Refracturing candidate selection for MFHWs in tight oil and gas reservoirs using hybrid method with data analysis techniques and fuzzy clustering

The selection of refracturing candidate is one of the most important jobs faced by oilfield engineers. However, due to the complicated multi-parameter relationships and their comprehensive influence, the selection of refracturing candidate is often very difficult. In this paper, a novel approach combining data analysis techniques and fuzzy clustering was proposed to select refracturing candidate. First, the analysis techniques were used to quantitatively calculate the weight coefficient and determine the key factors. Then, the idealized refracturing well was established by considering the main factors. Fuzzy clustering was applied to evaluate refracturing potential. Finally, reservoirs numerical simulation was used to further evaluate reservoirs energy and material basis of the optimum refracturing candidates. The hybrid method has been successfully applied to a tight oil reservoir in China. The average steady production was 15.8 t/d after refracturing treatment, increasing significantly compared with previous status. The research results can guide the development of tight oil and gas reservoirs effectively.

Geological characteristics of unconventional tight oil reservoir (10^(9) t): A case study of Upper Cretaceous Qingshankou Formation, northern Songliao Basin, NE China

The Daqing exploration area in the northern Songliao Basin has great potential for unconventional oil and gas resources,among which the total resources of tight oil alone exceed 109 t and is regarded as an important resource base of Daqing oilfield.After years of exploration in the Qijia area,Songliao Basin,NE China,tight oil has been found in the Upper Cretaceous Qingshankou Formation.To work out tight oil’s geological characteristics,taking tight oil in Gaotaizi oil layers of the Upper Cretaceous Qingshankou Formation in northern Songliao Basin as an example,this paper systematically analyzed the geological characteristics of unconventional tight oil in Gao3 and Gao4 layers of the Qijia area,based on the data of the geological survey,well drilling journey,well logging,and test.It is that three sets of hydrocarbon source rocks(K2qn1,K2qn2+3,and K2n1)develop in the examined area,and exhibit excellent type I and II kerogens,high organic matter abundance,and moderate maturity.The reservoir is generally composed of thin-bedded mudstone,siltstone,and sandstone,and presents poor porosity(average 8.5 vol.%)and air permeability(average 4 mD).The main reservoir space primarily includes intergranular pores,secondary soluble pores,and intergranular soluble pores.Three types of orifice throats were identified,namely fine throat,extra-fine throat,and micro-fine throat.The siltstone is generally oil-bearing,the reservoirs with slime and calcium become worse oil-bearing,and the mudstone has no obvious oil-bearing characteristics.The brittleness indices of the sandstone in the tight oil reservoir range from 40%to 60%,and those of the mudstone range from 40%to 45%,indicating a better brittleness of the tight oil reservoir.Based on the study of typical core hole data,this paper gives a comprehensive evaluation of the properties of the tight oil and establishes a tight oil single well composite bar chart as well as the initial evaluation system with the core of properties in the tight oil reservoir.This study has theoretical guiding significance and practical application value for tight oil exploration and evaluation in the Qijia area.

Development characteristics and orientation of tight oil and gas in China

Through reviewing the development history of tight oil and gas in China,summarizing theoretical understandings in exploration and development,and comparing the geological conditions and development technologies objectively in China and the United States,we clarified the progress and stage of tight oil and gas exploration and development in China,and envisaged the future development orientation of theory and technology,process methods and development policy.In nearly a decade,relying on the exploration and development practice,science and technology research and management innovation,huge breakthroughs have been made.The laws of formation,distribution and accumulation of tight oil and gas have been researched,the development theories such as"multi-stage pressure drop"and"man-made reservoirs"have been established,and several technology series have been innovated and integrated.These technology series include enrichment regions selection,well pattern deployment,single well production and recovery factor enhancement,and low cost development.As a result,both of reserves and production of tight oil and gas increase rapidly.However,limited by the sedimentary environment and tectonic background,compared with North America,China’s tight oil and gas reservoirs are worse in continuity,more difficult to develop and poorer in economic efficiency.Moreover,there are still some gaps in reservoir identification accuracy and stimulating technology between China and North America.In the future,Chinese oil and gas companies should further improve the resource evaluation method,tackle key technologies such as high-precision 3D seismic interpretation,man-made reservoir,and intelligent engineering,innovate theories and technologies to enhance single well production and recovery rate,and actively endeavor to get the finance and tax subsidy on tight oil and gas.

