Balanced steam injection method for enhanced recovery of extra-heavy oil reservoirs

Abstract

The application provides a balanced steam injection method for improving the recovery of extra-heavy oil reservoirs, comprising the following steps: step 1, injecting wet saturated steam under the condition that the pressure is not higher than the fracture pressure of the extra-heavy oil reservoir; step 2, comprehensively considering the variable mass flow and variable dryness of the injected steam in the oil layer section and the reservoir properties of the oil layer section, determining the required division of the steam injection section in the oil layer section; step 3, establishing a process calculation method for balanced steam injection, and determining the shunt injection amount of each injection distributor; step 4, forming a balanced steam injection process string of the injection distributor + heat insulation tubing for balanced steam injection; and step 5, forming a high-temperature steam cavity and a drainage cavity which are balanced and expanded along the oil layer section. The balanced steam injection method for improving the recovery of extra-heavy oil reservoirs forms a high-temperature steam cavity and a drainage cavity which are balanced and expanded along the oil layer section in the reservoir, thereby avoiding the problem that the heated crude oil cannot effectively flow back due to the steam path during the steam injection process of the extra-heavy oil, and reducing the water recovery rate, the oil production and the recovery rate.

Description

Technical Field

[0001] This invention relates to the field of oilfield development technology, and in particular to a balanced steam injection method for enhancing the recovery rate of extra-heavy oil reservoirs. Background Technology

[0002] Heavy oil resources are widely distributed, mainly including the United States, Venezuela, and Canada, with estimated geological reserves of approximately 4000 × 10⁻⁶. 8 The heavy oil resources in China are also widely distributed. Although the resource volume is far smaller than that of foreign heavy oil resources, it still plays a crucial role in stabilizing China's oil reserves. Due to the advantages of high well-controlled reserves and large drainage areas, horizontal wells have become an important means of increasing and stabilizing production. However, the development of conventional thermal recovery horizontal wells is affected by reservoir heterogeneity, resulting in uneven utilization of the horizontal section and significant steam channeling issues. This prevents the full utilization of the high production advantage of thermal recovery horizontal wells, thus affecting the level of heavy oil thermal recovery development. Currently, the main technologies for developing heavy oil thermal recovery horizontal wells in China include steam huff and puff, steam drive, and SAGD. Among these, the thermal, agent, and gas combination technology utilizing the horizontal well structure is mainly applied to extra-heavy oil reservoirs, achieving good oil production enhancement effects.

[0003] However, as heavy oil thermal recovery progresses through multiple rounds of huff and puff, reservoir heterogeneity, edge and bottom water intrusion, and unreasonable steam injection processes lead to uneven steam absorption profiles and significant differences in utilization levels in horizontal sections. High water cuts persist throughout the cycle, failing to effectively leverage the large drainage area of ​​horizontal wells and severely impacting their development performance. With increasing huff and puff cycles, reservoir heterogeneity intensifies, and steam channeling and high water cuts become increasingly complex. Steam mainly flows ineffectively in areas with low oil saturation, leaving near-wellbore enriched oil that is difficult to effectively utilize, resulting in low steam utilization. Therefore, achieving balanced steam injection in oil-bearing sections during the steam injection process in thermal recovery horizontal wells to improve uneven utilization, enhance steam utilization and overall recovery rates in ultra-heavy oil reservoirs, and ultimately improve the quality and efficiency of ultra-heavy oil reservoir development has become a key research concern.

[0004] In the steam injection technology of horizontal sections of thermal recovery wells, the conventional calculation method in the petroleum industry is to study the steam flow law using a pipe flow mode with constant mass flow in the horizontal section. This involves calculating the parameter changes of the steam flowing within the pipe based on the principle that the mass remains constant from the heel to the toe of the horizontal well. However, this calculation method does not match the actual variable mass and dryness steam injection conditions in the field, and cannot achieve balanced steam injection in thermal recovery horizontal wells. Therefore, steam injection tools such as injectors should be used to divert the steam during injection, and calculations should be performed according to a variable mass flow mode to improve the accuracy of the steam injection parameter calculations for the horizontal section.

[0005] In 2012, Yang Hong et al. published a study on balanced steam injection technology for horizontal wells in heavy oil thermal recovery in Petroleum Geology and Engineering. In response to the problems of uneven steam distribution and uneven utilization of the horizontal section of the oil layer caused by the two-point steam injection method in horizontal wells for heavy oil thermal recovery, they established a heating analysis model for the horizontal section of the two-point steam injection method that conforms to the actual field conditions. They used a self-regulating steam distributor to achieve balanced steam distribution at multiple points in the horizontal section of the horizontal well, which partially improved the utilization of the horizontal section. However, the tubing structure of the balanced steam injection process was not perfect, and the placement of the distributor did not take into account the reservoir properties of the oil layer.

[0006] Chinese patent application No. 202111455571.0 discloses a method for adjusting the steam absorption profile of a heavy oil horizontal well. This method analyzes the degree of mobilization based on the deviation between the original well temperature and the measured curve, establishes a single temperature model, and uses the area method to uniformly distribute steam in the horizontal section, achieving the purpose of adjusting the steam absorption profile and uniform steam injection. This method assumes that the steam injection volume of each injector remains constant, and then adjusts the length of the horizontal section corresponding to each injector. Although it achieves uniform steam injection, it does not consider the reservoir properties of the oil layer and the thermophysical parameters of the injected steam, making it difficult to address the need for segmented steam injection in the oil layer as required, and thus failing to achieve balanced steam injection in thermal recovery horizontal wells.

[0007] Chinese patent application CN201610269609.8 discloses a method for improving the development effect of heavy oil reservoirs through stratified steam injection. This method includes: Step 1, selecting the optimal well location for stratified steam injection; Step 2, determining the optimal oil layer for stratified steam injection, and determining the number of layers and the specific location of the packers; Step 3, determining the steam injection volume for each layer, comprehensively considering the physical properties and effective thickness of each layer, and establishing a steam injection volume optimization model with the injection volume as the optimization variable; Step 4, designing the injection nozzle orifice to achieve uniform steam injection; Step 5, carrying out stratified steam injection operations based on the calculated steam injection volume for each layer and the determined stratified steam injection method. This method improves the development effect of heavy oil reservoirs by calculating and determining different injection volumes for different oil layer thicknesses to achieve balanced utilization of different layers, thereby expanding the steam injection coverage of heavy oil reservoirs, improving the utilization rate of injected steam, and significantly improving the development effect of heavy oil reservoirs. However, the steam injection rate optimization model established by this method only considers the steam injection rate, effective oil layer thickness, number of days of steam injection, control area of ​​the steam injection well, and steam injection intensity, without considering the influence of the thermal properties of the injected steam and the degree of steam drying.

[0008] In summary, current methods for enhancing oil recovery in extra-heavy oil reservoirs through balanced steam injection have certain problems. There is an urgent need for an efficient and optimized balanced steam injection huff and puff method and implementation process based on thermal recovery wells. This would address the need for on-demand, segmented steam injection under varying mass flow and dryness conditions in oil-bearing sections, thereby improving the overall recovery rate and economic benefits of heavy oil reservoirs. Summary of the Invention

[0009] The purpose of this invention is to provide a balanced steam injection method for enhancing the recovery rate of ultra-heavy oil reservoirs, which solves the need for on-demand segmented steam injection under varying mass flow and dryness of oil layers, thereby improving the overall recovery rate and economic benefits of heavy oil reservoirs.

[0010] The objective of this invention can be achieved through the following technical measures: a balanced steam injection method for enhancing oil recovery in extra-heavy oil reservoirs, comprising:

[0011] Step 1: Inject wet saturated steam at a pressure not exceeding the fracture pressure of the extra-heavy oil reservoir;

[0012] Step 2: Taking into account the variable mass flow and dryness of the injected steam in the oil reservoir section, as well as the reservoir properties of the oil reservoir section, determine the segmented steam injection areas of the oil reservoir section as needed.

