Research advances on the mechanisms of reservoir formation and hydrocarbon accumulation and the oil and gas development methods of deep and ultra-deep marine carbonates

Based on the new data of drilling, seismic, logging, test and experiments, the key scientific problems in reservoir formation, hydrocarbon accumulation and efficient oil and gas development methods of deep and ultra-deep marine carbonate strata in the central and western superimposed basin in China have been continuously studied.(1) The fault-controlled carbonate reservoir and the ancient dolomite reservoir are two important types of reservoirs in the deep and ultra-deep marine carbonates. According to the formation origin, the large-scale fault-controlled reservoir can be further divided into three types:fracture-cavity reservoir formed by tectonic rupture, fault and fluid-controlled reservoir, and shoal and mound reservoir modified by fault and fluid. The Sinian microbial dolomites are developed in the aragonite-dolomite sea. The predominant mound-shoal facies, early dolomitization and dissolution, acidic fluid environment, anhydrite capping and overpressure are the key factors for the formation and preservation of high-quality dolomite reservoirs.(2) The organic-rich shale of the marine carbonate strata in the superimposed basins of central and western China are mainly developed in the sedimentary environments of deep-water shelf of passive continental margin and carbonate ramp. The tectonic-thermal system is the important factor controlling the hydrocarbon phase in deep and ultra-deep reservoirs, and the reformed dynamic field controls oil and gas accumulation and distribution in deep and ultra-deep marine carbonates.(3) During the development of high-sulfur gas fields such as Puguang, sulfur precipitation blocks the wellbore. The application of sulfur solvent combined with coiled tubing has a significant effect on removing sulfur blockage. The integrated technology of dual-medium modeling and numerical simulation based on sedimentary simulation can accurately characterize the spatial distribution and changes of the water invasion front.Afterward, water control strategies for the entire life cycle of gas wells are proposed, including flow rate management, water drainage and plugging.(4) In the development of ultra-deep fault-controlled fractured-cavity reservoirs, well production declines rapidly due to the permeability reduction, which is a consequence of reservoir stress-sensitivity. The rapid phase change in condensate gas reservoir and pressure decline significantly affect the recovery of condensate oil. Innovative development methods such as gravity drive through water and natural gas injection, and natural gas drive through top injection and bottom production for ultra-deep fault-controlled condensate gas reservoirs are proposed. By adopting the hierarchical geological modeling and the fluid-solid-thermal coupled numerical simulation, the accuracy of producing performance prediction in oil and gas reservoirs has been effectively improved.

Progress and development directions of stimulation techniques for ultra-deep oil and gas reservoirs

By reviewing the development history of stimulation techniques for deep/ultra-deep oil and gas reservoirs,the new progress in this field in China and abroad has been summed up,including deeper understanding on formation mechanisms of fracture network in deep/ultra-deep oil and gas reservoir,performance improvement of fracturing fluid materials,fine stratification of ultra-deep vertical wells,and mature staged multi-cluster fracturing technique for ultra-deep and highly deviated wells/horizontal wells.In light of the exploration and development trend of ultra-deep oil and gas reservoirs in China,the requirements and technical difficulties in ultra-deep oil and gas reservoir stimulation are discussed:(1)The research and application of integrated geological engineering technology is difficult.(2)The requirements on fracturing materials for stimulation are high.(3)It is difficult to further improve the production in vertical profile of the ultra-deep and hugely thick reservoirs.(4)The requirements on tools and supporting high-pressure equipment on the ground for stimulation are high.(5)It is difficult to achieve efficient stimulation of ultra-deep,high-temperature and high-pressure wells.(6)It is difficult to monitor directly the reservoir stimulation and evaluate the stimulation effect accurately after stimulation.In line with the complex geological characteristics of ultra-deep oil and gas reservoirs in China,seven technical development directions are proposed:(1)To establish systematic new techniques for basic research and evaluation experiments;(2)to strengthen geological research and improve the operational mechanism of integrating geological research and engineering operation;(3)to develop high-efficiency fracturing materials for ultra-deep reservoirs;(4)to research separated layer fracturing technology for ultra-deep and hugely thick reservoirs;(5)to explore fracture-control stimulation technology for ultra-deep horizontal well;(6)to develop direct monitoring technology for hydraulic fractures in ultra-deep oil and gas reservoirs;(7)to develop downhole fracturing tools with high temperature and high pressure tolerance and supporting wellhead equipment able to withstand high pressure.