Sedimentology and Ichnology of Upper Montney Formation Tight Gas Reservoir, Northeastern British Columbia, Western Canada Sedimentary Basin

Several decades of conventional oil and gas production in Western Canada Sedimentary Basin (WCSB) have resulted in maturity of the basin, and attention is shifting to alternative hydrocarbon reservoir system, such as tight gas reservoir of the Montney Formation, which consists of siltstone with subordinate interlaminated very fine-grained sandstone. The Montney Formation resource play is one of Canada’s prime unconventional hydrocarbon reservoir, with reserve estimate in British Columbia (Natural Gas reserve = 271 TCF), Liquefied Natural Gas (LNG = 12,647 million barrels), and oil reserve (29 million barrels). Based on sedimentological and ichnological criteria, five lithofacies associations were identified in the study interval: Lithofacies F-1 (organic rich, wavy to parallel laminated, black colored siltstone);Lithofacies F-2 (very fine-grained sandstone interbedded with siltstone);Lithofacies F-3A (bioturbated silty-sandstone attributed to the Skolithos ichnofacies);Lithofacies F-3B (bioturbated siltstone attributed to Cruziana ichnofacies);Lithofacies F-4 (dolomitic, very fine-grained sandstone);and Lithofacies F-5 (massive siltstone). The depositional environments interpreted for the Montney Formation in the study area are lower shoreface through proximal offshore to distal offshore settings. Rock-Eval data (hydrogen Index and Oxygen Index) shows that Montney sediments contains mostly gas prone Type III/IV with subordinate Type II kerogen, TOC ranges from 0.39 - 3.54 wt% with a rare spike of 10.9 wt% TOC along the Montney/Doig boundary. Vitrinite reflectance data and Tmax show that thermal maturity of the Montney Formation is in the realm of “peak gas” generation window. Despite the economic significance of the Montney unconventional “resource-play”, however, the location and predictability of the best reservoir interval remain conjectural in part because the lithologic variability of the optimum reservoir lithologies has not been adequately characterized. This study presents lithofacies and ichnofacies analyses of the Montney Formation coupled with Rock-Eval geochemistry to interpret the sedimentology, ichnology, and reservoir potential of the Montney Formation tight gas reservoir in Fort St. John study area (T86N, R23W and T74N, R13W), northeastern British Columbia, western Canada.

Optimization of operational strategies for rich gas enhanced oil recovery based on a pilot test in the Bakken tight oil reservoir

Horizontal well drilling and multistage hydraulic fracturing have been demonstrated as effective approaches for stimulating oil production in the Bakken tight oil reservoir.However,after multiple years of production,primary oil recovery in the Bakken is generally less than 10%of the estimated original oil in place.Gas huff‘n’puff(HnP)has been tested in the Bakken Formation as an enhanced oil recovery(EOR)method;however,most field pilot test results showed no significant incremental oil production.One of the factors affecting HnP EOR performance is premature gas breakthrough,which is one of the most critical issues observed in the field because of the presence of interwell fractures.Consequently,injected gas rapidly reaches adjacent production wells without contacting reservoir rock and increasing oil recovery.Proper conformance control is therefore needed to avoid early gas breakthrough and improve EOR performance.In this study,a rich gas EOR pilot in the Bakken was carefully analyzed to collect the essential reservoir and operational data.A simulation model with 16 wells was then developed to reproduce the production history and predict the EOR performance with and without conformance control.EOR operational strategies,including single-and multiple-well HnP,with different gas injection constraints were investigated.The simulation results of single-well HnP without conformance control showed that a rich gas injection rate of at least 10 MMscfd was needed to yield meaningful incremental oil production.The strategy of conformance control via water injection could significantly improve oil production in the HnP well,but injecting an excessive amount of water also leads to water breakthrough and loss of oil production in the offset wells.By analyzing the production performance of the wells individually,the arrangement of wells was optimized for multiple-well HnP EOR.The multiwell results showed that rich gas EOR could improve oil production up to 7.4%by employing conformance control strategies.Furthermore,replacing rich gas with propane as the injection gas could result in 14%of incremental oil production.

