Research and Application of the Fourth Generation Stratified Water Injection Technology in Daqing Oilfield

Stratified water injection is the most important characteristic technology in Daqing oilfield development, providing technical support for efficient oilfield development. With the oilfield entering the ultra-high water cut stage, the remaining oil is highly dispersed, the oil-water relationship is complex, the qualified rate of water injection decreases rapidly, and the difficulty of stabilizing oil and controlling water is increasing. In order to solve the contradiction between the increasing workload of water injection well testing and the limited testing team, and mee t the needs of oilfield precision development, the cable-controlled layered water injection technology has been developed. In this technology, the pressure monitoring system, flow monitoring system and flow control system are placed in the preset cable intelligent water distributor. At the office end, the technicians send control instructions on the server software, and send them to the ground control box through the oilfield production wireless network. The cable carrier technology is used to transmit the instructions to the intelligent water distributor through the cable, so as to achieve real-time communication, obtain the downhole layering parameter information, and control the downhole layering injection volume. Ten intelligent injection technology demonstration areas have been built to verify the process adaptability under different development contradictions. The cable-controlled layered water injection technology improves the proportion of water absorption thickness, effectively controls the water cut rise rate and natural decline rate, and has obvious oil increase effect, promoting the digital transformation and intelligent development of the oilfield.


Introduction
Water injection development is the most important development method of oil fields.This method injects water into the formation through water injection wells to drive underground crude oil to production wells to achieve the purpose of oil production.Water injection wells often adopt stratified water injection method, which is an effective water injection method to improve crude oil recovery [1][2][3][4][5][6] .
The oilfield has entered an ultra-high water cut stage, the remaining oil is highly dispersed, the oil-water relationship is complex, the qualified rate of water injection decreases rapidly, and the difficulty of stabilizing oil and controlling water is increased [7-8] , resulting in three problems and challenges for stratified water injection technology.(1) The contradiction between the increasing workload of water injection well testing and adjustment and the limited testing team is increasingly prominent.(2) The point test data of conventional separate injection process is difficult to meet the needs of accurate oilfield development.(3) The conventional separate injection process cannot support the digital transformation and intelligent development of the oilfield [9-16] .In order to solve the above problems, Daqing Oilfield has studied the cable-controlled layered water injection technology and carried out demonstration application.

Process Design Of Cable-Controlled Layered Water Injection Technology
The cable-controlled layered water injection technology is to place the pressure monitoring system, flow monitoring system and flow control system in the preset cable intelligent water distributor.At the office end, the technicians send the control instructions on the server software, and send them to the ground control box through the oilfield production wireless network.The cable carrier technology is used to transmit the instructions to the intelligent water distributor through the cable to achieve real-time communication and obtain the downhole layered parameter information, Control the injection rate of downhole layering.The system realizes no manual operation, and the office remotely monitors the changes of various parameters in real time and controls the underground stratified flow and pressure.It has the functions of automatic measurement and adjustment, automatic data storage, out-of-tolerance alarm, remote seal inspection and pressure measurement, and standard report output (Figure 1) [17][18][19][20][21].

Figure 1. Schematic diagram of cable-controlled intelligent layered water injection technology
The single-core steel pipe cable passes through the cable packer and is connected to the wellhead from the bottom to the top from the lowest level of the underground cable-controlled intelligent water distributor.After passing through the wellhead seal, it is connected to the underground power supply/carrier wiring port of the ground control box.The underground cable-controlled intelligent water distributor can be powered by the wellhead of the well, or by the temporary power supply of the test vehicle, solar power generation and other ways.The whole system takes the computer as the measurement and control center, and the computer communication interface and the ground control box communication interface carry out various measurement and control operation instructions transmission, and then the underground power supply/carrier interface of the ground control box is sent to the underground target measurement and control layer cable-controlled intelligent water distributor through a single-core cable.The water distributor can measure and control according to the received measurement and control instructions.It can not only measure and collect the flow, formation pressure, string pressure, valve opening, adjust the working current of the motor, cable head working voltage and other information related to the current level of the well, but also adjust and control the flow regulating valve in the cable-controlled intelligent water distributor to realize the manual or automatic allocation of the underground stratified water injection flow; At the same time, relevant measurement and control information is sent to the wellhead ground control box through single-core cable.The ground control box transmits the underground measurement and control data and the working voltage and current of the ground control box to the computer, and the computer automatically or manually decides the next measurement and control command according to the judgment results.

