After more than a decade of large-scale development, the Daniudi gas field has built more than 2000 gas wells. According to the early understanding of composite sand gas reservoirs, the deployment of encrypted adjustment wells in the gas field has no place to stand, especially from 2017 to 2018, with an annual newly built production capacity of only 100-150 million cubic meters, which cannot support stable production of the gas field. "At the end of 2018, we began to initiate research on single sand body characterization technology and successfully applied it, reaching an international leading level, reversing the rapid decline trend of gas fields. We built a new annual production capacity of 1.27 billion cubic meters, supporting the hard and stable production of 3 billion cubic meters of gas fields," said Zhang Jiawei.

New technologies bring new opportunities

The Daniudi gas field has characteristics of low pressure, low permeability, low abundance, and low production. It belongs to an unconventional tight sandstone gas reservoir, which is difficult to stabilize production and has a high comprehensive decline rate. In the early stage of gas field development, researchers divided the gas field into 7 gas reservoirs, including He1, He2, Shan1, and Shan2, with 14 sets of composite sand bodies. Based on the principle of "fertilizing first and then thinning", they effectively developed high-quality Class I and II reservoirs using "vertical well single production" and "vertical well multi-layer combined production". Subsequently, they applied horizontal well technology to secondary Class III and IV reservoirs and also achieved beneficial development. The annual gas production of Daniudi gas field has been increasing year by year, with a gas production of 4 billion cubic meters in 2014.

"During this period, we have been using the concept of composite sand bodies to deploy well locations and have achieved expected production." Li Xiaohui, director of the Daniudi Natural Gas Institute of the North China Oil and Gas Exploration and Development Research Institute, introduced that as oil and gas development continues to deepen, the distribution of residual gas inside the reservoir becomes more scattered and complex, and the utilization of interlayer and planar reserves is unclear. The characterization of composite sand bodies can no longer meet the needs of development adjustments.

In 2017, a batch of new wells were deployed in North China for oil and gas, but the results did not meet expectations. At the end of 2018, research on single sand body characterization technology was fully launched.

In 2019, researchers accelerated the detailed description of single sand bodies in the entire gas field and all layers. They sorted and analyzed data from more than 1800 wells in the gas field, summarized the data of sand body characterization, and completed the division of single sand bodies. They subdivided the vertical 14 small layers into 29 single sand bodies, achieving the closure of the entire area and completing the characterization of single sand bodies in the main target layer. This is equivalent to re dividing and renovating an entire building, and the difficulty and complexity can be imagined.

Based on the characterization of individual sand bodies, researchers have evaluated that the uncontrolled reserves of the well network are 680 million cubic meters, and the encrypted adjustment can increase the annual production capacity by 37 million cubic meters; The evaluation shows that the well controlled untouched reserves are 3.3 billion cubic meters, and the combined drilling and production can increase the annual production capacity by 55 million cubic meters.

With the continuous revision and improvement of this technology, the upgrade from point to line, surface, and finally to three-dimensional has been completed, forming a three-dimensional configuration that objectively displays the distribution direction of gas reservoirs and provides support for well deployment.

Precise positioning and encryption adjustment well

Opening the distribution map of gas wells in the Daniudi gas field, the well locations are densely packed, making it difficult to deploy encrypted adjustment wells in the middle of these wells.

The Da66 well area is a superimposed area of Class II and III reserves, characterized by multiple vertical layers, thin single-layer gas layers, and poor connectivity. In the early stages of development, the main method was the vertical well "multi-layer combined production". Overall, the well network control was high, but there were significant differences in interlayer utilization. After large-scale development, the remaining gas was distributed in a fragmented manner.

