Numerical study on gas production via a horizontal well from hydrate reservoirs with different slope angles in the South China Sea

It is important to study the effect of hydrate production on the physical and mechanical properties of low-permeability clayey–silty reservoirs for the largescale exploitation of hydrate reservoirs in the South China Sea.In this study,a multiphysical-field coupling model,combined with actual exploration drilling data and the mechanical experimental data of hydrate cores in the laboratory,was established to investigate the physical and mechanical properties of low-permeability reservoirs with different slope angles during 5-year hydrate production by the depressurization method via a horizontal well.The result shows that the permeability of reservoirs severely affects gas production rate,and the maximum gas production amount of a 20-m-long horizontal well can reach186.8 m3/day during the 5-year hydrate production.Reservoirs with smaller slope angles show higher gas production rates.The depressurization propagation and hydrate dissociation mainly develop along the direction parallel to the slope.Besides,the mean effective stress of reservoirs is concentrated in the near-wellbore area with the on-going hydrate production,and gradually decreases with the increase of the slope angle.Different from the effective stress distribution law,the total reservoir settlement amount first decreases and then increases with the increase of the slope angle.The maximum settlement of reservoirs with a 0°slope angle is up to 3.4 m,and the displacement in the near-wellbore area is as high as2.2 m after 5 years of hydrate production.It is concluded that the pore pressure drop region of low-permeability reservoirs in the South China Sea is limited,and various slope angles further lead to differences in effective stress and strain of reservoirs during hydrate production,resulting in severe uneven settlement of reservoirs.

Advances in enhanced oil recovery technologies for low permeability reservoirs

Low permeability oil and gas resources are rich and have great potential all over the world, which has gradually become the main goal of oil and gas development. However, after traditional primary and secondary exploitation, there is still a large amount of remaining oil that has not been recovered.Therefore, in recent years, enhanced oil recovery(EOR) technologies for low permeability reservoirs have been greatly developed to further improve crude oil production. This study presents a comprehensive review of EOR technologies in low permeability reservoirs with an emphasis on gas flooding, surfactant flooding, nanofluid flooding and imbibition EOR technologies. In addition, two kinds of gel systems are introduced for conformance control in low permeability reservoirs with channeling problems. Finally,the technical challenges, directions and outlooks of EOR in low permeability reservoirs are addressed.

Development of the theory and technology for low permeability reservoirs in China

The development theories of low-permeability oil and gas reservoirs are refined, the key development technologies are summarized, and the prospect and technical direction of sustainable development are discussed based on the understanding and research on developed low-permeability oil and gas resources in China. The main achievements include:(1) the theories of low-permeability reservoir seepage, dual-medium seepage, relative homogeneity, etc.(2) the well location optimization technology combining favorable area of reservoir with gas-bearing prediction and combining pre-stack with post-stack;(3) oriented perforating multi-fracture, multistage sand adding, multistage temporary plugging, vertical well multilayer, horizontal and other fracturing techniques to improve productivity of single well;(4) the technology of increasing injection and keeping pressure, such as overall decreasing pressure, local pressurization, shaped charge stamping and plugging removal, fine separate injection, mild advanced water injection and so on;(5) enhanced recovery technology of optimization of injection-production well network in horizontal wells. To continue to develop low-permeability reserves economically and effectively, there are three aspects of work to be done well:(1) depending on technical improvement, continue to innovate new technologies and methods, establish a new mode of low quality reservoir development economically, determine the main technical boundaries and form replacement technology reserves of advanced development;(2) adhering to the management system of low cost technology & low cost, set up a complete set of low-cost dual integration innovation system through continuous innovation in technology and management;(3) striving for national preferential policies.

Porosity Calculation of Tight Sand Gas Reservoirs with GA-CM Hybrid Optimization Log Interpretation Method

Tight sand gas reservoirs are our country’s fairly rich unconventional natural gas resources, and their exploration and development is of prime importance. Sulige Gas Field which located in the northern Ordos Basin is tight sand gas reservoirs. It is typically featured by low porosity and low permeability, and the error of porosity calculation by traditional methods is larger. Multicomponent explanation model is built by analyzing the thin slice data, and the objective function is got according to the concept of optimization log interpretation method. This paper puts the Genetic Algorithm and the Complex Algorithm together to form the GA-CM Hybrid Algorithm for searching the optimal solution of the objective function, getting the porosity of tight sandstone gas reservoirs. The deviation got by this method is lesser compared with the core porosity, with a high reliability.

