Fracture propagation,proppant transport and parameter optimization of multi-well pad fracturing treatment

This paper establishes a 3D multi-well pad fracturing numerical model coupled with fracture propagation and proppant migration based on the displacement discontinuity method and Eulerian-Eulerian frameworks,and the fracture propagation and proppant distribution during multi-well fracturing are investigated by taking the actual multi-well pad parameters as an example.Fracture initiation and propagation during multi-well pad fracturing are jointly affected by a variety of stress interference mechanisms such as inter-cluster,inter-stage,and inter-well,and the fracture extension is unbalanced among clusters,asymmetric on both wings,and dipping at heels.Due to the significant influence of fracture morphology and width on the migration capacity of proppant in the fracture,proppant is mainly placed in the area near the wellbore with large fracture width,while a high-concentration sandwash may easily occur in the area with narrow fracture width as a result of quick bridging.On the whole,the proppant placement range is limited.Increasing the well-spacing can reduce the stress interference of adjacent wells and promote the uniform distribution of fractures and proppant on both wings.The maximum stimulated reservoir volume or multi-fracture uniform propagation can be achieved by optimizing the well spacing.Although reducing the perforation-cluster spacing also can improve the stimulated reservoir area,a too low cluster spacing is not conducive to effectively increasing the propped fracture area.Since increasing the stage time lag is beneficial to reduce inter-stage stress interference,zipper fracturing produces more uniform fracture propagation and proppant distribution.

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.

Proppant transport law in multi-branched fractures induced by volume fracturing

To further clarify the proppant transport and placement law in multi-branched fractures induced by volume fracturing, proppant transport simulation experiments were performed with different fracture shapes, sand ratios, branched fracture opening time and injection sequence of proppants in varied particle sizes. The results show that the settled proppant height increases and the placement length decreases in main fractures as the fracturing fluid diverts gradually to the branched fractures at different positions. The flow rate in branched fractures is the main factor affecting their filling. The diverion to branched fractures leads to low flow rate and poor filling of far-wellbore branched fractures. The inclined fracture wall exerts a frictional force on the proppant to slow its settlement, thus enhancing the vertical proppant distribution in the fracture. The increase of sand ratio can improve the filling of near-wellbore main fracture and far-wellbore branched fracture and also increase the settled proppant height in main fracture. Due to the limitation of fracture height, when the sand ratio increases to a certain level, the increment of fracture filling decreases. When branched fracture is always open(or extends continuously), the supporting effect on the branched fractures is the best, but the proppant placement length within the main fractures is shorter. The fractures support effect is better when it is first closed and then opened(or extends in late stage) than when it is first opened and then closed(or extends in early stage). Injecting proppants with different particle sizes in a specific sequence can improve the placement lengths of main fracture and branched fracture. Injection of proppants in an ascending order of particle size improves the near-wellbore fracture filling, to a better extent than that in a descending order of particle size.

Numerical Modelling of Proppant Transport in Hydraulic Fractures

The distribution of proppant injected in hydraulic fractures significantly affects the fracture conductivity and well performance.The proppant transport in thin fracturing fluid used during hydraulic fracturing in the unconventional reservoirs is considerably different from fracturing fluids in the conventional reservoir due to the very low viscosity and quick deposition of the proppants.This paper presents the development of a three-dimensional Computational Fluid Dynamics(CFD)modelling technique for the prediction of proppant-fluid multiphase flow in hydraulic fractures.The proposed model also simulates the fluid leak-off behaviour from the fracture wall.The Euler-Granular and CFD-Discrete Element Method(CFD-DEM)multiphase modelling approach has been applied,and the equations defining the fluid-proppant and inter-proppant interaction have been solved using the finite volume technique.The proppant transport in hydraulic fractures has been studied comprehensively,and the computational modelling results of proppant distribution and other flow properties are in good agreement with the published experimental study.The parametric study is performed to investigate the effect of variation in proppant size,fluid viscosity and fracture width on the proppant transport.Smaller proppants can be injected early,followed by larger proppants to maintain high propping efficiency.This study has enhanced the understanding of the complex flow phenomenon between proppant and fracturing fluid and can play a vital role in hydraulic fracturing design.

