Experimental and numerical modeling of sulfur plugging in carbonate reservoirs

Sour gas, mainly in the form of hydrogen sulfide, is produced in large amounts from many oil and gas reservoirs in the United Arab Emirates. In addition to creating problems in production lines, the precipitation of elemental sulfur in vicinity of the wellbore is often reported to cause wellbore damage. While there have been several studies performed on the role of solid deposition in gas reservoirs, the role of sulfur deposition in oil reservoirs has not been investigated. This paper presents experimental results along with a comprehensive wellbore model that predicts sulfur precipitation as well as plugging. Two separate sets of experiments, one for a gas phase system and another for a crude oil system, were conducted to investigate the deposition of elemental sulfur in (linear) carbonate cores. The gas flow tests were conducted with elemental sulfur being carried with nitrogen through limestone cores. Changes in gas flow rate were monitored while the injection pressure was held constant. A series of experiments generated valuable data for plugging with elemental sulfur. X-ray diffraction tests provided evidence of sulfur deposition along the cores. The oil flow tests were carried out to observe sulfur precipitation and plugging in a carbonate core. The crude oil was de-asphalted before conducting these tests in order to isolate the effect of asphaltene plugging. Significant plugging was observed and was found to be dependent on flow rate and initial sulfur concentration. This information was used in a phenomenological model that was incorporated in the wellbore numerical model. The data for the numerical model were obtained from both test tube and oil flow experiments. By using a phenomenological model, the wellbore plugging was modeled with an excellent match (with experimental results).     

A NOVEL TECHNIQUE OF ASPHALTENE DEPOSITION TREATMENT USING ULTRASONIC IRRADIATION

The need for better understanding of asphaltene behavior in the crude oil and treatment techniques of its deposition in porous medium has been recognized but still requires extensive research and experimental activities. The ambitious goals of this study are to investigate: (1) influences of using ultrasonic irradiation on asphaltene behavior in the UAE crude oil, with consideration of solvent and temperature effects; and (2) influences of ultrasonic irradiation characteristics such as frequency and time interval on damaged oil permeability (due to asphaltene deposition) of carbonate reservoir rocks. To achieve the above-mentioned goals, three groups of experiments using ultrasonic irradiation were carried out. In the first group, 18 identical crude oil samples of 2.47 wt% initial asphaltene content were subjected to different time intervals of ultrasonic irradiation of 0, 5, 10, 15, 25 and 30 min and under different temperatures of 25, 40 and 60°C, respectively. Oil viscosity was measured and microscopic images of the centrifugated oil samples of asphaltene clusters were obtained. In the second set of experiments, 12 identical crude oil samples with different toluene concentrations of 5, 10, and 15 vol% were subjected to four different durations of ultrasonic irradiation and under different temperatures. In the third group, seven actual carbonate core samples of damaged oil permeability due to the injection of 60 pore volumes of asphaltic crude oil were subjected to different ultrasonic time interval of 5, 10, 15, 20, and 25 min and frequencies of 10, 15 and 20 kHz, respectively. Both the oil permeability of these samples was measured and the scanning electron microscope (SEM) images were carried out before and after the ultrasonic irradiation process. The results showed that subjection of the UAE crude oil to ultrasonic irradiation decreases the size of asphaltene clusters. Consequently, this effect reduces asphaltene tendency to precipitate at 10 min or more time interval of ultrasonic irradiation. In addition, similar results were obtained with solvent effect, but with more reduction in oil viscosity. The results also indicated that the increase of ultrasonic time interval and/or frequency drastically improve(s) damaged oil permeability.     

高速非达西流动时元素硫沉积模型研究

元素硫沉积是含硫气藏开发过程中广泛存在而又必须解决的难题之一。高含硫气藏一旦投入开发，地层压力会逐步下降，使得元素硫溶解度下降而沉积下来，沉积的元素硫会堵塞地层孔隙，降低渗流通道，影响气井产能。考虑了高含硫气体在近井地带作高速非达西流动，建立了硫的沉积模型，并利用该模型对实际气藏进行预测和对比研究，再通过实例计算发现，硫非瞬时平衡沉积比瞬时平衡沉积对地层的伤害更严重，高速非达西流动沉积比达西流动时对地层伤害更严重，认为在开发高含硫气藏时，必须合理选择开发速度，有效防止元素硫沉积。     

