The role of natural fracture activation in hydraulic fracturing for deep unconventional geo-energy reservoir stimulation

The presence of sealed or semi-sealed,multiscale natural fracture systems appears to be crucial for the successful stimulation of deep reservoirs.To explore the reaction of such systems to reservoir stimulation,a new numerical simulation approach for hydraulic stimulation has been developed,trying to establish a realistic model of the physics involved.Our new model successfully reproduces dynamic fracture activation,network generation,and overall reservoir permeability enhancement.Its outputs indicate that natural fractures facilitate stimulation far beyond the near-wellbore area,and can significantly improve the hydraulic conductivity of unconventional geo-energy reservoirs.According to our model,the fracture activation patterns are jointly determined by the occurrence of natural fractures and the in situ stress.High-density natural fractures,high-fluid pressure,and low effective stress environments promote the formation of complex fracture networks during stimulation.Multistage or multicluster fracturing treatments with an appropriate spacing also increase the stimulated reservoir area(SRA).The simulation scheme demonstrated in this work offers the possibility to elucidate the complex multiphysical couplings seen in the field through detailed site-specific modeling.

Interpretation of Temperature Profiles during Soak Periods in Steam-stimulated Wells

To address the problems existing in testing steam injection profiles in a steam-stimulated well during steam injection and production periods, this paper proposes that the temperature profile in the completion interval could be tested during the soak period. A mathematical model for calculating the vertical distribution of temperature in a single layer reservoir is established based on the temperature characteristics of steam stimulated reservoirs, and the vertical distribution of temperature in a single layer reservoir could be obtained and heat loss could be calculated. The temperature, which is disturbed by thermal conduction in a multilayer reservoir, and heat loss could be derived based on the superposition principle of temperature potential. This paper establishes a multilayer testing temperature profile interpretation method and interprets the actual test temperature profile of Well Gao 3-7-66. The results indicate that the temperature profile in the soak period can reflect the thermal absorption conditions in various reservoir beds.

Progress and development directions of shale oil reservoir stimulation technology of China National Petroleum Corporation

By reviewing the development history of shale oil reservoir stimulation technology of PetroChina Company Limited(PetroChina),we have systematically summarized the main progress of shale oil reservoir stimulation technology of CNPC in five aspects:reservoir stimulation mechanism,fracture-controlled fracturing,geological-engineering integrated reservoir stimulation design platform,low-cost materials,and large well-pad three-dimensional development mode.It is made clear that the major stimulation technology for shale oil reservoir is the high density multi-cluster and fracture-controlled staged fracturing aiming to increase fracture-controlled reserves,lower operation costs and increase economic benefits.Based on comprehensive analysis of the challenges shale oil reservoir stimulation technology faces in three-dimensional development,stimulation parameter optimization for fracture-controlled fracturing,refracturing and low-cost stimulation technology,we proposed five development directions of the stimulation technology:(1)Strengthen the research on integration of geology and engineering to make full use of reservoir stimulation.(2)Deepen the study on fracture-controlled fracturing to improve reserves development degree.(3)Promote horizontal well three-dimensional development of shale oil to realize the production of multiple layers vertically.(4)Research refracturing technology of shale oil reservoir through horizontal well to efficiently tap the remaining reserves between fractures.(5)Develop low-cost stimulation supporting technology to help reduce the cost and increase economic benefit of oilfield development.

Progress and development directions of stimulation techniques for ultra-deep oil and gas reservoirs

