Application of Fracturing To New Recovery Schemes
Authors: Anderson H.A. 
Published: March 2010

SINCE the first hydraulic fracturing treatment was conducted in 1974, fracture stimulation has become a standard completion technique for conventional crude and natural gas wells. In that 28-year period, nearly one million wells have been treated in zones from a few hundred feet deep to over twenty thousand. New techniques are continually being developed to handle new problems a. the method is applied more widely.

The theoretical aspects of fracturing are the subject of a large body of literature, but, because of the lack of direct evidence, many theories are largely unverified.

The purpose of the present paper is to examine the potential of hydraulic fracturing to improve in-situ tar sand recovery, heavy oil recovery, in-situ coal gasification and geothermal energy recovery. Three questions are asked and methods of determining answers are presented:Can a fracture be createdIs the fracture horzontal or vertical, and if vertical what direction does it run?How far does the fracture go?

Fracture Initiation

Hubbert and Willis(2) developed many of the original concepts regarding rock failure. It is generally conceded that failure occurs on a plane perpendicular to the least stress. The direction of that stress and even its magnitude are very difficult to predict with confidence. Some semi-empirical approaches have shown limited success.

A number of authors(5) conceive rock failure to be tensile inelastic rupture after the overcoming of tectonic stresses. However, at the very narrow tip of the fracture, with almost zero area, there are such great stresses that failure is immediate and inescapable regardless of the tensile strength of the rock. Because nature insists on finite forces, the pressure in the fracture must drop to zero some short distance before the tip; instead of an infinitely sharp liquid chisel, the fracturing wedge is blunt and does not penetrate the entire fracture length(3).

There is some contention that the drilling of a borehole affects the formations in the immediate vicinity of the wellbore, increasing stress concentration(1). This would explain the occurrence of initial breakdown pressures higher than the extension pressures. Fenner and Westergaard'" propose that there is a plastic zone surrounding the borehole some 80 wellbore diameters across. Plastic deformation in this area could also account for the increase in breakdown pressures over extension pressures.

Fracture Orientation

So far we have left untouched the matter of orientation. Hubbert and Willis(2) reasoned that in areas characterized by normal faulting, vertical fractures should predominate, because horizontal stresses are less than overburden stress. Where active thrust faulting occurs, horizontal fractures should occur; fracture pressures will be equal to or greater than the weight of overlying formations. In geologically simple areas, fractures will generally be vertical and roughly of the same strike, because horizontal stresses are usually less than the overburden stress. Fracture-orientation determinations have been made on a variety of oil and gas formations by direct measurement with impression packers(7,8), borehole photography(10) and coring(6), and indirectly- from inter-well pressure interference tests(9), water-flood performance (11), acoustic logs(8), aerial photography(7,13) and failure- pressure studies.