Highly deviated and horizontal wells represent a major new thrust in the Petroleum Industry. While the drilling and completion of these wells presentinteresting problems, their stimulation has proven even more challenging.
If these wells are drilled in formations that are ordinarily candidates forhydraulic fracturing, then several issues arise:
Can a horizontal well substitute a fractured vertical well?
If a horizontal well is to be fractured. what should be its direction vis avis the expected fracture azimuth?
How will the fracture initiate?
How can zonal isolation be accomplished?
In the comparison of the production performance of fractured vertical fracturedhorizontal wells, a new skin effect describing the choke resulting from mecontact between horizontal wells and vertical fractures is introduced.Furthermore, criteria are established for the decision to complete the well inan optimum manner, so that either orthogonal or longitudinal fractures arecompleted. The concept of the net present value (NPV) is used to calculate therequired number of induced hydraulic fractures in a horizontal well at least toequal the NPV of a fractured vertical well.
Fracturing of highly deviated and horizontal wells poses new challenges, results in certain advantages and creates new possibilities.
Although there are some examples of large-volume hydraulic fracturingoperations (i,e., Overbey et al., 1988), success in the fracturing ofhorizontal and high angle wells has not always been met. In the past, fracturing of deviated wells has been unsuccessful or not aggressively pursuedfor a number of reasons, including the following.
Concern as to what methods would be economically effective for isolatingindividual stages and whether or not simultaneous injection into multiplefractures is an effective stimulation procedure. The later will be discussedfurther.
Failure of some hydraulic fracturing stimulations associated withinattention to the unique stress conditions around boreholes. Some of thesetreatments have been performed under the false premise that fracture initiationand propagation would not differ from that for a vertical well. The directionof fracture initiation may not be the same as the ultimate direction ofpropagation (preferentially perpendicular to the minimum principal stress, unless overridden by in-situ discontinuities). Hence, induced hydraulicfractures may not be planar, they will initiate in a direction governed by adynamic interaction between the stress conditions prevailing at the wellborewall and the rate-viscosity characteristics of the treatment, later propagatingin a direction perpendicular to the minimum in-situ stress component.
These aspects and the associated economic considerations should be addressedin planning the fracturing of high-angle and horizontal wells.
Success depends on a comprehension of the characteristics of initiation andpropagation of hydraulically induced fractures in arbitrarily orientedwellbores. To appreciate this issue, consider two limiting cases (Fig. 1). Theupper panel of Fig. 1 shows a longitudinal fracture, associated with a wellboredrilled in the s2 or s1 direction. The lower panel ofthis figure represents a transverse fracture. This wills preferentially occurwhen the wellbore is aligned with the direction of the minimum principalstress.