Theoretical Progress and Key Technologies of Onshore Ultra-Deep Oil/Gas Exploration

Oil/gas exploration around the world has extended into deep and ultra-deep strata because it is increasingly difficult to find new large-scale oil/gas reservoirs in shallow–middle buried strata. In recent years, China has made remarkable achievements in oil/gas exploration in ultra-deep areas including carbonate and clastic reservoirs. Some (ultra) large-scale oil and gas fields have been discovered. The oil/gas accumulation mechanisms and key technologies of oil/gas reservoir exploration and development are summarized in this study in order to share China’s experiences. Ultra-deep oil/gas originates from numerous sources of hydrocarbons and multiphase charging. Liquid hydrocarbons can form in ultradeep layers due to low geothermal gradients or overpressures, and the natural gas composition in ultra-deep areas is complicated by the reactions between deep hydrocarbons, water, and rock or by the addition of mantle- or crust-sourced gases. These oils/gases are mainly stored in the original highenergy reef/shoal complexes or in sand body sediments. They usually have high original porosity. Secondary pores are often developed by dissolution, dolomitization, and fracturing in the late stage. The early pores have been preserved by retentive diageneses such as the early charging of hydrocarbons. Oil/gas accumulation in ultra-deep areas generally has the characteristics of near-source accumulation and sustained preservation. The effective exploration and development of ultra-deep oil/gas reservoirs depend on the support of key technologies. Use of the latest technologies such as seismic signal acquisition and processing, low porosity and permeability zone prediction, and gas–water identification has enabled the discovery of ultra-deep oil/gas resources. In addition, advanced technologies for drilling, completion, and oil/gas testing have ensured the effective development of these fields.

Practice and theoretical and technical progress in exploration and development of Shunbei ultra-deep carbonate oil and gas field,Tarim Basin,NW China

In this review on the exploration and development process of the Shunbei ultra-deep carbonate oil and gas field in the Tarim Basin, the progress of exploration and development technologies during the National 13th Five-Year Plan of China has been summarized systematically, giving important guidance for the exploration and development of ultra-deep marine carbonate reservoirs in China and abroad. Through analyzing the primary geological factors of “hydrocarbon generation-reservoir formation-hydrocarbon accumulation” of ancient and superposed basin comprehensively and dynamically, we point out that because the Lower Cambrian Yuertusi Formation high-quality source rocks have been located in a low-temperature environment for a long time, they were capable of generating hydrocarbon continuously in late stage, providing ideal geological conditions for massive liquid hydrocarbon accumulation in ultra-deep layers. In addition, strike-slip faults developed in tectonically stable areas have strong control on reservoir formation and hydrocarbon accumulation in this region. With these understandings, the exploration focus shifted from the two paleo-uplifts located in the north and the south to the Shuntuoguole lower uplift located in between and achieved major hydrocarbon discoveries. Through continuing improvement of seismic exploration technologies for ultra-deep carbonates in desert, integrated technologies including seismic acquisition in ultra-deep carbonates,seismic imaging of strike-slip faults and the associated cavity-fracture systems, detailed structural interpretation of strike-slip faults, characterization and quantitative description of fault-controlled cavities and fractures, description of fault-controlled traps and target optimization have been established. Geology-engineering integration including well trajectory optimization,high efficiency drilling, completion and reservoir reformation technologies has provided important support for exploration and development of the Shunbei oil and gas field.

Efficient development strategies for large ultra-deep structural gas fields in China

Through analyzing the development of large ultra-deep structural gas fields in China,strategies for the efficient development of such gas fields are proposed based on their geological characteristics and production performance.According to matrix properties,fracture development degree and configuration between matrix and fractures,the reservoirs are classified into three types:single porosity single permeability system,dual porosity dual permeability system,and dual porosity single permeability system.These three types of gas reservoirs show remarkable differences in different scales of permeability,the ratio of dynamic reserves to volumetric reserves and water invasion risk.It is pointed out that the key factors affecting development efficiency of these gas fields are determination of production scale and rapid identification of water invasion.Figuring out the characteristics of the gas fields and working out pertinent technical policies are the keys to achieve efficient development.The specific strategies include reinforcing early production appraisal before full scale production by deploying high precision development seismic survey,deploying development appraisal wells in batches and scale production test to get a clear understanding on the structure,reservoir type,distribution pattern of gas and water,and recoverable reserves,controlling production construction pace to ensure enough evaluation time and accurate evaluation results in the early stage,in line with the development program made according to the recoverable reserves,working out proper development strategies,optimizing pattern and proration of wells based on water invasion risk and gas supply capacity of matrix,and reinforcing research and development of key technologies.

