Hydraulic Fracturing: Loss of Treatment Effectiveness due to Fracture Complexity
The internet is full of articles and tutorials that describe hydraulic fracturing. You will find tons of information on introduction to the concept of hydraulic fracturing, designing of frac treatments, selecting materials (fluid & proppants) for hydraulic frac jobs. However it is difficult to find quality articles on the problems and common issues found in hydraulic fracturing – despite the technology being the forefront of the oil & gas industry in recent years.
With our introduction & detailed blog series on mini-frac treatments (with tips & tutorial), we decided to write on a topic rarely discussed in hydraulic fracturing, Pressure loss due to fracture complexity. We, in the stimulation industry are hardwired to imagine a hydraulic fracture in the formation as a beautiful elliptical bi-wing crack. Commercial simulators have these 3D colorful graphics of the frac cross-section that try its best to deceive our imagination. However, a frac, is everything but a perfect symmetry.
Further, when we talk about high treating pressures, conventionally we refer to wellbore, perforation & near-wellbore pressure losses. However, an interesting concept to understand besides pipe, perforation and near wellbore friction is pressure losses due to fracture complexity.
What is Fracture Complexity?
Think of a car covering a distance 10 miles on the straight highway, now think of the same distance covered by the same car in an urban downtown with bumpy road, traffic intersections, pedestrians and roadside hawkers. Intuitively, the effort put by the vehicle in terms horsepower would be greater in downtown & more fuel will be consumed. Similarly, as fracture growth becomes complex, resultant net pressure required to keep it open also increases.
Fracture growth and complexity scenarios (SPE 119890)
As the above pictorial suggests, complex fracture growth is analogous to downtown drive in rush hour. The turning and twisting of complex fractures in the extended or mid field region and then re orienting in the direction of the principal planes creates a high fracture pressure that does not diminish instantly when the treatment is shut-in. The Figure illustrates the conceptual complex fracture behavior in the mid-field and the far field (Weng. 1993 and Weijers et al. 2002). A Frac engineer, while also considering the effects of elevated pressures due to pipe, perforation and NWB frictions, can take a hint of Frac complexity if the pressure plot shows a high ISIP (subsequently high frac gradient). This effect is due to a pressure choke further beyond the near wellbore area in the fracture.
How does it affect a frac treatment?
Frac complexity brings along loss in frac width and subsequently improper proppant placement. The anisotropy and interaction between a propagating frac and preexisting natural fractures also contributes to fracture complexity resulting in relatively high apparent net pressures.
High treatment pressures, high frac gradients, mid field and far field treatment pressure losses and subsequently fracture complexity can be due to a wide range of reasons which mistakenly may not be perceived as “life-threatening” for stimulation effectiveness. However, a variety of scenarios are possible where an interplay of one or more reasons including creation and/or propagation of multiple fractures, interaction of induced fractures with natural fractures and re-orientations of induced fractures can lead to unfavorable outcomes. These scenarios, grouped as “fracture complexity”, are detrimental to treatment due to excessively high treating pressures, poor fracture connectivity, high leak off, inefficient proppant placement, reduced effective half-length and consequently sub premise post frac performance. Relevant case studies show controlling roles of tectonic settings of basin (normal, strike slip and reverse fault regimes), lithologies (Sandstone, Tight Sandstone and Shale) and rock fabric (stiffness and nature/ orientation of existing planes of weaknesses) on induced fracture complexities.
Why is it important to understand its role in treatment success?
Contemporary frac stimulation theories are characterizing the “Initiation and Propagation of Planer Bi-wing Fracture” as an oversimplified approach even for tight sandstone rocks as it does not take into account the key players dealing with inherent rock complexity. The information on frac complexity can help in capitalizing it by focusing its cause and effect during frac candidate selection, treatment design, modeling, performance prediction and treatment execution.
In the next addition of this blog, we will discuss how frac treatment data can be utilized to predict presence of fracture complexity and what remedial treatments or mitigations can be implemented to avoid entering the mess of fracture complexity.
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