Evaluation of numerical schemes for capturing shock waves in modeling proppant transport in fractures

In petroleum engineering, the transport phenomenon of proppants in a fracture caused by hydraulic fracturing is captured by hyperbolic partial differential equations(PDEs). The solution of this kind of PDEs may encounter smooth transitions, or there can be large gradients of the field variables. The numerical challenge posed in a shock situation is that high-order finite difference schemes lead to significant oscillations in the vicinity of shocks despite that such schemes result in higher accuracy in smooth regions. On the other hand, first-order methods provide monotonic solution convergences near the shocks,while giving poorer accuracy in the smooth regions.Accurate numerical simulation of such systems is a challenging task using conventional numerical methods. In this paper, we investigate several shock-capturing schemes.The competency of each scheme was tested against onedimensional benchmark problems as well as published numerical experiments. The numerical results have shown good performance of high-resolution finite volume methods in capturing shocks by resolving discontinuities while maintaining accuracy in the smooth regions. Thesemethods along with Godunov splitting are applied to model proppant transport in fractures. It is concluded that the proposed scheme produces non-oscillatory and accurate results in obtaining a solution for proppant transport problems.

Simulation of proppant transport at intersection of hydraulic fracture and natural fracture of wellbores using CFD-DEM

Proppants transport is an advanced technique to improve the hydraulic fracture phenomenon,in order to promote the versatility of gas/oil reservoirs.A numerical simulation of proppants transport at both hydraulic fracture(HF)and natural fracture(NF)intersection is performed to provide a better understand-ing of key factors which cause,or contribute to proppants transport in HF-NF intersection.Computational fluid dynamics(CFD)in association with discrete element method(DEM)is used to model the complex interactions between proppant particles,host fluid medium and fractured walls.The effect of non-spherical geometry of particles is considered in this model,using the multi-sphere method.All interaction forces between fluid flow and particles are considered in the computational model.Moreover,the inter-actions of particle-particle and particle-wall are taken into account via Hertz-Mindlin model.The results of the CFD-DEM simulations are compared to the experimental data.It is found that the CFD-DEM sim-ulation is capable of predicting proppant transport and deposition quality at intersections which are in agreement with experimental data.The results indicate that the HF-NF intersection type,fluid velocity and NF aperture affect the quality of blockage occurrence,presenting a new index,called the blockage coefficient which indicates the severity of the blockage.

粗糙壁面压裂裂缝内支撑剂运移铺置特征

采用铸模工艺对岩石露头粗糙劈裂裂缝面进行立体重建,搭建大型可视化粗糙裂缝实验装置并开展支撑剂运移实验,研究粗糙壁面裂缝内支撑剂运移铺置的典型特征及其内在机制,分析裂缝倾斜程度、裂缝宽度以及压裂液黏度对支撑剂在粗糙裂缝内运移铺置的影响规律。研究表明:裂缝的粗糙特征会引起流道形状和流体流动模式的改变,导致支撑剂运移时易发生桥接堆积并形成未充填区,携砂液出现窜流、反向流和绕流等多种复杂流动形式,并影响砂丘堆积的稳定性;支撑剂在倾斜粗糙裂缝内具有更高的铺置率,实验中相较于垂直裂缝其铺置率最大增加了22.16个百分点,但砂丘的稳定性差;缝宽减小会加剧支撑剂的桥接堆积并导致更高的泵注压力;增大压裂液黏度能够减轻裂缝面粗糙特征引起的支撑剂桥接堆积。

