From the point of view of kinematics, geological structure phenomenon is an important record of historical tectonic movement events. As a strain image, it reflects the result of tectonic stress field activity in a certain period of tectonic movement in a certain way, and these tectonic geological phenomena, whether fold structural system or fault structural system, mostly play an important role in controlling the migration state of fluid. Therefore, in all kinds of important tectonic movements, it is the most essential method and way to study the characteristics of tectonic stress field and the mechanical process of its evolution in these fold or fracture systems, that is, tectonic stress driving is an important factor that can not be ignored in fluid migration and accumulation.
The stress distribution and stress state in the regional tectonic stress field reflect the tectonic stress drive. At present, in the study of ore field structure, different levels of tectonic stress field are often simulated and calculated, and the distribution law of ore field, vein or ore body is explored by means of its plane position relationship with known ore points. In the study, we try to directly connect the tectonic stress field driven by tectonic stress with the material field of fluid migration, and discuss the direction and speed of fluid migration. Therefore, the concept of migration potential field is introduced into geomechanics simulation laboratory, and the partial differential equation of the relationship among stress drive, fluid stress and migration potential field is established.
1. The tectonic stress provides the driving force for fluid migration.
By studying the relationship between tectonic stress and oil and gas accumulation, foreign scholars point out that due to the periodic tectonic dynamic action, the transformation process of organic matter and hydrocarbon generation speed are accelerated, and as a result, oil and gas accumulation can be fully formed in oil and gas basins. Some domestic scholars (Huang et al.,1989; Zhang Zhimin et al., 1993) discussed the relationship between tectonic stress field and oil and gas accumulation from the analysis of geological structural system, and thought that tectonic stress could promote oil and gas accumulation. In fact, as early as the early 1960s, Professor Li Siguang clearly put forward the research direction of "stress-driven and fluid movement in rocks". Since then, some scholars have put forward the theory and method of stress-driven and oil-gas migration on the basis of oilfield geological work and with the help of physical simulation and mathematical simulation, and think that the local low stress value area may be a high oil-gas enrichment area (Shen Shumin et al.,1989; Deng et al., 1993).
The direct driving force of fluid migration is the tectonic stress field that produces structural deformation and the in-situ stress field activity that has not caused deformation at present. Research shows that all kinds of tectonic movements, especially those that cause obvious deformation, play a vital role in fluid migration and accumulation. From the microscopic mechanism, the tectonic stress field that produces various deformations acts on the rock skeleton, causing it to produce elastic deformation, plastic flow and brittle fracture. At the same time, it also acts on the fluid in the rock gap, making it migrate to the high tectonic space with low fluid potential through various channels such as capillaries, pores and fracture surfaces until it stays.
On the other hand, today's geostress does not cause obvious deformation, but it also plays a very important role in fluid migration. As we all know, the permeability of rock is not only related to factors such as rock porosity, but also affected by the state of in-situ stress. The two horizontal stresses are not equal, that is, the anisotropy of horizontal stress will cause the anisotropy of rock permeability. Carlson (1986) found that the ratio of two horizontal stresses in smark, Sweden is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where: σH is the maximum horizontal principal stress; σh is the minimum horizontal principal stress.
The conductivity of liquid in rock is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where: BH is the conductivity along the direction of maximum horizontal principal stress; Bh is the conductivity along the direction of minimum horizontal principal stress.
It can be seen that the conductivity along the direction of the maximum horizontal principal stress is large, and the liquid flows more easily in this direction. This in-situ stress feature can be used to guide the deployment of production wells and water injection wells and improve oil production efficiency.
In a word, under the action of tectonic stress field, fluid moves from high-pressure and low-porosity zone to low-pressure and high-porosity zone at the high part of the structure, and the stress is released in the low-value area of local stress, and the pressure and energy are also reduced.
2. Tectonic movement provides a channel for fluid migration.
As the product of tectonic movement, the formation of micro-cracks, cracks and cracks of different scales provides a good channel for fluid migration.
