How to enhance heavy oil recovery to meet the oil consumption is a popular issue around the world, and it has attracted widespread attention. A two-dimensional visualized model was adopted to study the pore-scale mechanisms and development effects of foam for enhancing oil recovery in steam injection processes for heavy oil. Experimental images visually presented that small bubbles gather together to form bigger foams, thus blocking the small pores and throats and leading to fluid diversion in porous media. As a result, the sweep efficiency was improved from 46.18% to 77.93% after foam injection. Foams could effectively improve the mobility ratio between oil and water and decreased water cut after foam injection, which was significant for decaying the decline of oil production. As for the pore-scale level, after foams were injected into the visualized model, the residual oil caused by steam flooding entered into the main streamline under the disturbance of foams and was carried out by the following displacement fluid. The heavy oil was emulsified into O/W emulsions that had lower viscosity under the action of foams; hence, more trapped oil was mobilized and displaced. As a result, the micro oil displacement efficiency increased from 72.76% to 84.01%. In order to provide a reference for the choice of foam injection, experiments that investigated development effects of cold foam and hot foam were also conducted. Compared with the incremental of oil recovery caused by cold foam, that induced by hot foam was 41.51% higher, demonstrating that the coinjection of steam and foam was more advantageous to heavy oil production.
As a temperature-sensitive non-Newtonian fluid, the seepage of heavy crude oil in porous media shows the non-linear characteristics. The flowing behavior of three heavy oils through porous media is experimentally investigated, and the influence of temperature and pressure-drop on this flowing process is also described. Thereafter, based on the flowing behavior of heavy crude oil, the new models of productivity of the thermal producers (including vertical well and horizontal well) are proposed. In these models, both the threshold pressure gradient (TPG) and thermal effect are taken into account. The flowing experiments of heavy oil in porous media indicate that the pressure gradient and temperature have the significant influence on the flowing process because of the existence of threshold temperature and TPG. Heavy crude oil begins to flow only when the pressure gradient is in excess of TPG, and there dose not actually exist TPG above the threshold temperature. The viscosity-temperature curves demonstrate that the viscosity of heavy crude oil has an obvious feature of two straight-lines on semilog coordinate. On account of the damage of overlapping phenomena of asphaltenes in crude oil, when temperature is higher than the critical temperature, the reducing trend of TPG (with the increase of temperature) will be lessened. Furthermore, on the basis of flowing process of heavy crude oil, the concepts of threshold temperature and certain production temperature of thermal wells are introduced. That heavy oil with a higher viscosity would have a higher threshold temperature, as well as the certain production temperature. The application of horizontal wells tremendously increases the oil recovery rate in comparison with the vertical wells. This investigation could be used as a tool to study the flowing process of heavy oil and productivity calculation of thermal wells in heavy oil reservoirs.
Foam flow experiments were carried out to study the influence factors such as surfactant concentration, foam quality, injection rate of liquid and gas, permeability of porous media, temperature, and oil saturation on blocking ability and flowing characteristics of steady foams in porous media. Foam blocking mechanisms and flowing characteristics were summarized according to the experimental results and foam migration behavior. The results showed that the pressure distribution of flowing foams was linearly descending in porous media at steady state. The results further showed that the foam size and quality in pores along the sand pack were almost uniform, that is, foam generation and destruction gradually reached dynamic equilibrium at steady state. In porous media, the blocking ability of steady foams increased with the concentration of the foaming agent and the increase in the permeability of porous media, but the blocking ability decreased with the increase in the temperature, the shearing rate, and the oil saturation of the porous media. Foam resistance factor reached maximal value at the foam quality of 85% in porous media. List of symbols CMC Critical micelle concentration of surfactant in solution, wt% AOSAlpha olefin sulfonate, which is a kind of surfactant BS-12 Dodecyl dimethyl betaine, which is a kind of surfactant SDS Sodium dodecyl sulfate, which is a kind of surfactant V fm Foaming volume of foaming agent, ml
With the increasing demand of energy, technical research of how to enhance oil recovery of steam flooding for heavy oil reservoirs has attracted widespread attention at present. Air-injection is an effective technology that has been used to improve the development effect of steam flooding in heavy oil reservoirs. The low-temperature oxidation (LTO) reaction and the high-temperature combustion reaction are the main mechanisms of air-injection technology. The high-temperature combustion reaction can decompose the heavy component in heavy oil, but it requires a higher-temperature condition, which the steam flooding process cannot offer. In addition, the LTO reaction between air and heavy oil consumes the O 2 , so the safety risk of explosion caused by the mixture of O 2 and hydrocarbon gas can be eliminated. Nevertheless, oil viscosity will increase. During the steam flooding process, an aquathermolysis reaction occurs between heavy oil and high-temperature water, which decreases the content of the heavy component in heavy oil. Besides, catalyst MnO 2 promotes the reaction by decreasing the activation energy of the reaction. In this paper, several static oxidative decomposition experiments are conducted to study the change characteristics of pressure, gas composition, oil composition, and oil viscosity after the reactions with different temperatures, pressures, and water saturations. In addition, four dynamic displacement experiments are conducted to compare the displacement effect of different displacement methods, including N 2 -injection displacement, air-injection displacement, steam flooding, and airinjection-assisted steam flooding. Experimental results show that air-injection can effectively improve the development effect of steam flooding in heavy oil reservoirs. Upgrading and viscosity reduction for heavy oil by the combination of the LTO reaction and the aquathermolysis reaction can slow down steam channeling and increase production rate, thereby enhancing the ultimate recovery of steam flooding.
Summary Microhole-drilling technology is a high-efficiency and low-cost technology that has developed rapidly in recent years. However, during microhole-horizontal-well drilling, cuttings are easy to deposit at the bottom of the wellbore because of gravity and nonrotation of drillpipe. Inadequate drill-cuttings removal can cause costly problems such as excessive drag, and even mechanical pipe sticking. Therefore, many laboratory studies as well as field observations have been directed toward addressing the cuttings-transport problem. In the present study, a full-scale horizontal-cuttings-transport flow loop was set up and a total of 136 experiments were conducted. By analyzing the cuttings volumetric concentration and the dimensionless height of the cuttings bed, the effects of flow rate (0.00058–0.00078 m3/s), cuttings diameter (0.0003–0.005 m), rate of penetration (ROP) (0.00211–0.00636 m/s), eccentricity (0–0.8), and wellbore diameter (0.04–0.08 m) on wellbore-cleaning efficiency were obtained. It was found that cuttings-transport efficiency increased first and then decreased as cuttings diameter increased. Flow rate was the main parameter. Higher flow rate, lower ROP, lower eccentricity, and smaller drillpipe/wellbore-diameter ratio all led to higher wellbore-cleaning efficiency in microhole horizontal wells. In addition, a model for estimating the cuttings volumetric concentration and the cuttings-bed height was proposed by dimensional analysis dependent on the thorough understanding of the effects of various variables. The predictions were good when they were compared with the experimental data obtained. Major factors influencing cuttings transport in the field during microhole drilling and conventional rotary drilling were compared, and the reasons for their differences were discussed. The limitations of the proposed model in this study were also discussed. These results could provide a factual basis for improving microhole-drilling hydraulics.
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