[0013] Step 3: Establish a process calculation method for balanced steam injection and determine the diversion and distribution volume of each injector;

[0014] Step 4: Form a balanced steam injection process string for horizontal well thermal recovery using an injector and insulated tubing for balanced steam injection;

[0015] Step 5: Form a high-temperature steam chamber and an oil drain chamber that extend evenly along the oil layer.

[0016] The objective of this invention can also be achieved through the following technical measures:

[0017] In step 1, the fracturing pressure of extra-heavy oil reservoirs, also known as formation strength, refers to the minimum pressure that causes existing fractures in the formation to expand and extend or causes fractures to form in unfractured formations. Normal formation fracturing pressure generally increases with well depth. Deep formations are subjected to greater pressure from overlying strata, which can compress them into a very dense structure, resulting in high fracturing pressure.

[0018] In step 1, the reservoir fracture pressure P f The standard calculation formula is:

[0019] P f =10 -3 ρ f gH (1)

[0020] Among them, P f ρ represents the burst pressure, in MPa; f This represents the equivalent density under burst pressure, expressed in g / cm³. 3 g represents gravitational acceleration, m / s² 2 H represents vertical depth, in meters (m).

[0021] For extra-heavy oil reservoirs, the equilibrium steam injection pressure should not exceed the reservoir fracture pressure, at which point the fracture pressure P...f The formula for calculation is:

[0022]

[0023] Where v represents Poisson's ratio, a decimal; α represents the pore pressure contribution coefficient, which is related to formation porosity; P o P represents the pressure of the overlying strata, in MPa; p σ represents formation pore pressure, MPa; T Represents tectonic stress in the formation, MPa; σ t Represents the tensile strength of rock, in MPa;

[0024] At this point, the fracturing pressure of the extra-heavy oil reservoir is also the maximum steam injection pressure of the equilibrium steam injection process, which is the upper limit of the steam injection pressure.

[0025] In step 2, the oil-bearing section is divided into segmented steam injection zones as needed. During the steam injection process in the thermal recovery horizontal well, as steam is continuously absorbed by the oil layer and transfers heat to it, the steam mass flow rate along the horizontal section decreases, and the steam dryness also gradually changes. The steam pressure, temperature, velocity, heat, and dryness will decrease and change continuously during the injection process along the horizontal section. Different oil-bearing sections exhibit heterogeneous characteristics due to differences in porosity and permeability. Areas with higher permeability have larger pressure differentials, resulting in better permeation and absorption of steam mass and heat. This affects the steam injection effect of other oil-bearing sections, leading to uneven utilization of other oil-bearing sections due to insufficient injected enthalpy.

[0026] In step 2, taking into account the variable mass flow and dryness of the injected steam in the oil layer section, as well as the reservoir properties of the oil layer section, the steam injection pressure difference is controlled. The oil layer section is divided into segmented steam injection areas as needed. The steam injection volume and steam dryness are optimized according to the required steam injection volume of each oil layer section to ensure that the injection enthalpy of each layer meets the heating requirements of the oil layer.

[0027] In step 3, the process calculation method for balanced steam injection considers the physical properties of the oil layer and the thermophysical properties of the injected steam. A distributor is used to achieve on-demand steam diversion and injection, thereby achieving balanced steam injection in the oil layer. Based on the thermodynamic parameters of the injected wet saturated steam, including the steam dryness, pressure, gas specific volume, liquid specific volume, gas phase isobaric specific heat, gas phase isovolumetric specific heat, liquid specific heat at the distributor inlet, as well as the distributor outlet pressure, orifice diameter, and flow coefficient, the flow rate of a single orifice of the distributor is calculated. Based on the designed steam injection rate, the number and diameter of the distributor orifices are optimized.

[0028] Step 3, the process calculation method for equalization steam injection, includes:

[0029] Step 31: The flow process of wet saturated steam through the nozzle of the dispenser can be regarded as a polytropic process, and the polytropic index n needs to be determined.

[0030] Step 32: Calculate the specific volume and density of the wet saturated steam at the inlet and outlet of the dispenser nozzle;

[0031] Step 33: Determine the outlet flow rate u of the wet saturated steam at the nozzle. m2 Calculate the steam mass flow rate G;

[0032] Step 34, the critical pressure ratio is the pressure P corresponding to the flow velocity at the nozzle outlet reaching the speed of sound. cr The ratio to the inlet pressure P1 is based on the critical pressure ratio R. cr Determine the status of the nozzle;

[0033] Step 35: When the nozzle of the dispenser reaches the critical flow state, i.e., 0 ≤ P2 / P1 ≤ P cr / P1, the orifice discharge rate reaches its maximum, calculate the steam injection rate of the distributor at this time;

[0034] Step 36, when the dispenser nozzle is in a subcritical flow state, i.e., P cr / P1<P2 / P1<1, the orifice discharge flow rate has not reached the maximum, calculate the steam injection rate of the distributor at this time;

[0035] Step 37: Based on the designed steam injection rate, repeat steps 31 to 36 to continuously optimize the number and diameter of the injection device and verify whether the structural parameters of the injection device meet the production requirements during the horizontal well steam injection process.

[0036] In step 31, the formula for calculating the polytropic index n is:

[0037]

[0038] Where, x g c represents the mass fraction of the gas phase, as a decimal. pg1 c vg1 and c liq represents the specific heat of gas phase at constant pressure, the specific heat of gas phase at constant volume, and the specific heat of liquid phase, respectively, in kJ / (kg·K).

[0039] In step 32, the formulas for calculating the specific volume and density of the wet saturated steam at the inlet and outlet of the dispenser nozzle are as follows:

[0040]

[0041] Among them, v g v liq and v m Let m represent the specific volumes of the gas phase, liquid phase, and wet saturated vapor, respectively. 3 / kg; ρ m This represents the density of wet saturated steam, in kg / m³. 3P represents pressure, MPa; subscript 1 and subscript 2 represent the inlet and outlet of the dispenser nozzle, respectively.

[0042] In step 33, the formula for calculating the steam mass flow rate G is:

[0043]

[0044]

[0045] Where C represents the nozzle flow coefficient, a decimal; A represents the nozzle outlet area, A = πd 2 / 4, d indicates the nozzle diameter of the dispensing device, in mm.

[0046] In step 34, the critical pressure ratio R cr The calculation formula is:

[0047]

[0048] Among them, parameter β is related to the gas phase mass fraction, gas phase volume, and liquid phase volume, and its expression is:

[0049]

[0050] In step 35, the formula for calculating the steam injection rate of the distributor is as follows:

[0051]

[0052] Holes represents the number of holes in the dispenser.

[0053] In step 36, the formula for calculating the steam injection rate of the distributor is as follows:

[0054]

[0055] Holes represents the number of holes in the dispenser.

[0056] In step 4, the formed steam distributor uses either circumferential or spiral perforation to distribute steam. The flow rate of each outlet is calculated and designed using the model from step 3 to ensure that the flow rate of each steam outlet is distributed according to the designed flow rate. This ensures that the entire horizontal section is designed with a variable dryness and variable flow rate steam injection profile, achieving the goal of balanced steam injection.

[0057] In step 4, the process tubing for the thermal recovery horizontal well is constructed using a combination of a distributor and insulated tubing. The distributor is installed on the insulated tubing to control the injection pressure differential. Steam is injected evenly at a pressure not exceeding the reservoir fracture pressure. Following the principle of segmented steam injection based on the required oil layer sections, the distributor is strategically placed at the locations of the horizontal sections requiring steam injection. Different horizontal sections are separated using in-tube packers. During injection, steam is injected from the insulated tubing, enters the horizontal section tubing, flows through the distributor, and enters the annulus through the distributor's orifice. Following the principle of balanced steam injection, the placement of the distributor in the horizontal section, the number of orifices, and the orifice diameter are all optimized to ensure that the injection enthalpy of each section meets the oil layer heating requirements. This creates a high-temperature steam chamber and a drainage chamber that evenly expands along the oil layer sections within the reservoir, achieving efficient utilization of the oil layer sections and improving oil recovery.

