Effect of Formation Modulus Contrast on Hydraulic Fracture Height Containment
- Hongren Gu (Schlumberger) | Eduard Siebrits (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- May 2008
- Document Type
- Journal Paper
- 170 - 176
- 2008. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 2.5.2 Fracturing Materials (Fluids, Proppant), 3 Production and Well Operations, 2.4.5 Gravel pack design & evaluation, 1.2.2 Geomechanics, 2.5.1 Fracture design and containment, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.8.3 Coal Seam Gas
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Much study has been conducted on the effect of formation Young's modulus and in situ stress on hydraulic fracture height containment in layered formations. It has been well documented that in situ stress contrast is the dominant parameter controlling fracture height growth, and that Young's modulus contrast is less important. However, a recent study pointed out that modulus contrast can have significant implications on fracture geometry and proppant placement (Smith et al. 2001). To expand on this topic, we consider the combined effects of modulus contrast and in situ stress contrast on fracture geometry. A pseudo 3D (P3D) hydraulic fracture simulator with a rigorous layered modulus formulation is used in this study. The fracture height calculated based on uniform modulus versus layered modulus, under the same in situ stress contrast conditions, is compared.
The results are analyzed and explained, based on fracture mechanics fundamentals as well as the coupled fluid pressure effect in hydraulic fracturing. One important finding is that low-modulus layers can also contain fracture height. The results from this study can be applied to hydraulic fracturing treatments in formations with moderate to significant modulus contrast. The mechanisms studied in this work can also partially explain some recent results from microseismic or tiltmeter mapping that show more fracture height containment than that predicted by commonly used P3D hydraulic fracturing simulators based on averaged modulus.
Because fracture height is recognized as one of the critical factors that can determine the success or failure of a hydraulic fracturing treatment, many studies have been conducted on the effects of formation Young's modulus, in situ stress, fracture toughness, and layer interfaces on hydraulic fracture height containment in layered formations (Smith et al. 2001; Simonson et al. 1978; Daneshy 1978; van Eekelen 1982; Warpinski et al. 1982, 1998; Teufel and Clark 1984; Thiercelin et al. 1989; Wang and Clifton 1990). Because of these studies, it is now well known that in situ stress contrast is the dominant parameter controlling fracture height growth and that Young's modulus contrast is less important. When studying different height-containment mechanisms, modulus contrast is often considered separately from stress contrast to isolate the effect of each parameter. In reality, formation layers of different moduli are likely to have different in situ stresses (Teufel and Clark 1984) and the contributions of both must be considered together.
With the development of tiltmeter and microseismic mapping services, more direct measurements or estimates of hydraulic fracture geometry are available. It has been observed that sometimes the fracture is more contained in height than predicted by simulators. Some new mechanisms and explanations have been given, including the "composite layer effect,?? "shear dampening,?? and fracture behavior at layer interfaces, for the unexpected height containment (Warpinski et al. 1998; Barree and Winterfeld 1998; Wolhart et al. 2004).
Alternatively, more advanced numerical models have been developed for hydraulic fracture simulators (Smith et al. 2001; Siebrits et al. 2001), and the combined effect of height-containment mechanisms can now be studied with fewer approximations for hydraulic fracturing conditions. The study of the layered modulus effect has been investigated using a finite element method that can rigorously account for different moduli in a hydraulic fracture simulator (Smith et al. 2001). Two effects of high-modulus layers on fracture height containment were provided and explained. The shortcomings of using an averaged modulus were pointed out by comparing simulation results of averaged modulus with that of layered modulus.
To expand further on the shortcomings of using an averaged modulus, we consider the combined effect of modulus contrast and in situ stress contrast on fracture geometry, and show that modulus contrast can have a significant effect on fracture height. Height growth can be contained by low-modulus layers because of different mechanisms than those already discussed in the literature for high-modulus layers (Smith et al. 2001).
In this paper, fracture height-containment mechanisms are briefly reviewed. A parametric study using a hydraulic fracture simulator that rigorously accounts for variable modulus is conducted for various combinations of stress contrast, modulus contrast, and fluid viscosity. The results are analyzed, and the reasons for limited height growth are explained based on fracture mechanics fundamentals and the coupled fluid pressure effect in hydraulic fracturing.
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