By reviewing the development history of stimulation techniques for deep/ultra-deep oil and gas reservoirs,the new progress in this field in China and abroad has been summed up,including deeper understanding on formation mechanisms of fracture network in deep/ultra-deep oil and gas reservoir,performance improvement of fracturing fluid materials,fine stratification of ultra-deep vertical wells,and mature staged multi-cluster fracturing technique for ultra-deep and highly deviated wells/horizontal wells.In light of the exploration and development trend of ultra-deep oil and gas reservoirs in China,the requirements and technical difficulties in ultra-deep oil and gas reservoir stimulation are discussed:(1)The research and application of integrated geological engineering technology is difficult.(2)The requirements on fracturing materials for stimulation are high.(3)It is difficult to further improve the production in vertical profile of the ultra-deep and hugely thick reservoirs.(4)The requirements on tools and supporting high-pressure equipment on the ground for stimulation are high.(5)It is difficult to achieve efficient stimulation of ultra-deep,high-temperature and high-pressure wells.(6)It is difficult to monitor directly the reservoir stimulation and evaluate the stimulation effect accurately after stimulation.In line with the complex geological characteristics of ultra-deep oil and gas reservoirs in China,seven technical development directions are proposed:(1)To establish systematic new techniques for basic research and evaluation experiments;(2)to strengthen geological research and improve the operational mechanism of integrating geological research and engineering operation;(3)to develop high-efficiency fracturing materials for ultra-deep reservoirs;(4)to research separated layer fracturing technology for ultra-deep and hugely thick reservoirs;(5)to explore fracture-control stimulation technology for ultra-deep horizontal well;(6)to develop direct monitoring technology for hydraulic fractures in ultra-deep oil and gas reservoirs;(7)to develop downhole fracturing tools with high temperature and high pressure tolerance and supporting wellhead equipment able to withstand high pressure.

A novel approach of tight oil reservoirs stimulation based on fracture controlling optimization and design

To deal with the stress interference caused by simultaneous propagation of multiple fractures and the wettability reversal and physical property changes of the reservoir caused by fracturing fluid getting in during large-volume fracturing of tight oil reservoirs through a horizontal well, a non-planar 3D fracture growth model was built, wettability reversal characterizing parameters and change of relative permeability curve were introduced to correct the production prediction model of fractured horizontal well, a fracturing design optimization software(Fr Smart) by integrating geological and engineering data was developed, and a fracturing design optimization approach for tight oil reservoirs based on fracture control was worked out. The adaptability of the method was analyzed and the fracture parameters of horizontal wells in tight oil reservoirs were optimized. The simulation results show that fracturing technology based on fracture control is suitable for tight oil reservoirs, and by optimizing fracture parameters, this technology makes it possible to produce the maximum amount of reserves in the well-controlled unit of unconventional reservoirs. The key points of fracturing design optimization based on fracture control include increasing lateral length of and reducing the row spacing between horizontal wells, increasing perforation clusters in one stage to decrease the spacing of neighboring fractures, and also avoiding interference of old and new fracturing wells. Field tests show that this technology can increase single well production and ultimate recovery. Using this technology in developing unconventional resources such as tight oil reservoirs in China will enhance the economics significantly.

Numerical Simulation Study on the Effect of Horizontal Well Reservoir Stimulation for Gas Hydrate Production

A new gas hydrate reservoir stimulation method of in-situ fracturing with transient heating is proposed, in line with analysis of the technological bottlenecks faced by marine gas hydrate production. This method injects the developed chemical reagents into a hydrate reservoir through hydraulic fracturing, releasing heat during the chemical reaction to increase the hydrate decomposition rate. The chemical reaction product furthermore has a honeycomb structure to support fractures and increase reservoir permeability. Based on the geological model of natural gas hydrate in the South China Sea, three development methods are simulated to evaluate hydrate production capacity, consisting of horizontal well, fractured horizontal well and in-situ fracturing with transient heating well. Compared with the horizontal well, the simulation results show that the cumulative gas production of the fractured horizontal well in one year is 7 times that of the horizontal well, while the cumulative gas production of in-situ fracturing with transient heating well is 12 times that of the horizontal well, which significantly improves daily efficiency and cumulative gas production. In addition, the variation patterns of hydrate saturation and temperature-pressure fields with production time for the three exploitation plans are presented, it being found that three sensitive parameters of fracture conductivity, fracture half-length and fracture number are positively correlated with hydrate production enhancement. Through the simulations, basic data and theoretical support for the optimization of gas hydrate reservoir stimulation scheme has been provided.

