AN EXPERIMENTAL APPROACH OF ELEMENTAL SULFUR DEPOSITION IN CARBONATE OIL RESERVOIRS

The existence of sulfur compounds in crude oils creates many problems of sulfur deposition in the vicinity of the wellbore hole, in well completion and/or production equipment, and in producing reservoir rocks. The major objectives of this experimental study are to investigate the influences of oil flow rate, initial sulfur concentration of crude oil, and reservoir rock permeability on elemental sulfur plugging in carbonate oil reservoirs. To achieve these objectives, actual crude oils were de-asphaltened to eliminate the effect of asphaltene deposition. Ten dynamic flow experiments were conducted using two actual crude oils of 0.78 and 1.67% sulfur concentrations. Viscosity of crude oils of different sulfur concentrations was measured under different conditions of temperature. The crude oils were flooded through actual carbonate cores of different permeability in the range of 2.34-28.16 millidarcy and under different flow rates of 0.5, 1.0, 1.5, and 2.0 cc/min. In-situ sulfur deposited was measured using Scanning Electron Microscopy (SEM) to provide the amount of sulfur deposited along the core samples. The results indicated that crude oil of higher sulfur concentration has higher viscosity than that of one of lower concentration. The deposition of elemental sulfur does not take place at the low rate of 0.50 cc/min, starts at 1.0 cc/min and increases as the flow rate increases up to 1.50 and 2.00 cc/min, respectively. In addition, the higher sulfur concentration of the crude oil increases the deposition of sulfur in carbonate oil reservoirs. The results also showed that permeability of carbonate reservoir rocks has a severe effect on sulfur deposition since carbonate rocks of higher permeability do not experience the problem of elemental sulfur deposition while the problem is more severe for lower permeability rocks. In addition, the depositional rate is accelerated rapidly as the rock permeability decreases. The obtained results of this study have important interest in identification of the most important factors affecting the elemental sulfur precipitation in heterogeneous carbonate oil reservoirs and robust implications in the development of reservoir simulation models.     

鄂尔多斯盆地长7段致密油成藏物性下限研究

鄂尔多斯盆地延长组长7段致密砂岩储层在湖盆中心大面积分布,成藏期的储层物性下限是决定油气是否充注储层的重要参数。运用恒速压汞和纳米CT扫描技术分析了长7段湖盆中心渗透率小于0.3×10-3 μm2、孔隙度小于12%的致密砂岩储层的物性及微观孔喉特征。结果表明,其平均孔隙半径为160μm,喉道半径不超过0.55μm,均值为0.33μm。在分析致密油成藏期储源压差、原油物理性质及盆地流体特征的基础上,结合致密储层油气驱替模拟实验及最小流动孔喉半径法,综合确定了研究区长7段致密油成藏期油气开始充注时的孔喉下限为14 nm,孔隙度下限为4.2%,渗透率下限为0.02×10-3 μm2,要达到含油饱和度超过40%而实现致密油的大面积连续分布,孔喉半径下限应为0.12μm,孔隙度下限为7.3%,渗透率下限值为0.07×10-3μm2。      

致密油储层微观特征及其形成机理——以鄂尔多斯盆地长6-长7段为例

鄂尔多斯盆地陇东地区长（延长组）6段和长7段富含大量致密油,但由于对其微观特征与成因缺乏深入了解,阻碍了该区致密油的有效开采。为此,根据岩石孔隙铸体薄片、场发射扫描电镜等技术,对研究区长6段和长7段致密油储层微观特征及其成因进行了深入研究。结果表明：研究区致密油储层形成于三角洲前缘远端远砂坝-席状砂及半深湖-深湖重力流沉积环境,岩石粒度细（主要为极细砂岩、粉砂岩、泥质粉砂岩及粉砂质泥岩）、杂基含量高（8%~10%）,几种储集岩的孔隙均极不发育,面孔率低,平均1.8%,孔径小（平均30 μm）,喉道细（平均0.08 μm）,平均孔隙度9%,渗透率基本上都低于0.3×10^-3 μm²,物性差。孔隙类型主要为粒间杂基微孔、长石及岩屑溶孔、胶结物晶间微孔。不同岩石类型其微观特征存在差异。沉积环境决定了其粒度细、粘土杂基高,细粒高粘土杂基岩石抗压性差,强烈的压实作用导致大量的原生孔隙损失,孔喉变得更加细小;孔喉细小的岩石由于孔隙中各种流体离子的半渗透膜效应引起强烈的碳酸盐和粘土矿物胶结,尤其是伊利石搭桥状和丝网状胶结,使岩石孔隙度渗透率进一步变差,后期酸性流体也难以进入发生溶蚀作用;云母与水云母杂基及碳酸盐胶结物对石英的强烈交代导致岩石抗压性变差以及固体体积增加,最终导致岩石的致密化。     

