Effect of stress sensitivity on displacement efficiency in CO2 flooding for fractured low permeability reservoirs

Carbon dioxide flooding is an effective means of enhanced oil recovery for low permeability reservoirs. If fractures are present in the reservoir, CO2 may flow along the fractures, resulting in low gas displacement efficiency. Reservoir pore pressure will fluctuate to some extent during a CO2 flood, causing a change in effective confining pressure. The result is rock deformation and a reduction in permeability with the reduction in fracture permeability, causing increased flow resistance in the fracture space. Simultaneously, gas cross flowing along the fractures is partially restrained. In this work, the effect of stress changes on permeability was studied through a series of flow experiments. The change in the flowrate distribution in a matrix block and contained fracture with an increase in effective pressure were analyzed. The results lead to an implicit comparison which shows that permeability of fractured core decreases sharply with an increase in effective confining pressure. The fracture flowrate ratio declines and the matrix flowrate ratio increases. Fracture flow will partially divert to the matrix block with the increase in effective confining pressure, improving gas displacement efficiency.     

Exploitation of fractured shale oil resources by cyclic CO2 injection

With shale oil reservoir pressure depletion and recovery of hydrocarbons from formations, the fracture apertures and conductivity are subject to reduction due to the interaction between stress effects and proppants. Suppose most of the proppants were concentrated in dominant fractures rather than sparsely allocated in the fracture network, the fracture conductivity would be less influenced by the induced stress effect. However, the merit of uniformly distributed proppants in the fracture network is that it increases the contact area for the injection gas with the shale matrix. In this paper, we address the question whether we should exploit or confine the fracture complexity for CO₂-EOR in shale oil reservoirs. Two proppant transport scenarios were simulated in this paper: Case 1—the proppant is uniformly distributed in the complex fracture system, propagating a partially propped or un-propped fracture network; Case 2—the proppant primarily settles in simple planar fractures. A series of sensitivity studies of the fracture conductivity were performed to investigate the conductivity requirements in order to more efficiently produce from the shale reservoirs. Our simulation results in this paper show the potential of CO₂ huff-n-puff to improve oil recovery in shale oil reservoirs. Simulation results indicate that the ultra-low permeability shales require an interconnected fracture network to maximize shale oil recovery in a reasonable time period. The well productivity of a fracture network with a conductivity of 4 mD ft achieves a better performance than that of planar fractures with an infinite conductivity. However, when the conductivity of fracture networks is inadequate, the planar fracture treatment design maybe a favorable choice. The available literature provides limited information on the relationship between fracture treatment design and the applicability of CO₂ huff-n-puff in very low permeability shale formations. Very limited field test or laboratory data are available on the investigation of conductivity requirements for cyclic CO₂ injection in shale oil reservoirs. In the context of CO₂ huff-n-puff EOR, the effect of fracture complexity on well productivity was examined by simulation approaches.     

氢监测技术在含硫输气管线上的应用

采用腐蚀挂片、缓蚀剂残余浓度分析以及氢监测技术,在含硫输气管线上研究了缓蚀剂应用效果,确定了缓蚀剂的有效保护时间。结果表明,氢监测技术能够无损、实时、全周相地监测含硫输气管线腐蚀状况,缓蚀剂残余浓度分析及氢监测结果显示缓蚀剂有效保护时间达到1个月以上。     

延长油田致密砂岩油藏CO2驱油机理研究

CO_2驱是提高低渗透油田产量、缓解温室效应的有效途径。针对鄂尔多斯盆地油藏压力系数低、原油轻质组分含量高的特点,通过PVT和最小混相压力等测试分析方法,揭示了低压、低孔、低渗油藏CO_2驱提高采收率主要机理。开展了CO_2注入储层与无机、有机物作用后的沉淀研究,表明CO_2在无机盐溶液中不会形成沉淀堵塞孔隙,CO_2与有机质作用后沉积点高于油藏压力,且注入压力越高,CO_2在地层原油中的溶解能力越强,目标区块CO_2注入后不易形成沥青质沉淀。物模驱替实验结果表明,均质岩心的采出程度明显高于非均质岩心,且随着岩心非均质性的增加,水驱采出程度、气驱采出程度及最终采出程度均明显下降。     