Progress in China's Unconventional Oil & Gas Exploration and Development and Theoretical Technologies

The new century has witnessed a strategic breakthrough in unconventional oil ＆ gas.Hydrocarbon accumulated in micro-/nano-scale pore throat shale systems has become an important domain that could replace current oil ＆ gas resources.Unconventional oil ＆ gas plays an increasingly important role in our energy demand.Tight gas,CBM,heavy oil and asphaltic sand have served as a key domain of exploration ＆ development,with tight oil becoming a ＇bright spot＇ domain and shale gas becoming a ＇hotspot＇ domain.China has made great breakthroughs in unconventional oil ＆ gas resources,such as tight gas,shale gas,tight oil and CBM,and great progress in oil shale,gas hydrate,heavy oil and oil sand.China has an estimated（223-263）×10~8t of unconventional oil resources and（890-1260）×l0^（12）m^3 of gas resources.China has made a breakthrough for progress in unconventional oil ＆ gas study.New progress achieved in fine-grained sedimentary studies related to continental open lacustrine basin large-scale shallow-water delta sand bodies,lacustrine basin central sandy clastic flow sediments and marine-continental fine-grained sediments provide a theoretical basis for the formation and distribution of basin central reservoir bodies.Great breakthroughs have been made in unconventional reservoir geology in respect of research methodology ＆ technology,multi-scale data merging and physical simulation of formation conditions.Overall characterization of unconventional reservoirs via multi-method and multi-scale becomes increasingly popular and facilitates the rapid development of unconventional oil ＆ gas geological theory,method and technology.The formation of innovative,continuous hydrocarbon accumulation theory,the establishment of the framework of the unconventional oil ＆ gas geological theory system,and the determination of the implications,geological feature,formation mechanism,distribution rule and core technology of unconventional oil＆ gas geological study lays a theoretical foundation for extensive unconventional oil ＆ gas exploration and development.Theories and technologies of unconventional oil ＆ gas exploration and development developed rapidly,including some key evaluation techniques such as ＇sweet spot zone＇ integrated evaluation and a six-property evaluation technique that uses hydrocarbon source,lithology,physical property,brittleness,hydrocarbon potential and stress anisotropy,and some key development ＆engineering technologies including micro-seismic monitoring,horizontal drilling ＆ completion and ＂factory-like＂ operation pattern, ＂man-made reservoir＂ development,which have facilitated the innovative development of unconventional oil ＆ gas.These breakthroughs define a new understanding in four aspects：①theoretical innovation;② key technologies;③ complete market mechanism and national policy support;and ④ well-developed ground infrastructure,which are significant for prolonging the life cycle of petroleum industry,accelerating the upgrade and development of theories and technologies and altering the global traditional energy structure.

Geological characteristics and ‘‘sweet area'' evaluation for tight oil

Tight oil has become the focus in exploration and development of unconventional oil in the world, especially in North America and China. In North America, there has been intensive exploration for tight oil in marine. In China, commercial exploration for tight oil in conti- nental sediments is now steadily underway. With the dis- covery of China＇s first tight oil field--Xin＇anbian Oilfield in the Ordos Basin, tight oil has been integrated officially into the category for reserves evaluation. Geologically, tight oil is characterized by distribution in depressions and slopes of basins, extensive, mature, and high-quality source rocks, large-scale reservoir space with micro- and nanopore throat systems, source rocks and reservoirs in close contact and with continuous distribution, and local ＂sweet area.＂ The evaluation of the distribution of tight oil ＂sweet area＂ should focus on relationships between ＂six features.＂ These are source properties, lithology, physical properties, brittleness, hydrocarbon potential, and stress anisotropy. In North America, tight oil prospects are distributed in lamellar shale or marl, where natural fractures are fre- quently present, with TOC 〉 4 %, porosity 〉 7 %, brittle mineral content 〉 50 %, oil saturation of 50 %-80 %, API 〉 35~, and pressure coefficient 〉 1.30. In China, tight oil prospects are distributed in lamellar shale, tight sand- stone, or tight carbonate rocks, with TOC 〉 2 %, poros- ity 〉 8 %, brittle mineral content 〉 40 %, oil saturation of 60 %-90 %, low crude oil viscosity, or high formation pressure. Continental tight oil is pervasive in China and its preliminary estimated technically recoverable resources are about （20-25） × lO8^ t.

