Abstract

The fracturing of horizontal wells has recently gained wide acceptance as a viable completion option to maximize the return on investment. This is especially true in the case of tight gas formations.

Horizontal wells have unique rock mechanics and operational aspects that affect fracture creation and propagation and control fracture orientation with respect to the horizontal well. The fracture orientation greatly affects the productivity and well testing aspects of the fractured horizontal wells. Depending on stress orientation relative to the wellbore, the fractures may be transverse or longitudinal.

This paper presents a model for fractured horizontal wells operating under constant pressure conditions. This condition is most suitable for producing tight gas reservoirs. The model considers the presence of either transverse or longitudinal fractures.

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 highly conductive 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 outer fractures outperform the inner fractures. However, in most practical cases, more than two fractures are necessary to efficiently produce the reservoir. A simplified economic analysis supports this conclusion.

Introduction

As evident from many actual field applications, drilling of horizontal wells in naturally fractured reservoirs and in reservoirs with gas or water-coning problems has been very successful. However, there are certain situations where fracturing a horizontal well is an economically attractive completion 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; i.e. cemented and cased or open hole. Fracturing a horizontal well may take place under several scenarios, some of which are listed below:

  1. Restricted vertical flow caused by low vertical permeability or the presence of shale streaks,

  2. Low formation productivity due to low formation permeability,

  3. The presence of natural fractures in a direction different from that of induced fractures. Thus, induced fractures have a high chance of intercepting the natural fractures, or

  4. There is 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.

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