本文關(guān)鍵詞：頁巖氣輸運機理的微納尺度力學(xué)研究　出處：《中國科學(xué)技術(shù)大學(xué)》2017年博士論文　論文類型：學(xué)位論文

  更多相關(guān)文章： 頁巖氣 蒙特卡洛法 分子動力學(xué) 吸附 解吸附滯后 毛細(xì)凝聚 注氣驅(qū)替 微流動 拋物線流 平推流

【摘要】：隨著人類社會的快速發(fā)展,傳統(tǒng)的煤和石油資源急劇消耗。這些化石燃料的過度使用,已嚴(yán)重污染了全球環(huán)境。在這種情況下,非常規(guī)油氣資源日益受到關(guān)注和重視。頁巖氣以其分布范圍廣、資源儲量大、污染低等優(yōu)點,成為當(dāng)前油氣勘探開發(fā)的熱點。頁巖氣是從頁巖層中開采出來的天然氣,主要成分是甲烷。不同于常規(guī)油氣藏,頁巖基質(zhì)含有大量的微納米孔隙,具有低孔隙度和低滲透率的特點。實際地層條件下,頁巖氣具有多樣的儲存方式(吸附、游離、溶解等)和多尺度的滲流方式(解吸附、擴散、非達西流、達西流等)。相應(yīng)地,頁巖氣的勘探開發(fā)涉及一系列多尺度多學(xué)科交叉的問題。當(dāng)前,頁巖氣的勘探開發(fā)仍存在以下幾個關(guān)鍵問題。吸附是頁巖氣的主要儲存方式之一,但對納米孔隙中甲烷吸附機理和吸附結(jié)構(gòu)的認(rèn)識還不夠全面。實際地層影響吸附的因素很多,還沒有準(zhǔn)確表征頁巖吸附能力機理的理論模型。由于大量的甲烷吸附于孔隙中,開采時主要為吸附的逆過程解吸附。對于觀測到的解吸附滯后現(xiàn)象,仍未得到合理的解釋。受解吸附滯后現(xiàn)象的影響,傳統(tǒng)的降壓開采方式效率低下。工程上引入注氣驅(qū)替的新方法,但具體的驅(qū)替機理和驅(qū)替過程尚不明確。此外,微納米孔隙中甲烷的流動存在微尺度效應(yīng),需建立準(zhǔn)確描述其流動行為機理的模型。針對這些問題,本文采用蒙特卡洛和分子動力學(xué)方法,開展了系統(tǒng)的研究。研究發(fā)現(xiàn),由于壁面與甲烷存在分子間的相互作用力,納米孔隙的存儲容量高于相同體積的自由態(tài)。當(dāng)甲烷吸附于壁面時,其勢能降低。隨著孔隙寬度的增加,吸附態(tài)甲烷的結(jié)構(gòu)從單個吸附層轉(zhuǎn)變?yōu)樗膫�吸附層。對比不同孔隙對應(yīng)的等溫吸附線發(fā)現(xiàn),低壓時小孔隙反而能存儲更多的甲烷,這表明小孔隙的吸附作用更強。開采時主要為吸附的逆過程解吸附,工程和實驗上通�？梢杂^察到明顯的解吸附滯后現(xiàn)象。目前解釋此現(xiàn)象的機制主要有兩種:毛細(xì)凝聚和孔喉尺寸變化,本文對這兩種機制分別進行了模擬和分析。結(jié)果表明:毛細(xì)凝聚引起的解吸附滯后主要發(fā)生在甲烷的臨界溫度以下,高溫高壓的地層條件下不會發(fā)生毛細(xì)凝聚現(xiàn)象。進一步研究發(fā)現(xiàn),甲烷的吸附會導(dǎo)致頁巖基質(zhì)的腫脹,繼而引起孔喉尺寸的收縮。解吸附時,吸附的甲烷需要更高的能量才能通過縮小的孔喉,部分甲烷被困在孔隙中從而引起滯后。基于此機制,研究了壓強和溫度對解吸附滯后的影響規(guī)律。為驅(qū)替吸附的甲烷并提高開采效率,工程上通常采用注入氣體的方法,常見的注入氣為二氧化碳或氮氣。模擬研究了這兩種氣體的驅(qū)替機理:二氧化碳可以直接替換出吸附的甲烷而氮氣通過降低甲烷的分壓促進其解吸附。進一步,對比了相同工況下這兩種氣體的驅(qū)替過程:注入二氧化碳時,突破時間長,驅(qū)替面急劇。而注入氮氣時,突破時間短,驅(qū)替面平緩。頁巖基質(zhì)含有大量的微納米孔隙,甲烷在這些孔隙中流動時存在微尺度效應(yīng)。首先,利用平衡分子動力學(xué)方法計算了受限孔隙內(nèi)甲烷的密度分布和自擴散系數(shù)。研究發(fā)現(xiàn):吸附層中甲烷的密度較大,自擴散系數(shù)較小。在此基礎(chǔ)上,利用非平衡分子動力學(xué)方法模擬了寬度從1到10 nm孔隙中甲烷的流動行為。結(jié)果表明:甲烷的流動速度分布與孔隙寬度有關(guān)。隨著寬度的減小,速度曲線由拋物線轉(zhuǎn)變?yōu)橹本�。在相對較大的孔隙中,拋物線流可用滑移邊界修正的Navier-Stokes方程描述。相應(yīng)地,計算了有效粘度和滑移長度。隨著孔隙寬度的減小,表面擴散機制逐漸變得顯著。在小孔隙中,中心位置速度是均勻的而壁面附近速度線性增加。為描述這種流動規(guī)律,采用分段多項式進行擬合。對比發(fā)現(xiàn),這種情況下表面擴散機理能顯著增加總流量。本文利用分子模擬揭示了頁巖氣關(guān)鍵問題的微觀機理,得到的結(jié)果對頁巖氣的勘探開發(fā)具有重要意義。
[Abstract]:With the rapid development of human society, the traditional coal and petroleum resource. The excessive use of fossil fuels, the global environment has been seriously polluted. In this case, unconventional oil and gas resources has been paid more and more attention. With its wide distribution of shale gas resources, storage capacity, low pollution, become the current hot oil and gas exploration. Shale gas is mined from shale gas, mainly methane. Different from the conventional oil and gas reservoir, shale matrix contains lots of micro nano pores, with low porosity and low permeability characteristics. The actual reservoir conditions of shale gas has a variety of storage methods (the adsorption of free dissolved, etc.) and multiscale flow pattern (desorption, diffusion, non Darcy flow and the Darcy flow etc.). Accordingly, the exploration and development of shale gas involves a series of multi scale and multi discipline cross problem. At present, shale The exploration and development of gas still has the following key issues. The adsorption is one of the main storage methods of shale gas, but the understanding of methane adsorption and adsorption mechanism of nano pore structure in the actual formation is not comprehensive enough. Many factors affecting the adsorption mechanism, theoretical model is not accurately characterize shale adsorption ability. Due to the large amount of methane adsorption on the pore, mining mainly for the inverse process of adsorption desorption. The desorption observed with hysteresis, has not been properly explained. Influenced by solution adsorption hysteresis, the traditional way of depressurization efficiency. On the introduction of new methods for the construction of gas injection, but the specific mechanism of flooding and flooding for the