In order to "extract and clean" the remaining gas, researchers have innovated their thinking and carried out research on "small well group, small grid" modeling, digital and analog integration of remaining gas. They have analyzed the differences in interlayer utilization and plane wave effects of multi-layer co production gas reservoirs, explored the formation of a "small area centralized research, multi-layer system three-dimensional deployment, and ultimate utilization of reserves" approach, deployed two horizontal wells and one directional well respectively, and achieved full control of remaining reserves in the well group area, such as the He3, He1, Shan2, Tai2, and Tai1 layers, to fully capture the remaining gas. Up to now, the Da66 well area has produced a total of 33.78 million cubic meters of gas, with an average daily gas production of 81000 cubic meters, using a three well mixed well infill well group.

The Da 12 well area is a horizontal well densification zone. During the construction process of a horizontal well, there must be a deviated section before entering the horizontal section, and there is usually a target front distance of 300-350 meters between the wellhead and the landing point, with residual gas present. In 2022, North China Oil and Gas deployed the infill well D12P-95 in the second layer between the horizontal well DPT-30 and DPT-48, and achieved a daily gas production of 20000 cubic meters.

There is also a situation where well spacing is increased. In the early deployment of horizontal wells with a spacing of 500 meters, there may be inadequate fracturing and transformation at the end of the horizontal section. In order to tap into the remaining gas potential, in 2022, researchers deployed infill wells D12P-74 in DPT-16 and DPT-42 for the Tai2 layer, using a 1000 meter horizontal displacement design and adjusting the well spacing to 300 meters, achieving the expected production.

In the expansion block, researchers have determined that there are sufficient remaining resources based on the distribution direction of single sand bodies. They have deployed horizontal well groups D70-P1, D70-P2, and D70-P3 in the Da 70 well area. The target layers in the horizontal section are Shan 1-1, Shan 1-2, and Shan 1-3, all of which are located in narrow river channels. The single sand body characterization technique characterizes these 1-5-meter-thick "desserts", providing support for well deployment.

Measures to tap potential and "awaken" inefficient wells

The developed reserves of the Daniudi gas field are gradually decreasing, and some inefficient and ineffective gas wells are produced every year. Identifying gas wells with potential reserves has become an important task for refined exploration and development. With the gradual maturity of single sand body characterization technology, it becomes clear where and what types of residual gas exist.

In 2019, the Daniudi Natural Gas Institute of North China Oil and Gas Exploration and Development Research Institute sorted out a large number of inefficient and shut down old wells for potential exploration.

D1-1-144 well is a gas well that was put into operation in 2006. The original production layers were the He3 and Tai2 layers. Researchers believe that other unused gas layers have good reserve potential through research, so they have decided to re insert casing and perform fracturing and transformation on the unused gas layers of He1, He2, and Shan1. After the well is put into operation, it will increase daily gas production by 4400 cubic meters.

Wang Paiying, Deputy Director of the Production Technology Department at North China Gas Production Plant 1, said, "The effect of using this method to treat gas wells is sometimes equivalent to constructing a new well, but the cost is only 50% of the new well."

DK13-FP8 well is a directional well deployed in the Lower Paleozoic in 2020. Due to low gas production, it has not been put into operation and has been in a shutdown state. Based on the principles of geological evaluation with potential, engineering technology with countermeasures, and economic evaluation with benefits, researchers analogize the characteristics of adjacent well reservoirs and believe that the Upper Paleozoic gas reservoir in this well has potential. By sealing the Lower Paleozoic gas layer and opening the Upper Paleozoic Shan1 and He3 gas layers, the daily gas production of well DK13-FP8 is 23000 cubic meters.

"For this type of well with intact shutdown conditions, our next plan is to use window drilling technology to drill a new wellbore in the wellbore, directly to the target layer," said Wang Paiying.

Supporting three-dimensional development of gas fields

"This is the 83rd well group in Daniudi and Dongsheng gas fields. Multiple single wells can be constructed on a well group platform, and the trajectory of these wells is like the roots of a big tree, penetrating into different gas reservoir layers and extracting natural gas through three-dimensional development." Xing Dawei, the captain of the North China Wupu Drilling Team, who is currently constructing the well group, introduced.