[0058] In step 5, the leading edge of the steam chamber, which extends uniformly along the oil layer, is divided into a series of units in both the horizontal and vertical directions. The expression for the crude oil heating thickness γ of different units at different time periods is then given:

[0059]

[0060] Where Δt represents the time step; the superscript j indicates the j-th time step; α represents the reservoir thermal diffusivity, m 2 / s; U represents the velocity of the leading edge of the steam chamber, m / s;

[0061] The formula for calculating the oil production rate per unit well length in a thermal recovery horizontal well within different time periods is as follows:

[0062]

[0063] Where K represents absolute penetration rate, m / s; K ro Represents relative permeability, a decimal; θ represents the angle at the leading edge of the steam chamber, in degrees; m represents the dimensionless viscosity-temperature coefficient, which varies with the steam chamber pressure; v os The kinematic viscosity of crude oil at steam temperature, m 2 / s;

[0064] According to the principle of mass balance, when the steam chamber expands uniformly along the oil layer, the moving velocity U of different units at the leading edge of the steam chamber is... ix and U iy They are respectively:

[0065]

[0066] in, Let m represent the width of the i-th unit; The height of the i-th element is represented in meters (m); Φ represents the formation porosity, which is dimensionless; ΔS oIndicates the saturation of movable oil, dimensionless;

[0067] Based on the calculated velocity of the leading edge of the steam chamber, the position and angle θ of the leading edge of the steam chamber for each element at different times can be obtained, as expressed below:

[0068]

[0069] Among them, X i Y represents the horizontal distance (m) from the i-th unit to the production well; i The value represents the vertical height (m) of the i-th unit from the horizontal plane of the production well.

[0070] The objective of this invention can also be achieved through the following technical measures: a balanced steam injection system for enhancing oil recovery in extra-heavy oil reservoirs, characterized in that the balanced steam injection system for enhancing oil recovery in extra-heavy oil reservoirs adopts a balanced steam injection method for enhancing oil recovery in extra-heavy oil reservoirs, providing a process calculation method for balanced steam injection and a process tubing for implementation.

[0071] This invention presents a balanced steam injection method for enhancing oil recovery in extra-heavy oil reservoirs. During steam injection in thermal recovery horizontal wells, it provides a technology for on-demand segmented steam injection and reservoir mobilization under conditions of variable mass flow and varying dryness in heterogeneous oil-bearing sections. This invention utilizes a uniformly expanded high-temperature steam chamber and drainage chamber within the oil-bearing section to maximize and maximize the heating of the oil layer, achieving balanced mobilization of the oil-bearing section. While meeting the fracture pressure of extra-heavy oil reservoirs, steam is injected in a split manner using injection tools such as a steam distributor. Simultaneously, calculations are performed according to a variable mass flow model to improve the accuracy of steam injection parameters for the horizontal section. A balanced steam injection process string for thermal recovery horizontal wells is formed, consisting of "insulated tubing + an injection device with optimized orifice diameter and number of holes + a forced segmented steam chamber string."

[0072] The balanced steam injection method for enhancing oil recovery in extra-heavy oil reservoirs, as described in this invention, can improve the development effect of thermal recovery wells and overcome problems such as on-demand segmented steam injection and uneven utilization of oil layers caused by varying mass flow, varying dryness, and heterogeneity in oil-bearing sections during steam injection, especially in horizontal thermal recovery wells. This results in improved overall recovery and economic benefits for extra-heavy oil reservoirs. This invention primarily uses balanced steam injection at a pressure not exceeding the fracture pressure of the extra-heavy oil reservoir. By dividing the oil-bearing sections into segmented steam injection zones as needed, and optimizing the steam injection volume and dryness according to the required steam injection volume for each oil layer, it ensures that the injection enthalpy of each section meets the heating requirements of the oil layer. This forms a high-temperature steam cavity and drainage cavity that evenly expands along the oil-bearing sections within the reservoir, thus avoiding the common problems in extra-heavy oil steam injection where steam pathways prevent effective return of heated crude oil, resulting in low water recovery, low oil production, and low recovery rates. Attached Figure Description

[0073] Figure 1A flowchart illustrating a specific embodiment of the balanced steam injection method for enhancing oil recovery in extra-heavy oil reservoirs according to the present invention;

[0074] Figure 2 This is a schematic diagram of the dispensing device structure in a specific embodiment of the present invention;

[0075] Figure 3 This is a schematic diagram of the equalization steam injection process tubing in a specific embodiment of the present invention;

[0076] Figure 4 This is a schematic diagram of a high-temperature steam chamber and an oil drain chamber for the balanced expansion of the oil layer in a specific embodiment of the present invention;

[0077] Figure 5 This is a schematic diagram of the steam injection volume corresponding to the optimized orifice diameter and number of orifices of the injector in Embodiment 1 of the present invention;

[0078] Figure 6 This is a schematic diagram illustrating the flow pattern of steam within a horizontal well section pipeline in Embodiment 2 of the present invention;

[0079] Figure 7 This is a schematic diagram of the steam distribution in the horizontal section in Embodiment 3 of the present invention;

[0080] Figure 8 This is a schematic diagram of the steam distribution in the horizontal section in Embodiment 4 of the present invention;

[0081] In the diagram, 5-1 is the upper connector; 5-2 is the main pipe; 5-3 is the vent hole of the injector; 5-4 is the symmetrical steam outlet at both ends; 5-5 is the casing; 5-6 is the downstream nozzle; 5-7 is the lower connector; 1-casing; 2-downhole compensator; 3-insulated tubing; 4-thermal packer; 5-injector; 6-in-tube packer; 7-anti-sticking centralizer; 8-oil layer; 9-high-temperature steam chamber and drain chamber evenly extended along the oil layer section; 11-steam injection; 12-oil layer; 13-steam chamber vertically upward to the top of the oil layer; 14-high-temperature steam chamber and drain chamber evenly extended along the oil layer section. Detailed Implementation

[0082] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0083] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0084] This invention primarily focuses on balanced steam injection at pressures not exceeding the fracture pressure of extra-heavy oil reservoirs. By dividing the oil-bearing sections into segmented steam injection zones as needed, and optimizing the steam injection rate and dryness according to the required steam volume for each layer, it ensures that the injection enthalpy of each segment meets the heating requirements of the oil layer. This creates high-temperature steam chambers and drainage chambers that evenly extend along the oil-bearing sections within the reservoir. This avoids the common problem in extra-heavy oil steam injection where steam pathways prevent effective return of heated crude oil, resulting in low water recovery, low oil production, and low oil recovery rates. This invention provides a process calculation method for balanced steam injection and a corresponding process tubing. The key technical points of this invention include:

[0085] 1. In the process of steam injection in thermal recovery horizontal wells, a technology is provided for the on-demand segmented steam injection and oil-bearing segment activation process under the conditions of variable mass flow and variable dryness of oil-bearing segments caused by heterogeneity.

[0086] 2. By utilizing the high-temperature steam chamber and oil drain chamber that expand evenly in the oil layer section, the oil layer can be heated to the maximum extent and over the widest range, thus achieving balanced utilization of the oil layer section.

[0087] 3. To meet the fracture pressure of extra-heavy oil reservoirs, steam is injected in a split manner using steam injection tools such as a steam injector. At the same time, calculations are performed according to the variable mass flow mode to improve the accuracy of steam injection parameters in the horizontal section.

[0088] 4. Form a balanced steam injection process string for thermal recovery horizontal wells, which is a combination of "insulated tubing + injection device formed by optimized calculation of injection device diameter and number of holes + forced segmented steam chamber string".

[0089] The technical measures of this invention are mainly achieved through Figure 1 This is achieved through a specific process.

[0090] In step 101, wet saturated steam is injected at a pressure not exceeding the fracture pressure of the extra-heavy oil reservoir. The process then proceeds to step 102.

[0091] In step 102, taking into account the varying mass flow and dryness of the injected steam in the oil reservoir section, as well as the reservoir properties of the oil reservoir section, the oil reservoir section is divided into segmented steam injection zones as needed. The process then proceeds to step 103.

[0092] In step 103, a process calculation method for balanced steam injection is established to determine the diversion and distribution volume of each injector. The process then proceeds to step 104.