Permeability damage micro-mechanisms and stimulation of low-permeability sandstone reservoirs: A case study from Jiyang Depression, Bohai Bay Basin, China

According to the characteristics of"structural elements"(framework grain,interstitial material and pore throat structure)of low-permeability sandstone reservoir,the"step by step dissolution and separation"acidification and acid fracturing technology has been developed and tested in field.There are three main mechanisms affecting permeability of low-permeability sandstone reservoir:(1)The mud fillings between the framework grains block the seepage channels.(2)In the process of burial,the products from crystallization caused by changes in salinity and solubility and uneven migration and variation of the syn-sedimentary formation water occupy the pores and throat between grains.(3)Under the action of gradual increase of overburden pressure,the framework grains of the rock is compacted tighter,making the seepage channels turn narrower.The"step by step dissolution and separation"acidification(acid fracturing)technology uses sustained release acid as main acidizing fluid,supramolecular solvent instead of hydrochloric acid to dissolve carbonate,and a composite system of ammonium hydrogen fluoride,fluoroboric acid,and fluorophosphoric acid to dissolve silicate,and dissolving and implementing step by step,finally reaching the goal of increasing porosity and permeability.By using the technology,the main blocking interstitial material can be dissolved effectively and the dissolution residual can be removed from the rock frame,thus expanding the effective drainage radius and increasing production and injection of single well.This technology has been proved effective by field test.

基于微地震连续裂缝网络模型的SRV研究

在干热岩的开发和改造阶段,需要对储层进行水力压裂改造,储层改造体积(stimulated reservoir volume,SRV)是评价压裂效果的主要标准。微地震监测作为水力压裂的技术环节之一,可以对储层改造体积进行有效估算。本文探讨了基于微地震连续裂缝网络模型的SRV计算方法,以指导干热岩的水力压裂工作。首先,基于微震事件时空分布特征与多项震源参数进行连续裂缝网络建模;其次,提取裂缝网格长度,并选取适当的裂缝高度与裂缝宽度以计算储层改造体积;最后,选取某双井型干热岩微地震监测数据对该方法进行实际应用。实例应用结果表明,该方法可有效估算储层改造体积,为干热岩后续开发与改造提供依据。

Practice and development suggestions of hydraulic fracturing technology in the Gulong shale oil reservoirs of Songliao Basin, NE China

This paper reviews the multiple rounds of upgrades of the hydraulic fracturing technology used in the Gulong shale oil reservoirs and gives suggestions about stimulation technology development in relation to the production performance of Gulong shale oil wells.Under the control of high-density bedding fractures,fracturing in the Gulong shale results in a complex fracture morphology,yet with highly suppressed fracture height and length.Hydraulic fracturing fails to generate artificial fractures with sufficient lengths and heights,which is a main restraint on the effective stimulation in the Gulong shale oil reservoirs.In this regard,the fracturing design shall follow the strategy of"controlling near-wellbore complex fractures and maximizing the extension of main fractures"Increasing the proportions of guar gum fracturing fluids,reducing perforation clusters within one fracturing stage,raising pump rates and appropriately exploiting stress interference are conducive to fracture propagation and lead to a considerably expanded stimulated reservoir volume(SRV).The upgraded main hydraulic fracturing technology is much more applicable to the Gulong shale oil reservoirs.It accelerates the oil production with a low flowback rate and lifts oil cut during the initial production of well groups,which both help to improve well production.It is suggested to optimize the hydraulic fracturing technology in six aspects,namely,suppressing propagation of near-wellbore microfractures,improving the pumping scheme of CO_(2),managing the perforating density,enhancing multi-proppant combination,reviewing well pattern/spacing,and discreetly applying fiber-assisted injection,so as to improve the SRv,the distal fracture complexity and the long-term fracture conductivity.

Proppant transport in rough fractures of unconventional oil and gas reservoirs

A method to generate fractures with rough surfaces was proposed according to the fractal interpolation theory.Considering the particle-particle,particle-wall and particle-fluid interactions,a proppant-fracturing fluid two-phase flow model based on computational fluid dynamics(CFD)-discrete element method(DEM)coupling was established.The simulation results were verified with relevant experimental data.It was proved that the model can match transport and accumulation of proppants in rough fractures well.Several cases of numerical simulations were carried out.Compared with proppant transport in smooth flat fractures,bulge on the rough fracture wall affects transport and settlement of proppants significantly in proppant transportation in rough fractures.The higher the roughness of fracture,the faster the settlement of proppant particles near the fracture inlet,the shorter the horizontal transport distance,and the more likely to accumulate near the fracture inlet to form a sand plugging in a short time.Fracture wall roughness could control the migration path of fracturing fluid to a certain degree and change the path of proppant filling in the fracture.On the one hand,the rough wall bulge raises the proppant transport path and the proppants flow out of the fracture,reducing the proppant sweep area.On the other hand,the sand-carrying fluid is prone to change flow direction near the contact point of bulge,thus expanding the proppant sweep area.