Laboratory to field scale assessment for EOR applicability in tight oil reservoirs

Tight oil reservoirs are contributing a major role to fulfill the overall crude oil needs,especially in the US.However,the dilemma is their ultra-tight permeability and an uneconomically short-lived primary recovery factor.Therefore,the application of EOR in the early reservoir development phase is considered effective for fast-paced and economical tight oil recovery.To achieve these objectives,it is imperative to determine the optimum EOR potential and the best-suited EOR application for every individual tight oil reservoir to maximize its ultimate recovery factor.Since most of the tight oil reservoirs are found in wide spatial source rock with complex and compacted pores and poor geophysical properties yet they hold high saturation of good quality oil and therefore,every single percent increase in oil recovery from such huge reservoirs potentially provide an additional million barrels of oil.Hence,the EOR application in such reservoirs is quite essential.However,the physical understanding of EOR applications in different circumstances from laboratory to field scale is the key to success and similarly,the fundamental physical concepts of fluid flow-dynamics under confinement conditions play an important role.This paper presents a detailed discussion on laboratory-based experimental achievements at micro-scale including fundamental concepts under confinement environment,physics-based numerical studies,and recent actual field piloting experiences based on the U.S.unconventional plays.The objective of this paper is to discuss all the critical reservoir rock and fluid properties and their contribution to reservoir development through massive multi-staged hydraulic fracture networks and the EOR applications.Especially the CO_(2)and produced hydrocarbon gas injection through single well-based huff-n-puff operational constraints are discussed in detail both at micro and macro scale.

Exploring the potential of oil and gas resources in Sichuan Basin with Super Basin Thinking

Based on the contemporary strategy of Petro China and the“Super Basin Thinking”initiative,we analyze the petroleum system,the remaining oil and gas resource distribution,and the Super Basin development scheme in the Sichuan Basin with the aim of unlocking its full resource potential.We conclude that,(1)The three-stage evolution of the Sichuan Basin has resulted in the stereoscopic distribution of hydrocarbon systems dominated by natural gas.The prospecting Nanhua-rift stage gas system is potentially to be found in the ultra-deep part of the basin.The marine-cratonic stage gas system is distributed in the Sinian to Mid-Triassic formations,mainly conventional gas and shale gas resources.The foreland-basin stage tight sand gas and shale oil resources are found in the Upper Triassic-Jurassic formations.Such resource base provides the foundation for the implementation of Super Basin paradigm in the Sichuan Basin.(2)To ensure larger scale hydrocarbon exploration and production,technologies regarding deep to ultra-deep carbonate reservoirs,tight-sand gas,and shale oil are necessarily to be advanced.(3)In order to achieve the full hydrocarbon potential of the Sichuan Basin,pertinent exploration strategies are expected to be proposed with regard to each hydrocarbon system respectively,government and policy supports ought to be strengthened,and new cooperative pattern should be established.Introducing the“Super Basin Thinking”provides references and guidelines for further deployment of hydrocarbon exploration and production in the Sichuan Basin and other developed basins.

Sichuan super gas basin in southwest China

A sedimentary basin is classified as a super basin when its cumulative production exceeds 5 billion barrels of oil equivalent(6.82×10^(8) t of oil or 7931.66×10^(8) m^(3) of gas)and its remaining recoverable resources are at least 5 billion barrels of oil equivalent.By the end of 2019,the total output of oil and gas in Sichuan Basin had been 6569×10^(8) m^(3),the ratio of gas to oil was 80:1,and the total remaining recoverable resources reached 136404×10^(8) m^(3),which makes it as a second-tier super basin.Because the output is mainly gas,it is a super gas basin.The reason why the Sichuan Basin is a super gas basin is that it has four advantages:(1)The advantage of gas source rocks:it has the most gas source rocks(9 sets)among all the basins in China.(2)The advantage of resource quantity:it has the most total remaining recoverable resources among all the basins in China(136404×10^(8) m^(3)).(3)The advantage of large gas fields:it has the most large gas fields(27)among all the basins in China.(4)The advantage of total production:by the end of 2019,the total gas production had been 6487.8×10^(8) m^(3),which ranked the first among all the basins in China.There are four major breakthroughs in natural gas exploration in Sichuan Basin:(1)Breakthrough in shale gas:shale gas was firstly found in the Ordovician Wufeng-Silurian Longmaxi formations in China.(2)Breakthrough in tight sandstone gas:the Triassic Xu2 Member gas reservoir in Zhongba gas field is the first high recovery tight sandstone gas reservoir in China.(3)Breakthrough in giant carbonate gas fields.(4)Breakthrough in ultra-deep gas reservoir.These breakthroughs have led to important progress in different basins across the country.Super basins are classified according to three criteria:accumulative oil and gas production,remaining recoverable resources,tectonic attributes of the basin and the proportion of oil and gas in accumulative oil and gas production.

