Gives an insight into a panorama of China’s oil and gas exploration strategic policies, exploration principles, new theories, new technologies, new methods and managerial experiences to meet the demand for oil and gas exploration development both inside and outsideChina. Focusing on the key issues in oil and gas exploration, it provides a platform of discussion and mutual exchange for decision-makers, engineering and technological professionals and scientific researchers in an attempt to better serve the country’s oil and gas exploration.
Geology-engineering integration emphasizes the interaction between geological research and engineering operation. In China, the geological conditions of unconventional oil and gas fields are complex. As a result, geological modeling faces unique challenges in terms of data foundation and application requirements. On the one hand, the data is mainly from horizontal wells, so the data volume is large and the types are miscellaneous. On the other hand, the model is required to iterate quickly or even “timely” modeling during the operation process. Therefore, it is critical to make full use of various data to establish a high-quality geological model quickly. This paper focuses on the uniqueness of unconventional reservoir modeling, taking shale gas reservior as an example, and proposes specific workflows and methods. First, the geological modeling workflow of the horizontal well is systematically described. Through cycle correlation in true thickness domain, 2D geosteering section, and integration of seismic and well data, the difficulties of structural and attribute modeling of horizontal wells have been solved from one-dimension to three-dimension. Second, with the ant tracking technology as an example, natural fracture prediction and modeling methods are summarized. Fractures are widely developed in unconventional reservoirs. Based on the understanding of geological settings of fracture development, cross-validation from multidisciplinary data, such as FMI logging, drilling, and micro-seismic, is helpful to achieve reasonable fracture modeling. The third is the geological modeling workflow and application under different application requirements, such as well deployment optimization of multi-disciplinary integration, timely modeling supporting the geosteering, and fracturing engineering application.
Effective development of hard-to-recover oil reserves is an important measure to enhance oil development in China. The hard-to-recover reserves of low permeability and heavy oil in the Shengli Oilfield are nearly 6 × 10
8 t, accounting for more than 10% of the nation. Effective development faces many challenges such as complex oil-water system, lack of supporting technologies, and low productivity. Sinopec took Shengli Oilfield as a pilot project. After more than two years of cooperative development practice, it has explored a number of effective methods such as “innovation of cooperative mechanism”, “searching for sweet spots in low-grade reserves”, “geology–engineering integration”, “fine reservoir stimulation to increase productivity”. It has preliminarily achieved the goal of “management, recovery, and prodution”, the hard-to-recover oil reserves. The cumulative produced oil reserves are 4 860 × 10
4 t, established productivity is over 50 × 10
4 t, the drilling cycle is shortened by more than 60%, single well production is increased by about 40%, and the break-even oil price is dropped from 75/bbl to within 50/bbl. The practice of producing hard-to-recover oil reserves in Shengli Oilfield has initially formed a unique contracted productivity establishment plan and created an efficient model of geology–engineering integration of the whole industry chain, which has opened up a new direction for the sustainable development of mature oil areas with high degree of exploration.
The shale oil and gas revolution in the United States has brought about the rapid development of unconventional oil and gas industry. China National Offshore Oil Corporation (hereinafter referred to as CNOOC) follows the trend and actively opens up the unconventional oil and gas business. After acquiring China United Coalbed Methane Company, CNOOC was faced with a series of challenges in unconventional gas development. For coalbed methane, resource grade is poor; reserve abundance is low; production scale is small; and capacity of productivity construction is poor. For tight gas, the reserves are present but the production is low. Combined with the characteristics of coalbed methane and tight gas reservoirs and the technical requirements of exploration and development, through the management innovation of exploration and development integration, CNOOC has explored the path to achieve commercial development of unconventional gas and achieved preliminary results. Through the innovation of management mode and system construction, the rhythm and effect of unconventional gas exploration and development have been obtained, which realizes the goal of exploration and drilling, pipeline construction, pilot production, and gas sales in the same year, and greatly shortens the production cycle. It not only improves the exploration quality and investment efficiency but also avoids the risk of direct large-scale development and accelerates the pace of increasing reserves and production. It is hoped that through the exploration and demonstration from a single project and its corresponding system reform and innovation, the healthy and orderly development of the unconventional gas industry will be promoted in China.