Sichuan super gas basin in southwest China

A sedimentary basin is classified as a super basin when its cumulative production exceeds 5 billion barrels of oil equivalent(6.82×10^(8) t of oil or 7931.66×10^(8) m^(3) of gas)and its remaining recoverable resources are at least 5 billion barrels of oil equivalent.By the end of 2019,the total output of oil and gas in Sichuan Basin had been 6569×10^(8) m^(3),the ratio of gas to oil was 80:1,and the total remaining recoverable resources reached 136404×10^(8) m^(3),which makes it as a second-tier super basin.Because the output is mainly gas,it is a super gas basin.The reason why the Sichuan Basin is a super gas basin is that it has four advantages:(1)The advantage of gas source rocks:it has the most gas source rocks(9 sets)among all the basins in China.(2)The advantage of resource quantity:it has the most total remaining recoverable resources among all the basins in China(136404×10^(8) m^(3)).(3)The advantage of large gas fields:it has the most large gas fields(27)among all the basins in China.(4)The advantage of total production:by the end of 2019,the total gas production had been 6487.8×10^(8) m^(3),which ranked the first among all the basins in China.There are four major breakthroughs in natural gas exploration in Sichuan Basin:(1)Breakthrough in shale gas:shale gas was firstly found in the Ordovician Wufeng-Silurian Longmaxi formations in China.(2)Breakthrough in tight sandstone gas:the Triassic Xu2 Member gas reservoir in Zhongba gas field is the first high recovery tight sandstone gas reservoir in China.(3)Breakthrough in giant carbonate gas fields.(4)Breakthrough in ultra-deep gas reservoir.These breakthroughs have led to important progress in different basins across the country.Super basins are classified according to three criteria:accumulative oil and gas production,remaining recoverable resources,tectonic attributes of the basin and the proportion of oil and gas in accumulative oil and gas production.

Reservoir accumulation conditions and key exploration&development technologies for Keshen gas field in Tarim Basin

The Keshen gas field is located in the central part of Kuqa foreland thrust belt in Tarim Basin,and is another large gas field discovered in Kuqa depression after Kela 2 gas field.Since the breakthrough in 2008,a number of large and medium scale gas reservoirs including Keshen 2,Keshen 5 and Keshen 8 have been discovered,that are characterized by ultra depth,ultra-high pressure,ultra-low porosity,ultra-low permeability,high temperature and high pressure.With natural gas geological reserves of nearly trillion cubic meters and production capacity of nearly 5.5 billion cubic meters,the Keshen gas field is the main natural gas producing area in Tarim Oilfield.The Keshen gas field is located in a series of thrusting imbrication structures in the Kelasu structural belt of Kuqa foreland thrust belt.The salt roof structure,plastic rheology of salt beds and pre-salt faulted anticlinal structure constitute the large wedge-shaped thrust body.The thick delta sandstone of the Cretaceous Bashijike Formation is widely distributed,and it forms the superior reservoir-caprock combination with overlying Paleogene thick gypsum-salt bed.The deep Jurassic-Triassic oil and gas migrate vertically along fault system formed in Late Himalaya,break through the thick Cretaceous mudstone and move laterally along the fracture system of the pre-salt reservoirs,to form anticline and fault anticline high pressure reservoir groups.Through near ten years of studies,the three-dimensional seismic acquisition and processing technology for complex mountainous areas,extrusion salt-related structural modeling technology and fractured low-porosity sandstone reservoir evaluation technology have been established,which lay a foundation for realization of oil and gas exploration objectives.Logging acquisition and evaluation technology for high temperature,high pressure,ultra-deep and low-porosity sandstone gas reservoirs,and efficient development technology for fractured tight sandstone gas reservoirs have been developed,which provide a technical support for efficient exploration&development and rapid production of the Keshen gas field.