The role of in-situ stresses in controlling hydraulic-fracture geometry and extent has been widely recognized. This paper describes the results and applications of several research programs carried out over the past few years to optimize the design of hydraulic-fracture stimulation treatments using information pertaining to in-situ stress action within the reservoir.
Begun as fracture-mechanics-based theoretical studies of propagation and containment of hydraulically induced fractures, these programs have grown into full-scale field demonstrations of the deduced principles. A review is provided of field-measured in-situ stresses in the pay and confining formations. The existence of in-situ stress contrast between the pay zone and the bounding layers has been demonstrated in these field demonstrations. Furthermore, the results also showed the significant role of the in-situ contrasts in fracture containment. Unfortunately, however, great variability in the stress contrast from site to site has been observed. The field programs have been performed in both openhole and cased wells. Laboratory studies of hydraulically fractured large block samples have been carried out. Cubic samples up to 3.3 ft per side were subjected to triaxial stresses as high as 2,175 psi. The results of these tests have been used to support the field efforts.
The programs described in this paper indicate that successful stimulation design requires a knowledge of the in-situ stress field and contrasts within relatively narrow ranges at well depth where the stimulation treatment is performed. A general knowledge of the approximate regional stress fields and gradients is not a sufficient data base for design. Stimulation designs must be adapted to the in-situ stress contrasts to obtain deeply penetrating fractures. To minimize the costs of in-situ stress determinations on a well-by-well basis, a wireline-operated hydraulic-fracturing tool has been designed. The tool does not require a rig on the well; and because it is entirely self-contained, considerable cost savings will be possible compared with the costs of standard techniques of stress determination by hydraulic fracturing.
The design of a fracturing treatment is generally based on the assumption that the vertical height of the fracture is known and that this height remains a constant from the wellbore to the point of deepest lateral penetration. This fracture geometry may be quite accurate in the presence of strong barriers to vertical-fracture growth. In fact, massive hydraulic fracturing (MHF) results consistent with design predictions appear to be successful in reservoirs where the adjacent rock layers form effective barriers to vertical fracture growth. One must expect, however, to encounter many situations in which natural barriers to vertical fracture migration do not exist.
Application of the fundamental principles of fracture mechanics has led to rapid development of both quantitative and qualitative predictions of hydraulic-fracture growth and geometry based on knowledge of the in-situ material properties and in-situ stress. Recent studies of hydraulic fracturing have delineated those factors that affect fracture geometry and fracture containment within the pay zone. These factors include the contrast in
material properties,
in-situ stress, and
in-situ stress gradients and fracturing-fluid density.
Daneshy, Cleary, and Advani et al. further discussed the effect of the contrast in material properties (including the interface) on the created fracture geometry. The materialbarrier concept (contrast in mechanical properties) theoretically provides the basis for retarded vertical growth of a hydraulic fracture that approaches an interface between the pay zone and a bounding formation with stiffer elastic response. However, the effectiveness of such containment is negated by the high probability of intersecting preexisting flaws near the interface.
JPT
P. 1071^
