Document Type : Original Research Paper

Authors

Department of Chemical Engineering, Faculty of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran

Abstract

Water is critically important, because its supply is under stress. In oil fields, the ratio-of-water-to-oil (WCUT%) can be 95% or higher. Managing this produced water is a great challenge whereas the best opportunity to reduce costs, improve profitability and preserve the natural environment. The oil industry is looking for more effective ways to reduce water consuming and improve the recovery rates. Nano materials are an obvious place to look. This study provides new insights into ZnO nanoparticles effects on residual oil saturation (SOR) and WCUT% through steam assisted gravity drainage (SAGD) process by experimental work. Laboratory tests were conducted in two experiments through the use of 2 dimensional scaled SAGD cell from an Iranian heavy oil reservoir. In the first experiment, the SAGD cell was saturated with heavy oil and in the second one, the cell was flooded with nanoparticles before saturation with oil. The amount of recoveries were monitored during 12 hours. Results show that the ultimate oil recoveries increase from 52.43% to 87.93% by adding ZnO nanoparticles, respectively. The experimental results provide the nanoparticles ability to reduce produced water and minimize fresh water use can contribute to water conservation.

Highlights

Oil and gas producers need to understand their water consumption, the requirements and limitations of the areas in which they operate and the potential of nanotechnology investment to protect and enhance their profitability into the future. This study shows the capability of ZnO nanoparticles to reduce viscosity and increases heavy oil recovery whereas WCUT% is decreased through SAGD process. The catalytic chemical reaction of nanoparticles on breaking the bonds between carbon and sulfur is assumed to be the main reason of viscosity reduction. The most viscosity reduction happens at the lowest concentration of nanoparticles (0.2-0.5%wt). Based on ZnO nanoparticles characteristics on viscosity reduction, the final oil recovery is increased considerably hence, the SOR and WCUT% is definitely decreased.

Keywords

INTRODUCTION

The oil and gas industry is undergoing a series of dramatic shifts with one common outcome, extracting hydrocarbons is harder than ever before. Production from the world’s largest conventional fields is in decline while national oil companies continue to control the majority of the world’s oil reserves. Simultaneously, global demand for oil and gas continues to grow, fueled in large part by emerging economies.

