Experimental Investigation of Hydraulic Fracturing Through Perforations

Abstract

In this study it was found that breakdown pressures of hydraulic fractures decrease as the number of perforations increases. The existence of the casing and the perforations seems to have little influence on the direction of the created fracture, which is perpendicular to the least principal stress. Introduction The majority of hydraulic fracturing treatments in the oil and gas fields are performed through perforations. These perforations are usually created by perforations. These perforations are usually created by shaped charges, by hydrojets, or occasionally by bullets. They generally penetrate through the casing, through the cement, and several inches into the formation. Each perforation has approximately a cylindrical shape, with a diameter of about 1/8 to 1/2 in. Most of the known theoretical and experimental research on hydraulic fracturing has been performed in open holes. The reasons for this are that the openhole situation is easier to handle, and more important, the studies on open holes provide insight into the more complicated problem of fracturing cased holes. This paper appears to be the first known serious examination of hydraulic fracturing through perforations. Because of a lack of knowledge about the perforations. Because of a lack of knowledge about the stress concentrations around the borehole and the perforations, the studies reported here are mainly perforations, the studies reported here are mainly experimental and composed of observations of the types, orientations, and breakdown pressures of the induced fractures. The theoretical examination of the problem is limited to the calculation of the stresses around the casing and in the formation. However, the experimental results are quite interesting and shed new light on the influence of perforations on the created hydraulic fractures. Stress Distribution Around Cased Boreholes It is generally accepted that hydraulic fractures are created whenever the maximum tensile stress induced at the borehole wall exceeds the tensile strength of the formation. Thus the study of stress distribution around the borehole becomes an integral part of the examination of fracture initiation. Contrary to openhole situations, it is very difficult to derive analytical expressions for the stress distribution around perforated cased holes, mainly because of the complicated geometry. Thus, in examining the fracture initiation in cased holes, the only two alternatives appear to be experimentation or numerical simulation. The results reported here are derived from an experimental approach. The problem of stress distribution around a cased hole (without perforations) has been solved by Savin, for the generalized plane stress or plane strain conditions. Fig. 2 compares the stresses around open and cased holes. The symbols sigma 11, sigma 22, and sigma 33 denote the three in-situ principal stresses, and sigma, sigma 33, and sigma are used for the tangential, radial, and shear stress components around the borehole (Fig. 1). The borehole is assumed to be parallel to sigma 33. The curves in Fig. 2 show that the existence of the casing significantly alters the stresses induced by sigma 11, sigma 22, and sigma 33 around the borehole. When one adds to this the changes caused by the perforations and fluid leak-off, it becomes obvious why the breakdown pressure of perforated cased holes should be considerably perforated cased holes should be considerably different from that of open holes, JPT P. 1201