中国陆相湖盆致密油成藏主控因素综述

致密油是指致密储层中的石油聚集,储层主要为致密砂岩和碳酸盐岩2大类,覆压渗透率多小于0.1×10-3 μm2,一般无自然产能,需经技术改造方能获工业油流。我国主要发育陆相湖盆致密油藏,通过对比研究分析发现,发育优质烃源岩、存在"甜点区"、具备成藏原动力、近源聚集是我国陆相致密油成藏的主要条件。我国陆相致密油主要发育2类优质烃源岩:Ⅰ类烃源岩有机质类型好、丰度高,有机质成熟度高,生烃潜力大;Ⅱ类烃源岩生烃转化率较高。发育致密砂岩和碳酸盐岩2类储层,储层具有较强的非均质性,横向不连续,垂向叠置分布。生烃增压是我国陆相致密油成藏的主要原动力,强大的源储压差驱替生成的石油向紧邻优质烃源岩的致密储层中持续充注;其中,微裂缝沟通、微—纳米孔发育是致密油运移聚集的关键。微—纳米孔发育增大了致密储层的有效储集空间,微裂缝沟通为致密油的运移聚集提供了有效通道。我国陆相致密油资源丰富,初步预测其有利勘探面积约16×104 km2,地质资源量约(160~200)×108 t,有利勘探领域主要分布在鄂尔多斯、准噶尔、松辽、渤海湾、柴达木、四川等盆地。     

Investigation of supercritical carbon dioxide, aspheltenic crude oil, and formation brine interactions in carbonate formations

Asphaltene precipitation and rock dissolution can significantly hinder the success of carbon dioxide flooding of asphaltenic crude oil in carbonate heterogeneous formations. It is essential during CO₂ flooding when both processes exist to study separately the effect of each phenomenon on the flooding process. Ten core flooding experiments under similar reservoir conditions of 4000 psia pressure and 250 °F temperature were conducted to evaluate each phenomenon separately. Actual rock cores representing different areas of carbonate oil field saturated with actual fluids of filtrated brine and asphaltenic crude oil were used to evaluate the interaction between supercritical (SC)-CO₂, carbonate rock, and its contained fluid. Asphaltene content of the produced crude oil, water and mineralogical rock analyses, and scanning electron microscopic (SEM) photos of rock pores were performed to evaluate the effect of supercritical (SC)-CO₂ flood on the permeability and mineralogical variation characteristics of the limestone cores. Results indicated that calcite dissolution and/or precipitation is the major reason for permeability improvement and/or impairment. It is also proven that the amount of permeability damage depends on the fabric of the rocks, salinity of the brine, and initial core permeability. The results also indicated that CO₂ injection in fresh water saturated carbonate rock led to complete collapse of that rock. It is recommended that fluid assessment should be conducted to the different areas of the field.     

考虑非平衡过程元素硫沉积对高含硫气藏储层伤害研究

元素硫沉积是高含硫气藏开发有别于常规气藏开发的一个重要研究内容.当地层压力下降时,元素硫溶解度下降使其沉积下来,沉积的元素硫会堵塞地层,从而降低地层孔隙度和渗透率.为了定量研究硫沉积对地层孔隙度和渗透率的影响,首先建立了元素硫沉积的伤害模型,然后利用该模型对一个实际高含硫气藏开发时由于元素硫沉积引起的地层孔隙度、渗透率和沉积含硫饱和度的动态变化进行了计算求解.通过实例计算发现:地层渗透率越低时越容易发生硫沉积,且硫沉积主要在井筒附近发生,当生产时间越长时,硫沉积的量越多,从而对地层的伤害越严重.     