迂曲裂缝内支撑剂运移数值模拟

水力压裂后,储层形成较多的迂曲裂缝,支撑剂在迂曲裂缝中的运移和铺置规律尚不明确,在稠密颗粒流模型的基础上耦合颗粒物料仿真软件,模拟迂曲角度、支撑剂尺寸、注入速率、携砂液密度和支撑剂密度对迂曲裂缝中支撑剂运移和砂堵的影响。结果表明:大多支撑剂会堆积在裂缝第一转折角处,迂曲角度的变化会显著影响支撑剂的运移和砂堵的发生,迂曲角度大,容易发生砂堵且砂堵程度严重;密度大和粒径小的支撑剂发生砂堵时会形成致密的支撑剂堆积,加剧砂堵程度;注入速率大,支撑剂在裂缝中运移距离较远,但注入速度和携砂液密度的增加,会加剧砂堵程度。

Experimental investigation and correlations for proppant distribution in narrow fractures of deep shale gas reservoirs

Hydraulic fracturing is a crucial stimulation for the development of deep shale gas reservoirs.A key challenge to the effectiveness of hydraulic fracturing is to place small proppants in complex narrow fractures reasonably.The experiments with varied particle and fluid parameters are carried out in a narrow planar channel to understand particle transport and distribution.The four dimensionless parameters,including the Reynold number,Shields number,density ratio,and particle volume fraction,are introduced to describe the particle transport in narrow fractures.The results indicate that the narrow channel probably induces fluid fingers and small particle aggregation in a highly viscous fluid,leading to particle settlement near the entrance.The low viscous fluid is beneficial to disperse particles further into the fracture,especially in the high-speed fluid velocity.The linear and natural logarithmic laws have relationships with dimensionless parameters accurately.The multiple linear regression method developed two correlation models with four dimensionless parameters to predict the bed equilibrium height and covered area of small particles in narrow fractures.The study provides fundamental insight into understanding small size proppant distribution in deep reservoirs.

Numerical Simulation of Proppant Dynamics in a Rough Inclined Fracture

Although the dynamics of proppant(small ceramic balls used to prevent opened fractures from closing on the release of pressure)have been the subject of several numerical studies over recent years,large-scale inclined fractures exist in unconventional reservoirs for which relevant information is still missing.In the present study,this problem is investigated numerically considering the influence of several relevant factors such as the fracture roughness,inclination,the proppant particle size,the injection rate and the fluid viscosity.The results show that a rough wall enables the proppant to travel farther and cover larger areas.The inclination angle has little effect on the dune but a significant influence on the suspension zone.The area of this zone increases with a decrease in the inclination angle,and its value for an inclination of 15°is 20 times that at 90°.Small particle size,high injection rate,and high fracturing fluid viscosity have a beneficial influence on proppant transport;vice versa they hinder settling phenomena.

体积压裂多分支裂缝支撑剂运移规律

为进一步明确体积压裂多分支裂缝内支撑剂的运移铺置规律,开展了不同裂缝形态、砂比、分支缝开启时机和不同粒径支撑剂注入顺序条件下的支撑剂运移模拟实验。研究表明,不同位置分支缝的逐级分流使分支后主缝砂堤高度增加,铺置长度减小。分支缝的流量是影响其充填的主要因素,分流作用使得远井分支缝流量小、充填情况较差。倾斜裂缝壁面对支撑剂施加减缓其沉降的摩擦力,提高支撑剂在裂缝纵向的分布。砂比增加可改善近井主缝和远井分支缝的充填并使主缝砂堤长度增加,由于缝高的限制,砂比提高到一定值后裂缝充填情况改善幅度减小。分支缝常开(持续扩展)时分支缝最终的支撑效果最好,但主缝铺置长度短,先关后开(后期扩展)的支撑效果优于先开后关(前期扩展)。不同粒径支撑剂顺序注入可增加主缝和分支缝的铺置长度,先小后大注入可改善近井裂缝充填,先大后小注入时近井裂缝充填情况不如先小后大注入时。