As a result, producers have resorted to new techniques to bypass declining and inaccessible legacy sources of oil and gas. The last five years have seen a dramatic increase in production from unconventional sources. These source, shale, oil sands and deep water offshore, represented 47 percent of capital spending in oil industry in 2016. Producers are using more to get less, more labor, more energy, more time and more water which all lead to higher costs for both producers and consumers. From the water used to flood declining conventional and offshore wells to the water injected to fracture underground shale to the steam required for oil sands extraction. Water is the most important input to the oil and gas industry. By 2030, if current trends continue, global water requirements are expected to exceed supplies by forty percent. This trend is all the more relevant in oil and gas production, as many of the world’s largest reserves reside in the most water- starved regions. Oil and gas producers should be concerned with water not only as a proactive step to be more efficient, but also as a defensive step against declining water supplies. From chemical and mechanical conformance tools to custom water treatment, water challenges with processes and technologies that reduce unwanted water production and treat produced water for disposal or reuse while satisfying a broad range of reservoir management and environmental objectives are solved. Globally, oil wells produce about 220 million BWPD (barrels of water per day) roughly three barrels of water for every barrel of oil. In older fields, the WCUT% can be 95% or higher. Managing this produced water is a great challenge for operators. Rising prices for energy coupled with the increasing environmental awareness of consumers are responsible for a flood of products on the market that promise certain advantages for environmental and climate protection. Nanotechnology exhibits special physical and chemical properties that makes it interesting for novel and environmentally friendly products. In the chemical industry sector, nano materials are applied based on their special catalytic properties in order to boost energy and resource efficiency and nano materials can replace environmentally problematic chemicals in certain fields of application. High hopes are being placed in nanotechnology optimized products and processes for energy production and storage. These are currently in the development phase and are slated to contribute significantly to climate protection and solving our energy problems in the future [1]. The capability of nanotechnology usage in enhanced oil recovery processes is investigated. The rock pores may contain trapped oil, gas and water. Nanoparticles can be used to recover more residual oil. It is showed that nanotechnology affects on several parameters such as oil viscosity reduction [2]. High surface-to-volume ratio of nano fluids leads to improve thermal properties. The surface-to-volume ratio of nanoparticles may be 1000 times greater than of micro particles. It is tested a variety of particle sizes and types to find those best suited for plugging the rock pores, which turn out to be elastic nanoparticles made of polymer threads that retract into coils. Nanoparticles in solid form such as silica are less effective [3]. The effect of nano sized metal on decreasing viscosity through thermal process is reported. The results provide a good understanding of viscosity mechanism [4]. Silica nanoparticles fluid is flooded in water-wet sandstone and investigated the hydrophilic or hydrophobic monolayer role of them in the pore spaces. It is found that adsorption of SNPs can help to alter reservoir wettability [5]. Wettability can be changed depending on nanoparticles type through altering the chemical interactions in interfacial tension [6]. The oil recovery is increased about 4-5% compared to brine in the core flooding procedure by nano fluids. Hence, the potential of nanoparticles enhanced oil recovery is clear. The experimental results show that the IFT between water phase and oil phase can be reduced by nano fluids and the wettability of solid surface alters to more water wet and releasing oil drops by increasing capillary pressure is completely obvious [7]. Nano-size metal particles is used to reduce viscosity of heavy oil/bitumen through steam injection techniques. The experiments are a good proof of viscosity reduction by adding metal particles. The optimal concentration of metal particles are critical factors to affect on viscosity reduction [8]. The objectives of this study are to clarify the potential of ZnO nanoparticles on increasing oil recovery, decreasing the residual oil saturation (SOR) and WCUT%. By considering of environmental issues and characteristics of ZnO nanoparticles, the event of viscosity reduction by adding nanoparticles causes to use lower energy to supply steam in thermal process which helps to save more energy and water.

 

MATERIALS AND METHODS

A 2D sand pack model was designed for studying SAGD experiment. A single SAGD process well pair structure was deployed for steam injection and oil production. The vertical spacing between well pairs, l, set up to 15 cm. Two experiments were conducted which the major experimental condition sand purposes are listed in Table 1.

 

Table 1. Experimental conditions and purposes of two steps

 

Scaled Reservoir Model 

For investigating SAGD process, a rectangular physical model (Sand pack) designed for this study which was made of stain steel 316 with dimensions 20 × 20 × 4 cm3. As shown in Fig. 1, the physical model was consisted of different parts.

 

Materials 

The crude oil used in this study was taken from an Iranian heavy oil field. Heavy oil properties are given in Table 2. The water phase for all experiments as base fluid was distilled water with viscosity of 1cp at ambient condition. To study the effect of nanoparticle on SAGD process, zinc oxide was selected. ZnO nanoparticles specifications are listed in Table 3.

 

Table 2. Properties of the oil used

 

Table 3. Specifications of ZnO nanoparticles used

 

Selection of the best concentration of nanoparticles is one of the challenging issues. Fig. 2 shows the effect of different concentrations of ZnO nanoparticles on heavy oil viscosity at different temperatures. It seems that by adding nanoparticles, viscosity reduction happens more. Another feature of Fig. 2 is that by decreasing nano concentration, viscosity decreases more. It is presumed that the main reason of this viscosity reduction can be related to the catalytic characteristics of nanoparticles on breaking the carbon and sulfur bonds in a chemical reaction. The most viscosity reduction happens at the lowest concentration of nanoparticles (0.2-0.5%wt).

 

Fig. 1. The schematic of physical model;

1- The main body with inside dimensions 20204 cm.

2- Upper cap with dimensions of cm.

3- Lower cap with dimensions of cm.

4- Steam injector well in 15 cm from producer.

5- Steam injector well in 10 cm from producer.

6- Oil producer well in the bottom of model.

7- A total of 9 thermocouple (T1-T9) probes installed to the model. The thermocouples connected to the data acquisition instrument, which recorded and displayed the temperature profile within the model during the experiment.

