X. Zhang

Crude Oil Displacement Efficiency of Produced Gas Re-injection

CO2 flooding process has been a proven valuable tertiary enhanced oil recovery (EOR) technique. Meanwhile, it can also store greenhouse gas like CO2. However, in the late period of CO2 flooding process, mixed gases containing CO2 and hydrocarbon gases would be produced along with the production of crude oil. Usually, these mixed gases would be separated by gas processing plant. To prevent greenhouse effect, CO2 is re-injected to the reservoir while hydrocarbon gas is sold in the market. But the separation process is very costly. In this article, the feasibility of the produced gas re-injection without separation was investigated by experiment. Based on the produced oil and gas of Jilin oilfield, slim tube tests were conducted to study displacement efficiency of the crude oil by CO2 re-injection. In addition, components of the produced oil were analyzed according to the color and production stages. Finally, factors like displacement pressure and CO2 content were studied. It was found that produced gas re-injection without separation is a feasible method to save time and cost.

Preparation and performance of fluorescent polyacrylamide microspheres as a profile control and tracer agent

Polyacrylamide microspheres have been successfully used to reduce water production in reservoirs, but it is impossible to distinguish polyacrylamide microspheres from polyacrylamide that is used to enhance oil recovery and is already present in production fluids. In order to detect polyacrylamide microspheres in the reservoir produced fluid, fluorescent polyacrylamide microspheres P(AM-BA-AMCO), which fluoresce under ultraviolet irradiation, were synthesized via an inverse suspension polymerization. In order to keep the particle size distribution in a narrow range, the synthesis conditions of the polymerization were studied, including the stirring speed and the concentrations of initiator, Na2CO3, and dispersant. The bonding characteristics of microspheres were determined by Fourier transform infrared spectroscopy. The surface morphology of these microspheres was observed under ultraviolet irradiation with an inverse fluorescence microscope. A laboratory evaluation test showed that the fluorescent polymer microspheres had good water swelling capability, thus they had the ability to plug and migrate in a sand pack. The plugging rate was 99.8xa0% and the residual resistance coefficient was 800 after microsphere treatment in the sand pack. Furthermore, the fluorescent microspheres and their fragments were accurately detected under ultraviolet irradiation in the produced fluid, even though they had experienced extrusion and deformation in the sand pack.

An improved method to study CO2–oil relative permeability under miscible conditions

Relative permeability curves are crucial parameters for reservoir engineer and reservoir commercial simulator to predict reservoir performance throughout the life of a reservoir, but meet difficulties in laboratory to obtain reliable data under miscible conditions due to the lack of proper testing and formulation methods. Up to now, most relative permeability curves are measured in short core segments by core flooding, which can hardly display miscible flooding features for early gas breakthrough and insufficient contacting time between CO2 and oil. In addition, the commonly used analytical and semi-analytical data processing methods are not suitable for miscible flooding for ignoring the mechanism of vaporizing and dissolving mechanism. In this study, slim tubes (101 and 1,528xa0cm in length) and long composite cores (74.46xa0cm in length) instead of conventional core segments were used to acquire reliable experimental data of CO2 flooding under miscible or near miscible condition. Then, using improved empirical Corey model which assumes shape defining factor bog is a function of displacement pressure P combined with history-matching method to calculate relative permeability curves under near miscible and miscible conditions. Results indicated core length is another important parameter to simulate miscible flooding other than pressure, temperature and oil composition, and using long composite cores and improved data processing method more reliable data can be obtained compared with conventional measured method. It is found residual oil saturation in short slim tube is 16.25xa0% higher than that of long slim tube and CO2 relative permeability is lower in short slim tube/core segment than in long slime tube/long composite cores.

An Inhibition Properties Comparison of Potassium Chloride and Polyoxypropylene Diamine in Water-based Drilling Fluid

Polyoxypropylene diamine has been used as shale hydration inhibitor in drilling fluid and exhibited outstanding performance. The authors describe the inhibition characteristics of KCl and polyoxypropylene diamine and provided a comparative assessment. Through bentonite inhibition test, bulk hardness test, hot-rolling dispersion test, slake durability test, fluid loss test, linear swelling test and X-ray diffraction test, the inhibitive properties of potassium chloride, and polyoxypropylene diamine were evaluated. The results indicate that polyoxypropylene diamine is superior to potassium chloride in shale stabilization, which can effectively suppress the hydration and swelling of clay and play an important role in permanent stabilization with a relatively low concentration. Moreover, the mechanisms of the two inhibitors are briefly analyzed.