支撑剂在纳米改性VES压裂液中的运移特性

在压裂过程中,由于天然裂缝的存在,支撑剂在裂缝内的铺置较为复杂,直接影响了压裂增产的效果。为提高压裂液在裂缝中的携砂能力,在黏弹性表面活性剂(VES)压裂液中添加纳米氧化锌颗粒,形成纳米改性VES压裂液。通过自主研发的平板裂缝实验台对比分析清水与纳米改性VES压裂液的携砂效果。结果发现,纳米改性VES压裂液铺砂更加均匀,效果更好。通过应力旋转流变仪测定纳米改性VES压裂液的流变特性,采用Cross流体本构方程对实验结果进行拟合,得到相应的流变参数,并利用颗粒动力学的方法对支撑剂在纳米改性VES压裂液中的运移特性进行研究。模拟结果表明,随着压裂液砂质量分数的增加,支撑剂之间易于发生团聚,使得更多的颗粒发生沉降,达到砂堤平衡高度的时间减小,平衡高度增加;随着支撑剂颗粒粒径及密度的增大,支撑剂沉降速率增加,砂堤高度增加,且靠近入口,极易出现砂堵的现象。在实际压裂施工过程中,要尽可能选用小粒径、低密度的支撑剂。

Numerical simulations of supercritical carbon dioxide fracturing:A review

As an emerging waterless fracturing technology,supercritical carbon dioxide(SC-CO_(2))fracturing can reduce reservoir damage and dependence on water resources,and can also promote the reservoir stimulation and geological storage of carbon dioxide(CO_(2)).It is vital to figure out the laws in SC-CO_(2)fracturing for the large-scale field implementation of this technology.This paper reviews the numerical simulations of wellbore flow and heat transfer,fracture initiation and propagation,and proppant transport in SC-CO_(2)fracturing,including the numerical approaches and the obtained findings.It shows that the variations of wellbore temperature and pressure are complex and strongly transient.The wellhead pressure can be reduced by tubing and annulus co-injection or adding drag reducers into the fracturing fluid.Increasing the temperature of CO_(2)with wellhead heating can promote CO_(2)to reach the well bottom in the supercritical state.Compared with hydraulic fracturing,SC-CO_(2)fracturing has a lower fracture initiation pressure and can form a more complex fracture network,but the fracture width is narrower.The technology of SC-CO_(2)fracturing followed by thickened SC-CO_(2)fracturing,which combines with high injection rates and ultra-light proppants,can improve the placement effect of proppants while improving the complexity and width of fractures.The follow-up research is required to get a deeper insight into the SC-CO_(2)fracturing mechanisms and develop cost-effective drag reducers,thickeners,and ultra-light proppants.This paper can guide further research and promote the field application of SC-CO_(2)fracturing technology.

非常规油气储集层粗糙压裂裂缝内支撑剂运移机理

基于分形插值理论建立了具有粗糙壁面裂缝的生成方法,同时考虑颗粒-颗粒、颗粒-壁面、颗粒-流体的相互作用,建立了基于计算流体力学(CFD)-离散单元法(DEM)耦合的支撑剂-压裂液两相流动模型。经实验数据的检验,证实该模型可以较好地匹配粗糙裂缝内支撑剂的运移情况及堆积过程。经多个方案的数值模拟研究表明:与光滑平板裂缝相比,支撑剂在粗糙裂缝内输送,壁面粗糙凸起会显著影响支撑剂的运移与沉降,裂缝模型的粗糙程度越高,裂缝入口附近的支撑剂颗粒沉降速度越快,其水平运移距离越短,越倾向于在裂缝入口附近堆积,并在较短时间内形成缝内砂堵。裂缝壁面粗糙度在一定程度上可控制流体的运移路径,改变支撑剂填充裂缝的方式,一方面粗糙壁面凸起抬升了支撑剂运移轨迹,使支撑剂流出裂缝,导致覆盖率减小;另一方面携砂液易在粗糙壁面凸起接触点附近发生转向流动,可在一定程度上扩大支撑剂覆盖范围。