Design Of Ground Wireless Remote Control System
Daqing Oilfield has selected the existing production wireless network as the data transmission channel and developed a wireless transmission system to realize stable transmission of production data and control instructions of water injection wells.Each well is equipped with a wireless communication module on site, and the ground control box transmits the downhole layered data to the oilfield production wireless network through this device; Users of the enterprise network can access the server of the oilfield production network to realize remote testing and data query of intelligent separate injection wells [22].The network structure of the ground wireless remote control system is shown in Figure 2.
The data remote wireless transmission module includes the RS485 to RJ45 serial port server and the MEM wireless communication module.The field equipment is connected with the serial port server through the RS485 interface, and the serial port server is configured with RS485, IP port and other network configurations, and then the serial port server is connected with the wireless communication module through the RJ45 network port, and the wireless module is configured.The functional modules of the ground wireless remote control system are shown in Figure 3. the pressure in the tubing (i.e., the pressure before the nozzle), and the formation pressure (i.e., the pressure after the nozzle) without the need to run the instrument.At the same time, the regulating valve is opened and closed in the well, and the timing automatic measurement and adjustment, the pressure fluctuation is small, and the measurement and adjustment accuracy is high.See Figure 4 for comparison between conventional and intelligent survey and adjustment.

Construction and Effect Of Demonstration Area
At present, there are 1003 wireline controlled stratified water injection wells in Daqing Oilfield, with a maximum operation time of 5 years and a maximum of 8 intervals.The qualified rate of water injection can be maintained at more than 92% for a long time.In order to verify the process adaptability under different development contradictions, based on the principle of "problem orientation", adhering to the continuous innovation of technology and the guidance of technology demonstration, ten cable-controlled layered water injection technology demonstration zones have been built, and good oil increase effect has been achieved.
(1) For conventional water injection process, the test and adjustment period is generally 4 months.According to statistics, about 49.3% of water injection wells failed to maintain qualified injection allocation after 4 months, affecting the effect of water injection development.In view of the characteristics of small well spacing and frequent changes in water absorption performance of the interval in the demonstration area, using the technical advantages of real-time control, the testing and adjustment period was reduced from 4 months to 15 days, so that the water injection wells remained qualified for a long time, and the water injection effect was effectively improved.
(2) In view of the low degree of development of thin and poor reservoirs in the demonstration area, the advantage of wireline controlled layered water injection technology can monitor the pressure IOP Publishing doi:10.1088/1742-6596/2706/1/0120796 behind the mouth.By increasing the pressure difference between the front and back of the mouth of the low degree of development layers, the water absorption thickness of the sandstone is increased by 4.3 percentage points, of which the water absorption thickness of the off-surface reservoir is increased by 5.8 percentage points, and the development degree of thin and poor reservoirs is effectively improved.See Table 1 for the water absorption of oil layers in the demonstration area of wireline controlled stratified water injection technology.(3)In view of the serious problem of ineffective circulation in the demonstration area, the wireline controlled layered water injection is used for precise control, and the measurement and adjustment accuracy is increased from within ± 30% to within ± 10%, to ensure the precise control of water injection in high permeability layers, and the oil production and water cut of the connected oil wells are basically unchanged, so as to effectively control the inefficient and ineffective circulation.The production and operation curve of the demonstration area is shown in Figure 7. with cable direct control real-time monitoring technology.Ensure the qualified rate of water injection through precise regulation and control for a long time, and at the same time collect sufficient downhole data to provide data support for scientific adjustment of injection-production plan.After 4 years of application, the cumulative oil production is 468300 tons, with a year-on-year increase of 40300 tons, and the benefit is 606 million yuan.Based on the average daily injection of 50 tons of single well, the cumulative consumption of 9928000 tons of injected water is 55.104 million yuan, and the recovery rate is 11.00 yuan.The improvement effect is obvious.

