−11.15) ×109 tones. Taking into account of the future prospects of petroleum development, tight oil may become a realistic alternative to the conventional oil resources in China.

Introduction to the special issue of unconventional hydrocarbon research

Unconventional hydrocarbon research has developed rapidly in the recent years. In order to review the advances in unconventional hydrocarbon research in the areas of reservoir geology, geophysics, engineering, and economic evaluation, the Unconventional Natural Gas Institute (UNGI) of China University of Petroleum (Beijing) was invited to organize this special issue. Papers in this special issue are mainly about tight oil and gas, shale oil and gas, and coalbed methane, which are the main types of unconventional hydrocarbon. In the area of geology and accumulation mechanisms, Song et al. divided unconventional oil and gas into different types based on relationships between source rock evolution and different unconventional reservoirs and further noted that the fundamental difference between unconventional and conventional hydrocarbons is the driving force. Xiao et al. discussed the controlling factors and enrichment area evaluation of shale gas of the Lower Paleozoic marine strata in south China, and established geological and loss models of shale gas for uplifted/folded and faulted/folded areas. Jiang et al. identified three types of tight sandstone gas reservoirs according to the timing of coupling between the two phases of gas charging and reservoir tightening, and clarified the controlling factors, accumulation mechanisms and modes of different types of tight gas reservoirs. Zou et al. optimized the evaluation parameters and standards of the tight oil ‘‘sweet spots’’ which are significant for the exploration of tight oil in China. In the area of geophysical exploration, Yin et al. proposed a constrained basis pursuit inversion (BPI) method for stably estimating the Young’s modulus and Poisson’s ratio, and effectively predicting the brittleness of unconventional reservoirs. In the area of hydraulic fracturing, Zeng et al. used the finite element method to quantify the stress distribution and investigated the controlling mechanism of the stress distribution of multi-stage hydraulic fracturing and fracture expansion. Ge et al. discussed the interaction between hydraulic fracturing fluids and shale matrix, clarified the mechanism of low flowback efficiency of fracturing fluids, and established its relationship to gas productivity. In the area of development and economic evaluation, Hu et al. discussed the pore structure of typical American and Chinese shales and investigated the effect of pore structure on the decline of shale gas well production. Wang et al. discussed the influence of gas transport mechanisms on the productivity of multi-stage fractured horizontal wells in shale gas reservoirs. Zhang et al. proposed a model of hydraulic fracture in tight oil reservoirs by using a combined finite and discrete element method to direct the hydraulic fracturing of tight reservoirs. Tang et al. quantitatively divided the coalbed methane well-production process into four stages and quantitatively characterized the coal reservoir permeability change in different stages. Zhang et al. simulated the absolute adsorption and excess adsorption capacity of medium rank coal to CO2, CH4, and C2H6 gas using molecular dynamics, and provided a theoretical support for improving coalbed methane recovery using CO2 injection. Luo et al. applied the discounted cash & Yan Song sya@petrochina.com.cn

Discussion on effective development techniques for continental tight oil in China

Abstract Based on the main geological features and technical breakthroughs made in tight oil exploration, the major challenges facing tight oil development are analyzed, and the key technical trend for tight oil development is discussed in this paper. Mainly found in continental deposits, tight oil reservoirs in China feature small area, poor physical properties, big differences in geological characteristics between different basins, but low porosity, low permeability and pressure in general. In contrast to marine tight oil, tight oil in continental deposits faces such challenges as low production and recovery, and poor economics. Through nearly three years of research and pilot test, an integrated development mode with repeated fracturing of horizontal wells as the principal technique has been proposed, which includes integrated design, platform long horizontal well drilling, massive volume fracturing, re-fracturing stimulation, controlled production, factory-like operation, concentrated surface construction etc. It is recommended that study be strengthened on basic tight oil development theory, practical development technologies, and economic evaluation of tight oil development over the whole life cycle.

Major Breakthroughs in Geological Theory, Key Techniques and Exploration of Tight Oil in China

In recent years,U.S.tight oil exploration and development have achieved significant progress,with rapidly increasing production,which has significantly changed the U.S.A.energy supply pattern(Fig.1).Compared with the North American marine to marginal marine-continental transitional facies,China’s tight oil(Fig.1)