Formation fracturing

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Chapter 55

Formation Fracturing
S.J. Martinez, U. 01 TUIU * R.E. Steanson. DOM~~II Schlumhergcr A.W. Coulter. D oi~cil Schlumherrcr
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Introduction
Fracturing techniques were developed in 1948 and the first commercial fracturing treatments were conducted in 1949. The process rapidly gained popularity because of its high success ratio, and within a very few years. thousands ofwells per year were being stimulated by hydraulic fracturing treatments. Early treatments consisted of pumping 1,000 to 3.000 gal of fracturing fluid, containing about I Ibm of 20/40-mesh sand/gal. at rates of I to 2 bbbmin. Today. a single treatment can require several hundred thousand gallons of fluid and more than a million pounds of propping agent. Although irtjection rates have exceeded 300bblimin in some instances, rates of 20 to 60 bblimin are about average. Materials, equipment, and techniques have become highly sophisticated. A bibliography is presented at the end of this chapter for those interested in a detailed discussion of particular technologies. This discussion is limited to a generalized description of fracturing theory, materials, techniques, equipment, and treatmentplanning and design. low permeability if the connecting channels are very small and fluid flow is restricted. In the case of high permeability. drilling fluids may enter the flow channels and later impair flow into the wellbore. In the cast of low permeability, the flow channels may not permit enough flow into the wcllbore. In either case. the well may not be commercial because fluid cannot flowinto the wellborc fast enough. It then becomes necessary to create an artificial channel that w*ill increase the ability of the reservoir rock to conduct tluid into the wellbore. Such channels often can be crcatcd by hydraulic fracturing. During hydraulic fracturing treatments. what actually happens when a rock ruptures. or fractures. can be ex plained by basic rock mechanics. All subsurface rocksare stressed in three directions because of the weight of overlying formations and their horizontal reactions. Whether one of the horizontal stresses or the vertical stress ia the greatest will depend on the additional stresses imposed on the rock by prior folding. faulting. or other peological movement in the area. These tectonic stresses will control the direction of the fracture and determinewghethcr the fracture plane will be horizontal. vertical, or inclined. Every formation rock has some measure of strength depending on its structure. compaction, and cementation. It has tensile strength in both vertical and horizontal directions. The forces tending to hold the rock together are the stresses on the rock and the strength of the rock itself. When a wellbore is filled with fluid andpressure is applied at the surface, the pressure ofthe fluid in a perforation or even in the port spaces of the rock will increase. This hydraulic pressure is applied equally in all directions. If the pressure is increased, the forces applied by the fluid pressure in the rock will become equal to the forces tending to hold the rock together. Any additional pressure applied will cause the rock tosplit or fracture. The fracture will extend as long as sufficient pressure is applied by injection of additional fluids.

Hydraulic Fracturing Theory
Oil and gas accumulations occur in the pore spaces or natural fractures of a subsurface rock where structural and/or stratigraphic features form a trap. When a well is drilled into an oil-bearing rock. the fluids must flow through the surroundingrock into the wellbore before they can be brought to the surface. If the pore spaces of the rock are interconnected so that channels exist through which the oil can flow. the rock is “permeable.” The ease wzith which tlutd can how through a rock determines its degree of permeability. It has high permeability ifoil. gas, or water can flow easily through existing channels and

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