琼海凸起不同电阻率油藏原油特征及成藏过程

对比琼海凸起低电阻率与高电阻率油藏原油特征发现,相对于高电阻率油藏,低电阻率油藏原油的密度和含硫量较低,成熟度稍高,生烃母质含有较多陆源有机质且氧化程度更高,为富含黏土矿物。琼海凸起低电阻率与高电阻率油藏有着不同的成藏过程：早期生成的以文昌B凹陷文昌组原油为主的混合原油,优先充注到琼海凸起下部充注压力较小的高渗透储集层,形成了高电阻率油藏;随着高电阻率油藏逐渐趋于饱和,晚期大量生成的以文昌A凹陷恩平组原油为主的混合原油,充注到琼海凸起上部充注压力较大的低渗透储集层,形成了低电阻率油藏。琼海凸起广泛发育的低电阻率油藏指示其周缘生烃凹陷具有较大的生烃强度,是今后勘探的重点。     

考虑元素硫沉积的水平井产量预测方法研究

随着含硫气藏的深入开发,水平井的作用也变得越来越重要,准确预测水平井产量对于含硫气藏开发具有重要的意义。考虑元素硫沉积对储层渗透率的影响,建立含硫气藏水平井产量预测公式,并分析水平井长度、储层有效厚度、元素硫沉积和非均质性对产量的影响,其中元素硫沉积和非均质性对水平井产量影响较大。计算结果表明,考虑元素硫沉积的Joshi公式和陈元千公式计算结果与实际产量接近,可用于含硫气藏生产初期产量预测,该结论对类似气藏开发具有一定的借鉴意义。     

The effect of fracture characteristics on reduction of permeability by asphaltene precipitation in carbonate formation

Crude oils produced in many parts of the world contain asphaltene. Asphaltene plugging is a known cause of near-wellbore formation damage. The deposition phenomenon of asphaltene has been extensively investigated in homogenous porous media. However, we have not found any reported experimental work on asphaltene plugging in naturally or artificially fractured reservoirs. The main objective of this study is to investigate the effect of fracture characteristics on reduction of permeability by asphaltene deposition in carbonate formation.A single fracture system is made with different fracture characteristics such as porosity, permeability, and fracture angle using saw-cut limestone core samples with metallic plate. The fracture aperture was kept constant for most of the studied systems. The effect of flow rate, fracture angle, matrix to fracture permeability, asphaltene concentration, and the state of stress on the retained permeability is presented. Different fracture angle orientations of 45°, 90°, and 180° relevant to the flow axis were investigated. A noninvasive imaging technique—scanning electron microscopy (SEM)—was employed to visualize changes on the surface of the fracture as a result of aspheltenic crude oil flow through the system.     

A Novel Technique For Determining Microscopic Pore Size Distribution of Heterogeneous Reservoir Rocks

Fluid flow in porous media is affected by petrophysical properties and their distribution. Standard techniques of core analysis provide an average of varying petrophysical properties and pore size distribution (PSD) of porous rocks. However, these techniques have severe assumptions that may negatively influence the results obtained.

The major objectives of this study are to develop a new technique capable of providing the microscopic PSD of heterogeneous reservoir rocks and to apply this technique to interpret the formation damage problem due to sulfur deposition. The current methods of calculating the pore size distribution were reviewed and evaluated. To achieve the objectives of this study, 10 reservoir rock samples extracted from an actual oil reservoir were photographed using a scanning electron microscope (SEM) and analyzed using computer software to provide the PSD. In order to investigate the application of the proposed technique, actual sulfuric crude oil was flooded through an actual core sample and the in-situ deposited sulfur along the damaged core sample was calculated, plugged, and the opening pores of the core were quantified.

The results showed that a new technique capable of explaining and providing the required information about pore size distribution and its applications on a microscopic scale was developed. In addition, the influences of fluid flow characteristics of elemental sulfur through reservoir rock were presented. The newly proposed technique provided valuable data such as PSD, minimum and maximum pore diameters, plugged and nonplugged areas, and deposited amounts of damaging elements. The judicious applications of the attained results represent an impact in studying and/or evaluation of formation damage, designing the microbial enhanced oil recovery process, and development and validation of many oil reservoir simulation models.     

