Laboratory Studies of Depressurization with a High Concentration of Surfactant in Low-Permeability Reservoirs

To solve the problem of high pressure of water injection in low permeability reservoirs, a high concentration of surfactant system was developed in this article. With the solubilization of oil in aqueous surfactant solution as a criterion, a formula was screened from anionic and nonionic-anionic surfactants, and the optimal depressurized system was obtained as follows, 13.3% surfactant HEX +2.23% n-propanol +4.47% n-butanol, the solubilization capability being up to 0.66 g/g. This system had good salt tolerance, and it exhibited water external microemulsion in the range of of 1 to 200 g/L NaCl. Core flooding results show that this high concentration of surfactant system formed water external microemulsion with the residual oil in the core, reducing the displacement pressure over 35%. Meanwhile, the effects of concentration and injection volume on depressurization were also investigated. It is indicated that a good depressurization effect was achieved after injecting 1 pore volume of the system with the salt concentration of 100 g/L.     

降压开采模拟试验的水合物分解阵面演化过程

水合物分解阵面演化过程与开采安全性和产气效率密切相关,是开采原位监测的重要组成部分。在玻璃砂样品中进行了甲烷水合物降压开采模拟试验,探讨了水合物饱和度对渗流阵面和水合物分解阵面演化过程的影响,结合已有理论模型,分析了水合物分解阵面传播速度的关键影响因素。结果表明:渗流阵面和水合物分解阵面的传播距离均与时间平方根呈近似线性关系,传播速度均随水合物饱和度的增加而减小;水合物分解阵面的传播速度随多孔介质的有效渗透率和降压幅度的增加而变快,随孔隙率的增加而变慢,粗砂质地层更有利于水合物降压分解阵面的传播。     

Numerical Investigation into the Development Performance of Gas Hydrate by Depressurization Based on Heat Transfer and Entropy Generation Analyses

The purpose of this study is to analyze the dynamic properties of gas hydrate development from a large hydrate simulator through numerical simulation. A mathematical model of heat transfer and entropy production of methane hydrate dissociation by depressurization has been established, and the change behaviors of various heat flows and entropy generations have been evaluated. Simulation results show that most of the heat supplied from outside is assimilated by methane hydrate. The energy loss caused by the fluid production is insignificant in comparison to the heat assimilation of the hydrate reservoir. The entropy generation of gas hydrate can be considered as the entropy flow from the ambient environment to the hydrate particles, and it is favorable from the perspective of efficient hydrate exploitation. On the contrary, the undesirable entropy generations of water, gas and quartz sand are induced by the irreversible heat conduction and thermal convection under notable temperature gradient in the deposit. Although lower production pressure will lead to larger entropy production of the whole system, the irreversible energy loss is always extremely limited when compared with the amount of thermal energy utilized by methane hydrate. The production pressure should be set as low as possible for the purpose of enhancing exploitation efficiency, as the entropy production rate is not sensitive to the energy recovery rate under depressurization.     

Appearance of Nucleation Shocks in a Boiling Liquid

Depressurization of high-pressure vessels filled with a liquid coming to the boil with a decrease in pressure is investigated. After depressurization, which takes less than 1 ms, a rarefaction wave propagates into the vessel. Experiments [1–5] demonstrated that the pressure behind the wave goes over to a constant value, which is independent of the vessel diameter and lies between the saturation point and atmospheric pressure.The correspondence between the experiments and different bubble formation theories (bubble formation on the vessel walls, due to rupture of the bonds between the water molecules or boiling on foreign particles) is analyzed. A “ mechanical nucleation” theory is proposed, in which it is assumed that the liquid comes to the boil on a limited number of foreign particles, and the bubbles formed on the nucleation centers undergo multiple fragmentation due to the instability developing under the action of centrifugal accelerations of the bubble surface in the course of bubble growth.The calculations demonstrated that, after depressurization, bubble fragmentation occurs at the vessel outlet. Due to the growth of the phase interface in the course of fragmentation, the boiling intensity increases, the pressure grows, and a shock wave propagates into the vessel, following the rarefaction wave. Multiple fragmentation of the bubbles occurs on the shock front. This wave is followed by a series of waves with smaller amplitudes. The pressure in the vessel attains a stable level, without any shocks. This level is characterized by the metastability or superheating of the liquid, i.e. the difference between the liquid temperature and the saturation point. It is demonstrated that the resultant pressure in the vessel is independent of the number of initial boiling centers or the initial pressure in the vessel and is determined only by the initial temperature. For water, the dependence of the superheating of the liquid on the initial temperature is found and compared with experimental data.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, 2005, pp. 103–117.Original Russian Text Copyright © 2005 by Ivashnev and Smirnov.     

