Abstract

In response to the problem of the need to restart heavy oil water ring transportation pipelines due to water ring instability and damage, based on a self-designed indoor small-scale restart ring simulation experimental device, four types of ordinary heavy oil in Lvda Oilfield were taken as the research object. The experimental study investigated the variation law of restart pressure drop with time when the heavy oil water ring transportation pipelines were restarted at a constant water flow rate after shutdown. The influence of oil holding rate, oil viscosity, shutdown time, and constant water flow rate on restart pressure drop was discussed. IBM SPSS software was used to perform regression analysis on the results of orthogonal restart experiments, establish a multiple nonlinear regression model for restart pressure drop, and verify its predictive reliability through 192 sets of experimental data. The experimental results show that the variation of restart pressure drop with time can be divided into two stages: pressure drop attenuation and pressure drop constant; The restart pressure drop is positively correlated with oil holding rate, oil viscosity, shutdown time, and constant water flow rate, but the effect of oil holding rate on constant water flow rate is most significant; The predicted values of the restart pressure drop model are in good agreement with the experimental measurement values, and their relative errors are all within the range of ±10%.

With the continuous growth of demand for oil resources and the increasing scarcity of conventional crude oil, the supply of world crude oil resources is shifting from light oil to heavy oil. The abundant reserves of heavy oil are bound to play an important role in the global energy market in the coming decades. However, heavy oil has special properties such as high viscosity, high density, and poor fluidity, which bring great difficulties and challenges to its extraction, storage, transportation, and processing. At present, the pipeline transportation of heavy oil is mainly achieved through three methods: viscosity reduction, drag reduction, and oil upgrading. Reducing viscosity refers to reducing the viscosity of crude oil, which can be achieved through methods such as heating (pre heating heavy oil or heating pipelines), dilution (mixing thinner with lower viscosity in heavy oil), emulsification (adding surfactants to heavy oil to form a water in oil emulsion), etc; Drag reduction refers to reducing the frictional resistance between heavy oil and the pipe wall. Measures that can be taken include adding drag reducing agents and forming a low viscosity annular flow structure that wraps around high viscosity oil cores; Oil upgrading refers to the process of upgrading heavy oil on the oil well site to produce synthetic crude oil with lower viscosity, higher API degree (specific gravity index), and lower asphaltene, heavy metals, and sulfur content. Among them, the low viscosity ring method, especially the water ring transportation method, has been widely studied by scholars in related fields worldwide due to its outstanding characteristics of low energy consumption and environmental friendliness. It is considered one of the most promising heavy oil transportation methods.

At present, domestic and foreign scholars have carried out a large amount of theoretical analysis, experimental research, and numerical simulation work in the field of heavy oil water ring transportation, but they are mainly focused on studying the problems of heavy oil water ring transportation under normal operating conditions, such as the optimization design of water ring generators and their auxiliary components, lubrication and drag reduction mechanism of heavy oil water ring transportation, flow pattern characteristics and pressure drop characteristics of heavy oil water ring flow, and strengthening measures for the stability of heavy oil water ring flow. However, there is little attention paid to the difficulties of restarting after planned maintenance or accidental shutdown and shutdown. Therefore, this article takes four types of ordinary heavy oil and tap water from Lvda Oilfield as the research object, and independently designs and develops a simulation experimental system for heavy oil water ring transportation shutdown and restart. The simulation study investigates the variation law of pressure drop with time during the restart process, analyzes the effects of oil holdup, oil viscosity, shutdown time, and constant water flow rate on the maximum restart pressure drop, and based on orthogonal restart experiments, establishes a multiple nonlinear regression model for the maximum restart pressure drop. The research results can provide theoretical support for developing reasonable restart plans for on-site shutdown pipelines and effectively avoiding safety risks.

1.Materials and Methods

1.1 Materials

Four representative ordinary heavy oils (numbered LD1, LD2, LD3, and LD4) from the Lvda Oilfield in the Bohai Sea were selected as experimental oil samples to conduct heavy oil water ring transportation pipeline shutdown and restart experiments. Using the pyknometer method, the densities of LD1, LD2, LD3, and LD4 at 20℃ were determined to be 902kg/m³, 913kg/m³, 921kg/m³, and 936kg/m³, respectively. Using the HAAKE Viscotester iQ Air rheometer, the rheological and viscosity temperature characteristics of Luda heavy oil were tested in the range of 20~70℃, as shown in Figure 1. From this, it can be seen that Lvda heavy oil exhibits Newtonian fluid characteristics within the test temperature range, and its viscosity shows a sharp decrease followed by a gentle trend with increasing temperature. The viscosity of LD1, LD2, LD3, and LD4 at 20℃ was measured to be 1.0553Pa· s, 2.038Pa·s, 2.55Pa·s, and 3.02Pa·s, respectively. The experimental water comes from Chengdu Water Supply Plant, and its density and viscosity at 20℃ are 998.2kg/m3 and 1.005mPa·s, respectively.