Formation conditions and accumulation characteristics of Bozhong 19-6 large condensate gas field in offshore Bohai Bay Basin

Based on the study of natural gas resource, low buried hill trap formation mechanism, high quality reservoir control factors and natural gas preservation conditions, the formation conditions and reservoir accumulation characteristics of Bozhong 19-6 large condensate gas field were summarized. Large gas generation potential of multiple sets of thick humic-sapropelic source rocks in high maturity stage in Bozhong depression was the basis of large gas field formation. The multi-stage tectonic evolution since Indosinian period formed large-scale buried hill traps. The Tanlu fault activity formed multi-type reservoirs, and buried hill metamorphic rock of Archean and sand-conglomerate of Kongdian Formation were high-quality reservoirs. Thick overpressure lacustrine mudstone and weak neotectonic movement provided good preservation conditions. Bozhong 19-6 gas reservoir was a condensate gas reservoir with very high condensate oil content, and the gas origin was humic-sapropelic and kerogen-cracking gas, and the gas field had large gas thickness, high gas column characteristics and the accumulation process was first oil and then gas. The buried hill reservoir was a massive reservoir and the Kongdian reservoir was a stratified reservoir. The gas field had multi-channel hydrocarbon intense charge from overpressure source rocks, atmospheric-weak overpressure reservoir favorable for accumulation, thick overpressure mudstone caprock favorable for preservation, and natural gas ultra-late rapid accumulation model.

BZ19-6低渗透储层反凝析污染及解除方法实验研究

BZ19-6凝析气田储量大、地露压差小、储层高温高压且低孔低渗,生产作业过程中极易产生反凝析污染。评价反凝析污染程度,采用合适的污染解除措施,对改善BZ19-6凝析气田反凝析污染具有重要意义。利用复配凝析气开展长岩心衰竭实验,模拟反凝析油污染,测试不同衰竭压力点对应的气相渗透率并评价反凝析污染程度;同时开展了注活性剂(TC281)、注甲醇、注甲醇+活性剂3组解除反凝析污染实验,及注甲醇+活性剂1组解除反凝析+水锁综合液相污染实验。实验结果表明:注活性剂(TC281)、注甲醇、注甲醇+活性剂3种方案对解除反凝析污染均有一定效果,注甲醇+活性剂1组解除反凝析污染效果最好,渗透率恢复率达84%;注甲醇解除反凝析污染渗透率恢复率为81%;注活性剂1组解除反凝析渗透率恢复率为54%;注甲醇+活性剂1组解除反凝析+水锁综合液相污染,渗透率恢复率达到80%。实验为BZ19-6凝析气田解除反凝析污染提供了方案指导。

深层低渗凝析气藏气驱适应性研究

针对深层低渗凝析气藏由衰竭开采转变为气驱开发过程中适应性不明确的问题,综合采用PVT(地层流体高压物性)分析仪、长岩心驱替物理模拟技术和数值模拟计算方法,开展了一系列评价研究。通过对比分析注CO_(2)、天然气(伴生气、CH_(4))、N_(2)对凝析气体系高压物性影响与提高凝析油采出程度情况,明确了深层低渗凝析气藏气驱的适应性。实验结果表明:CO_(2)在凝析油中的溶解度和溶解气油比最大,具有较强降低凝析气藏饱和压力和露点压力的特点,提高凝析油采出程度的效果最佳。同时,采用长岩心物理模拟技术针对CO_(2)驱进行了注入时机、注入方式、注气速度优化研究,明确了在露点压力以上脉冲式注气效果更好,为注气开发技术政策及现场方案的实施提供数据支撑。

Feasibility of Gas Drive in Fang-48 Fault Block Oil Reservoir

The Fang-48 fault block oil reservoir is an extremely low permeability reservoir, and it is difficult to produce such a reservoir by waterflooding. Laboratory analysis of reservoir oil shows that the minimum miscibility pressure for CO2 drive in Fang-48 fault block oil reservoir is 29 MPa, lower than the formation fracture pressure of 34 MPa, so the displacement mechanism is miscible drive. The threshold pressure gradient for gas injection is less than that for waterflooding, and the recovery by gas drive is higher than waterflooding. Furthermore, the threshold pressure gradient for carbon dioxide injection is smaller than that for hydrocarbon gas, and the oil recovery by carbon dioxide drive is higher than that by hydrocarbon gas displacement, so carbon dioxide drive is recommended for the development of the Fang-48 fault block oil reservoir.