[0093] In step 104, a balanced steam injection process string consisting of an injector and insulating tubing is formed for the thermal recovery horizontal well to perform balanced steam injection. The process then proceeds to step 105.

[0094] In step 105, a high-temperature steam chamber and an oil drain chamber are formed that extend evenly along the oil layer section.

[0095] In step 101, the fracturing pressure of the extra-heavy oil reservoir, also known as formation strength, refers to the minimum pressure required to expand and extend existing fractures in the formation or to induce fractures in unfractured formations. Normal formation fracturing pressure generally increases with well depth. Deep formations are subjected to greater pressure from overlying strata, resulting in very dense rock and high fracturing pressure. The reservoir fracturing pressure P... f The standard calculation formula is:

[0096] P f =10 -3 ρ f gH (1)

[0097] Among them, P f ρ represents the burst pressure, in MPa; f This represents the equivalent density under burst pressure, expressed in g / cm³. 3 g represents gravitational acceleration, m / s² 2 H represents vertical depth, in meters (m).

[0098] For extra-heavy oil reservoirs, the equilibrium steam injection pressure should not exceed the reservoir fracture pressure, at which point the fracture pressure P... f The formula for calculation is:

[0099]

[0100] Where v represents Poisson's ratio, a decimal; α represents the pore pressure contribution coefficient, which is related to formation porosity; P o P represents the pressure of the overlying strata, in MPa; p σ represents formation pore pressure, MPa; T Represents tectonic stress in the formation, MPa; σ t This represents the tensile strength of rock, expressed in MPa.

[0101] At this point, the fracturing pressure of the extra-heavy oil reservoir is also the maximum steam injection pressure of the equilibrium steam injection process, which is the upper limit of the steam injection pressure.

[0102] In step 102, the oil-bearing section is divided into segmented steam injection zones as needed. During steam injection in the thermal recovery horizontal well, as steam is continuously absorbed by the oil layer, transferring heat to it, the steam mass flow rate along the horizontal section decreases, and the steam dryness also gradually changes. The steam pressure, temperature, velocity, heat, and dryness decrease and change continuously during the injection process along the horizontal section. Different oil-bearing sections exhibit heterogeneous characteristics due to differences in porosity and permeability. Areas with higher permeability have larger pressure differentials, resulting in better permeation and absorption of steam mass and heat. This affects the steam injection effect of other oil-bearing sections, leading to uneven utilization of other oil-bearing sections due to insufficient injected enthalpy. Therefore, taking into account the variable mass flow and dryness of the injected steam in the oil layer section, as well as the reservoir properties of the oil layer section, the steam injection pressure difference is controlled, and the oil layer section is divided into segmented steam injection areas as needed. The steam injection volume and steam dryness are optimized according to the required steam injection volume of each oil layer section to ensure that the injection enthalpy of each layer meets the heating needs of the oil layer.

[0103] In step 103, the process calculation method for balanced steam injection considers the physical properties of the oil reservoir and the thermophysical properties of the injected steam. It utilizes a distributor to achieve on-demand steam diversion and injection, thereby achieving balanced steam injection in the oil reservoir. Based on the thermodynamic parameters of the injected wet saturated steam, including the steam dryness, pressure, gas specific volume, liquid specific volume, gas phase isobaric specific heat, gas phase isovolumetric specific heat, liquid phase specific heat at the distributor inlet, as well as the distributor outlet pressure, orifice diameter, and flow coefficient, the flow rate of a single orifice of the distributor is calculated. The number and diameter of the distributor orifices are optimized according to the designed steam injection rate. According to a preferred embodiment of the present invention, the process calculation method for balanced steam injection includes:

[0104] 1-1 The flow process of wet saturated steam through the nozzle of the dispenser can be regarded as a polytropic process, and the polytropic index n needs to be determined:

[0105]

[0106] Where, x g c represents the mass fraction of the gas phase, as a decimal. pg c vg and c liq represents the specific heat of gas phase at constant pressure, the specific heat of gas phase at constant volume, and the specific heat of liquid phase, respectively, in kJ / (kg·K).

[0107] 1-2 Calculate the specific volume and density of the wet saturated steam at the inlet and outlet of the dispenser nozzle:

[0108]

[0109] Among them, v g v liq and v m Let m represent the specific volumes of the gas phase, liquid phase, and wet saturated vapor, respectively.3 / kg; ρ m This represents the density of wet saturated steam, in kg / m³. 3 P represents pressure, MPa; subscript 1 and subscript 2 represent the inlet and outlet of the dispenser nozzle, respectively.

[0110] 1-3 Determine the outlet velocity u of the wet saturated steam at the nozzle. m2 Calculate the steam mass flow rate G:

[0111]

[0112] Where C represents the nozzle flow coefficient, a decimal; A represents the nozzle outlet area, A = πd 2 / 4, d indicates the nozzle diameter of the dispensing device, in mm.

[0113] 1-4 The critical pressure ratio is the pressure P corresponding to the flow velocity at the nozzle outlet reaching the speed of sound. cr The ratio to the inlet pressure P1 is based on the critical pressure ratio R. cr Determine the state of the nozzle.

[0114]

[0115] Among them, parameter β is related to the gas phase mass fraction, gas phase volume, and liquid phase volume, and its expression is:

[0116]

[0117] 1-5 When the dispenser nozzle reaches the critical flow state, i.e., 0≤P2 / P1≤P cr / P1, the orifice discharge rate reaches its maximum. At this time, the steam injection rate of the distributor is:

[0118]

[0119] 1-6 When the dispenser nozzle is in a subcritical flow state, i.e., P cr / P1<P2 / P1<1, the orifice discharge flow rate has not reached its maximum. At this time, the steam injection rate of the distributor is:

[0120]

[0121] Holes represents the number of holes in the dispenser.

[0122] 1-7 Based on the designed steam injection rate, repeat steps 1-1 to 1-6 to continuously optimize the number and diameter of the injector holes and verify whether the structural parameters of the injector meet the production requirements during the horizontal well steam injection process.

[0123] In step 104, the implemented dispenser uses either circumferential or spiral perforation for steam distribution, as shown in the diagram. Figure 2 The flow rate of each outlet is calculated and designed using the above mathematical model to ensure that the flow rate of each steam outlet is distributed according to the designed flow rate. This ensures that the entire horizontal section is designed with a variable dryness and variable flow rate steam injection profile, achieving the goal of balanced steam injection. The connection relationship of each part is as follows: the upper connector 5-1 connects to the upstream steam injection string, the main pipe 5-2 connects to the upper connector 5-1, and the main pipe 5-2 has a distributor vent hole 5-3 designed according to the calculation model. In order to avoid the steam from the distributor vent hole 5-3 directly impacting the casing or filter pipe and extending the service life of the casing or filter pipe, a steam outlet hole 5-4 with two symmetrical steam outlet holes at both ends and a protective pipe 5-5 are involved. At the same time, in order to better match the pressure balance of each segment, a downstream nozzle 5-6 is designed to perform fracturing balance with the subsequent steam injection unit, and is connected to the downstream steam injection unit through the lower connector 5-7. The steam flows and is distributed in the inner pipe.

[0124] The aforementioned process tubing, specifically the balanced steam injection process tubing for thermal recovery horizontal wells, is constructed using a combination of an injector and insulating tubing, such as... Figure 3 As shown. The process tubing consists of casing, downhole compensator, insulated tubing, thermally sensitive packer, injector, in-tube packer, and anti-sticking centralizer. The injector is installed on the insulated tubing to control the steam injection pressure differential. Steam is injected evenly at a pressure not exceeding the reservoir fracture pressure. Following the principle of segmented steam injection based on the required oil layer sections, the injector is strategically placed at the horizontal sections requiring steam injection. Different horizontal sections are separated using in-tube packers. During steam injection, steam is injected from the insulated tubing, enters the horizontal section tubing, flows through the injector, and enters the annulus through the injector's orifice. Following the principle of balanced steam injection, the placement of the injector in the horizontal section, the number of orifices, and the orifice diameter are optimized to ensure that the injection enthalpy of each layer meets the oil layer heating requirements. This creates a high-temperature steam chamber and drainage chamber that evenly expands along the oil layer sections within the reservoir, achieving efficient utilization of the oil layer sections and improving oil recovery.