With the expansion of global oil and gas exploration toward deep, subtle, and unconventional reservoirs, new breakthroughs have been made in oil and gas exploration in China. The Mahu tight conglomerate reservoirs in the Junggar Basin, Xinjiang are rich in oil resources and have great potential for large-scale construction. However, the factors that affect the oil flow capacity in the reservoir are complex and diverse. Also, there are a number of challenges during the development. For example, the reservoir heterogeneity is strong; the determination and classified evaluation of “sweet spot” are difficult; the difference of horizontal principal stress between the two directions is large; the formation and extension mechanism of hydraulic fractures is unclear; the cost is high; the geological conditions are complex. Guided by geology-engineering integration, combining the geological characteristics of the study area with the production practice, we have established the classified evaluation standard of “sweet spots” for this type of reservoirs. By the method of volume fracturing with intensive segments, stereoscopic development technologies and tests of “long horizontal sections, intensive segments, small well spacing, large well clusters and industrialized production” have been explored, forming a set of technical systems for efficient development of tight conglomerate reservoirs and achieving good results. The improvements of accuracy in description of “sweet spots” and 3D geological models have significantly increased the efficiency of drilling and completion engineering. The average number of adjustment times of horizontal section trajectories decreased from 5 m to 2 m and the penetration rate of the oil layer reached up to more than 90%. Moreover, the average cluster spacing in Ma-131 well block decreased from 67 m to 35 m, and the average cumulative production increased by 37.5% in 300 days.
The Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin, Xinjiang is rich in continental shale oil resources, with an estimated resource more than 15.8 × 10
8 t. In order to realize the effective development of shale oil, based on the detailed analysis of geological characteristics, engineering technologies, production characteristics and laws of continental shale oil in the Lucaogou Formation, and combined with the development practice, we have obtained the following achievements and understandings in recent years: (1) The source rocks of the Lucaogou shale oil in the Jimsar Sag are from the continental shale of saline lacustrine fine-grained deposit, which are high-quality source rocks with large thickness and characterized by integrated sources and reservoirs. Oil is accumulated in situ in the shale intervals, and in the “sweet spot” intervals, hydrocarbons were mainly supplied from adjacent source rocks with self-generation as auxiliary. All these characteristics indicated the typical continental shale oil. (2) It is clear that the free porosity and free oil reserve abundance in the fracture-controlled areas created by volume fracturing from horizontal wells constitute the basis of high production for the shale oil. (3) With the increase in buried depth, the difference of horizontal two-direction stress increases, and the fracture complexity decreases, which is the main reason for the low production of some horizontal wells with good oil-bearing property in the “upper sweet spot interval”. (4) The oil viscosity is the key control factor of the production from horizontal wells in the “lower sweet spot interval”. (5) The penetration length of high-quality “sweet spots” and the proppant volume per meter during hydraulic fracturing are the key engineering factors for the high production from shale oil horizontal wells in the Jimsar Sag. (6) Due to the imbibition and replacement between the fracturing fluid and the shale oil in the matrix pores, the production can be improved by proper soaking after fracturing. (7) Due to the limitation of hydraulic fracture length, the appropriate well spacing should not be larger than 200 m. Through continuous research and development practice, shale oil development has obtained good results. The maximum annual production of a single horizontal well has been more than 1.3 × 10
4 m
3. Since 2019, it has basically entered the stage of large-scale trial production, which accumulated rich experiences for the industrial development of continental shale oil in China.