Rheological property of low-damage,ideal packing,film-forming amphoteric/sulfonation polymer drilling fluids

Low-permeability dense reservoirs,including micro-fractured reservoirs,are commonly characterized by high content of clay substances,high original water saturation,high sensitivity to invasive fluids,high capillary pressure,complicated structure and anisotropic,high flow-resistance and micro pore throats etc,.Generally they also have lots of natural micro fractures,probably leading to stress sensibility.Main damaging factors in such reservoirs are water-sensibility and water-blocking caused by invasive fluids during drilling and production operations.Once damaged,formation permeability can rarely recovered.Numerous studies have shown that damaging extent of water-blocking ranges from 70% to 90%.Main damaging mechanisms and influencing factors of water-blocking were systematically analyzed.Also some feasible precaution or treating approaches of water-blocking were put forward.In a laboratory setting,a new multi-functional drilling fluid composed mainly of amphion polymer,sulfonation polymer,high effectively preventive water-blocking surfactants,ideal packing temporary bridging agents(TBA) and film-forming agents,etc.,were developed.New low-damage drilling fluids has many advantages,such as good rheological properties,excellent effectiveness of water-blocking prevention,good temporary plugging effect,low filtration and ultra-low permeability(API filtration≤5 mL,HTHP filtration≤10 mL,mud cake frictional coefficient≤0.14,permeability recovery>81%),can efficiently prevent or minimize damage,preserve natural formation and enhance comprehensive development-investment effect in TUHA Jurassic dense sandstone reservoir formation with low-permeability,the only one developing integrated condense gas field.Some references can be provided to similar reservoir formations.

中国海上气田开发与提高采收率技术

中国海上目前在生产气田共计有30个、气井总数为288口,主要分布在莺歌海盆地、琼东南盆地、珠江口盆地、东海陆架盆地和渤海湾盆地。海上气田的勘探成本、工程建造成本、钻完井和生产操作费用均较高,加之开发调整及生产措施实施难度大,因而陆上气田开发采用的相关方法和技术在海上推广应用便受到诸多限制。为了提高中国海上气田开发的效益,有必要归纳和总结我国海上气田在开发技术与提高天然气采收率技术方面的研究成果与实施经验。按照气藏特点,将中国目前的海上气田分为凝析气藏、低渗透气藏、边底水气藏、高温高压含酸气气藏、深水气藏等5种类型;在此基础上,对其地质特征、开发特点及存在的问题分别进行了探讨;进而结合典型案例对各类气藏的配套开发技术与提高天然气采收率技术及实施经验进行了总结;最后对中国海上天然气开发技术的发展方向及潜力给予了展望。

Key Problems and Countermeasures in CO_(2)Flooding and Storage

Based on literature research in combination with the practice of CO_(2)flooding and storage in Jilin Oilfield,this study assesses the key problems in CO_(2)flooding and storage,proposing the corresponding countermeasures from five aspects of CO_(2)gas source condition,namely geological condition evaluation,scheme design incoordination with other production methods,economic and effectiveness evaluation,together with dynamic monitoring and safety evaluation.The results show that CO_(2)flooding is the most economic and effective CO_(2)storage method.In eastern China,inorganic origin CO_(2)gas reservoirs are widely developed and are especially the most enriched in the Paleozoic carbonate rock strata and the Cenozoic Paleogene–Neogene system,which provide a rich resource base for CO_(2)flooding and storage.In the future,CO_(2)generated in the industrial field will become the main gas source of CO_(2)flooding and storage.The evaluation of geological conditions of oil and gas reservoirs is the basis for the potential evaluation,planning scheme design and implementation of CO_(2)flooding and storage.CO_(2)storage should be below the depth of 800 m,the CO_(2)flooding and storage effects in lowpermeability oil reservoirs being the best.CO_(2)geological storage mechanisms primarily consist of tectonic geological storage,bound gas storage,dissolution storage,mineralization storage,hydrodynamic storage and coalbed adsorption storage.The practice of CO_(2)flooding and storage in Jilin Oilfield demonstratesthat the oil increment by CO_(2)flooding is at least 24%higher than by conventional water flooding.The most critical factor determining the success or failure of CO_(2)flooding and storage is economic effectiveness,which needs to be explored from two aspects:the method and technology innovation along with the carbon peaking and carbon neutrality policy support.After CO_(2)is injected into the reservoir,it will react with the reservoir and fluid,the problem of CO_(2)recovery or overflow will occur,so the dynamic monitoring and safety evaluation of CO_(2)flooding and storage are very important.This study is of great significance to the expansion of the application scope of CO_(2)flooding and storage and future scientific planning and deployment.