The obvious deformation and fracture caused by tectonic movement can be used as a channel for fluid migration. Generally speaking, tensile cracks have opening function and compressive cracks have closing function, but not necessarily. Their opening and closing are not only related to mechanical properties, but also to fracture activities. As far as pure fracture surface is concerned, it generally plays a shielding role and blocks fluid migration. Because faults juxtapose rock layers with different capillary characteristics and fluid pressures. Only under certain conditions, the fracture surface itself provides a smooth passage for the fluid. E.C.D. Hooper (1991) put forward the periodic flow theory after studying the fluid migration along the growth fault. The theory holds that when the fracture is active, the permeability and fluid potential increase, and the fluid can move upward along the fracture, while when the fracture is inactive, the permeability decreases and the flow stops. With the activity of fault and the concentration of fluid flow, a fluid potential gradient is formed between the fault surface and surrounding rock. If the fluid is sufficiently concentrated, it is likely to cause fluid migration. Periodic flow will also lead to periodic changes in the direction of fluid flow in the compacted basin. In the low permeability period, only the lateral flow through the fault is of great significance, while in the active fault period, oil and gas can not only cross laterally, but also migrate horizontally and nearly vertically along the fault plane. It is a well-known fact that overthrust fault zone controls the sealing of oil and gas. For example, a series of oil and gas fields have been discovered under the huge Cordillera overthrust fault zone in western North America. Large oil and gas fields controlled by overthrust fault zones have been discovered in Ordos and Karamay. However, the periodic activity of overthrust fault zone can also become the main channel of oil and gas migration, such as Luntai fault in Tabei Oilfield.
3. Fluid migration potential
Like other underground fluids, fluid migration in underground rocks follows Darcy's law. The purpose of fluid migration is to achieve a new state of energy balance. In order to describe the energy state of fluid at a certain point underground, people introduced the concept of fluid potential (Hubbert, 1940). The so-called fluid potential is simply the mechanical energy of a unit mass of fluid at any position. To be exact, the potential energy of a point on any surface in a fluid is the total mechanical energy generated by doing work when the fluid with unit mass is transferred from its reference surface to any surface. Its mathematical expression is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where φ is the fluid potential at a certain point; Z is the elevation of the point; G is the acceleration of gravity; P is the fluid pressure at this point; ρ is the fluid density; Q is the velocity of the fluid at this point.
Fluid migration potential is used to describe the energy of fluid at a certain point underground, and fluid migration potential field is used to analyze and study the law of fluid migration (Shen Shumin et al., 1989). In fact, whether the fluid can migrate does not depend on the absolute size of its fluid potential somewhere, but on whether there is a fluid potential difference. Similarly, the existence of fluid migration potential difference is the fundamental reason for fluid migration. In this sense, it is more important to study the fluid migration potential difference than to study the fluid migration potential at a certain point alone. Therefore, the study of fluid migration by using migration potential field can make the study of fluid migration move from traditional static and qualitative analysis to dynamic, semi-quantitative and quantitative research, thus describing fluid migration more accurately and comprehensively, especially the study of stress-driven and fluid migration potential field, and opening up new research ways and methods for fluid migration research.
4. Theoretical equation of fluid migration potential
Because stress plays an irreplaceable role in fluid migration, the traditional fluid potential equation can no longer accurately describe the fluid migration process, so it is necessary to find a new fluid migration potential state equation. To this end, we made three assumptions:
1) When the medium is saturated, the volume coefficients of rock skeleton, fluid and gas are m, n and s respectively, so there is a relationship between them: m+n+s =1;
2) The rock skeleton is rigid, and the compression of liquid phase can be ignored compared with other variables;
3) The gas phase is in a relatively closed environment and moves with the medium.
For the convenience of research, take a hexahedral dxdydz, and if it is only a plane problem, dz= 1.
According to the law of conservation of mass, there are three continuous equations in the element during deformation and motion.
The continuous equation of the liquid phase part is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where Ux and Uy represent the velocity components of liquid in X and Y directions, respectively.
The continuous equation of the solid part is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where Vx and Vy represent the velocity components of solids in the x and y directions, respectively.
Set in dt time, the mass of gas secreted from the liquid phase is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where: ρ is the gas density; μ is the unit secretion coefficient, so the continuous equation of closed gas is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where Wx and Wy are the components of gas migration velocity in x and y directions respectively.
Because Wx and Wy are generally small, the second-order WeChat service is omitted. The above formula is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Because the gas is enclosed in the medium, there are:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
that is
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Synthesize the formulas (5-5), (5-6), (5-8) and (5- 10) and sort out:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
According to the Darcy-Gerscher Valov relation in the x and y directions:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where i0 is the initial head.