As a large superimposed basin with rich oil and gas, the Ordos basin develops multiple sets of Permian and Triassic continental shale strata and is abundant in continental shale gas resources. Compared with the marine shale, the continental shale is more complicated. There are many challenges during the exploration and development of continental shale, which restrict its economic exploitation. In this paper, in terms of the geological characteristics, resources background, and technologies of exploration and development, the achievements in the practice of the exploration and development of continental shale gas in the Ordos basin were summarized. In addition, the current technical difficulties of continental shale gas were analyzed to effectively utilize opportunities and timely approach challenges in the development process. According to comprehensive analyses, the shale gas parameters in the Yanchang exploration area, such as the continental shale thickness, total organic carbon content, maturity, and gas content, have reached above the lower limit of favorable intervals of shale gas stipulated by national standards. Therefore, it meets the geological conditions for accumulation of shale gas. Under the control factors of macro and micro heterogeneous, the continental shale gas has different enrichment characteristics in terms of phase state and scale. There are two types of accumulation modes, which are adsorption accumulation and adsorption + free composite accumulation. The understanding of different accumulation mechanisms guides the selection of the favorable zone and the production from continental shale gas wells. A number of key supporting technologies have been formed during the exploration practice of continental shale gas. Logging evaluation methods suitable for strong heterogeneous continental shale have been established. The water-based drilling fluid system and a series of supporting equipment for drilling and completion have been developed. The CO
2 fracturing technology system and the recovery-utilization technology of wellsite waste fluid have been formed. Meanwhile, there are still many challenges in terms of precise prediction of “sweet spot”, improvement of drilling rate and efficiency, environmental protection and efficiency improvement of fracturing, and cost reduction. At present, the exploration and development of continental shale gas in China is still in an early stage, with high costs and low initial production. It is still necessary to carry out pilot tests for the exploration and development of continental shale gas to achieve scientific and technological breakthroughs. Besides, it can form a resource replacement and promote the continental shale gas industry to obtain economic benefits.
Large-scale development of shale gas in China is at the initial stage. The unscientific valuation method is a grave problem of optimization and management of drilling investment. This paper proposes a set of optimized analysis methods of drilling investment, as “integrated valuation method + standard well management”. The integrated valuation method is to establish a valuation method system for drilling engineering to meet the needs of the whole process management of oil and gas exploration and development projects. The premise is to establish unified and standardized calculation rules of bill of quantities for drilling engineering, cost composition of drilling engineering projects, and valuation standard system of the whole process of drilling engineering. Standard well management is to establish several standard well projects for scientific investment decision-making and organization of drilling production. There are six main functions. The above-mentioned methods were used in the calculation of development well investment and the optimized analysis of cost reduction and benefit increase for CNPC’s special planning for shale gas development from 2021 to 2030. This paper introduces the analysis of drilling productivity, preparation of budget quotas, rough-calculation quotas, rough-calculation indicators, estimation indicators, reference indicators, and the methods and results of drilling investment calculation.
In January 2020, light crude oil was obtained from Lower Cambrian dolomite with burial depth greater than 8,200 m in Well Luntan-1, which was located in the Lunnan low bulge in the Tabei uplift, indicating a major breakthrough in ultra-deep oil and gas exploration in old craton of Tarim Basin. This paper describes the background of well Luntan-1 discovery and analyzes the exploration history of the ultra-deep Cambrian pre-salt oil reserves, aiming to provide experience and reference for the exploration of new zones, new strata, and new types in other similar areas. According to the drilling results, Well Luntan-1 penetrated a set of high-quality source rocks in Cambrian Yuertusi Formation and 2 sets of reservoir-cap assemblages, which are evaporite gypsum cap rocks in the Awatage Formation and dolomite reservoir in the Shayilike-Wusonggeer Formations, and mudstone cap in the Yuertusi Formation and dolomite weathering crust reservoir in the Sinian Qigebulake Formation. The production layer in well Luntan-1 is the Wusonggeer Formation with volatile oil at normal temperature and pressure systems. In addition, trace natural gas was obtained from the weathering crust of the Sinian Qigebulake Formation. The exploration of the platform-basin area in the Tarim Basin has experienced 2 strategic shifts from clastic rocks to carbonate rocks, and then to pre-salt dolomite. Well Luntan-1 is an important sign of the second strategic shift, which is of milestone significance. The favorable oil accumulation conditions of well Luntan-1 are the successive development of stable paleo-uplift and the high-quality source-reservoir-cap assemblages. By analogy, this paper analyzes the exploration prospect of the Cambrian pre-salt dolomite in Tarim Basin and points out that the Tazhong-Gucheng area, the south slope of the Tabei area and the northern Maigaiti slope-Keping area are the favorable and key zones for further exploration.