Effectiveness and sensitivity analysis of solution gas re-injection in Baikouquan tight formation,Mahu sag for enhanced oil recovery

To address the fast productivity decline of the horizontal wells and low oil recovery during natural depletion in Baikouquan formation,the approach of solution gas re-injection was proposed with the primary objective of further developing this formation.Herein,a field-scale numerical compositional reservoir model was built up based on the formation properties and then the effects of permeability,fractures and formation stress on the production dynamics were thoroughly investigated.A sensitivity analysis,which can correlate the oil recovery with these parameters,was also performed.The results showed that the re-injection of solution gas could remarkably retard the production depletion of the horizontal wells thereby improving the oil production.The oil recovery rate increased with permeability,fracture half-length,fracture conductivity,and formation dip.With regard to the fracture distribution,it was found that the interlaced fracture outperformed the aligned fracture for the solution gas re-injection.The influence of the formation stress should be carefully considered in the production process.Sensitivity analysis indicated that the formation dip was the paramount parameter,and the permeability,fracture half-length,and fracture conductivity also played central roles.The results of this study supplement earlier observations and provide constructive envision for enhanced oil recovery of tight reservoirs.

Feasibility study on application of volume acid fracturing technology to tight gas carbonate reservoir development

How to effectively develop tight-gas carbonate reservoir and achieve high recovery is always a problem for the oil and gas industry.To solve this problem,domestic petroleum engineers use the combination of the successful experiences of North American shale gas pools development by stimulated reservoir volume(SRV)fracturing with the research achievements of Chinese tight gas development by acid fracturing to propose volume acid fracturing technology for fractured tightgas carbonate reservoir,which has achieved a good stimulation effect in the pilot tests.To determine what reservoir conditions are suitable to carry out volume acid fracturing,this paper firstly introduces volume acid fracturing technology by giving the stimulation mechanism and technical ideas,and initially analyzes the feasibility by the comparison of reservoir characteristics of shale gas with tight-gas carbonate.Then,this paper analyzes the validity and limitation of the volume acid fracturing technology via the analyses of control conditions for volume acid fracturing in reservoir fracturing performance,natural fracture,horizontal principal stress difference,orientation of insitu stress and natural fracture,and gives the solution for the limitation.The study results show that the volume acid fracturing process can be used to greatly improve the flow environment of tight-gas carbonate reservoir and increase production;the incremental or stimulation response is closely related with reservoir fracturing performance,the degree of development of natural fracture,the small intersection angle between hydraulic fracture and natural fracture,the large horizontal principal stress difference is easy to form a narrow fracture zone,and it is disadvantageous to create fracture network,but the degradable fiber diversion technology may largely weaken the disadvantage.The practices indicate that the application of volume acid fracturing process to the tight-gas carbonate reservoir development is feasible in the Ordovician Majiagou Formation of lower Paleozoic,which is of great significance and practical value for domestic tight-gas carbonate reservoir development and studies in the future.