柴达木盆地西部地区古近系湖相碳酸盐岩储层特征

柴达木盆地西部地区古近系下干柴沟组上段(E₃²)至上油砂山组(N₂²)广泛发育湖相碳酸盐岩。为研究其分布及储层发育特征,开展岩心观察、薄片鉴定、扫描电镜及X射线衍射分析等工作。结果表明：(1)研究区湖相碳酸盐岩具有岩石类型多、相变快、陆源碎屑发育、普遍白云石化等特点;主要岩石类型有块状碳酸盐岩、纹层状碳酸盐岩、藻灰岩、颗粒碳酸盐岩,其分布与沉积微相及古环境密切相关。(2)研究区块状碳酸盐岩储集空间以晶间孔为主,局部发育裂缝,纹层状碳酸盐岩顺层微裂缝发育,藻灰岩储集空间以藻格架孔为主,非均质性强,颗粒碳酸盐岩受胶结作用影响显著。(3)块状碳酸盐岩平均孔隙度为10.6%,平均基质渗透率为0.03 mD,排驱压力为10.0 MPa,平均孔喉半径为0.038μm;纹层状碳酸盐岩平均孔隙度为7.4%,平均渗透率为0.76 mD,排驱压力为11.5 MPa,平均孔喉半径为0.071μm;藻灰岩平均孔隙度为10.1%,平均渗透率为9.09 mD,排驱压力为3.2 MPa,平均孔喉半径为0.117μm;颗粒碳酸盐岩...     

PETROPHYSICAL CHARACTERISTICS OF SOURCE AND RESERVOIR ROCKS IN THE HISTRIA BASIN, WESTERN BLACK SEA

The petroleum system in the Histria Basin, Western Black Sea, includes Oligocene source rocks and Upper Cretaceous — Eocene reservoir rocks. Here we report on the petrophysical characteristics of these source and reservoir rocks using mercury intrusion porosimetry data from 14 core samples collected from five wells drilled on the East Lebada, West Lebada and Pescarus structures. Samples were in general dominated by carbonate lithologies with minor shales. Petrophysical parameters analyzed were: median pore-throat radius, average pore-throat radius, apparent porosity, pore-throat size distribution, pore-throat type, pore-throat sorting, maximum threshold entry radius, pore-throat radius at 35% mercury saturation (R35), and air permeability. Reservoir rock quality was estimated using a permeability / porosity / pore-throat type plot.
The Oligocene samples showed little petrophysical variation. Samples were relatively homogenous and had the same pore-throat type (nano), were well sorted, had unimodal pore-throat distribution (suggesting the existence of a single fluid phase), had similar values for median and average pore-throat radius, and similar values for R35 and maximum threshold entry radius. Upper Cretaceous – Eocene samples were more heterogeneous in terms of petrophysical properties, and reservoir quality was in general higher than in the Oligocene interval. Average porosity and calculated air-permeability values were 18.4% and 0.37 mD, respectively for Upper Cretaceous samples; and 11.8% and 27.11 mD, respectively for Eocene samples. A case study of Oligocene and Cretaceous – Eocene samples from well West Lebada 817 is presented.
This paper represents the first petrophysical study of source and reservoir rocks in the Histria Basin, Western Black Sea. The results will help to establish the links between petrophysical characteristics, age and depositional environment for source and reservoir rocks in other basins bordering the Black Sea.     