深部地热能系统主要挑战与耦合储能的增强型创新开发模式

中国在中低温地热能直接利用方面早已领先全球,但在深部地热能发电方面却发展缓慢。深部高温高压环境下岩石渗透性降低,深部地热能开采需要建立工程型地热系统(Engineered Geothermal System,EGS),通过水力压裂对储层进行改造,以获得具有较高渗透性的人工地热储层。由于目前常用的深部地热能储层改造技术主要借鉴油气增产领域的水力压裂工艺,在热储改造效果、地震风险控制、高效取热等方面受到限制。首先总结深部地热能水力压裂的特点为:裂缝破坏主要以剪切机理为主;冷水回灌引起的温差效应产生拉应力会促使裂缝向更远处扩展;持续的注水使注入井井筒压力高于地层压力,有助于保持裂缝处于张开状态。因此,EGS水力压裂不需要使用支撑剂,与依靠支撑剂的油气井增产压裂完全不同。同时,系统剖析EGS面临的发电产能低、注采连通差、诱发破坏性地震以及无补贴难盈利4大难题与挑战。从创新压裂和循环利用层面提出耦合储能的增强型创新开发模式(Regenerative Engineered Geothermal System,REGS),通过数值模拟研究REGS的优点。结果表明,采用水平井非等距、非等面积、非等注水量分段压裂,可以提高注水井与生产井的连通能力。通过优化压裂工艺,采用多段压裂模式,每次压裂初始期间快速提高注水速率,而后期缓慢降低注水速率,避免井筒压力的突然波动,可以达到控制诱发地震震级的目的,避免产生实质性伤害地震。结合可再生能源大规模地下存储,既能实现多能互补,又能提高REGS项目生产寿命和盈利能力。研究成果有助于为我国深部地热能热电联产和储能一体化技术的试点和标准化推广奠定基础。

基于储层甜点特征的致密砂砾岩油气藏压裂增产技术研究

中拐凸起南斜坡二叠系佳木河组二段储层为一套含浊沸石的致密砂砾岩。早期,在有利构造上钻探的直井产量均较低,2014年钻探该区第一口水平井用于提产,但因未能有效控制缝高,致深部水体锥进过快,长期试采效果差。近年来,地震地质综合研究发现,储层存在局部甜点。因下部并无隔层,压裂时需严格控制裂缝下延。通过对储层甜点特征进行梳理,综合评价了各断块甜点的优质程度,为试油段优选和新井的部署提供依据。为了实现控缝高压裂,提高单井产量和试采效果,开展人工隔层导流能力室内实验、完成人工隔层支撑剂粒径的优选,进行压裂数值模拟研究、确定了研究区形成有效厚度人工隔板的临界施工参数。研究认为,前置液阶段,改变液体黏度和排量、铺置有效人工隔层能够抑制水力裂缝向下扩展;携砂液阶段,加入一定浓度纤维,可进一步提升支撑剂对上部优质储层的支撑。这种组合控缝高工艺技术投入该区压裂实施后,有效控制了裂缝下延高度,甜点层段得到充分改造,试油及后续试采期间维持了较高油气产能,压裂提产效果明显。