页岩气藏流动机理与产能影响因素分析

为研究气体在页岩储层中的流动机理并分析影响页岩气藏产能的控制因素,基于广泛的文献调研,描述了页岩气在页岩储层中流动主要经历的3个过程：解吸附、扩散和渗流,分析了其影响因素和适用条件。在此基础上,利用数值模拟方法分析了吸附气含量、Langmuir体积、Langmuir压力、扩散系数、基质渗透率、微裂缝渗透率和压裂诱导裂缝导流能力等因素对页岩气水平井产能的影响情况。结果表明：①天然气地质储量保持不变时,随吸附气含量增高,水平井日产气量和相同开发时间累积产气量逐渐降低,地层平均压力下降速度加快;②相同吸附气浓度条件下,随Langmuir体积和Langmuir压力的增加,水平井日产气量和相同开发时间累积产气量逐渐降低,初期产量递减速度加快;③气体扩散系数对产能影响较小;④基质渗透率介于1.0×10^-9～1.0×10^-6 mD时,基质渗透率是控制水平井产能的主要因素,随基质渗透率增加,日产气量和累积产气量迅速增加;⑤基质渗透率大于1.0×10^-6 mD 时,基质渗透率和微裂缝渗透率均是控制水平井产能的主要因素,日产气量和累积产气量随基质渗透率和微裂缝渗透率的增加而增加;⑥随压裂诱导裂缝导流能力增加,水平井累积产气量逐渐增加,累积产气量增幅逐渐减小,压裂诱导裂缝存在着最优导流能力。     

裂缝对气藏储层渗透率及气井产能的贡献

裂缝是气藏储层渗流的重要通道，对储层渗透率的贡献十分明显，但目前难以量化评价。针对这一难题，综合考虑裂缝尺度（缝高、缝宽和裂缝贯通程度），通过对岩心进行人工定量造缝后开展气测渗透率实验测试，分别研究了贯通和非贯通（贯通程度分别为20%，40%，60%，80%）2种情景下裂缝对岩石渗透率的贡献。结果表明，贯通和非贯通裂缝对地层岩石渗透率均有贡献。贯通裂缝对岩心渗透率贡献十分明显，可提高岩石渗透率80%以上，其作用大小与裂缝开度（缝高×缝宽）密切相关；非贯通裂缝对岩石渗透率也存在一定的贡献，对基质起到沟通作用，改善储层整体的渗流能力，其作用大小与裂缝贯通程度关系密切。在实验测试的基础上，以单井为研究对象，综合考虑裂缝导流能力、裂缝沟通能力和基质供气能力三方面因素，建立了裂缝对气井产能贡献数学模型，结合气田实际气井的基础参数评价了裂缝对气井产能的贡献。      

页岩压裂裂缝渗透率的测试与分析

为了提高页岩气体积压裂的有效性,有必要对不同类型页岩压裂裂缝渗透率及其影响因素开展进一步的评价研究。为此,在研究页岩矿物组构特征、储层孔隙与裂缝特征的基础上,对3种不同类型(原位闭合型、剪切自支撑型、单层支撑型)的页岩压裂裂缝渗透率进行测试,并系统研究了页岩储层压裂裂缝类型、裂缝面粗糙度、碳酸盐矿物含量、页岩层理、围压等因素对页岩压裂裂缝渗透率的影响。结果表明:(1)原位闭合型裂缝渗透率与压力的关系符合Walsh理论,随围压增大而减小,其渗透率介于0.13~16.75m D,该类型裂缝不论有没有支撑剂充填和错位,对于提高页岩气储层产能都同样的重要、有效;(2)相对于原位闭合型裂缝,剪切自支撑型裂缝渗透率可提高1~2个数量级(渗透率介于7.53~88.48 m D)、单层支撑型裂缝渗透率可提高2~3个数量级(渗透率介于9.98~771.82 m D);(3)裂缝面粗糙度越大,裂缝渗透率越高,裂缝面分维数与渗透率具有较好的正相关关系;(4)剪切自支撑、单层支撑型裂缝渗透率和压力的关系与Walsh理论存在着一定的偏差,该偏差反映了自支撑点破碎、支撑剂嵌入和破碎等的影响。结论认为,该实验研究成果对于页岩气压裂工艺优选、参数优化具有指导意义。     