[0125] In step 105, the high-temperature steam chamber and oil drain chamber that expand evenly in the oil layer section, such as Figure 4 As shown, 11 represents steam injected from the wellhead; 12 represents the oil layer where the horizontal well is located; 13 represents the irregular vertical steam cavity formed by conventional steam injection that extends to the top of the oil layer; 14 represents the high-temperature steam cavity and oil drain cavity that are evenly extended along the oil layer segment formed by the balanced steam injection of the present invention.

[0126] Conventional steam injection creates a rapidly moving and irregular steam chamber that extends to the top of the oil layer, causing ineffective heat loss in areas with low oil saturation. This severe steam channeling problem hinders effective water recovery. In contrast, the high-temperature steam chamber and drainage chamber of this invention, which extend evenly along the oil layer, can heat the oil layer to the maximum extent and range, achieving balanced utilization of the oil layer. This avoids the common problem in extra-heavy oil steam injection where steam pathways prevent effective return of heated crude oil, resulting in low water recovery, low oil production, and low oil recovery rates. The leading edge of the evenly extended steam chamber along the oil layer is divided into a series of units in both the horizontal and vertical directions. The expression for the crude oil heating thickness γ of different units at different time periods is:

[0127]

[0128] Where Δt represents the time step; the superscript j indicates the j-th time step; α represents the reservoir thermal diffusivity, m 2 / s; U represents the speed at which the leading edge of the steam chamber moves, in m / s.

[0129] The formula for calculating the oil production rate per unit well length in a thermal recovery horizontal well within different time periods is as follows:

[0130]

[0131] Where K represents absolute penetration rate, m / s; K ro Represents relative permeability, a decimal; θ represents the angle at the leading edge of the steam chamber, in degrees; m represents the dimensionless viscosity-temperature coefficient, which varies with the steam chamber pressure; v os The kinematic viscosity of crude oil at steam temperature, m 2 / s.

[0132] According to the principle of mass balance, when the steam chamber expands uniformly along the oil layer, the moving velocity U of different units at the leading edge of the steam chamber is... ix and U iy They are respectively:

[0133]

[0134] in, Let m represent the width of the i-th unit; The height of the i-th element is represented in meters (m); Φ represents the formation porosity, which is dimensionless; ΔS o It represents the saturation of movable oil and is dimensionless.

[0135] Based on the calculated velocity of the leading edge of the steam chamber, the position and angle θ of the leading edge of the steam chamber for each element at different times can be obtained, as expressed below:

[0136]

[0137] Among them, X i Y represents the horizontal distance (m) from the i-th unit to the production well; i The value represents the vertical height (m) of the i-th unit from the horizontal plane of the production well.

[0138] The technical advantages of this invention are as follows: This invention solves the need for on-demand segmented steam injection in oil-bearing sections under varying mass flow and dryness during steam injection in thermal recovery horizontal wells, achieving balanced steam injection in oil-bearing sections; it can independently design and optimize the distribution of the injectors in the steam injection process string, as well as the number and diameter of the injectors, to meet the production needs of thermal recovery horizontal wells.

[0139] The following are several specific embodiments of the application of the present invention.

[0140] Example 1

[0141] In a specific embodiment 1 of the present invention, well A is a thermal recovery horizontal well in an extra-heavy oil reservoir with a reservoir porosity of 28%, a permeability of 350 mD, and an oil layer viscosity of 6,820,000 mPa·s. The specific reservoir parameters are shown in Table 1.

[0142] Table 1A Reservoir Parameters

[0143]

[0144]

[0145] Well A has undergone five rounds of steam injection development using the huff and puff method. However, due to the lack of consideration for segmented steam injection into the oil-bearing sections during development, the number of injection nozzles was excessive, resulting in an overly large flow area. This led to uneven steam injection at the front of the horizontal section and insufficient injection at the rear, resulting in poor injection efficiency, uneven oil layer utilization, low formation water recovery, and increasing water accumulation. Therefore, the sixth round of huff and puff development requires a more rational distribution of injection nozzles, optimization of the number and diameter of nozzles, and an increase in steam injection volume at the rear of the horizontal section to achieve balanced steam injection throughout the entire horizontal section and improve injection efficiency.

[0146] The optimization of balanced steam injection in Well A was carried out using efficient and balanced steam injection huff and puff optimization methods and processes for extra-heavy oil reservoirs. Specific technical measures include... Figure 1 As shown, uniform steam injection is carried out at a pressure not exceeding the fracture pressure of extra-heavy oil reservoirs. By dividing the oil-bearing sections into segmented steam injection zones as needed, and optimizing the steam injection volume and dryness according to the required steam injection volume of each oil layer, it is ensured that the injection enthalpy of each section meets the heating needs of the oil layer. This forms a high-temperature steam cavity and oil drainage cavity that are evenly extended along the oil-bearing sections in the reservoir, thereby avoiding the problems of ineffective return of heated crude oil caused by the steam path in the steam injection process of extra-heavy oil, resulting in low water recovery rate, low oil production, and low recovery rate.

[0147] In step 101, wet saturated steam is injected at a pressure not exceeding the fracture pressure of the extra-heavy oil reservoir. The process then proceeds to step 102.

[0148] In step 102, taking into account the varying mass flow and dryness of the injected steam in the oil reservoir section, as well as the reservoir properties of the oil reservoir section, the oil reservoir section is divided into segmented steam injection zones as needed. The process then proceeds to step 103.

[0149] In step 103, a process calculation method for balanced steam injection is established to determine the diversion and distribution volume of each injector. The process then proceeds to step 104.

[0150] In step 104, a balanced steam injection process string consisting of an injector and insulating tubing is formed for the thermal recovery horizontal well to perform balanced steam injection. The process then proceeds to step 105.

[0151] In step 105, a high-temperature steam chamber and an oil drain chamber are formed that extend evenly along the oil layer section.

[0152] In step 101, the fracturing pressure of the extra-heavy oil reservoir, also known as formation strength, refers to the minimum pressure required to expand and extend existing fractures in the formation or to induce fractures in unfractured formations. Normal formation fracturing pressure generally increases with well depth. Deep formations are subjected to greater pressure from overlying strata, resulting in very dense rock and high fracturing pressure. The reservoir fracturing pressure P... f The standard calculation formula is:

[0153] P f =10 -3 ρ f gH (1)

[0154] Among them, P f ρ represents the burst pressure, in MPa; f This represents the equivalent density under burst pressure, expressed in g / cm³. 3 g represents gravitational acceleration, m / s² 2 H represents vertical depth, in meters (m).

[0155] For extra-heavy oil reservoirs, the equilibrium steam injection pressure should not exceed the reservoir fracture pressure, at which point the fracture pressure P... f The formula for calculation is:

[0156]

[0157] Where v represents Poisson's ratio, a decimal; α represents the pore pressure contribution coefficient, which is related to formation porosity; P o P represents the pressure of the overlying strata, in MPa; p σ represents formation pore pressure, MPa; T Represents tectonic stress in the formation, MPa; σ tThis represents the tensile strength of rock, expressed in MPa.

[0158] At this point, the fracturing pressure of the extra-heavy oil reservoir is also the maximum steam injection pressure of the equilibrium steam injection process, which is the upper limit of the steam injection pressure.

[0159] In step 102, the oil-bearing section is divided into segmented steam injection zones as needed. During steam injection in the thermal recovery horizontal well, as steam is continuously absorbed by the oil layer, transferring heat to it, the steam mass flow rate along the horizontal section decreases, and the steam dryness also gradually changes. The steam pressure, temperature, velocity, heat, and dryness decrease and change continuously during the injection process along the horizontal section. Different oil-bearing sections exhibit heterogeneous characteristics due to differences in porosity and permeability. Areas with higher permeability have larger pressure differentials, resulting in better permeation and absorption of steam mass and heat. This affects the steam injection effect of other oil-bearing sections, leading to uneven utilization of other oil-bearing sections due to insufficient injected enthalpy. Therefore, taking into account the variable mass flow and dryness of the injected steam in the oil layer section, as well as the reservoir properties of the oil layer section, the steam injection pressure difference is controlled, and the oil layer section is divided into segmented steam injection areas as needed. The steam injection volume and steam dryness are optimized according to the required steam injection volume of each oil layer section to ensure that the injection enthalpy of each layer meets the heating needs of the oil layer.