神府区块深部煤层气高效开发技术研究

【目的】深部煤层气是保障国家油气安全、实现碳达峰碳中和目标的重要接替资源之一,具有广阔的开发前景。但在不同地质条件下,深部煤层气开发技术的通用性较差,在某些区块成功应用的技术不能直接推广到其他区块。目前,新区块前期勘探开发时井型、井网部署还存在一定的盲目性。因此,需要研发针对神府区块深部煤层气高效开发的储层工程改造适配性技术。【方法】首先,以鄂尔多斯盆地神府区块79口生产井实际数据为基础,从产能、经济性2个方面对直井和水平井两种井型的开发效果进行比较;其次,以神府地区深部煤层地质条件为输入参数,对深部煤层水平井的合理井长进行模拟分析;最后,通过地质气藏、钻完井、工程和经济评价等多专业结合,以单位面积经济效益为优化目标,得出神府区块深部煤层气高效开发的储层工程改造具体工艺参数。【结果和结论】结果表明:神府区块深部煤层气高效开发的适合井型为水平井;可实现效益最大化时的水平井长度范围为800~1500 m;开发的最优井距为300 m、簇间距为15~20 m、半缝长为120 m,对应的压裂总液量应为(1.8~2.4)万m^(3),总砂量(0.25~0.35)万m^(3)。在上述研究的基础上,以“段内多簇+缩短簇间距+高排量大规模注入+变黏滑溜水造缝携砂+高强度加砂+全尺度裂缝支撑+等孔径限流射孔+造复杂缝网”为核心,建立了一套适合神府区块深部煤层储层改造工艺,实现了现阶段神府区块深部煤层气的高效开发,为现阶段国内其他盆地深部煤层气资源开发提供借鉴。

济阳页岩油开发“三元”储渗理论技术与实践

基于济阳坳陷页岩油储层上万米岩心资料与60余口水平井的开发实践,提出济阳页岩油“储元”、“缝元”、“压元”的“三元”储渗理论。“三元”协同支撑济阳页岩油的富集高产:“储元”控制页岩油的富集,咸化湖盆无机孔-缝及灰-泥优质纹层组构发育利于页岩油储集,高生烃能力、高游离烃含量为高产提供物质基础;“缝元”控制页岩油的渗流,天然裂缝为页岩油的运移和聚集提供渗流通道,压裂改造缝沟通天然裂缝形成复杂缝网,为高产提供渗流基础;“压元”控制页岩油的高产稳产,高地层压力为油气运聚提供原始动力,压裂增能可进一步提高岩石及流体弹性能,强化孔缝原油渗吸置换,减缓应力敏感性,为长期稳产提供能量基础。基于“三元”储渗理论,形成了以立体井网优化、立体均衡压裂、全周期优化调控为核心的立体开发技术,有效指导了现场生产实施,为济阳页岩油规模化效益开发提供技术支撑。

深层页岩气储层水力压裂裂缝扩展影响机理

现有中浅层页岩气压裂取得的认识难以解释深层页岩气储层压裂过程中水力裂缝扩展的影响机理。为了研究四川盆地深层页岩气储层水力压裂裂缝扩展机理,文中以泸州深层页岩气区LS1平台为研究对象,进行基于地质-工程一体化的水力压裂裂缝扩展数值模拟与分析。首先,根据泸州区块目标井区储层的地质概况,建立地质模型,明确地质力学属性;然后,基于微地震和蚂蚁体数据,建立符合真实储层构造特性的离散天然裂缝网络;在此基础上,根据现场实际施工数据,建立了基于DFN的水力压裂复杂裂缝扩展模型,并基于微地震监测结果对模型进行了验证;最后,还研究了4个单因素对储层改造体积的影响特点。研究结果对深层页岩气储层水力压裂复杂缝网研究有一定的指导作用。

储层工程地质力学导论

全球范围内的减少碳排放和实现可持续发展,都亟需提升清洁能源的消费占比并加大地热和页岩油气等非常规能源的开发力度。由于非常规储层具有致密超低渗以及油气赋存状态等自身工程地质特性,而且存在于地下深部而具有人不可入的特点,其开发必须通过钻孔对储层实施改造改性。储层工程地质力学则是研究储层工程地质特性的一门理论和应用科学,是地质学、力学与地球物理学的一个交叉学科。如果说储层地质力学是连接油气地质和油气开发的纽带,则储层工程地质力学进一步综合了工程地质力学、岩石力学以及储层地质力学的相关理论和方法,评价储层的工程地质特性与所处物理环境力场,研究储层改造改性原理与技术,为深部能源资源开发提供科学依据和技术支持。储层工程地质力学更强调将储层地质力学理论与储层改造改性相结合,解决深部能源资源开发中的储层改造改性关键技术难题,聚焦于储层压裂、储层改性、流体流动以及THMC强耦合过程,服务于清洁能源开发与减碳固碳,助力于我国“碳达峰”和“碳中和”目标的实现。

海域天然气水合物储层改造增产技术研究进展

天然气水合物是以甲烷为主的烃类气体在高压、低温条件下形成的固态可燃烧的结晶化合物,主要分布在温度和压力条件适合的陆域冻土区和海底,其中有超过90%的天然气水合物分布在海底的沉积物中(黄鑫等,2022;王志刚等,2023)。当前已探明的天然气水合物储量是全球石化燃料资源的2倍,如果能够开采出15%的天然气水合物.