井平台多井压裂裂缝扩展、支撑剂运移规律与参数优化

基于位移不连续方法和欧拉-欧拉法建立了井平台多井压裂模式下三维裂缝扩展与支撑剂运移耦合模型,以实际井平台参数为例研究了多井压裂裂缝扩展形态与缝内支撑剂铺置规律。研究表明:井平台压裂模式下裂缝起裂及扩展受段内、段间及井间等多种应力干扰的共同影响,裂缝展布形态具有各簇非均衡、两翼非对称以及跟端倾向等特征;裂缝形态及宽度对缝内支撑剂运移能力影响显著,支撑剂主要铺置在近井缝宽较大区域,而缝宽较窄处容易快速桥接形成高浓度砂堆,整体支撑剂铺置范围有限;增大井间距离有利于降低邻井干扰作用,促进两翼裂缝和支撑剂均匀展布,以储集层有效改造面积或者多裂缝扩展均衡程度最大为目标,井平台存在最优井间距离;降低簇间距离能够提升储集层改造程度,但簇间距离过低并不能有效增加缝内支撑面积;增大压裂段间隔时长有助于降低段间应力干扰,拉链压裂模式下段内各簇裂缝扩展和支撑剂铺置更加均衡。

基于PIV/PTV的平板裂缝支撑剂输送试验研究

为了解水力压裂过程中水力裂缝内支撑剂的铺置规律,基于平板裂缝开展了支撑剂输送试验,分析了泵注排量、压裂液黏度、注入位置、支撑剂类型对支撑剂铺置过程的影响;运用PIV/PTV技术,测试了压裂液-支撑剂两相运动速度,从颗粒运动角度分析了不同因素对最终砂堤形态的影响。试验发现:平板单缝内支撑剂铺置存在“裂缝前端先堆积至平衡高度,再稳定向后端铺置”和“砂堤整体纵向增长,稳定向后端铺置”2种典型模式,2种模式可以在泵注的不同阶段出现并转换;砂堤不同位置形态主控因素存在差异,注入位置与排量主要控制前缘形态,黏度与排量主要控制中部形态,黏度主要控制后缘形态;在裂缝远端,支撑剂沉降存在“回流式”和“直接式”2种模式,前者受涡流控制,后者则仅依靠重力沉降;现场施工时可考虑“定向射孔+大排量中高黏70/140目石英砂(主体支撑剂)+40/70目陶粒架桥+大排量中高黏70/140目石英砂长距离输送+排量尾追40/70目陶粒”,兼顾缝长方向远距离铺置和近井地带裂缝与井筒的高连通性。平板裂缝内支撑剂运移与铺置规律试验结果可以为页岩储层压裂主裂缝内支撑剂高效铺置及储层改造工艺参数优化提供参考。

薄煤层水力裂缝宽度变化对支撑剂运移的影响

水力压裂仍是低渗透性储层提高渗透性且获得经济产能的主要途径,尤其针对薄煤层,不同裂缝宽度裂缝内支撑剂运移规律受多个因素影响。文中采用数据建模、数值模拟和工程试验的方式,研究了压裂液参数、支撑剂参数对不同裂缝宽度支撑剂的铺砂面积和有效支撑裂缝长度的影响。结果表明:裂缝宽度影响和制约了支撑剂的运移,对于裂缝较发育且缝宽较宽储层,提高施工排量、压裂液黏度和优化支撑剂粒径与砂比,能有效地增大裂缝内支撑剂的铺置面积和有效裂缝长度。针对裂缝不发育且裂缝宽度较小储层,优化支撑剂的粒径和砂比,有利于增大裂缝的有效支撑裂缝长度。针对薄—中厚煤层,采用“高前置液占比、小粒径支撑剂、低砂比、短段塞式加砂方式”,能够有效提高其压裂波及范围和有效支撑效果,研究成果成功应用于贵州薄煤层煤层气开发。