The three-dimensional development of cluster wells has many advantages, such as significant cost reduction, intensive environmental protection, and increased storage capacity in multiple layers. However, the tight and low-permeability oil and gas reservoirs in the Ordos Basin have strong heterogeneity, narrow river channels, and scattered distribution of "sweet spots".

"Due to the difficulty of overall utilization, we will further promote the three-dimensional development mode of well groups, tackle key technologies such as drilling trajectory control, precise guidance, and reservoir protection, so that they can adapt to gas reservoir development and accurately thread the scattered 'pearls'," said Wang Xiang, President of North China Oil and Gas Engineering Technology Research Institute.

"We establish configuration models for different sedimentary types of sand bodies based on single sand body characterization research, forming three-dimensional fine characterization techniques to make the spatial distribution of gas reservoirs more intuitive and achieve targeted well deployment." Yan Shuhong, an oil and gas field development expert at the North China Oil and Gas Exploration and Development Research Institute, introduced.

At present, the Daniudi gas field can deploy various complex trajectory development wells such as 3D horizontal wells and fishhook wells on the same well group platform, achieving maximum reserve utilization.

Guiding Fracturing Renovation with "One Stage, One Strategy"

Fracturing operation is an effective means to increase single well production of tight sandstone gas. When deploying well positions based on the concept of composite sand bodies, researchers believe that as long as fracturing is carried out in the sand body, the gas reservoir can be connected, but in reality, gas bearing sand bodies may be missed due to the presence of interlayers.

After the application of single sand body characterization technology, fracturing operations become more targeted. Based on the external shape, volume, internal configuration, etc. of the "sweet spot" of a single sand body, parameters such as interval between fracturing sections, fracturing scale, control of fracture height, and formation of long fractures can be designed in a targeted and directional manner. "In the early design, the interval between horizontal sections of horizontal wells was about 100 meters. Later, it was discovered that there were interlayers between gas layers, resulting in insufficient fracturing and poor results. Later, the interval was optimized to 30-50 meters according to the size of the sand body configuration." Yan Shuhong said.

Through continuous exploration, they aimed to fully utilize the remaining gas and summarized the construction mode of "one strategy at a time, reasonable joint arrangement": when encountering vertically stacked layers, they appropriately increased the pressure crack height and tried to communicate with more gas layers as much as possible; When the gas layer extends horizontally for a long time, fracturing techniques that can create long fractures are used; When encountering gas layers with poor display effects, reduce the fracturing scale or do not perform fracturing.

Based on the characterization of individual sand bodies, targeted "large-scale, dense cutting" fracturing methods were implemented, and significant results were achieved in the fracturing transformation. The average daily production of a horizontal well reached 35000 cubic meters, and the controlled reserves of a single well increased by 50%.

Wang Guozhuang, Chief Expert of North China Oil and Gas and Manager of Oil and Gas Development Management Department

The Party Branch Secretary and Deputy Manager of the North China Exploration and Development Research Center of the Petroleum Exploration and Development Research Institute are rigorous

Q: What is tight gas? What are the characteristics?

Wang Guozhuang: Tight sandstone gas reservoirs (abbreviated as tight gas) are widely distributed in major oil and gas basins around the world and are one of the important types of natural gas reservoirs. According to the results of the fourth oil and gas resource evaluation, the total onshore tight gas resources in China are 21.85 trillion cubic meters, of which the Upper Paleozoic tight gas resources in the Ordos Basin are 13.3 trillion cubic meters, accounting for 60% of the national total.

China has strong heterogeneity in tight reservoirs, with small individual effective sand bodies and an isolated non-uniform distribution. Low porosity thin layers make it difficult to predict and describe three-dimensional seismic reservoirs, making it difficult to discover high-quality reservoirs.