Geologic characteristics, controlling factors and hydrocarbon accumulation mechanisms of China’s Large Gas Provinces of low porosity and permeability

Based on the analysis of the geological characteristics and controlling factors, we analyzed the formation mechanism of different types of gas reservoirs. The main characteristics of gas provinces with low porosity and permeability are mainly as follows: large area, low abundance, small gas pools and large gas provinces; widely distributed excellent hydrocarbon source rocks with closely contacted source-reservoir-cap association; development mainly in large continental depressions or in paralic shallow-river delta systems; many kinds of traps coexisting in large areas, dominantly para-layered lithologic, digenetic and capillary pressure traps; double fluid flow mechanisms of Darcy flow and non-Darcy flow; complicated gas and water relations; and having the resource distribution of highly productive "sweet spots", banding concentration, and macroscopically large areas integrated. The main controlling factors of large sandstone gas provinces with low porosity and permeability are stable dynamic backgrounds and gentle structural frameworks which control the extensive distribution of alternate (interbedded) sandstones and mudstones; weak hydropower of large gentle lake basins controlling the formation of discontinuous, low porosity and permeability reservoirs in shallow-water deltas; regionally differential diagenesis and no homogeneous digenetic facies controlling the development of favorable reservoirs and digenetic traps; and weak and dispersive reservoir-forming dynamic forces leading to the widely distributed small traps with low abundance. Low porosity and permeability gas provinces with different trap types have different formation mechanisms which include fluid diversion pressure difference interactive mechanism of lithologic-trap gas accumulations, separated differential collection mechanism of digenetic-trap gas accumulations, and the Non-Darcy flow mechanism of capillary-pressure gas accumulations.

Productivity model for gas reservoirs with open-hole multi-fracturing horizontal wells and optimization of hydraulic fracture parameters

Multi-fractured horizontal wells are commonly employed to improve the productivity of low and ultra-low permeability gas reservoirs.However,conventional productivity models for open-hole multi-fractured horizontal wells do not consider the interferences between hydraulic fractures and the open-hole segments,resulting in significant errors in calculation results.In this article,a novel productivity prediction model for gas reservoirs with open-hole multi-fractured horizontal wells was proposed based on complex potential theories,potential superimposition,and numerical analysis.Herein,an open-hole segment between two adjacent fractures was regarded as an equivalent fracture,which was discretized as in cases of artificial fractures.The proposed model was then applied to investigate the effects of various parameters,such as the angle between the fracture and horizontal shaft,fracture quantity,fracture length,diversion capacity of fractures,horizontal well length,and inter-fracture distance,on the productivity of low permeability gas reservoirs with multi-fractured horizontal wells.Simulation results revealed that the quantity,length,and distribution of fractures had significant effects on the productivity of low permeability gas reservoirs while the effects of the diversion capacity of fractures and the angle between the fracture and horizontal shaft were negligible.Additionally,a U-shaped distribution of fracture lengths was preferential as the quantity of fractures at shaft ends was twice that in the middle area.

对低渗气藏渗流机理实验研究的新认识

低渗致密气藏储量大但开发困难,注重开发过程中的机理研究则有利于合理地制定技术政策。为此,重点讨论了低渗气藏的机理问题:①启动压力是否存在,如何测试?其对开发的影响如何;②应力敏感是否存在,应当如何测试才更有代表性;③低渗致密气藏束缚水饱和度较高,而原始含水饱和度低,但在开发中仍会产出地层水的原因是什么;④凝析水析出会不会对地层造成污染;⑤低渗致密气藏衰竭过程中凝析油污染情况如何?所探讨的问题对我国低渗致密气藏的开发有着指导意义。

低渗透凝析气藏储层损害特征及钻井液保护技术

低渗透凝析气藏储层孔喉细小,渗透性差,黏土含量高,非均质性严重,因而容易因各种入井工作液侵入储层造成水敏、水锁等损害,并经常发生其特有的反凝析损害。结合吐哈油田巴喀低渗透凝析气藏,系统分析了低渗透凝析气藏的储层损害特征。室内评价实验表明,水锁是该储层的主要损害因素之一,渗透率损害率为65.23%-91.01%。在该区原用聚磺钻井液基础上,优选可有效降低油/水界面张力的表面活性剂,应用成膜技术与理想充填＂协同增效＂的储层保护新方法,优化设计出保护储层低损害钻井液;通过开展配伍性、动态污染等实验对该钻井液的储层保护特性进行了评价。室内实验结果表明,该钻井液具有良好的流变性与配伍性,可使储层岩心的渗透率恢复值从63.2%增至84.7%,具有明显的储层保护效果。

