Although horizontal wells may offer significant production improvement over vertical wells, it may be necessary to fracture horizontal wells to maximize their return on investment. This is especially true in the case of tight gas formations.
This paper presents a model for fractured horizontal wells operating under constant pressure conditions, which is most suitable for producing tight gas reservoirs. The created fracture may be longitudinal or transverse. In this paper we examine the factors involved in determining the optimum number of transverse fractures for both finite and infinite reservoirs. For a group of transverse fractures, the rate distribution for each fracture is presented and analyzed. The effect of uneven fracture length is briefly presented.
The performance of a longitudinal fracture is examined and compared to a fractured vertical well and to a transverse-fractured horizontal well. A comparison of longitudinal versus transverse fractures from reservoir and operational points-of-view is presented. Also included is a short discussion of field examples.
Because performance of a longitudinal fracture is almost identical to that of a fractured vertical well, the existing solutions for fractured vertical wells may be applied to longitudinal fractures with a high degree of confidence. This approximation is valid for moderate to high dimensionless conductivity. In the case of transverse fractures, the outerfractures outperform the inner fractures. However, more than two fractures are necessary to efficiently produce the reservoir for most cases. A simplified economical analysis supports this conclusion.
Although unstimulated horizontal wells in naturally fractured reservoirs and in reservoirs with gas or water-coning problems have been very successful, there are certain situations where fracturing a horizontal well is a viable option. Because of the dependence of fracture orientation on well direction with respect to the stress field, the possibility of fracturing a horizontal well must be considered before the well is drilled. The appropriate contingency plans should be made to anticipate the possible low production from an unstimulated well. It should be remembered that fracturing a horizontal well may also dictate how the well may be completed, cemented, and cased versus open hole. Fracturing a horizontal well may take place in one of the following situations:
Restricted vertical flow caused by low vertical permeability or the presence of shale streaks
The presence of natural fractures in a direction different from that of induced fractures
Low formation productivity, or
Low stress contrast between the pay zone and the surrounding layers. In this case, a large fracturing treatment of a vertical well would not be an acceptable option since the fracture would grow in height as well as length. Drilling a horizontal well and creating either transverse or longitudinal fractures would allow rapid depletion of the reservoir through the fracture(s).
Although fundamentally similar to fracturing vertical wells, fracturing horizontal wells has unique aspects that require very special attention if successful treatment is to be secured. Differences between horizontal and vertical wells exist in areas of rock mechanics, reservoir engineering, and operational aspects.
Depending on well orientation with respect to minimum horizontal stress and length of perforated interval, either a transverse or longitudinal fracture may be created.