Conclusion
(1) Wire-controlled stratified water injection technology can realize real-time monitoring of downhole flow, pressure and other parameters, and provide data support for the adjustment of injection-production relationship of oil reservoirs.This technology can be applied to key monitoring wells requiring continuous layered monitoring data.Obtain continuous monitoring data such as formation pressure, flow and temperature, provide new data support for fine geological analysis, guide block development, and effectively improve development effect and benefit.(2)The cable-controlled layered water injection technology has realized "three leaps": the production mode of survey and adjustment has changed from artificial machinery to intelligent survey and adjustment, the well site survey and adjustment has changed to indoor remote survey and adjustment, the operators have changed from "blue-collar" to "white-collar".The fourth generation of layered water injection technology with this technology as the main body has great application prospects.(3)In the next step, it is recommended to tackle the joint commissioning and control technology of intelligent injection-production well groups, optimize the schemes of separate injection and production, drilling and shut-in adjustment, casing damage prevention and control, carry out the research on intelligent technology of reservoir analysis, promote the continuous upgrading of intelligent technology, and provide strong support for the digital transformation and intelligent development of the oilfield.

Figure 2 .Figure 3 .
Figure 3. Function module of ground wireless remote control system Time/sTesting time a.Conventionalmeasurement and adjustment b.Intelligent measurement and adjustment

Figure 4 .Figure 5 .
Figure 4. Comparison of conventional and intelligent survey and adjustment curves (2) The wireline controlled layered water injection technology realizes the online sealing inspection of the packer without the need for the testing vehicle to run the instrument, improves the sealing inspection efficiency and reduces the testing cost.Two seal inspection methods are formed: (1) fast and remote seal inspection.Open and close the control valve of the water distributor in the target layer, and quickly judge the sealing status of the packer through the pressure change relationship between the layer and the adjacent layer.(2) Standard wellhead seal inspection.The sealing inspection of standard packer can be realized by closing the control valve of water distributor at the interval and "opening closing opening" operation at the wellhead.Take Well A as an example, there are 3 layers in total, which are partial I, partial II and partial III.The sealing of the packer between BiaoII and BiaoIII shall be checked by means of fast remote sealing inspection.In the process of closing the control valve of partial II layer, the pressure in front of the nozzle of partial II layer increased and the pressure behind the nozzle gradually decreased, indicating that the packer between partial II and partial III was sealed.The standard sealing inspection method is used to check the sealing of the packer between bias I and bias II.After closing the control valve of the partial II layer, perform the "open close open" operation on the water injection valve at the wellhead.The pressure behind the mouth of the partial I layer changes with the "open close open" of the wellhead.The pressure behind the mouth of the partial II layer does not change with the "open close open" of the wellhead, indicating that the packer between the partial I and the partial II is sealed.See Fig. 5 for the fast remote seal inspection and standard wellhead seal inspection curves of Well A.

Figure 6 .
Figure 6.Static pressure test curve of deviated layer II of Well A

Figure 7 .
Figure 7. Production and operation curve of the demonstration area(4) In the intelligent water injection demonstration area, more than 200 wells have been applied with cable direct control real-time monitoring technology.Ensure the qualified rate of water injection through precise regulation and control for a long time, and at the same time collect sufficient downhole data to provide data support for scientific adjustment of injection-production plan.After 4 years of application, the cumulative oil production is 468300 tons, with a year-on-year increase of 40300 tons, and the benefit is 606 million yuan.Based on the average daily injection of 50 tons of single well, the cumulative consumption of 9928000 tons of injected water is 55.104 million yuan, and the recovery rate is 11.00 yuan.The improvement effect is obvious.

Table 1 .
Water absorption status of reservoir in the demonstration area of wireline controlled stratified water injection technology