不稳态窜流部分打开压裂井试井分析模型建立及应用

由于火山岩气藏具有产层厚、岩石类型多、岩性和岩相复杂多变、非均质性强、天然裂缝发育等特点,在压裂井开发过程中,存在压裂裂缝难以全部压开储集体的情况,天然裂缝以垂向斜交缝和高角度缝为主,并且大量试井资料显示介质间具有明显的不稳态窜流特征。针对上述特征,考虑压裂裂缝部分打开储集体、将基质简化为柱状基质和天然裂缝呈交替均匀分布特征,建立了不稳态窜流部分打开压裂井试井分析模型,利用点源模型、分离变量法等方法进行求解,并绘制了典型曲线图版,典型曲线分为井储段、过渡段、线性流段、球形流段、不稳态窜流段和边界反映段6个主要的流动段,其中边界包括无限大边界和封闭边界,并对裂缝半长等参数进行敏感性分析。以克拉美丽火山岩气藏的压裂井为例,分析其试井曲线,并进行试井解释,研究结果表明：该压裂井的水平渗透率较小,垂向渗透率特别大,部分打开程度较小,分析结果与气井的静态、动态资料基本一致。     

超深高含硫气藏气—液硫两相渗流实验

超深高含硫气藏开发过程中会在储层中出现气—液硫同流的现象,其对气井产能的影响目前还缺乏实验数据的验证。为此,研制了一套适用于高温高压条件下气—液硫两相驱替实时测试的装置,并制订了相应的测试流程,选取四川盆地元坝气田的取样岩心开展气—液硫两相驱替实验,并采用非稳态法计算气、液硫两相相对渗透率,得到气—液硫相对渗透率曲线,进而开展气—液硫两相渗流规律的定量化研究。结果表明:(1)气、液硫两相共渗区较窄,当液硫临界饱和度高于40%时,井筒附近的液硫饱和度达到液硫临界流动饱和度,从而阻碍井筒附近气体的流动;(2)围压的变化会引起气—液硫相对渗透率曲线的变化,当围压增大时,气相相对渗透率及液硫相对渗透率均下降;(3)随着驱替压差增大,气体流速加快,携硫能力增强,气相相对渗透率及液硫相对渗透率均有所上升。结论认为,气—液硫两相相对渗透率曲线的获得,实现了对气—液硫两相渗流规律的定量化研究,可用于超深含硫气井的产能评价。     

压裂多级裂缝内流量分布规律

当前,以页岩油气为主的非常规油气藏开发力度日益加大,水力压裂是开发该类储层的核心技术。在该类储层的开发过程中,往往采用多级复杂裂缝压裂技术,但目前针对压裂过程中各级裂缝内流量分布规律研究甚少,而该规律对认识裂缝、指导压裂方案至关重要。为了研究压裂过程中多级裂缝内的流量分布规律,自主研制了多级裂缝系统有效输砂模拟实验装置,在模拟多级裂缝情况下,开展了不同压裂液黏度、支撑剂粒径、注入排量、砂比等因素对各级裂缝内流量的影响规律实验研究。研究结果表明,各级裂缝中流量占比逐级减小,主裂缝占比平均为64.63%,一级分支缝平均为22.14%,二级分支缝平均为13.23%;各级裂缝中流量分布比例主要受总流量大小影响,流量越大,主裂缝中流量占比越高,分支缝中流量占比越低,其次依次为支撑剂粒径、压裂液黏度和砂比。通过研究形成了一套多级裂缝内流量分布规律评价方法,揭示了各级裂缝内流量分布规律,为认识裂缝、优化压裂设计方案提供了依据。      