The High-Pressure Physical Property of Gas With CO2 in the Changshen Gas Reservoir

The natural gas in Changshen gas reservoir has high CO2 content. There are no conventional methods to calculate the physical parameters of natural gas. In view of this specificity, the physical parameters are determined and analyzed by PVT laboratory experiments. Experimental studies show that, below 20 MPa, the compressibility factor, volume factor, density, and isothermal compressibility have strong sensitivity to pressure, but the steam content contributes little and the absolute viscosity maintains strong pressure sensitivity. The compressibility factor, density, absolute viscosity, and steam content increase violently stronger than volume factor with the increase of CO2, and the isothermal compressibility is not sensitive to CO2. The variations of the high-pressure physical properties of Changshen gas reservoir depend on pressure and CO2 content, which should be considered in the reservoir development.

Nonequilibrium Features of N2 and Oil System

Nonequilibrium state is a universal phenomenon in gas injection development. However, in previous researches and applications, it tends to be regarded as quasi-equilibrium state, or even equilibrium state. Based on research work by pioneers, a simple model for measuring diffusivity of N2 in oil has been developed. In addition, laboratory experiments were performed in this study. Results indicate that an infinitely thin layer exists between N2 zone and oil zone in the pressure-volume-temperaturacelda (PVT) cell. Gas and oil molecules will slowly diffuse into one another across the sharp interface by convection, dispersion, and diffusion. Viscosity and gas oil ratio (GOR) are two important parameters representing the phase behavior features of the N2 and oil system. These dependent variables are related to parameters like pressure, temperature, and oil viscosity.

Mechanism of Produced Gas Reinjection During CO2 Flooding by Chromatographic Analysis

CO2 flooding process has been a proven valuable method that could not only enhance oil recovery but also store greenhouse gas. However, CO2 source greatly restrict its application in China. In this article, based on the produced oil and gas of Jilin oilfield, slim tube tests were conducted to study the feasibility of the produced gas reinjection without separation. In addition, according to the phenomenon of the experiment, displacement process was divided into three stages. Chromatographic analysis was conducted to study the mechanism of production gas reinjection during CO2. Results indicate that components of the produced oil change along with CO2 content, displacement pressure and production stages.

A New Modeling Approach for Bubble Growth in Foamy Oil

Abstract Some of the heavy oil reservoirs show high primary recovery and high flow rate under solution gas drive. The foamy oil theory has been proposed for the anomalous behavior. There have been numerous studies of diffusion-induced growth or collapse of bubbles in a fluid. However, there are some limits about them such as the determination of initial bubbles radius. Pointed to the problem that how to determine the initial bubble radius, this article proposes a new method base on the hydrodynamic equation, and then regards the bubble as a production well with source and sink theory based on the similarity between the bubbles concentration distribution and the pressure distribution of production well. According to the model established, the bubble growth by pressure and time is predicted, that is, how the amount and size of bubbles change during solution-gas drive is presented, which provided the guidance to determine the parameters of foamy oil due to bubbles change, such as foamy oil viscosity, compressibility, permeability, and so on. This model proved to be reasonable and correct by the agreement between the bubble concentration distribution and the pressure distribution during solution-gas drive.

Study on Acoustic Cavitation and Magnetic Coupling Wax Control Technology

Abstract To solve the wax deposition problem of high-pour-point and high-waxy oil wells, a new acoustic cavitation and magnetic coupling shaft wax control technology was proposed. Experimental research and field tests were conducted. This wax control technology can achieve the purpose of freezing point depression, viscosity reduction by emulsification, and wax control through the combined effect of cavitation, acoustic vibration, and magnetic intensification. Laboratory experiments showed that after acoustic cavitation and magnetic coupling treatment, the viscosity reduction ratio of high-pour-point oil was over 25.5% at 30°C; the freezing point was decreased from 32°C to 27°C, a 15.6% reduction; the wax crystal structure of the oil samples was changed from crude, thick, and dense to fine, thin, and sparse. Field tests showed that the acoustic cavitation and magnetic coupling wax control device can significantly extend the wells wax deposit cycle by more than 30 days and reduce the wax thickness (Yumen Oilfield). Using this technology, the flushing period was extended more than two times, with a maximum up to nine times, and the normal production time was also prolonged (Zhundong Oilfield in Xinjiang). The results are useful for the popularization and application of the new shaft wax control technology for high-freezing-point and high-waxy oil wells.

Effect of different molecular weight amphiphilic polymers on emulsifying behavior

Abstract Amphiphilic polymers applied in oil fields are polymer mixtures with different molecular weight. Stability of O/W emulsion prepared by amphiphilic polymers with different molecular weight was studied by stability analyzer. The difference in stability was clarified according to the external phase and interfacial properties. Results showed that PI(micro-molecular polymer) formed a tighter network structure in solution which exhibit higher viscoelasticity and more resistant to shear; and was more favorable to the emulsion stability than PII(macro-molecular polymer) due to higher surface activity. This work provides references for the molecular weight distribution optimization of amphiphilic polymer.