Integrated numerical simulation of hydraulic fracturing and production in shale gas well considering gas-water two-phase flow

Based on the displacement discontinuity method and the discrete fracture unified pipe network model,a sequential iterative numerical method was used to build a fracturing-production integrated numerical model of shale gas well considering the two-phase flow of gas and water.The model accounts for the influence of natural fractures and matrix properties on the fracturing process and directly applies post-fracturing formation pressure and water saturation distribution to subsequent well shut-in and production simulation,allowing for a more accurate fracturing-production integrated simulation.The results show that the reservoir physical properties have great impacts on fracture propagation,and the reasonable prediction of formation pressure and reservoir fluid distribution after the fracturing is critical to accurately predict the gas and fluid production of the shale gas wells.Compared with the conventional method,the proposed model can more accurately simulate the water and gas production by considering the impact of fracturing on both matrix pressure and water saturation.The established model is applied to the integrated fracturing-production simulation of practical horizontal shale gas wells.The simulation results are in good agreement with the practical production data,thus verifying the accuracy of the model.

Numerical simulation of gas transport mechanisms in tight shale gas reservoirs

Due to the nanometer scale pore size and extremely low permeability of a shale matrix,traditional Darcy＇s law can not exactly describe the combined gas transport mechanisms of viscous flow and Knudsen diffusion.Three transport models modified by the Darcy equation with apparent permeability are used to describe the combined gas transport mechanisms in ultra-tight porous media,the result shows that Knudsen diffusion has a great impact on the gas transport and Darcy＇s law cannot be used in a shale matrix with a pore diameter less than 1 μm.A single porosity model and a double porosity model with consideration of the combined gas transport mechanisms are developed to evaluate the influence of gas transport mechanisms and fracture parameters respectively on shale gas production.The numerical results show that the gas production predicted by Darcy＇s law is lower than that predicted with consideration of Knudsen diffusion and the tighter the shale matrix,the greater difference of the gas production estimates.In addition,the numerical simulation results indicate that shale fractures have a great impact on shale gas production.Shale gas cannot be produced economically without fractures.

Fracture network types revealed by well test curves for shale reservoirs in the Sichuan Basin,China

Pressure buildup testing can be used to analyze fracture network characteristics and conduct quantitative interpretation of relevant parameters for shale gas wells,thus providing bases for assessing the well productivity and formulating proper development strategies.This study establishes a new well test interpretation model for fractured horizontal wells based on seepage mechanisms of shale reservoirs and proposes a method for identifying fracturing patterns based on the characteristic slopes of pressure buildup curves and curve combination patterns.The pressure buildup curve patterns are identified to represent three types of shale reservoirs in the Sichuan Basin,namely the moderately deep shale reservoirs with high pressure,deep shale reservoirs with ultra-high pressure,and moderately deep shale reservoirs with normal pressure.Based on this,the relationship between the typical pressure buildup curve patterns and the fracture network types are put forward.Fracturing effects of three types of shale gas reservoir are compared and analyzed.The results show that typical flow patterns of shale reservoirs include bilinear flow in primary and secondary fractures,linear flow in secondary fractures,bilinear flow in secondary fractures and matrix,and linear flow in matrix.The fracture network characteristics can be determined using the characteristic slopes of pressure buildup curves and curve combinations.The linear flow in early secondary fractures is increasingly distinct with an increase in primary fracture conductivity.Moreover,the bilinear flow in secondary fractures and matrix and the subsequent linear flow in the matrix occur as the propping and density of secondary fractures increase.The increase in the burial depth,in-situ stress,and stress difference corresponds to a decrease in the propping of primary fractures that expand along different directions in the shale gas wells in the Sichuan Basin.Four pressure buildup curve patterns exist in the Sichuan Basin and its periphery.The pattern of pressure buildup curves of shale reservoirs in the Yongchuan area can be described as 1/2/→1/4,indicating limited stimulated reservoir volume,poorly propped secondary fractures,and the forming of primary fractures that extend only to certain directions.The pressure buildup curves of shale reservoirs in the main block of the Fuling area show a pattern of 1/4/→1/2 or 1/2,indicating greater stimulated reservoir volume,well propped secondary fractures,and the forming of complex fracture networks.The pattern of pressure buildup curves of shale reservoirs in the Pingqiao area is 1/2/→1/4→/1/2,indicating a fracturing effect somewhere between that of the Fuling and Yongchuan areas.For reservoirs with normal pressure,it is difficult to determine fracture network characteristics from pressure buildup curves due to insufficient formation energy and limited liquid drainage.