低渗透油藏裂缝动态渗吸机理实验研究

根据低渗透油藏裂缝与基质交渗流动的理论模型和物理模型,建立了裂缝与基质之间动态渗吸的实验方法并就裂缝内驱替速度、油水黏度比、润湿性,初始含水饱和度等参数对动态渗吸效果的影响进行了实验研究.对于低渗透裂缝性油藏,在压力梯度作用下水在裂缝内流动,同时由于毛细管力作用水渗吸到基质内,渗吸到基质中的水将油替换出来渗流到裂缝中,注入水再将裂缝中的油驱替到出口端,这就是裂缝与基质之间的交渗流动.动态渗吸实验结果表明:在本实验条件下,存在一个最佳驱替速度(3.0 mL/h),渗吸效率最高为35.5%;在一定的驱替速度范围内,由干毛细管力与黏性力的共同作用,渗吸效果最好.亲水岩心的动态渗吸效果最好.油水黏度比越小,动态渗吸效果越好.初始含水饱和度越高,毛细管力越小,动态渗吸效果越差.图6参20     

钻井液对裂缝性地层气侵的影响模拟研究

裂缝性地层钻进过程中,容易发生重力置换气侵,裂缝内气体进入井筒后,控制不当易引发井涌、井漏、井喷等复杂情况,气侵发生时裂缝附近的压力波动容易导致井壁失稳。在塔中北坡顺南区块裂缝特征参数分析基础上,使用Gambit建模软件进行前期建模和网格划分,建立了井筒与裂缝的三维模型;运用ANSYS Fluent流体力学软件,建立多相流模型,分析置换过程中裂缝开口处动压对裂缝开口处井壁稳定性的影响;以现场某井的数据为参考,模拟分析钻井液的密度、黏度、排量对重力置换气侵的影响。结果表明,置换气侵发生时,由于压差较大,瞬间流速较大,在0.1 s内降低至较低水平;动压的最大值在裂缝的上下两端,最小值在裂缝中上部,这3个位置的裂缝口缝宽容易增加,引起井壁失稳;钻井液的密度、黏度变化对气侵时裂缝口的动压影响较小,变化量小于6%,对气侵速度的稳定值影响也较小;而排量对于气侵时裂缝口的动压影响较大,变化量达到66.9%,降低钻井液排量有利于井壁稳定。      

裂缝性低渗油藏注气提高采收率实验研究

HH油田为典型的低孔特低渗油藏,部分地层伴有裂缝发育,现面临衰竭式开采后注水开采压力高的难题.通过基质单管岩心驱替实验,探究目标地层进行注气开发的可能性,明确何种注入气体及驱替方式具有较好提高采收率效果.通过基质+裂缝双管并联岩心驱替实验,确定目标地层中微裂缝对提高采收率效果影响.结果表明,CO2较减氧空气是更好的注入...     

特低渗透油藏水驱规律及最佳驱替模式

为了进一步改善特低渗透油藏开发效果，提高水驱采收率，通过大量特低渗透油藏水驱开采特征研究，揭示了特低渗透油藏的水驱规律：在注水开发过程中，特低渗透油藏会首先沿现今最大水平主应力方向注、采井间开启注水动态裂缝，随着注水压力的升高，或将开启与之成最小角度的注采井连线方向裂缝，导致注入水沿裂缝方向注采井无效循环，造成油藏水驱开发效果很差。等值渗流阻力法计算结果也证明了面积驱替径向渗流转为裂缝线性侧向驱替平行流后可大大降低渗流阻力。由此提出了“沿现今最大水平主应力方向注水动态裂缝线性注水、侧向基质驱替”的井网转换模式。井网模式的转换避免了注水动态裂缝导致的注入水无效循环，消除了动态裂缝对储层非均质性的影响，减小了渗流阻力，扩大了水驱波及程度。现场应用效果显著，单井产能增加了一倍，平面波及系数提高了43.2%，水驱采收率提高了19.3%。     