Find the first-order WeChat service of X and Y from the above two formulas, and add them to get:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
The sorting formulas (5- 1 1) and (5- 13) are as follows:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Under the condition of constant temperature, the equation of state of gas is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where P0 is the initial pressure.
Take its first-order WeChat service as an example:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
manufacture
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
because
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where e is porosity, so
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
By (5- 15), (5- 16), (5- 17), (5- 19) formula:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Under the action of tectonic stress, there is a functional relationship between void ratio and effective total stress in the process of medium deformation, namely:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
rule
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
In order to simplify the derivation process, the linear compression curve equation is adopted, then:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Let the constant be b, that is:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where: a is the compression coefficient; ξ is the lateral pressure coefficient.
Substitute formulas (5-22) and (5-24) into formula (5-20) and arrange them to obtain:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
For the effective total stress θ of porous media, when doing plane problems, there are:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where: is the axial effective stress; σ 1 and σ2 are the axial stress of bone and the total stress of bone.
Substituting formula (5-26) into formula (5-25), where A=-B, we get:
Mineralization and Lithospheric Tectonic Dynamics in Lanping-Weixi Area, Yunnan Province
Where θ is the tectonic stress; P is the internal pressure of the fluid; H is potential (head height); I0 is the initial head; Kx and Ky are the media permeability in x and y directions respectively; E is the porosity of the medium; A and b are constants.
This is a new partial differential equation that describes the state of fluid migration potential, and expresses the differential relationship between tectonic stress (θ), fluid internal pressure (P) and fluid migration potential. The right end of the equal sign is the migration potential, and the first term at the left end of the equal sign represents the change of fluid flow caused by the change of tectonic stress, and the second term represents the contribution of the change of internal pressure of liquid and gas to the flow, that is, the migration potential of fluid consists of these two parts. The formula shows that the internal pressure of fluid in porous media is controlled by the crustal stress state, that is, the stress change controls the fluid migration state. Generally speaking, the movement trend of fluid is the low value area of migration potential, and it is an effective method to predict the area where fluid may stop according to the distribution law of migration potential field and actual geological conditions.
We can also know that to simulate the migration potential field of mineral liquid, we must first simulate the tectonic stress field, and the key to simulate the migration potential field is to simulate the tectonic stress field correctly. Based on this formula, it is discretized, and combined with the tectonic stress field program, the simulation program of mineral fluid migration potential field can be compiled.
After analyzing the characteristics of each deposit in this ore field, it is determined that the typical structural profile in this area is a complex imbricate structure (see Figure 5-3c). Its two faults with the same dip and a shovel fault with the opposite dip form a complex fault combination style, forming a multi-level structural sliding system, and this shovel fault is often the main sliding surface.
Through simulation calculation, the stress field is characterized by the distribution of stress isolines along the fault, forming extreme stress areas at the end and intersection of the fault. The maximum stress gradient zone is located in the upper wall of faults, and there is a low stress zone between faults (Figure 5- 17 and Figure 5- 18).
Input the stress field calculation results into the migration potential field calculation program, get the migration potential field results, and draw the contour map. The characteristics of migration potential field are as follows: when faults are active, the low migration potential area of maximum principal stress has two I-level areas and one II-level area (Figure 5-19); The migration potential of the maximum shear stress is also clearly reflected in the II-level low potential area, and the role of faults as migration channels of mineral fluids is very obvious (Figure 5-20).
Figure 5- 17 Isogram of Maximum Principal Stress
Figure 5- 18 maximum shear stress contour map
According to the characteristics of stress field and migration potential field and their corresponding relationship, it can be seen that low stress is a necessary condition for forming low migration potential; Faults play an obvious role in mineral liquid migration. Tectonic stress not only acts on rocks, but also on mineral liquid, driving the flow of mineral liquid. Due to the complexity of tectonic development and the difference of tectonic activities, the stress state in geological bodies is quite different, so the migration potential of mineral liquids is also different, which may form a low potential area. According to the principle of conservation of energy, in general, the mineral liquid flows from the high migration potential area to the low migration potential area, and the maximum possible flow direction is the direction with the largest migration potential gradient. Therefore, the area with low migration potential is the most likely place where the ore liquid stops, that is, the place where the deposit is formed.
Figure 5- 19 migration potential contour map corresponding to maximum principal stress
Figure 5-20 Isogram of Migration Potential Corresponding to Maximum Shear Stress