页岩气压裂管汇冲蚀损伤行为研究

材料冲蚀是油气行业常见的失效模式。在页岩气水力压裂过程中,地面高压管汇持续受管内压裂液支撑剂固体颗粒高速冲击。在苛刻工况下,高压管汇敏感部位易由冲蚀损伤而导致穿孔甚至爆裂。然而,对页岩气压裂管汇冲蚀损伤机理尚不完全清晰,导致对冲蚀行为难以预测。对高压管汇材料开展了固液两相流冲蚀磨损试验,分析了携砂液冲击角度、流速对靶材冲蚀磨损的影响机制,然后基于试验参数开展了颗粒冲蚀过程中的冲击动力学仿真,对不同工况下材料冲蚀过程进行分析。结果表明:冲击角度和流速会严重影响冲蚀磨损程度,冲蚀率随冲击角度增大呈现出先增大后减小的趋势;随着冲击角增大,靶材处接触应力的各个分量变化较大,导致不同角度的冲击颗粒对靶材产生的切削、唇片、凹坑等微观损伤形貌的差异。随着流速增大,由于靶材接触应力影响区的范围和应力值大幅增加,冲蚀率呈幂函数增加。

Investigation of low water recovery based on gas-water two-phase low-velocity Non-Darcy flow model for hydraulically fractured horizontal wells in shale

Various mechanisms are employed to interpret the low water recovery during the shale-gas production period,such as extra-trapped water in the fracture network,water imbibition due to osmotic pressure and capillary pressure.These lead to the difficulty of water flow,which could be described by lowvelocity non-Darcy's law known as threshold pressure gradient(TPG).In this paper we firstly employ the low-velocity non-Darcy's law to describe the water flow and use Darcy flow accounting for slip flow and free molecular flow mechanisms to model gas flow in the shale formation.The sensitive study using numerical simulation shows that the proposed flow model could model the low fracturing liquid recovery and that large pseudo TPG leads to lower fracturing liquid recovery.Thus,the proposed model would give new insight to model the low water recovery in shale formations.

页岩气藏大规模体积压裂后井筒快速排液方法优选

页岩气藏孔隙度低,地层压力低,经过大规模体积压裂后,返排率低一直是试油工艺的一个难题。压裂后,流体在产层中停留时间过长,会损害产层,很难达到预期的压裂效果。如何高效快速排液对页岩气藏的开发具有十分重要的意义,文章探讨了目前常用的几种页岩气体积压裂后快速排液方法,为页岩气储层改造后快速排液提供了科学依据。

页岩气压裂高压管汇失效风险分析及防控措施

针对大排量、高压力页岩气压裂施工作业中存在的高压管汇失效风险,分析了某公司近年来的高压管汇失效事故事件,识别出了常见的失效形式及部位,构建了基于计算流体力学的固液两相流射流模型,模拟计算了失效的破坏力和伤害范围。研究结果表明:刺漏是高压管汇最常见失效形式,弯头、三通等应力集中区是刺漏失效的高危部位;100 MPa工况时刺漏影响范围可达100 m,离刺漏点1 m处,流体流速为260 m/s,打击压力达3.5 GPa,远超防护钢板抗剪切强度。根据风险分析及事故模拟结果,从本质安全、工艺改进、管理、信息化提升4个维度提出了防控措施,为页岩气压裂施工作业安全防控措施的制定提供了参考。

页岩气井区压裂集中供水方案的研究与设计

为满足日益发展的页岩气产业需求,依据页岩气勘探开发的技术支撑,结合我国页岩油气资源多位于水资源缺乏地区而开采页岩气又离不开水的特点,针对页岩气开发过程的压裂用水提出了一套合理、高效的集中供水方案,通过合理布置永久管网和临时管网,灵活调配已建生产井站返排液,既缩短了工程建设周期,又减少了工程投资,达到节约水资源的目的。通过各区块压裂供水试验证明:集中供水方案能有效调整、规划各区块平台井站投产后的返排液,最大程度地降低清水使用量,解决了各区块水资源缺乏问题,具有较好的实际应用价值,可推广于各页岩气田使用。

用于页岩气压裂液的可折叠软体储液罐研制

页岩气藏开发需进行大规模分段压裂改造,动用的液罐数目庞大,传统钢制储液罐只能"一车一罐"运输,井场占地面积大、施工成本高。为了降低液罐综合使用费用、减少液罐占地面积,自主研制了一种可折叠软体储液罐。基于有限元模拟,首先进行了结构优化设计、结构参数的优化和材料及加工工艺优选等,使其具有优异的力学性能和便于作业的技术参数。试制及评价试验结果表明,该储液罐有限元模拟结果和罐体可靠性试验数据一致性好,软体内罐、吊网的强度设计能满足实际使用需要,可靠性、安全性高;加入防旋流设计、快速排液和排空等设计理念,使其进一步具备应用于页岩气压裂液的技术优势。目前该储液罐已成功用于JS10、XY1-HF、XY2-HF等井的现场施工,先导性试验结果表明其完全满足页岩气压裂改造的大液量储液需要,大幅度降低了运输成本和施工费用,减小了井场占地面积,为保障页岩气藏安全高效开发、降低施工成本提供了技术支持。