地质工程一体化在玛湖凹陷致密砂砾岩水平井开发中的实践

准噶尔盆地玛湖凹陷东斜坡的玛东2井区致密砂砾岩储层埋藏深,物性差,水敏、压敏现象严重,储层岩体砾径分选性差,非均质性强,脆性差,导致其效益开发难度大。在吸取邻区经验的基础上,将地质认识和工程技术紧密结合,对玛东2井区内第一口评价水平井(玛东1号)的工程设计进行了大量研究论证,并引进先进技术保障实施效果。开展大量岩心实验和精细测井解释,摸清了区块砂砾岩的岩性和物性;采用精细地震采集、解释和建立砂砾岩储层分类方法,优选了目的层和轨迹;采用先进地质导向工具并利用模型重构技术实时调整,确保了钻遇率和钻井效率;通过试油分析和数值模拟分析影响砂砾岩储层压裂增产的因素,进行了工艺技术优化等。通过多学科的协调配合,玛东1号水平井在含水稳定后,采用3mm油嘴生产时的平均产油量达到53.9t/d,取得了远超区块直井的提产增效成果。

试油数据在估算致密砂砾岩储层渗透率中的应用

利用试油数据估算渗透率是致密砂砾岩储层渗透性评价的新途径。试油、试井数据均能综合反映储层流体流动能力,其中由试井测得的试井流体系数本身就是渗透率的函数,但试井作业周期长、费用高,而试油资料较易获取。文中基于压裂试油获取的累计产液量、入井压裂液总量、压力等参数,构建了试油物性指数,进而开展试油物性指数与试井流动系数对比分析。结果显示:研究区试油物性指数和试井流动系数有较好的相关性,可以通过试油资料计算试井流动系数;根据试井流动系数理论计算公式,建立了以试油资料为基础的渗透率计算式,以达到渗透率估算的目的。经验证,利用该公式估算的致密砂砾岩储层渗透率与生产数据吻合较好。

砂砾岩致密油藏超临界二氧化碳吞吐适应性分析

为研究砂砾岩致密油藏超临界CO2吞吐效果,基于M油田砂砾岩致密油藏岩储层条件室内模拟超临界CO2吞吐饱和油岩心、含束缚水饱和油岩心。通过吞吐前后岩心中采出油量、采出油组分变化、吞吐前后油相渗透率变化得出:束缚水存在增加了超临界CO2吞吐采出流体量但降低了原油的采收率,同时减弱了CO2对原油的萃取能力,使得采出油组分变轻,CO2萃取原油组分区间为C12~C21;CO2与地层水作用产生沉淀现象是导致吞吐后岩心油相渗透率下降的主要原因,CO2与水作用强度大于与原油作用强度。

径向钻孔技术在大民屯低渗透油藏的应用

随着油气田开发进入中后期,低渗透甚至特低渗油藏在油气田开发中的地位日趋重要。辽河油田大民屯沙四段致密砂砾岩储层物性较差,平均孔隙度为9.8%、平均渗透率仅0.5m D。储层微观孔隙以粒间孔、溶蚀孔为主,且孔隙结构以细—微细喉道不均匀为主,易受钻井泥浆污染堵塞孔喉,同时存在储层改造破裂压力梯度高、有效率低等问题,严重制约了单井产能。结合区域储层特性,引进了径向钻孔技术。通过建立该地区油藏数值模拟地质模型,研究分析不同钻孔角度以及钻孔长度下对储层渗流场、压裂人工裂缝、油水驱替等的影响。研究发现,在180°钻孔角度和一定钻孔长度下,储层改造可形成多条裂缝,改善地层渗流能力,从而大大提高人工裂缝的控制范围和泄油面积。该技术在本区块共实施3井次,对比常规射孔压裂,单井产能提高幅度较大。

乌里雅斯太凹陷苏布构造带油气成藏特征与勘探方向

针对苏布地区油源条件、砂体沉积模式、有效储层展布以及成藏模式认识不清等问题,运用地球化学分析、精细砂体刻画、岩矿分析等多种手段,从生油岩、储层条件、储盖组合3个方面分析了该区的油气成藏条件,阐明了垂向上发育多个异常孔隙带的原因。研究表明:该区发育常规油藏横向构造聚集,有利砂体富集、非常规致密砂砾岩油气藏以及泥页岩油气藏3种成藏模式,纵向上中浅层为常规油藏,深层为非常规油气藏,呈现由浅至深为油藏带、油气混合带、气藏带分布规律。研究成果有效指导了该区勘探,可为乌里雅斯太凹陷南洼槽指明下步勘探方向。