[0160] In step 103, the process calculation method for balanced steam injection considers the physical properties of the oil reservoir and the thermophysical properties of the injected steam. It utilizes a distributor to achieve on-demand steam diversion and injection, thereby achieving balanced steam injection in the oil reservoir. Based on the thermodynamic parameters of the injected wet saturated steam, including the steam dryness, pressure, gas specific volume, liquid specific volume, gas phase isobaric specific heat, gas phase isovolumetric specific heat, liquid phase specific heat at the distributor inlet, as well as the distributor outlet pressure, orifice diameter, and flow coefficient, the flow rate of a single orifice of the distributor is calculated. The number and diameter of the distributor orifices are optimized according to the designed steam injection rate. According to a preferred embodiment of the present invention, the process calculation method for balanced steam injection includes:

[0161] 1-1 The flow process of wet saturated steam through the nozzle of the dispenser can be regarded as a polytropic process, and the polytropic index n needs to be determined:

[0162]

[0163] Where, x g c represents the mass fraction of the gas phase, as a decimal. pg c vg and c liq represents the specific heat of gas phase at constant pressure, the specific heat of gas phase at constant volume, and the specific heat of liquid phase, respectively, in kJ / (kg·K).

[0164] 1-2 Calculate the specific volume and density of the wet saturated steam at the inlet and outlet of the dispenser nozzle:

[0165]

[0166] Among them, v g v liq and v m Let m represent the specific volumes of the gas phase, liquid phase, and wet saturated vapor, respectively. 3 / kg; ρ m This represents the density of wet saturated steam, in kg / m³. 3 P represents pressure, MPa; subscript 1 and subscript 2 represent the inlet and outlet of the dispenser nozzle, respectively.

[0167] 1-3 Determine the outlet velocity u of the wet saturated steam at the nozzle. m2 Calculate the steam mass flow rate G:

[0168]

[0169] Where C represents the nozzle flow coefficient, a decimal; A represents the nozzle outlet area, A = πd 2 / 4, d indicates the nozzle diameter of the dispensing device, in mm.

[0170] 1-4 The critical pressure ratio is the pressure P corresponding to the flow velocity at the nozzle outlet reaching the speed of sound. cr The ratio to the inlet pressure P1 is based on the critical pressure ratio R. cr Determine the state of the nozzle.

[0171]

[0172] Among them, parameter β is related to the gas phase mass fraction, gas phase volume, and liquid phase volume, and its expression is:

[0173]

[0174] 1-5 When the dispenser nozzle reaches the critical flow state, i.e., 0≤P2 / P1≤P cr / P1, the orifice discharge rate reaches its maximum. At this time, the steam injection rate of the distributor is:

[0175]

[0176] 1-6 When the dispenser nozzle is in a subcritical flow state, i.e., P cr / P1<P2 / P1<1, the orifice discharge flow rate has not reached its maximum. At this time, the steam injection rate of the distributor is:

[0177]

[0178] Holes represents the number of holes in the dispenser.

[0179] 1-7 Based on the designed steam injection rate, Well A is designed to have a steam injection rate of 220 t / d. Six injectors are installed, with designed orifice diameters of 6 mm and 10 mm respectively. Steps 1-1 to 1-6 are repeated to continuously optimize the number and diameter of the injectors, verifying whether the structural parameters of the injectors meet production requirements during horizontal well steam injection. The calculation results are shown in Table 2 and... Figure 5 As shown.

[0180] In step 104, the implemented dispenser uses either circumferential or spiral perforation for steam distribution, as shown in the diagram. Figure 2 The flow rate of each outlet is calculated and designed using the above mathematical model to ensure that the flow rate of each steam outlet is distributed according to the designed flow rate. This ensures that the entire horizontal section is designed with a variable dryness and variable flow rate steam injection profile, achieving the goal of balanced steam injection. The connection relationship of each part is as follows: the upper connector 5-1 connects to the upstream steam injection string, the main pipe 5-2 connects to the upper connector 5-1, and the main pipe 5-2 has a distributor vent hole 5-3 designed according to the calculation model. In order to avoid the steam from the distributor vent hole 5-3 directly impacting the casing or filter pipe and extending the service life of the casing or filter pipe, a steam outlet hole 5-4 with two symmetrical steam outlet holes at both ends and a protective pipe 5-5 are involved. At the same time, in order to better match the pressure balance of each segment, a downstream nozzle 5-6 is designed to perform fracturing balance with the subsequent steam injection unit, and is connected to the downstream steam injection unit through the lower connector 5-7. The steam flows and is distributed in the inner pipe.

[0181] The aforementioned process tubing, specifically the balanced steam injection process tubing for thermal recovery horizontal wells, is constructed using a combination of an injector and insulating tubing, such as... Figure 3 As shown. The process tubing consists of casing, downhole compensator, insulated tubing, thermally sensitive packer, injector, in-tube packer, and anti-sticking centralizer. The injector is installed on the insulated tubing to control the steam injection pressure differential. Steam is injected evenly at a pressure not exceeding the reservoir fracture pressure. Following the principle of segmented steam injection based on the required oil layer sections, the injector is strategically placed at the horizontal sections requiring steam injection. Different horizontal sections are separated using in-tube packers. During steam injection, steam is injected from the insulated tubing, enters the horizontal section tubing, flows through the injector, and enters the annulus through the injector's orifice. Following the principle of balanced steam injection, the placement of the injector in the horizontal section, the number of orifices, and the orifice diameter are optimized to ensure that the injection enthalpy of each layer meets the oil layer heating requirements. This creates a high-temperature steam chamber and drainage chamber that evenly expands along the oil layer sections within the reservoir, achieving efficient utilization of the oil layer sections and improving oil recovery.

[0182] In step 105, the high-temperature steam chamber and oil drain chamber that expand evenly in the oil layer section, such as Figure 4As shown, 11 represents steam injected from the wellhead; 12 represents the oil layer where the horizontal well is located; 13 represents the irregular, vertically extending steam cavity to the top of the oil layer formed by conventional steam injection; and 14 represents the high-temperature steam cavity and drain cavity that are evenly extended along the oil layer segment formed by the balanced steam injection of this invention. The steam cavity formed by conventional steam injection moves rapidly and irregularly, extending to the top of the oil layer, causing ineffective heat loss in areas with low oil saturation, resulting in severe steam leakage and ineffective water recovery. In contrast, the high-temperature steam cavity and drain cavity that are evenly extended along the oil layer segment formed by this invention can heat the oil layer to the maximum extent and range, achieving balanced utilization of the oil layer segment. This avoids the common problem in extra-heavy oil steam injection where the steam path prevents effective return of heated crude oil, leading to low water recovery rate, low oil production, and low recovery rate. The leading edge of the evenly extended steam cavity along the oil layer segment is divided into a series of units in both the horizontal and vertical directions. The expression for the crude oil heating thickness γ of different units at different time periods is:

[0183]

[0184] Where Δt represents the time step; the superscript j indicates the j-th time step; α represents the reservoir thermal diffusivity, m 2 / s; U represents the speed at which the leading edge of the steam chamber moves, in m / s.

[0185] The formula for calculating the oil production rate per unit well length in a thermal recovery horizontal well within different time periods is as follows:

[0186]

[0187] Where K represents absolute penetration rate, m / s; K ro Represents relative permeability, a decimal; θ represents the angle at the leading edge of the steam chamber, in degrees; m represents the dimensionless viscosity-temperature coefficient, which varies with the steam chamber pressure; v os The kinematic viscosity of crude oil at steam temperature, m 2 / s.