低渗致密气藏、凝析气藏开发难点与对策

我国低渗、致密气藏和凝析气藏的储量占相当大的比例,而其中相当部分处于低产状态,开发好这类气藏对石油工业持续稳定发展具有十分重要的意义。围绕深层低渗致密气藏、凝析气藏开发问题,首先分析了低渗致密气藏的地质特征和开发特征,提出了低渗致密气藏开发的十项配套工艺技术,最后重点建议了五项技术措施,即深度压裂改造技术,凝析气井井筒和近井地带积液的处理技术、开发后期低于最大凝析压力条件下的注气技术,低渗致密凝析气藏多孔介质油气体系相态分析技术和某些适合于低渗致密气藏和凝析气藏的气藏工程分析技术。

气藏提高采收率技术及其对策

虽然2000年以来我国的天然气产量排在世界前列,但目前我国的天然气产量却远远不能满足国民经济发展的需要,越来越多的气田已进入开发中后期,且绝大部分气藏属低渗透和水驱气藏,采收率低,如何提高气藏采收率已成为当前亟待解决的问题。关于油藏提高采收率的定义、剩余油描述方法与提高原油采收率(EOR)配套技术已较为成熟,但对气藏提高采收率(EGR)的定义与描述还未建立起来,对气藏剩余气分布规律的描述方法与EGR技术还不清楚。为此,在调研大量文献的基础上,对EGR进行了定义,提出按剩余气丰度的方法来进行剩余气分布的描述,分析了国内外已开发的3种主要类型气藏(低渗透气藏、凝析气藏、边底水气藏)的地质开发特征以及开发过程中遇到的问题,总结分析了提高采收率的相关技术与方法,并对EGR技术的发展提出了建议。该成果对气藏提高采收率有重要的借鉴和推广意义。

低渗致密砂岩凝析气藏液锁损害机理及防治——以吐哈油田丘东气藏为例

吐哈油田丘东低渗凝析气藏钻采过程中存在不同形式的液锁损害,系统分析了其损害机理和主要影响因素。液锁损害程度主要与岩心的渗透率、孔隙度、初始含水饱和度及油水界面张力有关,还与储集层岩性、胶结物类型及含量、孔隙结构特征和侵入流体的性质有关。针对钻/完井、生产、储集层改造及修井过程中液锁损害机理及特征,研发出防液锁低损害成膜钻井液及保压、增能、助排等综合防治技术,改进的钻井液可使受损害储集层岩心的渗透率恢复值由平均54%提高到81%,研发的助排剂可明显提高残酸的返排效果(助排率达88.9%),改进的修井液可使岩心渗透率恢复值提高16.1%,各种改进工艺在现场应用后,较好地保护了产层,提高了低渗透气藏的综合开发效益。

低渗凝析气藏凝析油临界流动饱和度实验研究

凝析油临界流动饱和度是近年研究的热点问题,但目前国内外还没有采用真实岩样和流体研究低渗凝析气藏凝析油临界流动饱和度的成果报道。文章介绍了采用超声波测试技术研究低渗凝析气藏凝析油临界流动饱和度的方法,选取Q75井的真实岩样和流体,通过长岩心衰竭实验测试了凝析油临界流动饱和度。将测试结果与国内外其它中高渗凝析气藏的凝析油临界流动饱和度进行了对比研究,分析了低渗凝析气藏凝析油饱和度较低的原因。研究结果表明:①超声波测试技术对研究深层高温高压低渗凝析气藏的凝析油临界流动饱和度具有较好的适应性;②桥口低渗凝析气藏的凝析油临界流动饱和度要比国内外其它中高渗凝析气藏的低得多;③较高的束缚水饱和度是导致低渗凝析气藏凝析油临界流动饱和度较低的主要原因,而高温高压低渗凝析气藏更低的油气界面张力也是重要的影响因素。此外,实验所用岩心的长度对凝析油临界流动饱和度的大小也会产生一定的影响。

白庙气田压裂改造技术研究与应用

针对白庙气田渗透率低、储层埋藏深、温度高、压裂难度大等特点,通过优选压裂液配方、压前压后处理及快速排液等措施,提高了白庙气田压裂的有效率。现场应用9口井,投入产出比为1:6,取得了较好的经济效益,对类似气田的压裂改造具有借鉴作用。