A NOVEL TECHNIQUE OF ASPHALTENE DEPOSITION TREATMENT USING ULTRASONIC IRRADIATION

ABSTRACT

The need for better understanding of asphaltene behavior in the crude oil and treatment techniques of its deposition in porous medium has been recognized but still requires extensive research and experimental activities. The ambitious goals of this study are to investigate: (1) influences of using ultrasonic irradiation on asphaltene behavior in the UAE crude oil, with consideration of solvent and temperature effects; and (2) influences of ultrasonic irradiation characteristics such as frequency and time interval on damaged oil permeability (due to asphaltene deposition) of carbonate reservoir rocks. To achieve the above-mentioned goals, three groups of experiments using ultrasonic irradiation were carried out. In the first group, 18 identical crude oil samples of 2.47 wt% initial asphaltene content were subjected to different time intervals of ultrasonic irradiation of 0, 5, 10, 15, 25 and 30 min and under different temperatures of 25, 40 and 60°C, respectively. Oil viscosity was measured and microscopic images of the centrifugated oil samples of asphaltene clusters were obtained. In the second set of experiments, 12 identical crude oil samples with different toluene concentrations of 5, 10, and 15 vol% were subjected to four different durations of ultrasonic irradiation and under different temperatures. In the third group, seven actual carbonate core samples of damaged oil permeability due to the injection of 60 pore volumes of asphaltic crude oil were subjected to different ultrasonic time interval of 5, 10, 15, 20, and 25 min and frequencies of 10, 15 and 20 kHz, respectively. Both the oil permeability of these samples was measured and the scanning electron microscope (SEM) images were carried out before and after the ultrasonic irradiation process. The results showed that subjection of the UAE crude oil to ultrasonic irradiation decreases the size of asphaltene clusters. Consequently, this effect reduces asphaltene tendency to precipitate at 10 min or more time interval of ultrasonic irradiation. In addition, similar results were obtained with solvent effect, but with more reduction in oil viscosity. The results also indicated that the increase of ultrasonic time interval and/or frequency drastically improve(s) damaged oil permeability.     

Effect of acid number of crude oil on oil recovery of smart water coupled with silica nanoparticles

Spontaneous imbibition (SI) is one of the most important mechanisms in reservoir engineering. In order to activate of this mechanism completely in carbonate reservoirs, wettability of these oil wet rocks should be altered to water-wet. Multi-step spontaneous imbibition tests were designed and conducted in this study. Results indicated that mixture of smart water and nano silica could increase oil recovery up to 5 percent. Effect of acid number of crude oil on oil recovery of this mixture was investigated by using three samples of crude oil during SI tests: A (0.38 mg KOH/g), B (0.25 mg KOH/g) and C (0.18 mg KOH/g). Maximum oil recovery of SI tests was recorded for oil sample C which had lowest acid number. Ion analysis of imbibing fluids indicated sharp reductions in concentration of sulfate ion in each step of SI tests which confirmed occurrence of ion exchange mechanism in the rock surface of core samples.     

特低渗透油藏源储配置与富集区优选测井评价方法

鄂尔多斯盆地西北部长8 油层组为典型的特低渗透、超低渗透油藏,储层参数的控制因素与平面分 布规律复杂,已成为油藏评价和开发建产中急需解决的问题。在单井精细解释基础上,通过对含油饱和度 分布规律和控制因素的分析,应用测井多井评价技术对特低渗透油藏开展多学科结合研究,建立了特低 渗透储层含油富集程度和烃源岩有机质丰度测井表征方法,提出烃源岩生烃能力与储层含油富集程度的 有效配置控制了有利富集区的分布。基于源储配置思路,研究了特低渗透油藏的富集规律,并优选出了富 集区。该研究对特低渗透油藏规避快速建产风险,加快勘探开发进程,提高整体效益具有重要意义和推广 价值,对致密油气等非常规领域的研究具有参考价值。     

前置蓄能压裂中的CO2在玛湖凹陷砾岩油藏中的作用

准噶尔盆地玛湖凹陷砾岩储集层致密,渗流能力差,油藏产量递减快,稳产难度大.CO2前置蓄能压裂比水力压裂效果好,但对CO2与玛湖凹陷原油及储集层岩石间的作用规律尚未有系统研究.因此,对CO2水溶液的置换能力及其对岩心矿物的溶蚀、岩心孔隙度和渗透率的改变等进行了研究.对于玛湖凹陷砾岩油藏,CO2水溶液置换原油率高于纯CO2...     

The Geological Characteristics of SEC Analogy Reservoirs in Jilin Oilfield

The study was aimed at carefully describing the geological characteristics of analogy reservoirs. By researching the geological characteristics of 199 SEC analogy reservoirs and summarizing the types of analogy reservoirs in Jilin oilfield, the authors have carefully depicted the geological characteristics of analogy reservoirs such as effective thickness, porosity, permeability, oil saturation, oil density, the viscosity of underground crude oil, and recovery. To sum up, there are eight types of SEC analogy reservoirs in Jilin oilfield and their reservoirs are respectively sandstone reservoir, fractured reservoir, and volcanic reservoir, whose porosity and permeability range from middle to low position, effective thickness is between 2.8 and 36.3 m, oil saturation is distributed from 47% to 65%, and crude oil is mainly about light and intermediate oil belonging to conventional oil.     