[0188] According to the principle of mass balance, when the steam chamber expands uniformly along the oil layer, the moving velocity U of different units at the leading edge of the steam chamber is... ix and U iy They are respectively:

[0189]

[0190] in, Let m represent the width of the i-th unit; The height of the i-th element is represented in meters (m); Φ represents the formation porosity, which is dimensionless; ΔS o It represents the saturation of movable oil and is dimensionless.

[0191] Based on the calculated velocity of the leading edge of the steam chamber, the position and angle θ of the leading edge of the steam chamber for each element at different times can be obtained, as expressed below:

[0192]

[0193] Among them, X i Y represents the horizontal distance (m) from the i-th unit to the production well; i The value represents the vertical height (m) of the i-th unit from the horizontal plane of the production well.

[0194] Table 2 Optimization of Filler Orifice Diameter and Number

[0195]

[0196] This invention optimizes the number and diameter of the injection nozzle in Well A. Compared to the designed steam injection rate of 220 t / d, the total steam injection rates for nozzles with diameters of 6 mm and 10 mm are 227.14 t / d and 224.13 t / d, respectively, with errors of 3.25% and 1.88%, both within acceptable ranges. Through the optimized balanced steam injection process of this invention, the recovery rate and water recovery rate of Well A are effectively improved, demonstrating the effectiveness and significant advantages of the proposed method in solving the problem of balanced steam injection in thermal recovery horizontal wells.

[0197] Example 2

[0198] Well B is a horizontal well for thermal recovery in an extra-heavy oil reservoir, with a horizontal section length of 400m. Superheated steam is injected into the horizontal section. Using the optimized balanced steam injection process calculation method and process tubing of this invention, the flow pattern of steam in the horizontal section of Well B is analyzed. The results are as follows: Figure 6 As shown, the injection units in the horizontal section of well B are configured at 10m / unit, with a designed steam injection rate of 9t / h, or 216t / d. The steam volume decreases in a stepwise manner throughout the horizontal section. Pressure and temperature gradually decrease, with the downward trend slowing down from the middle of the horizontal section until it remains essentially constant at the toe, exhibiting a trend of initial rapid decrease followed by slower decrease. Dryness gradually decreases, with the downward trend accelerating from the middle of the horizontal section, reaching its lowest point at the toe, exhibiting a trend of initial slow decrease followed by faster decrease. The preferred balanced steam injection process of this invention considers the variable quality and dryness characteristics of the oil-bearing reservoir, enabling the steam injection volume to be distributed as needed throughout the entire reservoir, thereby improving the recovery rate.

[0199] Example 3

[0200] In a specific embodiment 3 of the present invention, Well C is a horizontal well for thermal recovery in an extra-heavy oil reservoir, with a horizontal section length of 306.1m. According to a preferred embodiment of the present invention, uniform steam injection is performed at a pressure not exceeding the fracture pressure of the extra-heavy oil reservoir, with an injection pressure of 11MPa and an injection rate of 360t / d. Considering the variable mass flow rate and dryness of the injected steam in the oil layer, as well as the reservoir properties, the oil layer is divided into segmented steam injection zones as needed. The distribution of the injector in the horizontal section is optimized. In this embodiment, the injector adopts a 360° spiral perforation pattern. Using the preferred uniform steam injection process calculation method and process tubing of the present invention, combined with the designed steam injection rate, the injector orifice diameter is preferably 10mm. The number of orifices and the corresponding split steam injection volume of the injector are continuously optimized to verify whether the structural parameters of the injector meet the production requirements during horizontal well steam injection. The calculation results are shown in Table 3 and... Figure 7 As shown.

[0201] Table 3 Optimization of Number of Injector Holes and Steam Injection Volume

[0202]

[0203]

[0204] This invention optimizes the number of holes and corresponding steam injection rate of the injection device in Well C. Compared with the designed steam injection rate of 360 t / d, the total steam injection rate corresponding to the injection device is 358.26 t / d, with an error of 0.48%, which is within an acceptable range. Figure 7 It can be seen that the entire calculation process takes into account the influence of steam mass flow, and the steam quantity in the horizontal section decreases in a stepwise manner. Through the optimized balanced steam injection process of this invention, the recovery rate of Well C is effectively improved, demonstrating the effectiveness and outstanding advantages of the method proposed in solving the problem of balanced steam injection in thermal recovery horizontal wells.

[0205] Example 4

[0206] In a specific embodiment 4 of the present invention, Well D is a horizontal well for thermal recovery in an extra-heavy oil reservoir, with a horizontal section length of 300.73m. According to a preferred embodiment of the present invention, uniform steam injection is performed at a pressure not exceeding the fracture pressure of the extra-heavy oil reservoir, with an injection pressure of 13MPa and an injection rate of 480t / d. Considering the variable mass flow rate and dryness of the injected steam in the oil-bearing section, as well as the reservoir properties, the oil-bearing section is divided into segmented steam injection zones as needed. The distribution of the injector in the horizontal section is optimized. In this embodiment, the injector adopts a 360° spiral perforation pattern. Using the preferred uniform steam injection process calculation method and process tubing of the present invention, combined with the designed steam injection rate, the injector orifice diameter is preferably 10mm. The number of orifices and the corresponding split steam injection volume of the injector are continuously optimized to verify whether the structural parameters of the injector meet the production requirements during horizontal well steam injection. The calculation results are shown in Table 4 and... Figure 8 As shown.

[0207] Table 4 Optimization of Number of Injector Holes and Steam Injection Volume

[0208]

[0209]

[0210] This invention optimizes the number of holes and corresponding steam injection rate of the injection device in Well D. Compared with the designed steam injection rate of 480 t / d, the total steam injection rate corresponding to the injection device is 476.60 t / d, with an error of 0.71%, which is within an acceptable range. Figure 8 It can be seen that the entire calculation process takes into account the influence of steam mass flow. The steam volume in the horizontal section decreases in a stepwise manner, and the steam injection volume can be allocated as needed throughout the oil reservoir, thereby improving the recovery rate.

[0211] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0212] Except for the technical features described in the specification, all other technologies are known to those skilled in the art.

Claims

1. A method for enhancing oil recovery in extra-heavy oil reservoirs through balanced steam injection, characterized in that: The enhanced oil recovery methods for this extra-heavy oil reservoir include: Balanced steam injection. Step 1: Inject wet saturated steam at a pressure not exceeding the fracture pressure of the extra-heavy oil reservoir; Step 2: Taking into account the variable mass flow and dryness of the injected steam in the oil reservoir section, as well as the reservoir properties of the oil reservoir section, determine the segmented steam injection areas of the oil reservoir section as needed. Step 3: Establish a process calculation method for balanced steam injection and determine the diversion and distribution volume of each injector; Step 4: Form a balanced steam injection process string for horizontal well thermal recovery using an injector and insulated tubing for balanced steam injection; Step 5: Form a high-temperature steam chamber and an oil drain chamber that extend evenly along the oil layer section; Step 3, the process calculation method for equalization steam injection, includes: Step 31: The flow process of wet saturated steam through the nozzle of the dispenser is regarded as a polytropic process, and the polytropic index n needs to be determined. Step 32: Calculate the specific volume and density of the wet saturated steam at the inlet and outlet of the dispenser nozzle; Step 33: Determine the outlet flow rate u of the wet saturated steam at the nozzle. m2 Calculate the steam mass flow rate G; Step 34, the critical pressure ratio is the pressure P corresponding to the flow velocity at the nozzle outlet reaching the speed of sound. cr The ratio of the pressure to the nozzle inlet pressure P1 is based on the critical pressure ratio R. cr Determine the status of the nozzle; Step 35: When the nozzle of the dispenser reaches the critical flow state, i.e., 0 ≤ P2 / P1 ≤ P cr / P1, the orifice discharge rate reaches its maximum, calculate the steam injection rate of the distributor at this time; Step 36, when the dispenser nozzle is in a subcritical flow state, i.e., P cr / P1<P2 / P1<1, the orifice discharge flow rate has not reached the maximum, calculate the steam injection rate of the distributor at this time; Step 37: Based on the designed steam injection rate, repeat steps 31 to 36 to continuously optimize the number and diameter of the injector holes and verify whether the structural parameters of the injector meet the production requirements during the horizontal well steam injection process. In step 31, the formula for calculating the polytropic index n is: Where, x g c represents the mass fraction of the gas phase, as a decimal. pg1 c vg1 and c liq , respectively, represent the specific heat of gas phase at constant pressure, the specific heat of gas phase at constant volume, and the specific heat of liquid phase, in kJ / (kg·K); In step 32, the formulas for calculating the specific volume and density of the wet saturated steam at the inlet and outlet of the dispenser nozzle are as follows: Among them, v g v liq and v m Let m represent the specific volumes of the gas phase, liquid phase, and wet saturated vapor, respectively. 3 / kg; ρ m This represents the density of wet saturated steam, in kg / m³. 3 P represents pressure, MPa; subscript 1 and subscript 2 represent the inlet and outlet of the dispenser nozzle, respectively.

2. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 1, characterized in that, In step 1, the fracturing pressure of extra-heavy oil reservoirs, also known as formation strength, refers to the minimum pressure that causes existing fractures in the formation to expand and extend or to cause fractures in unfractured formations. Normal formation fracturing pressure increases with well depth. Deep formations are subjected to greater pressure from overlying strata, which can compress them into a very dense structure, resulting in high fracturing pressure.

3. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 2, characterized in that, In step 1, the reservoir fracture pressure P f The standard calculation formula is: P f =10 -3 ρ f gH (1) Among them, P f Represents reservoir fracture pressure, MPa; ρ f Expresses the equivalent density under burst pressure, in g / cm³. 3 g represents gravitational acceleration, m / s² 2 H represents vertical depth, in meters (m). For extra-heavy oil reservoirs, the equilibrium steam injection pressure should not exceed the reservoir fracture pressure, at which point the reservoir fracture pressure P... f The formula for calculation is: Where v represents Poisson's ratio, a decimal; α represents the pore pressure contribution coefficient, which is related to formation porosity; P o P represents the pressure of the overlying strata, in MPa; p σ represents formation pore pressure, MPa; T Represents tectonic stress in the formation, MPa; σ t Represents the tensile strength of rock, in MPa; At this point, the fracturing pressure of the extra-heavy oil reservoir is also the maximum steam injection pressure of the equilibrium steam injection process, which is the upper limit of the steam injection pressure.

4. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 1, characterized in that, In step 2, the oil-bearing section is divided into segmented steam injection zones as needed. During the steam injection process in the thermal recovery horizontal well, as steam is continuously absorbed by the oil layer and transfers heat to it, the steam mass flow rate along the horizontal section decreases, and the steam dryness also gradually changes. The steam pressure, temperature, velocity, heat, and dryness will decrease and change continuously during the injection process along the horizontal section. Different oil-bearing sections exhibit heterogeneous characteristics due to differences in porosity and permeability. Areas with higher permeability have larger pressure differentials, resulting in better permeation and absorption of steam mass and heat. This affects the steam injection effect of other oil-bearing sections, leading to uneven utilization of other oil-bearing sections due to insufficient injected enthalpy.

5. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 4, characterized in that, In step 2, taking into account the variable mass flow and dryness of the injected steam in the oil layer section, as well as the reservoir properties of the oil layer section, the steam injection pressure difference is controlled. The oil layer section is divided into segmented steam injection areas as needed. The steam injection volume and steam dryness are optimized according to the required steam injection volume of each oil layer section to ensure that the injection enthalpy of each layer meets the heating requirements of the oil layer.

6. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 1, characterized in that, In step 3, the process calculation method for balanced steam injection considers the physical properties of the oil layer and the thermophysical properties of the injected steam. A distributor is used to achieve on-demand steam diversion and injection, thereby achieving balanced steam injection in the oil layer. Based on the thermodynamic parameters of the injected wet saturated steam, including the steam dryness, pressure, gas specific volume, liquid specific volume, gas phase isobaric specific heat, gas phase isovolumetric specific heat, liquid specific heat at the distributor inlet, as well as the distributor outlet pressure, orifice diameter, and flow coefficient, the flow rate of a single orifice of the distributor is calculated. Based on the designed steam injection rate, the number and diameter of the distributor orifices are optimized.

7. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 6, characterized in that, In step 33, the formula for calculating the steam mass flow rate G is: Where C represents the nozzle flow coefficient, a decimal; A represents the nozzle outlet area, A = πd 2 / 4, d indicates the nozzle diameter of the dispensing device, in mm.

8. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 7, characterized in that, In step 34, the critical pressure ratio R cr The calculation formula is: Among them, parameter β is related to the gas phase mass fraction, gas phase volume, and liquid phase volume, and its expression is:

9. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 8, characterized in that, In step 35, the formula for calculating the steam injection rate of the distributor is as follows: Holes represents the number of holes in the dispenser.

10. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 9, characterized in that, In step 36, the formula for calculating the steam injection rate of the distributor is as follows:

11. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 1, characterized in that, In step 4, the formed steam distributor uses either circumferential or spiral perforation to distribute steam. The flow rate of each outlet is calculated and designed using the model from step 3 to ensure that the flow rate of each steam outlet is distributed according to the designed flow rate. This ensures that the entire horizontal section is designed with a variable dryness and variable flow rate steam injection profile, achieving the goal of balanced steam injection.

12. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 11, characterized in that, In step 4, the process tubing for the thermal recovery horizontal well is constructed using a combination of a distributor and insulated tubing. The distributor is installed on the insulated tubing to control the injection pressure differential. Steam is injected evenly at a pressure not exceeding the reservoir fracture pressure. Following the principle of segmented steam injection based on the required oil layer sections, the distributor is strategically placed at the locations of the horizontal sections requiring steam injection. Different horizontal sections are separated using in-tube packers. During injection, steam is injected from the insulated tubing, enters the horizontal section tubing, flows through the distributor, and enters the annulus through the distributor's orifice. Following the principle of balanced steam injection, the placement of the distributor in the horizontal section, the number of orifices, and the orifice diameter are all optimized to ensure that the injection enthalpy of each section meets the oil layer heating requirements. This creates a high-temperature steam chamber and a drainage chamber that evenly expands along the oil layer sections within the reservoir, achieving efficient utilization of the oil layer sections and improving oil recovery.

13. The method for enhancing oil recovery in extra-heavy oil reservoirs according to claim 1, characterized in that, In step 5, the leading edge of the steam chamber, which extends uniformly along the oil layer, is divided into a series of units in both the horizontal and vertical directions. The expression for the crude oil heating thickness γ of different units at different time periods is then given: Where Δt represents the time step; the superscript j indicates the j-th time step; δ represents the reservoir thermal diffusivity, m 2 / s; U represents the velocity of the leading edge of the steam chamber, m / s; Oil production rate Q per unit well length in thermal recovery horizontal wells over different time periods (j) The calculation formula is: Where K represents absolute penetration rate, m / s; K ro Represents relative permeability, a decimal; θ represents the angle at the leading edge of the steam chamber, in degrees; m represents the dimensionless viscosity-temperature coefficient, which varies with the steam chamber pressure; v os The kinematic viscosity of crude oil at steam temperature, m 2 / s; According to the principle of mass balance, when the steam chamber expands uniformly along the oil layer, the moving velocity U of different units at the leading edge of the steam chamber is... x and U y They are respectively: Where X represents the width of the element, in meters; Y represents the height of the element, in meters; Φ represents the formation porosity, which is dimensionless; ΔS o Indicates the saturation of movable oil, dimensionless; Based on the calculated velocity of the leading edge of the steam chamber, the position and angle θ of the leading edge of the steam chamber for each element at different times can be obtained, as expressed below: Among them, X i Y represents the horizontal distance (m) from the i-th unit to the production well; i The value represents the vertical height (m) of the i-th unit from the horizontal plane of the production well.

14. A balanced steam injection system for enhancing oil recovery in extra-heavy oil reservoirs, characterized in that: The balanced steam injection system for enhancing oil recovery in this extra-heavy oil reservoir uses the balanced steam injection method for enhancing oil recovery in extra-heavy oil reservoirs as described in any one of claims 1-13 to perform balanced steam injection process calculations and implementation.

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