二级裂缝倾角对复杂缝网中支撑剂输送的影响

支撑剂在复杂缝网中的分布情况直接决定了非常规油气的开发效果。为了探究体积压裂后复杂缝网中二级裂缝倾角对其支撑剂输送特征的影响,建立了可以改变二级裂缝位置和倾角的缝网模型。在分析携砂液在缝网内流动的基础上,建立了支撑剂在复杂缝网中的输送模型,并以曳力、弹性碰撞模型和Knudsen余弦散射定律分别表征支撑剂与携砂液、支撑剂颗粒间以及支撑剂与粗糙裂缝壁面的相互作用。数值模拟结果表明:改变二级裂缝倾角和位置,会严重影响缝网中主裂缝和二级裂缝内支撑剂的铺置情况;当二级裂缝水平时,其中的支撑剂分布范围最小,而增大二级裂缝到主裂缝入口的距离可以有效改善支撑剂在主裂缝中的分布;在二级裂缝垂直的缝网中支撑剂填充比例最大,当二级裂缝倾角为90°~120°时,支撑剂填充效果最差。研究成果为体积压裂中携砂液及缝网优化设计提供了理论指导。

基于CFD-DEM算法的压裂支撑剂缝内铺砂规律

为优化页岩气支撑剂的输送性能,采用计算流体力学-离散元(CFD-DEM)耦合方法对中等矩形裂缝进行数值模拟,考虑了重力、浮力以及颗粒-颗粒、颗粒-流体、颗粒-壁面之间的相互作用,采用单一因素控制变量法对支撑剂密度、携砂液流速、支撑剂质量分数以及压裂液黏度4个影响因素进行评价。研究结果表明:降低支撑剂与压裂液之间的质量分数差,可以提高86%的输送距离,确保支撑剂运移至裂缝深处;较高的携砂液流速会增大支撑剂的流化度,在距离入口较远处沉降;由于液体对颗粒的曳力作用,一定程度地提高压裂液的黏度会使支撑剂填充层的液体体积分数增加23%;增大支撑剂的砂比可以提前达到支撑剂的“平衡高度”,但在入口处有堆积,容易产生卡泵现象,可以采取变速加砂和梯度混砂的方式提高支撑剂的输送效果。研究结果可为现场压裂作业提供参考。

井地联合压裂长输管路支撑剂悬浮运移规律模拟研究

井地联合压裂是煤矿井下长钻孔分段压裂的发展趋势之一,压裂液经地面压裂泵加压后通过地面贯通井、煤矿井下长输管路进入煤矿井下长钻孔实施大排量压裂。支撑剂在长输管路中的悬浮运移规律对于优化设计加砂参数、避免管路中砂堵具有重要意义。通过室内实验评价压裂液的流变性能和携砂性能;基于欧拉-颗粒流理论构建数值模拟模型,研究水平管内支撑剂悬浮运移规律及其影响因素;探讨压裂液携带支撑剂运移的流态以及临界沉降流速的计算模型。结果表明:1%降阻剂的加入能够使活性水压裂液黏度提高3~5倍,支撑剂密度越小,压裂液黏度、砂比越高,支撑剂在压裂液中的沉降速度越小;支撑剂在水平管内的流动受到多因素的综合影响,压裂液流速越小,支撑剂密度和粒径越大,支撑剂在管道底部的沉积越严重,携砂效果越差;随着管路直径的增大,管道出口截面支撑剂体积分数最大的位置由管道中下部移动至管道底部,支撑剂流动对于管路的磨损加重;砂比越大,支撑剂间的相互作用越强,压裂液携砂能力降低;优选采用疏浚技术规范推荐的模型计算活性水携砂条件下的支撑剂临界沉降速度,随着管路直径的增大,所需的临界携砂排量呈指数式增大,提高压裂液黏度可降低携砂所需的临界排量。建立的携砂运移临界排量模型和总结的支撑剂运移规律可对管路直径和压裂液排量进行优化匹配,为井地联合压裂施工提供理论支撑。