Text | Edited by Tan Zhixu
As an important unconventional natural gas resource, the development and utilization of ultra-deep carbonate gas reservoir is of great significance to meet energy demand, however, due to the complexity of deep underground conditions, the development of ultra-deep carbonate gas reservoir faces many challenges, among which the characteristics of pore throat structure and its important impact on reservoir physical properties and reservoir development have an important impact.
Therefore, the purpose of my research is to provide strong support and assistance for the development of ultra-deep carbonate gas reservoirs by exploring and analyzing the influence of stress on the pore throat structure of ultra-deep carbonate gas reservoirs.
Overview of ultra-deep carbonate gas reservoirs
Characterization and classification of ultra-deep carbonate gas reservoirs
Ultra-deep carbonate gas reservoir is an important unconventional natural gas resource with high burial depth, usually located in deep strata thousands of meters or even tens of thousands of meters underground, which leads to high underground environmental parameters such as ground pressure, temperature and in-situ stress of ultra-deep carbonate gas reservoir, making development and utilization more complex and difficult.
Moreover, its reservoir rocks are mainly composed of carbonate rocks, such as limestone, dolomite and shale, etc., compared with conventional gas reservoirs, its reservoir porosity is relatively low, and the pore structure is complex, because carbonate rocks have a variety of pore types, including karst caves, fractures, granular gaps and dissolved pores, etc.
Due to the high temperature and high pressure environment in the deep strata, the physical and chemical properties of carbonate rocks have undergone obvious changes, such as mineral dissolution, recrystallization and cementation, which have a significant impact on the pore throat structure.
At the same time, it can also be classified according to its origin, geological structure and gas type and other characteristics, according to the cause, ultra-deep carbonate gas reservoir can be divided into biogenesis, chemical genesis and mechanical genesis gas reservoir, biogenic gas reservoir is a gas reservoir formed by biological action of organic substances in a specific environment, such as shale gas.
Chemically occurring gas reservoirs are gas reservoirs formed by dissolution inside or outside rocks, such as cave gas, and mechanical gas reservoirs are formed by the formation of rock fractures and cracks caused by tectonic movement, forming gas reservoirs.
The challenge of developing and exploiting ultra-deep carbonate gas reservoirs
At present, the development and utilization of ultra-deep carbonate gas reservoirs face a series of challenges, due to its extremely complex underground conditions, including high temperature, high pressure, high sulfur content and other special environments, these special environments for the exploration and development of gas reservoirs increase the technical difficulty, requiring developers to have a higher level of technical and engineering capabilities.
Moreover, the porosity, permeability and pore structure of the reservoir of ultra-deep carbonate gas reservoir change greatly, resulting in increased heterogeneity and uncertainty of the gas reservoir, which requires developers to conduct detailed research and analysis on the reservoir reservoir and formulate a reasonable development plan.
At the same time, ultra-deep carbonate gas reservoirs often have cracks and holes development, which have an important impact on the seepage performance and stability of gas reservoirs, making the development and adjustment of gas reservoirs more difficult.
Therefore, in order to effectively develop and utilize ultra-deep carbonate gas reservoirs, it is necessary to overcome technical difficulties and improve the feasibility and safety of exploration and development, which requires the comprehensive use of geological exploration, engineering technology and advanced oil and gas development technology, and continuous innovation and improvement of development methods to achieve efficient, safe and sustainable development of ultra-deep carbonate gas reservoirs.
Effect of stress on pore throat structure of ultra-deep carbonate rocks
The pore throat structure of ultra-deep carbonate gas reservoir refers to the characteristics and distribution of pores and throat channels, and stress is one of the important influencing factors of pore throat structure of ultra-deep carbonate gas reservoir, and the action of stress leads to changes in porosity, pore-throat connectivity and pore throat size, which in turn affects the physical properties of reservoirs and the development potential of gas reservoirs.
This is mainly because stress loading will cause changes in the porosity of ultra-deep carbonate rocks, and under high stress, pore pressure may occur, resulting in a decrease in porosity, which is caused by the compression of stress between rock particles, so that the pores are partially or completely filled, thereby reducing the porosity.
Conversely, under low stress, porosity may increase because the gap between rock particles expands.
Moreover, under high stress, the pore-throat connectivity may be affected by fragmentation and occlusion, resulting in weakened connectivity between pores, which is due to the fracture and deformation of the rock under stress, so that the originally connected pore throat is disconnected or filled, resulting in the flow of gas in the reservoir is restricted.
Under low stress, the pore-throat connection is relatively good, and the communication between pores is strong.
