Gas Science and Engineering ( IF 5.285 ) Pub Date : 2018-02-16 H.R. Nasriani, M. Jamiolahmady
Hydraulic fracturing, is a promising stimulation technique which is also known as hydrofracturing, hydrofracking and fracking. During the hydraulic fracturing (HF), the rock is cracked, i.e., fractured, by a high pressure injection of a fluid which is known as fracturing fluid (FF). The FF is mainly water, carrying suspended sand or another type of proppants into the well to initiate fractures in the reservoir rock, and consequently, hydrocarbon and FF will move towards the well more easily through fractures.
Hydro-fracturing is extensively used to increase the well productivity index, particularly in unconventional, tight and ultra-tight reservoirs. This expensive procedure, though, sometimes fails to meet expectations regarding the production enhancement. The leading explanations for this reduced performance is fracture clean-up inefficiency of the fracturing fluid (FF) that was primarily injected.
In this study, a parametric investigation of FF clean-up effectiveness of fractures was performed with 143360 simulations (in 35 different sets) including injection, shut-in and production stages. Because of the vast number of simulation runs which was required to be implemented by a reservoir simulator, a computer code was developed and utilised to routinely read input data, implement the simulation runs and produce output data. In each set (which consists of 4096 runs), instantaneous impacts of twelve different parameters (fracture and matrix permeability (i.e., Kf and Km) and capillary pressure (Pc), end points and exponents of gas and FF in the Brooks-Corey relative permeability correlation in both fracture and matrix) were investigated. To sample the domain of variables and to study the results, full factorial experimental design (two-level FFS) and linear surface methodology explaining the dependency of the loss in gas production, compared to the case there is no loss (i.e., 100% clean-up) to the related parameters at different production stages were investigated through he tornado charts of the response surface models, frequency of simulation runs with obtained Gas Production Loss, GPL, and saturation distribution maps of FF.
Results pointed out that in general, factors that control the mobility of FF inside the fracture had the most significant impact on cleanup efficiency. Conversely, in tight and ultratight sets, particularly when the applied pressure drawdown for the duration of production stage was small, the impact of fluid mobility within the matrix on gas production loss was more noticeable, i.e., it is crucial how fluids flow inside the matrix rather than how fast fracture is cleaned. In lower permeability matrix, in general, more gas production loss was detected and clean-up was slower. The impact of Pc on GPL minimisation was stronger when pressure drawdown was small and/or shut-in time was prolonged. As the formation becomes tighter, this observation was more pronounced, in other words, for such formations, the impact of a change in pressure drawdown and/or shut-in time on Pc and GPL was more noticeable.
Additionally, the results showed that as the length of the fracture reduced the impact of fracture pertinent parameters (i.e., fracture permeability and fluid (gas and FF) mobility pertinent parameters of Corey correlation in the fracture) on GPL reduced and the impact of those pertinent parameters in the matrix on GPL increased. The impact of Pc on minimising GPL is less noticeable in shorter fractures and vice versa. As the length of fracture reduced, quicker fracture clean-up was detected compared to those for longer fracture.
These discoveries help us to better understand the hydraulic fracturing process and can be used to settle issues regarding the performance of hydraulic fracturing and to improve the design of hydro-fracturing operations, which is an expensive but popular stimulation method for tight and ultra-tight reservoirs.
中文翻译:
在非常规领域中最大限度地提高压裂产能;水力压裂返排清理分析
水力压裂是一种有前途的增产技术,也被称为水力压裂,水力压裂和压裂。在水力压裂(HF)期间,岩石通过高压注入被称为压裂液(FF)的流体而破裂,即破裂。FF主要是水,将悬浮的沙子或其他类型的支撑剂带入井中,从而在储层岩石中引起裂缝,因此,碳氢化合物和FF将更容易通过裂缝向井移动。
水力压裂被广泛用于提高井的生产率指数,特别是在非常规,致密和超致密油藏中。但是,这种昂贵的过程有时无法满足对提高产量的期望。导致这种性能下降的主要原因是最初注入的压裂液(FF)的裂缝清理效率低下。
在这项研究中,通过143360模拟(在35套不同的设备中)进行了FF清理效果的参数研究,包括注入,关闭和生产阶段。由于需要由油藏模拟器进行大量的模拟运行,因此开发了计算机代码并将其用于例行读取输入数据,实施模拟运行并生成输出数据。在每组中(由4096次运行组成),瞬时影响包括十二种不同参数(裂缝和基质渗透率(即Kf和Km)和毛细管压力(Pc)),Brooks-Corey相对气体的终点和气体和FF的指数研究了裂缝和基质中的渗透率相关性。为了对变量域进行采样并研究结果,
结果指出,总体而言,控制FF在裂缝内流动性的因素对清理效率的影响最大。相反,在密闭和超密闭状态下,尤其是在生产阶段持续时间内施加的压降较小时,基质内流体流动性对气体产量损失的影响更为明显,即,至关重要的是流体如何在基质内流动而不是清理骨折的速度有多快。通常,在较低渗透率的基质中,检测到更多的产气量,净化速度较慢。当压降较小和/或关闭时间延长时,Pc对GPL最小化的影响会更大。随着地层变得更紧密,换句话说,对于这种地层,这种观察更加明显,
此外,结果表明,随着裂缝长度的减小,裂缝相关参数(即裂缝中Corey相关性的渗透率和流体(气体和FF)迁移率相关参数)对GPL的影响减小,而这些相关参数的影响GPL矩阵中的参数增加。在较短的骨折中,Pc对降低GPL的影响不太明显,反之亦然。随着骨折长度的减少,与更长的骨折相比,可以更快地清理骨折。
这些发现帮助我们更好地了解了水力压裂过程,可用于解决有关水力压裂性能的问题,并改善水力压裂作业的设计,这是一种用于致密和超致密油藏的昂贵但普遍的增产方法。