1.2 Experimental Setup

This article independently developed and designed a set of indoor small-scale heavy oil and water ring transportation shutdown and restart ring experimental device. The flowchart and physical diagram are shown in Figure 2. The device mainly consists of an oil-water supply system, a pipeline testing system, a blowing system, and a data acquisition system. It can achieve visual simulation of the entire process of heavy oil-water two-phase flow, shutdown, and restart in the pipeline.

The oil-water supply system consists of two parts: oil and water, which are used to supply the oil and water phases required for the experiment to the pipeline. The oil circuit mainly includes components such as a 50L oil storage tank, high-temperature slag oil pump, turbine flowmeter, valves, etc; The waterway mainly includes 50L water storage tank, vertical stainless steel multi-stage centrifugal pump, electromagnetic flowmeter, valves and other components. The pipeline testing system mainly consists of a self-made water ring generator, a testing pipe section, a 100L mixed liquid separation tank, valves, etc. The total length of the pipeline for the entire device is about 15m, with an inner diameter of 25mm. The material used is rigid polyvinyl chloride plastic (UPVC). Among them, the length of the test pipe section is 0.9m, and it is equipped with pressure relief holes at both ends. A differential pressure transmitter is connected to test the fluid pressure drop in this section. The purging system mainly consists of piston type air compressor, air storage tank, pressure regulating valve and other components. Its function is to purge the residual liquid in the pipeline after each experiment, in order to facilitate accurate measurement of pressure drop in subsequent experiments. The data acquisition system consists of four parts: measuring instruments, data collectors, data receivers, and computers, used for real-time automatic collection, display, and storage of parameters such as pressure, flow rate, and temperature. The main instruments and equipment used in the experiment are shown in Table 1.

1.3 Experimental Methods

1.3.1 Experimental Process

Before conducting shutdown and restart experiments on heavy oil and water ring transportation pipelines, it is necessary to first form a relatively stable oil-water ring flow pattern. According to the conditions and criteria for the stable existence of the annular flow structure proposed by Bannwart, this experiment sets the oil phase apparent flow velocity U_OS as 0.74m/s, and the water phase apparent flow velocity U_WS as 0.17m/s, 0.28m/s, 0.44m/s, 0.65m/s, 0.97m/s, and 1.43m/s, respectively. The corresponding inlet oil content C_o is 0.81, 0.73, 0.63, 0.53, 0.43, and 0.34. The formation, shutdown, and restart experiments of oil-water annular flow were conducted at room temperature of 20℃. The restart experiment used a constant water flow to push the stationary fluid, and the specific steps are as follows: Firstly, open the waterway valve and water pump; Secondly, open the oil circuit valve and oil pump; Next, adjust the oil and water flow rate to the set value through a frequency converter or bypass valve to generate a stable annular flow of oil and water; Subsequently, simultaneously close the oil pump, water pump, and corresponding valves, intercept the oil-water two-phase flow in the experimental pipeline and let it stand for a period of time; Finally, reopen the waterway valve and water pump, and push the static stratified oil-water two-phase fluid inside the pipe with a constant water flow rate to restore its flow. Repeat the above experimental steps to investigate the effects of different oil holding rates (H_o=0.26, 0.35, 0.45, 0.55, 0.66, 0.76), oil viscosity (μ_o=1.0553Pa·s, 2.038Pa·s, 2.55Pa·s, 3.02Pa· s), shutdown time (t_st=0.5h, 1h), and constant water flow velocity (U_cl=0.25m/s, 0.53m/s, 0.72m/s, 1.01m/s) on the characteristics of pipeline shutdown and restart behavior.

1.3.2 Uncertainty Analysis

To evaluate the accuracy and credibility of direct measurements such as temperature, pressure difference, and flow rate in the shutdown and restart experiment of heavy oil and water ring transportation pipelines, a B-class standard uncertainty u_B is introduced to measure it, and its calculation is shown in equation (1).

According to Table 1, the measurement accuracy of temperature transmitter and differential pressure transmitter are 0.1% and 0.25% respectively, and the accuracy level of turbine flowmeter and electromagnetic flowmeter are both 0.5 level; The accuracy of the data acquisition system used in the experiment is 0.2%, and the uncertainty of temperature, pressure difference, and flow rate can be obtained from equation (1) as 0.320%, 0.224%, and 0.539%, respectively.

1.4 Data Processing Methods

To verify the accuracy and reliability of the multiple nonlinear regression model for the pressure drop during shutdown and restart of heavy oil water ring transportation pipelines, the measured value Δp_max of the restart pressure drop is taken as the true value, and compared with the predicted value of the restart pressure drop Δp^'_max. Therefore, the relative error δ_Δp for predicting the pressure drop after shutdown and restart of the heavy oil water ring transportation pipeline was introduced, and its calculation is shown in equation (2).