大数据分析顺北油田SHB-X井试采产液量骤降原因

研究顺北油田SHB-X井试采过程中产液量急剧下降的原因，可以给整个顺北油田乃至深层碳酸盐岩油藏平稳采油提供工作制度选择依据。室内储层敏感性实验评价了3枚岩心柱塞，发现储层可能存在速敏和盐敏，但伤害程度差异较大，与生产实际差距也比较大，均不是产量骤降的主因。钻井液与储层岩石、地下原油配伍性评价表明，储层伤害程度不足以造成停产。遂利用单因素多元回归法分析SHB-X井所在井区7口井100 d生产数据，无人为干预地确定产液指数、产油指数2项产量参数与原油密度、原油黏度等12项影响因素定量关系。权重系数表明，井深大、采油速度高引起的结蜡、沥青质析出堵塞油管是产液量骤减的主控因素。因此，深层碳酸盐岩油气藏生产控制日产液量是平稳采油的关键。     

孤岛油田孤北1地区稠油油藏特征及储层敏感性分析

孤岛油田孤北1地区馆陶组Ng3-Ng5砂层组原油为重质稠油,原油具有高密度、高粘度、高含硫的特点,且分布规律受断层影响,具有越靠近断层原油更粘稠和上轻下重的特点.造成原油性质差异的主要原因是断层附近地层水的氧化作用和重力分异作用.研究区储层主要为河流相胶结疏松的砂岩储层,储层具有强速敏性和碱敏性,这是造成储层伤害,引起注汽效果不佳,造成采收率不高的主要因素.     

高含硫储层硫沉积微观特性

四川盆地川东北地区相继发现了一大批高含硫碳酸盐岩气藏,其储量占四川盆地天然气储量的一半以上。然而,硫沉积一直是制约高含硫气藏高效开发的一大难题。结合微米CT扫描、扫描电镜、能谱分析和气相色谱分析等实验手段,运用储层高含硫气体完成了碳酸盐岩岩样的硫沉积实验,获得了实验前后岩样微观孔隙结构和气体组分的定量变化,首次实现了储层岩石中硫沉积的三维可视化。硫沉积实验结果显示,高含硫气体中总硫含量减小12.58%,有机硫组分减小9.07%;硫微粒主要沉积在半径小于500μm的岩石孔隙中,导致岩石渗透率降低34.40%～67.80%,孔隙度减小3.77%～7.69%。研究结果进一步提升了对硫沉积微观特性的认识,为高含硫气藏增产措施的制定提供了支撑。     

Experimental Investigation of Contact Angles Under Oil-Microbial Solution on Carbonate Rocks

Contact angles are used to measure the wetting behavior of two immiscible fluids on a solid surface. Fluids are considered wettings if their contact angles with surface are less than 90°, and they are considered nonwetting, if their contact angles are greater than 90°. Because of its influence on other petrophysical properties of reservoir rocks, such as relative permeability, capillary pressure, and the residual oil saturation after a flood, wettability and its direct measure, the contact angle, play a significant role in affecting the recovery from both primary and improved recovery processes. In this work, contact angle alteration occurring in microbial enhanced oil recovery processes (MEOR) are quantified and described, along with a study of the factors that would enhance such contact angle alteration. An experimental method for the measurement of contact angles has been developed in which the contact angle is measured as a function of time. Measurements of contact angle and interfacial tension for four different types of UAE crude oil and four different mineralogical rock composition over a range of microbial concentration, salinity, and temperature are reported. Results showed that contact angles for the studied systems increased with temperature, crude oil sulfur concentration and microbial concentration up to a certain concentration, beyond which the bacteria concentration exhibited no effect on the contact angle. Crude oils containing low asphaltene concentration produced a stable contact angle and oils containing high asphaltene concentration produced surfaces with unstable wettability. The mineralogical composition of limestone rocks had no effect on the contact angle of microbial-oil system.