水流动强化天然气水合物降压分解研究

降压法被认为是最经济可行的天然气水合物开采方法,但开采后期驱动力不足、甚至产生水合物的二次生成,因此其应用受到限制。本文将降压法与水流动结合提升水合物分解驱动力,研究不同降压模式和水流动对天然气水合物分解特性的影响。发现当降压结合水流动时,压降为水合物分解提供初始驱动力,且压降越大水合物分解驱动力越大。同时水流动能够加快传热传质过程,为水合物分解提供额外的驱动力。在快速降压结合水流动模式中,较高背压下水流动为水合物分解提供主要的驱动力;在梯度降压结合水流动模式中,降压和水流动共同为水合物提供分解驱动力,对水合物分解的促进作用更加显著。     

新型降压槽结构设计及流动特性分析

针对三元复合驱分注过程中低视粘度损失和高压降不能同时实现的问题,设计出了一种新型降压槽,运用CFD软件对其环空流道的流动特性进行数值模拟,研究了新型降压槽内的压力、速度、湍动能、视粘度和应变率分布情况.同时将其与环形降压槽进行对比,结果表明,相同长度的降压槽,新型降压槽的压差更大,视粘度损失更小.     

海洋天然气水合物降压开采地层井壁力学稳定性分析

降压开采天然气水合物使其分解会导致储层孔隙度、渗透率、孔隙压力和岩层骨架有效应力发生改变,同时降低沉积物的胶结程度,使地层的抗剪强度和承载能力降低,从而引起井壁失稳、海底滑坡、海底面沉降等工程问题.为此,在地下多相非等温数值模拟软件TOUGH+Hydrate框架内,基于扩展的三维Biot固结理论,考虑水合物分解相变、传热(T)、流动(H)、岩土体变形(M)等过程及其相互耦合作用,建立了新的水合物开采传热-流动-力学(THM)耦合数学模型,并开发有限元程序对其进行数值求解.以中国南海神狐海域GMGS1航次SH2站位水合物储层条件为研究对象,构建了垂直井降压开采THM耦合地层井壁稳定性分析模型,预测了水合物开采过程中储层温-压-力场和水合物分解区的演化规律,揭示了地层优势出砂区域和海底面沉降趋势.结果表明:储层降压导致地层有效应力增大,进而引起井周地层发生沉降,且地层的沉降主要发生在降压开采前期,最大沉降位置位于井壁周围,向储层内部延伸地层沉降量快速减小;水合物分解导致井周地层力学强度降低,加剧了储层的沉降;井筒降压造成射孔段井壁应力集中最为明显,从而造成井壁破坏的潜在风险,这些区域正是水...     

水合物藏降压开采实验及数值模拟

建立了降压法开采水合物藏数学模型,考虑了气-水-水合物-冰相多相渗流、水合物相变及分解动力学过程、冰-水相变、热传导、对流过程、渗透率变化等对于水合物分解的影响。三维水合物藏模拟表明:在开采前期阶段,可采用降压法,但随着储层能量消耗,产气速度下降很快,需转变开采方式。分析了一些主要参数,如孔隙度、渗透率、饱和度、压力等对水合物开采的影响。     

多孔介质中天然气水合物注热+降压开采的实验研究

为研究注热与降压相结合的开采方式是否更加有利于天然气水合物的开采,在自制的天然气水合物开采模拟实验系统上进行了实验研究.在一维填砂模型中人工生成天然气水合物之后,进行先注热盐水然后再降低压力(注热+降压)的开采模拟实验,分析了开采过程中系统温度、电阻率变化规律以及产气量、能量效率等.结果表明:产气规律具有明显的阶段性,...     

High pressure phase transitions and depressurization amorphization of Bi2Fe4O9

High pressure structural behavior of Bi₂Fe₄O₉ has been studied by in situ angular-dispersive X-ray diffraction (ADXD) measurements up to 51.3 GPa. Two phase transitions have been observed at 7.6 and 22.6 GPa, respectively. A second high pressure structure (HP2) involving the tripling of lattice parameter c has been identified. An unusual amorphization occurs after releasing pressure. The high pressure phase transitions can be understood in terms of the increase in the coordination number of Fe³⁺ ion. The depressurization amorphization results from the appearance of the metastable HP2 and its collapse after releasing pressure. The results extend our understanding of pressure-induced amorphization.