8- The basis for rotating the model in different angels.

 

 

Fig. 2. The effect of different concentrations of ZnO nanoparticles on heavy oil viscosity

 

Experimental Procedure

Prior to all experiments, the model was assembled. The thermocouples were placed back into the model and the pressure test was conducted. Usually the model was left pressurized with gas for 24h to make sure that there was no pressure leak. In the second step, the physical model was packed with sand from the reservoir. During packing, the model was vibrated and held at several different angles to make sure no gap would be left behind and a homogenous packing was created. The packing and shaking process typically took 24h. Table 4 shows the measured porosity, absolute permeability and pore volume of packed model for all experiments. The third step was to evacuate the model to remove air from the pore space. Finally, the model was connected to a vacuum pump and evacuated for 16h. The model was disconnected from the vacuum pump and kept on vacuum for couple of hours to make sure that it held the vacuum. If high vacuum was maintained, it was ready for saturating. Fig. 3 shows the packed model saturated with oil. The oil saturation of model took around 72h. After passing these steps, the packed model was ready for SAGD process.

 

Table 4. Properties of packed model used

 

For doing SAGD test with ZnO nanoparticles, the model preparation was achieved in a step-wise manner as follows; a) clean the physical model, b) vacuum the pore space of packed model, c) flood the ZnO nanoparticles to packed model about 72h, d) dry the packed model about 72h, e) saturate the model with heavy oil about 72h. After finishing the previous test, the packed model cleaned with toluene during 42 days and dried in oven for 7 days. Then the packed model flooded with ZnO nanoparticles. Water was selected as base fluid. By using ultrasonic device UP200S (Hielscher Ultrasonic, 200W, 24 kHz), homogeneous nanoparticles solution was made. Maintaining the homogeneous solution during the experiment was one of the most important challenging issues. ZnO Nanoparticles tend to be disposed after around 7h. To avoid this problem and assure that the nanoparticles affected on the packed model directly, nanoparticles flooding procedure applied. First, the model was vacuumed and as shown in Fig. 4, flooding procedure was applied. Hence, the cell was ready to saturate with heavy oil for 72h the same as previous test.

Fig. 5 shows a schematic of the displacement apparatus used in this study. This apparatus included a water pump, steam generator, steam accumulator, 2-D scaled sand pack model, production control mechanism and the data acquisition system. Water was injected using nitrogen pressure via water accumulator cylinder. This positive pressure pump could inject at pressure range between 150 and up to 4000 psi. Steam generator heated water with an electrical element to vaporize water and injected the steam by nitrogen pressure pump with constant pressure. A temperature controller was used to inject the hot fluid at constant temperatures. The physical model was built from stainless steel with operating pressure up to 10000 psi and operating temperatures up to 500 °C. The physical model and measurement tools placed in a thermostatic oven as shown by dotted line in Fig. 5. In each experiment, the oven temperature fixed at 75°C and the system was allowed to reach thermal equilibrium. For steam injection, pressure was set in accumulator. When the system became steady state, oven and steam generation temperature and pressure injection were set and water became steam in vent tube. Once the oil production decreased below 1%, steam injection stopped. Mixture of heavy oil and produced condensate water collected in a sample bottle every 15 minutes. All oil production and condensed water measured in calibrated graduated cylindrical tubes. The oil and emulsion heated at 60 °C in the oven for 24h to break emulsion into oil and water and lately used centrifuged at 6000 RPM speed to separate the water and oil completely and the amount of separated oil was measured. The fluid cooled in a condenser before collection.

 

Fig. 3. Oil saturating set up; a) put the packed model inside the oven, b) fill the accumulator (600 ml) with heavy oil, c) connect the accumulator to injection pump and packed model, d) pump 2PV of heavy oil to the model to make sure it saturated with heavy oil completely

 

Fig. 4. Flooding the packed model with nanoparticles; a) fill the accumulator 600 ml with homogeneous ZnO nanoparticles solution, b) connect the accumulator to injection pump and packed model, c) pump 2PV of nanoparticle and flood the model