process is not clear. In addition, the micro pores of the methane flow in micro scale effect, need to establish an accurate description of its flow behavior mechanism model. To solve these problems, this paper uses the mask. Carlo and molecular dynamics methods, systematic studies were carried out. The study found that due to the wall and the methane interaction force between molecules, the storage capacity is higher than that of the same volume of nano pore free state. When the methane adsorbed on the wall surface, the potential energy decreases. With the increase of pore width, structure of adsorbed methane from a single the adsorption layer into four layers. The adsorption isotherm of different pore corresponding, but small pores can store more methane pressure, which indicates that the stronger adsorption of small pores. When mining mainly for the inverse process of adsorption desorption, engineering and experiment can usually be observed desorption hysteresis there are two main mechanisms. At present this phenomenon: capillary condensation and pore size changes, the two mechanisms were simulated and analyzed. The results showed that capillary condensation caused by solution Adsorption hysteresis occurs mainly in the following critical temperature of methane, high temperature and high pressure formation does not occur capillary condensation. Further study found that the adsorption of methane will lead to shale matrix swelling, followed by pore size shrinkage. Desorption, adsorption of methane requires more energy to pass through the pore throat narrowing. Some methane trapped in the pores causing lag. Based on this mechanism, studied the influence of pressure and temperature on the desorption hysteresis. The displacement for the adsorption of methane and improve the efficiency of mining, gas injection method is usually used in engineering, the common injection gas as carbon dioxide or nitrogen. Displacement mechanisms were studied the two gases: carbon dioxide can directly replace the adsorption of methane and nitrogen by reducing methane partial pressure to promote its desorption. Further, compared to the same condition of the two gases The displacement process: injection of carbon dioxide, the breakthrough time is long, sharp and surface displacement. When nitrogen is injected, the breakthrough time is short, flat surface displacement. The shale matrix contains lots of micro nano pore, micro scale effect of methane flow in these pores. First, the density distribution of limited pore methane and self the diffusion coefficients were calculated by equilibrium molecular dynamics method. The study found: methane adsorption layer in high density, self diffusion coefficient is smaller. On this basis, the flow behavior of width from 1 to 10 nm in the pore of methane was simulated using non-equilibrium molecular dynamics method. The results show that the flow velocity distribution and pore width of methane with the decreasing of the width, velocity curve by parabolic into line. In a relatively large pore, parabolic flow Navier-Stokes equation with slip boundary correction. Accordingly, effective viscosity calculation And the slip length. With the decrease of pore width, surface diffusion mechanism becomes more significant. In small pores, the center position of velocity is uniform and near wall velocity increases linearly. In order to describe the flow pattern, the piecewise polynomial fitting. By comparison, in this case the surface diffusion mechanism can significantly increase the total flow. The micro mechanism of key problems of shale gas is revealed by molecular simulation, the result has important significance for the exploration and development of shale gas.

【學(xué)位授予單位】：中國科學(xué)技術(shù)大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2017
【分類號】：TE37

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