用分形几何法预测毛管压力曲线

利用分形几何学的原理和方法,实现了利用常规岩心分析资料如孔隙度和渗透率等反演毛管压力曲线(建立孔喉体积分布预测模型),通过实际资料验证,该方法预测结果精度较高。而毛管压力曲线中包含了大量储集层信息,有一系列的地质应用,包括:计算储集层的相对渗透率;计算任意区间渗透率的贡献值;分析评价储集层的非均质性;确定流动孔喉半径下限值;划分储集层与非储集层并对储集层进行定量评价;对油层流动空间的有效性定量评价;计算储集层原始含油饱和度等,为非取心井段的孔隙结构研究及储集层研究与评价开辟了新途径。     

An Experimental Investigation of Permeability Impairment Under Dynamic Flow Conditions Due to Natural Depletion in an Iranian Oilfield

Asphaltene deposition is an issue that has received much attention since it has been shown to be the cause of major production problems. It leads to permeability reduction under the processes of natural depletion as well as hydrocarbon gas/CO₂ injection. Though a great deal of researches have focused on studying permeability impairment in reservoir rocks, little is known about the asphaltene deposition mechanisms that control the permeability reduction for Iranian reservoirs. In this work, an experimental effort is made to investigate the permeability impairment of core samples of Iranian oil reservoirs. The experiments are performed on both sandstone and carbonate rock types at reservoir temperature and pressure. The mass balance was used for evaluating of porosity reduction during the experiments. The results indicate that the dominant deposition mechanism changes as production proceeds. In addition, it has been found that the primary mechanism in permeability impairment is surface deposition. On the other hand, entrainment of asphaltene particles is manifested when outlet pressure drops from 4,200 to 3,800 Psig for both sandstone and carbonate samples. It can be drawn that asphaltene entrainment dependence to pressure is much more than that to the injected pore volume. This research illuminates the deposition mechanisms and determines dynamic parameters of asphaltene deposition, which are necessary to devise reliable prevention strategies.     

Dynamic Pore Scale Modeling of Asphaltene Deposition in Porous Media

Asphaltene deposition in porous medium is one of important factors in reducing the productivity of oil reservoirs. Reduction of permeability is the main factor, which is also due to reduced pores size or complete closure of them. The authors simulate phase one flow of oil in porous medium using a dynamic pore scale network model. Also asphaltene deposition process is considered based on a scaling equation. Because the greatest amount of precipitation occurs at bubble point pressure condition, we considered boundary conditions of the model in this pressure. The hypothetical model is only a very small element of a real reservoir rock therefore we assumed constant temperature in this process, consequently the main reason of asphaltene precipitation is pressure changes in the pores. Permeability reduction simulated was based on these steps: pore and throat pressure changes were due to fluid flow through the network and asphaltene deposited according to scaling equation. Applying a material balance for each pore/throat gives the volume reduction of pores/throats according to the deposited asphaltene. Due to this change in pores size permeability and porosity of the model is calculated. Repeating these steps over the time gives effect of asphaltene deposition on the primary properties of porous medium.     

鄂尔多斯盆地白豹—华池地区长6砂岩储层特征

根据铸体薄片、扫描电镜、碳氧同位素、物性、孔隙结构和声发射、岩心和测井分析和化验资料,对鄂尔多斯盆地白豹—华池地区长6砂岩储层特征进行的研究结果表明,该套储层储集空间以剩余原生粒间孔和次生粒间孔组合为主;储层物性较差,微裂缝对改善储层孔隙渗透性贡献不大;排驱压力和饱和度中值压力较高,孔喉半径小且分选性较差,歪度细,溶蚀作用对改善喉道连通性的效果不佳,孔隙结构参数总体较差。储层质量受沉积微相和成岩作用的综合影响,其中,由三角洲前缘滑塌形成的内-中扇较粗粒水道砂体为最有利储层发育的微相,成岩早期碳酸盐矿物和环边绿泥石胶结作用有利于部分原生孔隙和喉道的保存。利用储层物性和孔隙结构参数,将长6砂岩评价为4类储层,其中Ⅰ、Ⅱ类储层在目前的工艺技术条件下为可动用储层,是增储上产的首选储层。     

国外含油气盆地硅质岩储集层主要类型及勘探开发特点

结合近年来国外硅质岩油气藏勘探、开发实际情况,简要介绍硅质岩的成因、组成和分类,进而讨论硅质岩油气藏的圈闭分类、储集特性、岩石物理特征及钻井、完井和生产过程中所遇到的问题.硅质岩是指由生物作用、化学作用、生物化学作用和火山作用形成的富含SiO2的岩石,岩性致密、坚硬、易碎.硅质岩油气藏圈闭可分为裂缝型、古风化壳型、热液...     