Due to the small pore throat of tight reservoirs, a considerable proportion of natural gas is stored in nanoscale storage spaces or unconnected pores, making it difficult to produce and difficult to utilize reserves. At the same time, due to the tight reservoir and insufficient displacement of formation water during natural gas injection, there is not only traditional "bound water" in the gas reservoir, but also a large amount of capillary "trapped water". During the extraction process, it will become "movable water" as the production pressure difference increases, forming a gas-water two-phase flow in the gas reservoir, leading to a decrease in gas phase permeability. In addition, the inherent start-up pressure gradient, strong stress sensitivity, and water locking characteristics of tight reservoirs all lead to a decrease in oil recovery.

Rigorous: Global tight gas resources are abundant and widely distributed, making it one of the important fields for unconventional natural gas exploration and development. However, there is no unified tight gas standard and boundary in the world. Different countries determine their standards and boundaries based on resource conditions, technical and economic conditions, and tax policies in different periods. Tight gas in China refers to sandstone gas reservoirs with a permeability of less than or equal to 0.1 millidarcy in the overlying matrix. Generally, a single well has no natural production capacity or natural production capacity is lower than the lower limit of industrial gas flow, but under certain economic conditions and technical measures, industrial natural gas production can be obtained.

In 1927, tight gas was first discovered in the San Juan Basin of the United States. At present, about 70 basins with tight gas have been discovered worldwide, with a recoverable resource of 209.7 trillion cubic meters. Since the discovery of the Zhongba gas field in western Sichuan in 1971, China has been conducting research on tight gas. With the continuous progress of geological theory and development technology, significant progress has been made in the exploration and development of tight gas, which has become an important field for increasing natural gas storage and production.

Q: What progress has been made in the exploration and development of tight gas by Sinopec currently?

Rigorous: Sinopec's tight gas is mainly distributed in the Ordos and Sichuan Basin, with geological characteristics such as large burial depth, small gas layer thickness, strong heterogeneity, low gas saturation, and complex gas water relationship. Development has the characteristics of "three low, one fast, and one long". In recent years, Sinopec has aimed to increase single well production and reduce development costs. Drawing on foreign experience in tight gas development and combining with the geological characteristics of its own tight gas, through independent research and development innovation, it has developed key technologies such as reservoir "sweet spot" prediction and quantitative selection of favorable areas, optimization of multi-layer and multi well development, vertical well multi-layer fracturing and horizontal well segmented fracturing, drainage and gas extraction. It has also built large gas fields such as Xinchang, Daniudi, Zhongjiang, and Dongsheng, promoting the large-scale development of tight gas. A series of quantitative characterization and comprehensive adjustment optimization technologies for residual gas in tight gas reservoirs have been developed around long-term stable production in tight gas fields. These technologies mainly include precise description of complex sedimentary systems and single sand bodies, quantitative characterization of complex residual gas based on tight reservoir seepage characteristics, three-dimensional adjustment optimization technology for well networks, and integrated pressure utilization technology for the entire life cycle of gas reservoirs, wellbore, and surface, achieving long-term stable production in Daniudi and Xinchang tight gas fields.

Q: What is the next direction for tackling tight gas exploration and development?

Wang Guozhuang: Firstly, we will continue to strengthen the integration of 3D seismic acquisition, processing, and interpretation, effectively improve the quality of seismic data, and tackle the formation of different types of tight reservoirs and effective prediction technologies for "sweet spots"; The second is to strengthen the integration of seismic geology, continuously deepen the fine description of single sand bodies, establish a fine geological model that is close to the actual gas reservoir, clarify the spatial distribution characteristics of remaining gas, and take targeted measures such as new wells, side drilling, and hole filling to accurately tap the potential; The third is to deepen the research on the extraction laws of different types of gas wells, optimize reasonable extraction methods, continuously deepen the research and application of effective drainage and extraction technologies at different development stages, and reduce the pressure of abandoned formations.

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What is the characterization of a single sand body?

