Influence of Geologic Discontinuities on Hydraulic Fracture Propagation (includes associated papers 17011 and 17074 )
- N.R. Warpinski (Sandia Natl. Laboratories) | L.W. Teufel (Sandia Natl. Laboratories)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- February 1987
- Document Type
- Journal Paper
- 209 - 220
- 1987. Society of Petroleum Engineers
- 1.2.3 Rock properties, 3 Production and Well Operations, 5.9.2 Geothermal Resources, 2.5.2 Fracturing Materials (Fluids, Proppant), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.1.2 Separation and Treating, 2.5.1 Fracture design and containment, 5.1.2 Faults and Fracture Characterisation, 4.3.4 Scale, 2.4.3 Sand/Solids Control, 5.1.1 Exploration, Development, Structural Geology, 1.6.9 Coring, Fishing, 1.14 Casing and Cementing, 5.4.2 Gas Injection Methods
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Summary. Geologic discontinuities, such as joints, faults, and bedding planes, can significantly affect the overall geometry of hydraulic planes, can significantly affect the overall geometry of hydraulic fractures. This can occur by arresting the growth of the fracture, increasing fluid leakoff, hindering proppant transport, and enhancing the creation of multiple fractures. Results from mineback experiments and laboratory tests and analyses of these data are integrated to describe this complex fracture behavior.
Hydraulic fracturing has become a valuable technique for the stimulation of oil, gas, and geothermal reservoirs in a variety of reservoir rocks. In many applications, only short fractures are required for economic production, and for these, the constant-height, ideal-fracture models of Perkins and Kern, Geertsma and de Klerk, and Nordgren Perkins and Kern, Geertsma and de Klerk, and Nordgren may quite adequately represent the fracturing process. In low-permeability gas reservoirs, however, long, penetrating fractures are generally needed; in penetrating fractures are generally needed; in this case, many assumptions about the fracturing process need to be re-examined. In particular, the widely held assumption that the hydraulic fracture is an ideal, planar feature (usually of constant height) is probably untenable in many reservoirs because of geologic discontinuities.
Geologic discontinuities such as joints, faults, bedding planes, and stress contrasts are ubiquitous features whose planes, and stress contrasts are ubiquitous features whose effect on the hydraulic fracture depends on ancillary treatment and such reservoir parameters as, the treating pressure, in-situ stresses, orientations of the discontinuities, pressure, in-situ stresses, orientations of the discontinuities, and permeability. Previous analyses and laboratory and field data have shown the effects of some of these features but only hinted at others.
The effects of stresses, material properties, and unbonded bedding planes on fracture height are well documented and will not be discussed in detail in this paper. Clearly, the in-situ stress distribution is the paper. Clearly, the in-situ stress distribution is the primary factor controlling containment, but when the stress primary factor controlling containment, but when the stress contrasts are small, material property variations may have some effect. In addition, plasticity of the shale layers may restrict fracture height. Cohesionless interfaces can provide an excellent containment feature, but this is provide an excellent containment feature, but this is probably applicable mostly at shallow depths where the probably applicable mostly at shallow depths where the normal stress (in this case, the overburden) acting on the plane is small. plane is small. In a more general context, geologic discontinuities will influence the overall geometry and effectiveness of the hydraulic fracture by (1) arresting vertical propagation as described above; (2) arresting lateral propagation as at a fault or sand lens boundary where stresses may increase; (3) reducing total length by fluid leakoff; (4) reducing total length by facilitating the formation of multiple parallel-fracture systems; (5) hindering proppant transport and placement because of the nonplanarity of the fracture or fracture system; and (6) inducing additional fracture height growth from higher treating pressures because of many of the above features (e.g., Items 2, 4, and 5). The result may range from negligible to catastrophic, depending on the values of the ancillary parameters. parameters. We show examples of several of these features that were observed in mineback experiments at the U.S. DOE Nevada Test Site. In addition, we present laboratory data and analyses that give some guidelines as to when these features become important.
Examples From Mineback Experiments
The effects of many geologic discontinuities have been observed in mineback experiments conducted at DOE's Nevada Test Site. These facilities are ideal for hydraulic fracturing experiments because they provide an in-situ medium with the appropriate boundary conditions (in-situ stresses and no free surfaces) yet still allow for detailed examination of the created fractures and geological features through mineback (physical excavation of the rock to observe the fracture directly). A detailed physical description can be obtained through photography physical description can be obtained through photography and mapping, and this can be correlated with measured material properties, in-situ stress distributions, geologic discontinuities, fluid behavior, and the operational parameters of the test. parameters of the test. At tunnel level, there is approximately 1,400 ft [425 m] of overburden that provides a realistic in-situ stress distribution. The experiments were conducted mainly in ash-fall tuffs, which are soft, low-modulus, high-porosity, low-permeability tuffs that allow for easy excavation with a continuous-mining machine. Overlying the ash-fall tuff is an ash-flow tuff, which is much denser and has a higher modulus and lower porosity than the ash-fall tuff. The ash-flow tuff grades upward from an unwelded basal ash-flow tuff into a densely welded ash-flow tuff. Although the various volcanic tuffs in which these fractures are propagated are not the sandstones and shales usually encountered in gas reservoirs, proper application of rock mechanics principles allows the extrapolation of these results to principles allows the extrapolation of these results to gas well conditions.
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