Sensitivity assessment of strontium isotope as indicator of polluted groundwater for hydraulic fracturing flowback fluids produced in the Dameigou Shale of Qaidam Basin

Hydrogeochemical processes that would occur in polluted groundwater and aquifer system, may reduce the sensitivity of Sr isotope being the indicator of hydraulic fracturing flowback fluids(HFFF) in groundwater. In this paper, the Dameigou shale gas field in the northern Qaidam Basin was taken as the study area, where the hydrogeochemical processes affecting Sr isotope was analysed. Then, the model for Sr isotope in HFFF-polluted groundwater was constructed to assess the sensitivity of Sr isotope as HFFF indicator. The results show that the dissolution can release little Sr to polluted groundwater and cannot affect the εSr(the deviation of the 87 Sr/86 Sr ratio) of polluted groundwater. In the meantime, cation exchange can considerably affect Sr composition in the polluted groundwater. The Sr with low εSr is constantly released to groundwater from the solid phase of aquifer media by cation exchange with pollution of Quaternary groundwater by the HFFF and it accounts for 4.6% and 11.0% of Sr in polluted groundwater when the HFFF flux reaches 10% and 30% of the polluted groundwater, respectively. However, the Sr from cation exchange has limited impact on Sr isotope in polluted groundwater. Addition of Sr from cation exchange would only cause a 0.2% and 1.2% decrease in εSr of the polluted groundwater when the HFFF flux reaches 10% and 30% of the polluted groundwater, respectively. These results demonstrate that hydrogeochemical processes have little effect on the sensitivity of Sr isotope being the HFFF indicator in groundwater of the study area. For the scenario of groundwater pollution by HFFF, when the HFFF accounts for 5%(in volume percentage) of the polluted groundwater, the HFFF can result in detectable shifts of εSr(ΔεSr=0.86) in natural groundwater. Therefore, after consideration of hydrogeochemical processes occurred in aquifer with input of the HFFF, Sr isotope is still a sensitive indicator of the Quaternary groundwater pollution by the HFFF produced in the Dameigou shale of Qaidam Basin.     

新疆低渗透储层压后返排制度优化研究

压后返排作为压后管理的重要组成部分已经得到了压裂工程师的广泛关注,特别对于低渗透储层而言,如不能及时返排出储层中的压裂液,压裂液将会进入储层深部,对储层产生二次伤害。但如果返排速度过快,又会导致支撑剂发生回流,降低压裂效果,严重的会导致井筒及井口装置的破坏。因此,低渗透储层返排控制的一个重要关键技术就是优化合理的返排流速。返排见气前,本文基于物质平衡原理,利用渗流力学和工程流体力学理论,建立了支撑剂回流及裂缝闭合时间计算模型,得到了不同时刻井口压力与最佳油嘴匹配关系图。当返排见气后,研究并分析了见气后油嘴变大的原因,给出了具体的油嘴制定方法。以新疆低渗透储层X井为例,制定了返排优化方案,取得了较好的返排效果,该方法对同类储层具有一定的借鉴作用。     

Fracture closure modes during flowback from hydraulic fractures

Flowback analysis recently has been considered as a potential tool for assessing induced fractures through corresponding pressure analyses and rate transient analysis. In this paper, we study fracture closure mechanisms during the flowback of fracturing fluid after hydraulic fracturing treatments. Although it is known that flowback can be significantly affected by the geometry of the fractures and closure stress, there has not been any effort to understand the problem from the geomechanical point of view; rather, available studies assume that a fracture closes uniformly with constant fracture compressibility. The coupled geomechanics and fluid flow model presented in this paper help to elucidate how geostresses may affect fracturing fluid recovery under different conditions. We perform a scaling analysis to formulate the occurrence of different fracture closure modes and then use numerical analyses to verify our scaling parameters. The factors governing the flowback process include the mechanical and petrophysical properties of the rock as well as preexisting discontinuities such as natural fractures. Three different closure modes for fracture networks are described and numerically verified. The occurrence of each mode can dramatically affect fracturing fluid recovery. The role of fluid leakoff into the formation, fractures conductivity, and drawdown strategy are examined for each fracture closure scenario.     

FixAl及同位素方法在EGS返排液研究中的应用

增强地热系统(EGS)是开采低渗透率热岩体中热能的技术,属于广义的地热储工程。其中,作为换热介质被注入岩体并在换热后返回地表的返排液,不仅是岩体地球化学特征的信息载体,而且其物理化学行为直接影响着EGS系统的运行效果。FixAl化学热力学模拟和水同位素十三线图解在天然水热系统评价中得到了广泛应用,对返排液研究的实用性则是文章的核心问题。文中收集了全球主要EGS项目的返排液资料,基于FixAl方法分析矿物与返排液的化学平衡状态,并计算了流体在深部的热交换温度,用同位素模型验证了EGS系统中原生卤水的驱替过程。研究结果表明,上述方法在EGS返排液研究中是适用的。此外,返排液的化学特征对EGS的指示意义还包括厘定原生卤水在返排液中所占比例,识别岩浆挥发分溶解及储层改造时的添加剂残留,预测结垢趋势和流体腐蚀性等。未来需要通过更多的实验和模拟方法深入研究返排液的化学特征,建立EGS的热-水-力-化学(THMC)耦合模型,为科学开发深层地热能提供依据。     

页岩气开发的水资源与绿色发展面临的机遇与挑战

水与能源是全球关注的热点问题.水平井与分段压裂技术结合等新技术的发展,为实现页岩能源生产创造了可能性,同时增加能源对水资源的依赖性.水力压裂所需水量以及页岩气开采作业产生废水的水量和水质是页岩气开发的核心之一.本文以中国页岩气井开采先驱地区的川南页岩气开采区为例,分析页岩气开发对区域水资源系统及生态系统的可能影响,提出...