The geological conditions for reservoir formation in the Ordos Basin are complex, and in some areas, tight gas exhibits characteristics of multiple layers, thin layers, and relatively high permeability under low permeability background. Vertical subdivision of the strata can be achieved by subdividing composite sand or oil and gas layers into individual sand layers. The same single sand layer can be divided into several (isolated) single sand bodies on a plane. A single sand body refers to a sand body that is vertically and horizontally continuous within itself, separated from its upper and lower sand bodies by impermeable interlayers such as mudstone.

The study of single sand body configuration and characterization of single sand bodies are the focus of reservoir description in the middle and later stages of Daniudi gas field development. By dissecting the elements of different level units inside the reservoir, the heterogeneity inside the reservoir is described, and the geometric shape, scale, and stacking relationship of single sand bodies in the constituent units of different level reservoirs are depicted to achieve efficient tapping of remaining oil and gas potential.

In recent years, North China's oil and gas industry has comprehensively utilized digital platforms. Through well mapping navigation, a three-dimensional stratigraphic configuration has been formed through multi angle stratigraphic comparison of points, lines, surfaces, and bodies. The parameters such as the position, size, and direction of a single sand body have been clearly marked, allowing researchers to directly observe the distribution direction of the single sand body, clarify the planar distribution characteristics of various reservoirs, and create a "transparent" gas reservoir, providing technical support for remaining gas research and well deployment.

What are the contents of the single sand body characterization technology series?

Firstly, the vertical phase division technology utilizes a digital application platform to determine the sedimentary phase division scheme for a single sand body under the control of the stratigraphic framework, achieving consistent phase division for the entire gas field.

Secondly, the research technology of genesis mechanism, through sedimentary analysis and simulation, field outcrop observation, etc., clarifies that the sand body configuration of the Upper Paleozoic reservoir in the Daniudi gas field is mainly composed of braided rivers, delta plain distributary channels, and barrier island sedimentation, which points out the direction for dissecting the single sand body configuration.

Thirdly, the technique of establishing configuration patterns, based on the establishment of reservoir genesis models, clarifies the morphology, scale, stacking relationship, and internal configuration characteristics of different sedimentary types of single sand bodies, and finely characterizes the spatial distribution of reservoirs.

Fourthly, the technique of dissecting the dense well network configuration establishes the connection between outcrop core logging through the dissection of the dense well network, laying the foundation for the precise description of the position, morphology, scale, and stacking relationship of single sand bodies.

Fifthly, precise quantitative characterization technology has been developed through a large amount of modern sedimentation, field outcrop observation, core description research, and detailed comparison of over 1800 gas wells. A configuration characterization technology with "vertical staging, curve positioning, lateral boundary delineation, and core beach characterization" as the core has been formed, laying the foundation for the three-dimensional quantitative characterization of remaining gas in single sand bodies and the improvement of well networks.

What is the significance of the formation and application of this technology?

One is that the distribution of sand bodies can be inferred from known sources. By utilizing drilling data and conducting 3D geological modeling analysis, sedimentary simulation and dense well network dissection can be carried out to clarify the effective sand body distribution characteristics in the undrilled area, quantify the key parameters of different types of single sand body development scales, accurately predict the inter well reservoir distribution, and guide the actual drilling tracking and adjustment of well positions.

The second is to clarify the strategies for tapping the potential of remaining gas. Through precise characterization and numerical simulation of single sand bodies, residual gas can be subdivided into six categories based on the range of pressure waves and the characteristics of the intra layer flow resistance zone and inter layer barrier layer. These categories include well network uncontrolled type, inter well unobstructed type, seepage barrier type, inter layer uncontrolled type, inter layer unopened type, and inter layer interference type. Combined with formation pressure monitoring data, the distribution patterns and potential tapping directions of residual gas in different development units can be clarified.

The third is to guide the on-site construction of drilling and fracturing. Based on the results of single sand body description, analyze the effective sand body stacking characteristics, optimize the horizontal well trajectory design under different sand body stacking modes through dynamic simulation and benefit comparison through "modeling, numerical and modeling integration", and "geological engineering integration". Develop optimized geological and engineering "double sweet spot" fracturing transformation plans to improve single well productivity.