柴达木盆地西部地区下干柴沟组湖相白云岩晶间孔型储层物性下限的确定

物性下限是储层有效性评价及资源量计算的重要参数.以柴达木盆地英西地区古近系致密湖相碳酸盐岩为研究对象,在高压压汞测试及场发射扫描电镜分析确定纳米级白云石晶间孔孔喉分布范围的基础上,提出了一种确定储层物性下限的实验方法,该方法的核心是通过核磁共振多次离心测试来达到分离可动流体与束缚流体的目的,将可动流体与束缚流体分离点对...     

致密油储层岩石孔喉比与渗透率、孔隙度的关系

孔喉比是致密油储层岩石最重要的微观物性之一,对储层的剩余油分布与驱替压力影响很大。利用复合毛细管模型,考虑储层岩石的孔喉比、配位数、孔隙半径和喉道半径等孔隙结构参数,建立了致密油储层岩石的微观物性与宏观物性孔隙度、渗透率之间的理论关系式。并用44组板桥地区长6油层组致密油储层岩心的恒速压汞实验数据进行拟合。结果表明:致密油储层岩石孔隙度φ主要受孔隙半径影响,喉道半径控制岩石的渗透率k,孔喉比与φ0.5/k0.25间具有确定的函数关系。利用2组渗透率接近、孔隙度差异较大的岩心驱油实验,证实φ0.5/k0.25值大的致密砂岩,水驱油阻力大。     

早期碳酸盐胶结作用对恩平凹陷珠江组砂岩孔隙演化模式的影响

珠江口盆地恩平凹陷的碎屑岩储层中存在早期碳酸盐胶结,为分析胶结作用对储层孔隙演化的影响,以便为储层预测提供依据,采用普通薄片、铸体薄片、扫描电镜、压汞等分析方法,结合岩石物性研究和测井解释结果,分析了该区古近纪珠江组早期碳酸盐胶结砂岩的孔喉特征,发现基底式胶结的钙质砂岩,表现为“悬浮砂”结构,主要为低孔隙度、低渗透率储层;而孔隙式胶结的钙质砂岩,其渗透率一般超过100 mD,平均孔喉半径大于15 μm,为高孔隙度、高渗透率储层。在分析珠江组主要成岩作用类型的基础上,探讨了早期碳酸盐胶结作用对储层孔隙演化模式的影响,发现由于碳酸盐含量的不同,其影响具有双重性：①基底式胶结的钙质砂岩一般坚硬致密,难以被溶蚀改造,孔喉不发育;②孔隙式胶结利于原生孔隙保存和次生孔隙形成,其胶结的钙质砂岩在后期酸性流体溶蚀作用下可以产生特大溶孔,形成高孔、高渗储层。结论认为：该区早期碳酸盐胶结的海相砂岩,抗压实能力较强,在酸性流体作用下具备形成优质储层的潜能。     

An Experimental Investigation of Asphaltene Precipitation During Natural Production of Heavy and Light Oil Reservoirs: The Role of Pressure and Temperature

Abstract

Many oil reservoirs encounter asphaltene precipitation as a major problem during natural production. In spite of numerous experimental studies, the effect of temperature on asphaltene precipitation during pressure depletion at reservoir conditions is still obscure in the literature. To study their asphaltene precipitation behavior at different temperatures, two Iranian light and heavy live oil samples were selected. First, different screening criteria were applied to evaluate asphaltene instability of the selected reservoirs using pressure, volume, and temperature data. Then, a high pressure, high temperature filtration (HPHT) setup was designed to investigate the asphaltene precipitation behavior of the crude samples throughout the pressure depletion process. The performed HPHT tests at different temperature levels provided valuable data and illuminated the role of temperature on precipitation. In the final stage, the obtained data were fed into a commercial simulator for modeling and predicting purposes of asphaltene precipitation at different conditions. The results of the instability analysis illustrated precipitation possibilities for both reservoirs which are in agreement with the oil field observations. It is observed from experimental results that by increasing the temperature, the amount of precipitated asphaltene in light oil will increase, although it decreases precipitation for the heavy crude. The role of temperature is shown to be more significant for the light crude and more illuminated at lower pressures for both crude oils. The results of thermodynamic modeling proved reliable applicability of the software for predicting asphaltene precipitation under pressure depletion conditions. This study attempts to reveal the complicated role of temperature changes on asphaltene precipitation behavior for different reservoir crudes during natural production.     