准噶尔盆地南缘侏罗系深层致密砂岩储层裂缝及其有效性

准噶尔盆地南缘侏罗系致密砂岩储层埋深普遍大于4 500 m,储层物性差,裂缝是重要的储集空间和主要的渗流通道,影响深层致密砂岩的油气富集分布和产能。通过野外露头观测、岩心、薄片和实验资料,对侏罗系储层裂缝发育特征、控制因素及有效性进行了研究。准噶尔盆地南缘侏罗系储层以发育高角度构造裂缝为主,其次是成岩裂缝和异常高压裂缝。准噶尔盆地南缘构造裂缝的方位主要有NNE-SSW向、NNW-SSE向、NEE-SWW向和NW-SE向,但中段NNW-SSE向裂缝不发育,且由东向西,构造裂缝的发育程度逐渐降低。构造裂缝的形成和分布受应力场、构造、岩性、层厚及储层非均质性等因素的影响,而裂缝的渗流能力则与流体活动、异常高压和现今地应力分布等因素相关。研究区裂缝的有效性较好,有效裂缝比例占85%以上。高压渗流实验结果显示围压增加,裂缝渗透率呈负指数递减。当有效围压小于15 MPa（埋深>1 000 m）时,裂缝渗透率随围压的变化快;超过该围压（深度）,裂缝渗透率递减缓慢;至最大实验轴压115 MPa（埋深>8 000 m）,围压为65 MPa时,裂缝的渗透率仍有22.5×10^-3 μm²。表明即使在深层,天然裂缝仍然是有效的储集空间和渗流通道,是改善深层致密储层储渗性能和油气富集高产的重要因素。     

Black Oil and Compositional Reservoir Simulation for Increasing the Recovery Performance of an Iranian Fractured Carbonate Reservoir

Abstract

Naturally fractured reservoirs contain a significant amount of world oil reserves. Accurate and efficient reservoir simulation of naturally fractured reservoirs is one of the most important, challenging, and computationally intensive problems in reservoir engineering. Black oil and compositional reservoir simulators have been used to determine the reservoir management and production strategies to increase the oil recovery from a low-porosity, low-permeability fractured carbonate reservoir, with an average matrix permeability of 0.8 md, average fracture permeability of 500 md, and an average matrix porosity of 10%. This reservoir is a candidate for an enhanced oil recovery (EOR) process, because the reservoir production rate has been declined due to increasing the water cut as a result of rising the water oil contact. The injection techniques that have been considered in this study for black oil model include (a) gas injection, (b) water injection, and (c) simultaneous water alternating gas injection and for the compositional model include (a) dry gas injection, (b) CO₂ injection, and (c) N₂ injection. Simulation results show that CO₂ injection has the maximum oil recovery between the EOR scenarios.     

Gas Injection Into Fractured Reservoirs Above Bubble Point Pressure

Abstract

Among all enhanced oil recovery (EOR) scenarios, gas injection seems to be promising for implementation in naturally fractured reservoirs. The use of CO₂ has received considerable interest as a method of EOR but a major drawback is its availability and increasing cost. Therefore, an alternative gas like CH₄ or N₂ must be considered to meet the economic considerations. To investigate the efficiency of oil recovery by CO₂, N₂, and CH₄ injection in fractured carbonate rock, a series of experiments was designed. Both miscible and immiscible schemes for gas injection were carried out on a low-permeable outcrop carbonate rock that was surrounded by fracture, established with a novel experimental method. The experiments aimed to investigate the potential of oil recovery by secondary and tertiary gas injection under high-temperature conditions. The matrix block was saturated using a recombined mixture of Iranian live oil, and by pumping water into the annular space, the space between rubber sleeve and outer jacket, high overburden pressure was exerted to obtain the desired homogeneous saturation. Using a back-pressure regulator, the pressure was kept above the bubble point pressure. The inlet was attached to a constant pressure pump injecting gas or water above the bubble point pressure, and the overburden pressure was removed gradually and the inlet fluid inflated the rubber sleeve. The amount of produced water from the annular space was recorded to estimate the distance between the rubber sleeve and sand face. This distance creates the fracture surrounding the core. Gas was injected into the fracture at pressures above the bubble point of the oil. Oil recovery as a function of time was monitored during the experiments. Results from both secondary and tertiary gas injection experiments indicate that CO₂ injection at elevated pressure and temperature is more efficient than N₂ and CH₄ injection.     