大排列微地震实时监测技术在页岩油水力压裂中的应用

页岩油是重要的国家战略资源。在中国,以陆相页岩油为主,特别是济阳凹陷,页岩储层地质条件复杂,常用的裂缝识别技术包括蚂蚁体、相干体和曲率体等,无法满足精度要求,开采面临巨大挑战。为了加快页岩油的开发,在樊页1井组页岩油示范区开展基于大排列微地震实时监测,通过闭环优化过程进行施工参数迭代和技术方案优化。根据微地震震源参数,提出了计算储层改造体积和裂缝复杂性指数的新方法,用于定量评估储层改造波及范围和裂缝的复杂性。文中主要描述了在樊页1井组页岩油示范区开展的基于大排列微地震实时监测技术的地质工程一体化实践,所取得的认识和经验具有一定的借鉴价值。

密井网区叠前高分辨率反演及其在薄储层预测的应用

针对大庆长垣油田密井网区SⅡ油层组陆相河流-三角洲相沉积微相类型变化快、井间厚度2 m以下的窄小河道砂体预测误差大、河道走向预测多解性强、砂体间连通关系判别差异性大等问题,以长垣油田A区块为例,开展叠前井控道集处理技术、基于改进的Xu-White岩石物理模拟技术、基于“波形指示马尔科夫链-蒙特卡洛”随机模拟算法的叠前波形指示反演技术研究。结果表明:研究区砂岩储层具有低纵波阻抗、低纵横波速度比值的特征,纵波阻抗为3.8~8.2 Gg/(m^(2)·s),纵横波速度比为1.7~2.1;随时砂岩厚度的增加,其纵波阻抗及纵横波速度比有减小的趋势。通过70口后验井的定量分析及评价,在密井网区叠前波形指示反演对厚度为[1,2)m的薄互层砂体识别精度为80.7%,对厚度大于等于2 m的砂体识别精度为85.1%。研究成果对河流―三角洲相薄储层开发方案的合理制定及措施调整具有指导意义。

基于“扩缝串洞”理念的超深层小缝洞群碳酸盐岩储层改造数值模拟

随着四川盆地和塔里木盆地碳酸盐岩油气藏勘探开发逐渐迈向超深层领域,受地质条件影响,储层由大型缝洞向以厘米—毫米级小缝洞群转变,以往针对大型缝洞储层改造的酸压技术系列难以适应小缝洞群储层。为充分释放超深层小缝洞群碳酸盐岩储层的天然气产能潜力,基于小缝洞群碳酸盐岩储层特征,构建了多尺度离散缝洞方法,建立了考虑岩石变形、裂缝延伸、酸液流动反应、温度变化的热流固化多场耦合数值模型,最后提出了以多级转向扩面积、精准控黏串缝洞、定点生酸扩缝长为核心的“扩缝串洞”缝控改造理念,并对小缝洞群碳酸盐岩储层“扩缝串洞”缝控改造方式进行了优化分析。研究结果表明:①多级交替注入形成的黏性指进现象可以促进缝控改造面积的增加和裂缝非均匀刻蚀,从而提高改造后的产能,且缝控改造效果最佳;②酸液注入黏度比增加,缝控改造无因次产能指数呈现先增大后减小的规律,当黏度比超过30,缝控改造无因次产能指数开始迅速减小,缝控改造中酸液交替黏度比需控制在25~35区间;③酸液用量越大,缝控改造面积及改造后无因次产能指数越大,但对于一定地层参数和单井控制面积的缝控改造存在极限酸液用量,超过该酸液用量后缝控面积趋于极限值,而无因次产能指数仍随酸液用量的增加而稳定增大。结论认为,提高缝控改造面积和激活动用小缝洞群是小缝洞群碳酸盐岩储层提产的关键,该认识为超深层小缝洞群碳酸盐岩气藏的高效开发提供了理论指导。