鄂尔多斯盆地英旺油田长8 储层微观孔隙结构特征研究

针对鄂尔多斯盆地英旺油田长8 储层非均质性强、微观孔隙结构复杂及产量低等问题,利用扫描电 镜、铸体薄片、高压压汞和油水两相渗流实验等对长8 储层的岩石学特征、物性特征、微观孔隙结构及对 渗流能力和驱油效率的影响作了详细研究。结果表明：该区的孔隙结构可分为3 类,即粒间孔-溶 孔、溶孔-粒间孔和微孔-溶孔类,其中溶孔-粒间孔类为该区最好的储层,应重点开发;填隙物的类型和相 对含量、岩心孔隙度和平均孔喉半径是影响低渗透储层渗流能力的主要因素。     

Influence of pressure and CO2 content on the asphaltene precipitation and oil recovery during CO2 flooding

During CO₂ flooding, the crude oil is treated with CO₂, and meanwhile it is displaced by CO₂. Based on the two processes, the influence of pressure and CO₂ content on the asphaltene precipitation and oil recovery efficiency are systematically investigated by indoor simulation experiment. With the increase of the pressure or CO₂ content during CO₂ treatment, the amount of asphaltene precipitation can be increased to a certain value. Correspondingly, the degrees of the changes of oil-water interface, the compositions of crude oil, and reservoir permeability are positively correlated with the amount of asphaltene precipitation. However, during the process, the oil recovery has an optimal value due to the combined action of asphaltene precipitation and the improvement of flow performance of the crude oil. These conclusions can provide a basis for high efficiency development of low permeability oil reservoirs by CO₂ flooding.     

基于三水模型的储层分类方法评价低孔隙度低渗透率储层

低孔隙度低渗透率储层由于孔隙类型多样,结构复杂,往往孔隙度变化范围不大,而渗透率变化范围却很大.测井评价这类储层一直是难点.根据毛细管压力曲线形态,将研究区的储层分为4类,依据孔隙半径大小,计算出对应各类储层的自由孔隙度、微孔隙度和黏土孔隙度,把这3种孔隙度按各自占总孔隙的百分比投到孔隙分类三角图上,得到各类储层在三角图上的位置,根据岩心物性分析资料,分别建立孔渗关系.根据三水模型计算出各种孔隙度的大小,投到分类三角图上,得到各井连续的储层分类,选择相应类别的渗透率公式对储层进行综合评价.该方法在研究区应用中取得了满意的效果.     

羌塘盆地侏罗系碳酸盐岩储集层特征

在野外露头资料研究的基础上,运用岩石薄片、铸体薄片、扫描电镜、常规物性、毛细管压力曲线等手段,对羌塘盆地侏罗系碳酸盐岩储集层的岩石学、孔渗性及孔隙结构等特征作了详细研究。岩石类型主要为颗粒灰岩,包括内碎屑灰岩、鲕粒灰岩和生屑灰岩。储集空间包括孔隙和裂缝两种,孔隙主要为次生溶孔,裂缝为构造缝和溶蚀缝。孔隙的微观结构较差,基本上属于低孔低渗致密储集层。强烈的机械压实作用、胶结作用是导致储集层孔隙结构变差的主要因素,后期的溶解作用对储集层物性有一定的改善。构造作用使得岩石中裂隙发育,提高了岩石孔渗性,极大地改善了岩石的储集性能。     

CO2 miscible flooding in low permeability sandstone reservoirs and its influence on crude oil properties

Miscible CO₂ injection process has become widely used technique for the enhanced oil recovery in low permeability reservoirs. Core flooding experiments and field test of CO₂ miscible flooding in low permeability sandstone reservoirs and its influence on crude oil properties was studied. The results showed that CO₂ miscible flooding in low permeability sandstone reservoirs can enhance oil recovery both in laboratory study and field test. The permeability of sandstone reservoirs decreased during CO₂ miscible flooding due to the precipitation of asphaltene of crude oil. The precipitation of asphaltene lead to a reduction of asphaltene content and the apparent viscosity of crude oil. A further study on inhibitors and removers for asphaltene deposits from crude oil should be investigated to prevent and remove asphaltene deposits in low permeability sandstone reservoirs.     

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.