Compared with North America, the exploration and development of unconventional oil and gas resources in China started relatively late. Shale gas exploration and development began at the beginning of the “12th Five-Year Plan” period. The exploration and development of tight oil, tight gas, and shale oil started at the beginning of the “13th Five-Year Plan” and developed rapidly during the “13th Five-Year Plan.” In particular, the developments of shale gas in Sichuan and Chongqing, conglomerate oil field in Mahu sag in the Junggar Basin, and shale oil reservoir in Jimsar have entered the stage of large-scale development, becoming the most important field of oil and gas production increase in the end of the “13th Five-Year Plan” and the period of “14th Five-Year Plan” of CNPC. Compared with conventional oil and gas resources, unconventional oil and gas resources have poor reservoir conditions, which requires large-scale “horizontal well + volume fracturing” to achieve effective development. However, due to the limitations of rig equipment and the performance of horizontal well drilling tools, especially the limitations of the concepts and methods, the issues of low drilling efficiency and long well construction period are common in unconventional horizontal wells in China. Generally, the drilling period of horizontal well is 60–90 days, with vertical depth of 2 000–3 000 m and horizontal section length of 1 500–2 000 m. In North America, the drilling period of a horizontal well can be basically controlled within 15–25 days, with vertical depth of 2 000–3 000 m and horizontal section length of 2 000–3 000 m. A drilling rig in China can only drill 2–3 horizontal wells a year, while in North America a drilling rig can complete 15–20 horizontal wells a year. With the large-scale development of unconventional oil and gas resources in China, the contradiction of drilling rig shortage is increasing. Drilling efficiency has become a bottleneck problem that seriously restricts the rapid and large-scale production of unconventional oil and gas resources in China. There are two advantages for the great improvement of drilling efficiency of horizontal wells in North America: management and technology. For technology, there are mainly five aspects: The first is the continuous upgrading of drilling rig supporting capacity; the second is the improvement of the reliability and stability of downhole tools; the third is the large-platform factory operation; the fourth is the remote real-time support and decision-making system, and the fifth is the application of the concepts and methods of systematic drilling optimization. In 2018, Strait Energy Co., Ltd. cooperated with K&M drilling engineering consulting company of the United States to introduce internationally-advanced concepts and methods of systematic drilling optimization. The pilot test of drilling speed increase of horizontal wells was first carried out in Changning Shale Gas Co., Ltd. in Sichuan–Chongqing area, and in the conglomerate oil field in Mahu sag in Xinjiang, which greatly shortened the well construction period and verified the feasibility of systematic drilling optimization methods in the development of unconventional oil and gas resources in China. In order for further promotion, in 2019, the test of speed increase in the whole block was carried out in the conglomerate oil field in Mahu sag, which also achieved remarkable results. It further proves that the concepts and methods of systematically optimized drilling speed increase are scientific, universal and replicable, which is an important practice from experience drilling to scientific drilling, and shows a very promising popularization and application prospect.
The tight oil reservoirs of the Chang-7 member in the Longdong area, Ordos Basin, are mainly gravity flow sand bodies. The distribution of sand bodies is complex in both vertical and lateral directions, and the thickness of a single sand body is low, which poses a challenge to efficient drilling and production. In this paper, a set of geology-engineering integration methods is proposed, which combines multi-disciplinary knowledge such as near-bit measurement while drilling (MWD) technologies. On the basis of comprehensive geological research, 3D fine geology, reservoir, and geomechanics models are established for selection of well locations, design of factory-like platforms, drilling operations and optimization of geo-steering schemes, so as to design well trajectory scientifically and reasonably, enhance the penetration rate of the sand body during drilling, and ensure higher single well production in later production stage as well as the ultimate long-term accumulative production of the well block. The results show that high-quality reservoirs in the study area are mainly massive clastic-flow sandstones. And the key to improving penetration rate is to use the real-time transmission MWD data, and comprehensively analyze the drilling, well logging and mud logging data to determine the accurate bit location in the sedimentary cycle, so as to determine the geo-steering operation scheme. Under the guidance of this method, two horizontal wells are drilled, whose penetration rates of oil layers are 5%–10% higher than that of the surrounding wells. According to the early-stage numerical simulation based on geology and geomechanics models, combined with scientific well spacing during drilling and production practice, the optimized horizontal well spacing in the study area is finally defined as 400 m.