At the same time, stress also has an impact on the size of the pore throat of the ultra-deep carbonate gas reservoir, under the action of high stress, the stress transfer between the rock particles will lead to the shrinkage and deformation of the pore throat, so that the size of the pore throat is reduced, on the contrary, under the action of low stress, the pore throat size may increase, because the stress transfer between the rock particles is weak, and the pore throat can better maintain its original shape and size.
Therefore, the influence of stress on the pore throat structure of ultra-deep carbonate gas reservoir is mainly reflected in the changes of porosity, pore-throat connectivity and pore-throat size, which directly affect the physical properties of the reservoir and the development potential of the reservoir.
Influencing factors and mechanism of action
Effect and mechanism of stress on porosity
Stress loading will lead to elastic deformation and plastic deformation of the rock, thereby causing changes in porosity, in the elastic stage, the rock will undergo elastic strain, and the change of porosity is mainly caused by the change of pore volume and pore connectivity,
The decrease in pore volume may be due to the shrinkage or deformation of pores due to stress aggregation in the pore wall, or the reduction of pore volume due to the rearrangement of rock particles in the pore body, and the action of stress may also lead to friction and collision of rock particles in the pore body, further causing the reduction of pore volume.
In the plastic deformation stage, plastic strain will occur in the rock, and the change of porosity is mainly caused by the deformation and rearrangement of rock particles, and plastic deformation will lead to the compaction and rearrangement of rock particles, thereby reducing porosity.
At the same time, stress can also lead to the fracture and fragmentation of rock particles, further reducing the porosity, so stress loading will cause the porosity of ultra-deep carbonate gas reservoirs to decrease, which will have an important impact on the reservoir performance.
In addition, stress loading will change the connectivity of rock pores, that is, whether the channels between the pores are connected, higher stress will cause the rock particles in the pore body to accumulate more closely, reduce the connectivity between the pores, thereby reducing the permeability of the rock, which will limit the flow of natural gas in the rock and cause difficulties in the development and production of gas reservoirs.
Therefore, the influence of stress on the porosity of ultra-deep carbonate gas reservoirs is mainly achieved by changing the pore volume, pore connectivity and deformation and rearrangement of rock particles, and the understanding of these influence mechanisms is of great significance for accurately evaluating the reservoir characteristics and development potential of ultra-deep carbonate gas reservoirs.
Effect and mechanism of stress on pore-throat connectivity
Stress loading has a significant effect on pore-throat connectivity, which in turn affects rock permeability and reservoir physical properties, specifically, it causes the closure, deformation or expansion of the pore-throat, thereby altering the connectivity between orifice throats.
This is because when stress loading acts on ultra-deep carbonate rocks, the pores and throats in the rock are subjected to compressive forces, and the connectivity between the pores is limited, and higher stresses lead to the closure or contraction of the pores, reducing the permeability between the pores, thereby reducing the permeability of the rock,
This closure and contraction can cause the gas flow to be obstructed, limiting the ability of the gas to move through the pores.
Moreover, under the action of high stress, plastic deformation or fragmentation of pores and throat may occur, resulting in changes in the morphology of the pore throat, irregular shape of the pore throat after deformation, weakened connectivity, increasing the length of the seepage path of the gas in the rock, and causing greater resistance to gas transport.
At the same time, stress loading may also cause the expansion of the pore throat, in some cases, the rock under the action of stress may crack or expand the pore throat, which increases the connection between the pore throat, this expansion can improve the permeability of the rock, increase the gas migration channel, and facilitate the accumulation and recovery of gas.
Therefore, the influence of stress on the pore-throat connectivity of ultra-deep carbonate gas reservoirs is mainly realized through pore closure, deformation or expansion, which has an important impact on the permeability and gas transport capacity of rocks, which in turn affects the development and productivity evaluation of gas reservoirs.
Therefore, in the development of ultra-deep carbonate gas reservoirs, the influence of stress on pore-throat connectivity should be fully considered, and its influence mechanism should be further studied to guide reasonable development strategies and engineering design.
Influence and mechanism of stress on pore throat size and wall morphology
When stress loading acts on ultra-deep carbonate rocks, the pores and throat channels in the rock are subjected to compressive forces, and the connectivity between the pores is limited, and higher stress will lead to the closure or contraction of the pores, reducing the permeability between the pores, thereby reducing the permeability of the rock.
It may also cause microfracture expansion in ultra-deep carbonate gas reservoirs, resulting in changes in the morphology of the pore throat wall. At the same time, stress may also cause some existing pores in the rock to be closed or distorted, and stresses of different sizes and directions have different effects on the pore wall, thus forming different pore wall morphologies.
Therefore, stress is one of the important factors affecting the morphology of the pore wall of ultra-deep carbonate gas reservoirs, and its influence mechanism is complex. In actual mining, it is necessary to fully consider the action and change of stress, and adopt appropriate engineering measures to ensure the smooth exploitation of gas reservoirs.