川东北陆相储层裂缝特征差异性及对产能的影响

川东北地区陆相储层均为致密性储层,但不同区块、层位裂缝发育的特征及对产能的影响具有差异性。选取元坝东、元坝西、马路背3个区块,综合岩心、薄片、测井、试气及试采等资料,对各区块储层裂缝发育特征、裂缝对产能的影响进行对比研究。结果表明:元坝东、元坝西部主要发育剪切缝,均以低角度裂缝为主,宏观及微观裂缝密度均较低,其中元坝东部裂缝有效性较高,元坝西区块裂缝有效性较低。马路背地区发育剪切缝、张性缝2种力学成因的裂缝,以高角度缝为主,宏观及微观裂缝密度均较高,裂缝有效性较高。宏观裂缝的倾角、有效性、密度综合影响着气井的初期产能,集中发育的网状微观裂缝或微观裂缝溶蚀作用可改善储层基质,是气井稳产的关键。      

Anisotropic characteristics of electrical responses of fractured reservoir with multiple sets of fractures

In fractured reservoirs, the fractures not but also form the main flow channels which connect productivity of reservoirs. However, because of the only provide the storage space for hydrocarbons, the pores of the matrix, so fractures dominate the heterogeneity and randomness of the distribution of fractures, exploration and evaluation of fractured reservoirs is still one of the most difficult problems in the oil industry. In recent years, seismic anisotropy has been applied to the assessment of fractured formations, whereas electrical anisotropy which is more intense in fractured formations than seismic anisotropy has not been studied or used so extensively. In this study, fractured reservoir models which considered multiple sets of fractures with smooth and partly closed, rough surfaces were established based on the fractures and pore network, and the vertical and horizontal electrical resistivities were derived as a function of the matrix and fracture porosities according to Ohm＇s law. By using the anisotropic resistivity equations, variations of the electrical anisotropy of three types of fractured models under the conditions of free pressure and confining pressure were analyzed through the variations of the exerted pressure, matrix porosity, fracture aperture and formation water resistivity. The differences of the vertical and horizontal resistivities and the anisotropy between the connected and non-connected fractures were also analyzed. It is known from the simulated results that an increase of the confining pressure causes a decrease of electrical anisotropy because of the elasticity of the closed fractures and the decrease of the fracture aperture. For a fixed fracture porosity, the higher the matrix porosity, the weaker the electrical anisotropy in the rock formation.     

致密气藏压裂水平井反卷积试井模型

致密气储层的渗透率非常低,通常在0.1 mD以下,使得此类气井压裂后长时间无法达到地层径向流动状态,因此通过单个试井测试段数据无法解释出储层有效渗透率,进而无法确定水力裂缝有效半长;此外,井筒储集效应往往掩盖早期裂缝中的流态,导致裂缝导流能力这一关键完井参数求取困难。在缺失早期裂缝中流态和中期径向流态的情况下,如果强制拟合试井曲线,往往获得不唯一且偏差过大的参数解。针对这一问题,进行了反卷积试井分析方法研究。通过改进Schroeter反卷积数学模型并编制求解算法,实现了用短期试井数据和长期产量数据反演气藏瞬态压力响应历史的过程。数值试井算例验证了提出的反卷积模型及算法的正确性,反卷积后的瞬态压力响应去除了井筒储集效应,使裂缝线性流和地层径向流获得有效恢复。现场实例应用验证了提出的反卷积压力恢复试井分析方法的可行性,并证明短期的压力恢复试井数据亦能用于裂缝和储层动态参数计算,从而定量评价致密气藏水平井水力压裂施工效果。