Summary

This paper presents experimental results related to hydraulic fracturing of a horizontal well, specifically the nonplanar fracture geometries resulting from fracture initiation and propagation. Experiments were designed to investigate nonplanar fracture geometries which were categorized as: multiple parallel fractures, T-shaped fractures, and reoriented fractures. This paper discusses how these nonplanar fractures can be responsible for premature screen out and excessive treatment pressure when a horizontal well is hydraulically fractured. Reasons for unsuccessful hydraulic fracturing treatments of a horizontal well are presented and recommendations to ensure clear communication channel between the wellbore and the fracture are given.

Fracture initiation pressure is a function of the wellbore orientation relative to the maximum horizontal stress. It is maximum when the horizontal well is drilled in the direction of the minimum horizontal stress. Extension pressure, however, shows little variation with respect to the wellbore direction. Experimental data shows that a relationship between breakdown pressure and wellbore orientation exists and can be used for wellbore stability problems.

Microfrac analysis has been applied on the experimental data and showed that care must be exercised to determine closure pressure from a microfrac test performed on a horizontal well.

A new phenomenon, "Relief in Pressure (RIP)", is presented to help determine the fracture azimuth relative to the wellbore from observing the pressure behavior after breakdown.

The results of these openhole experiments can also be used for perforation design in cased holes to obtain a successful hydraulic fracturing treatment. Optimum perforated interval length and perforation phasing should follow the fracture azimuth at the wellbore as ft initiates from a given horizontal well.

Introduction

Horizontal wells have been drilled to increase the effective drainage area as compared to a vertical well, which can penetrate a smaller portion of the same reservoir. A horizontal well is fractured to improve productivity. Fracturing a horizontal well has sometimes been a dilemma during the last few years with occurrence of premature screenouts and high treatment pressures. This work identifies some of the problems, and suggests some solutions.

In most geological formations, the orientation angle of a horizontal well from the maximum horizontal stress plays a crucial role in achieving a successful stimulation treatment. The following three mechanisms related to wellbore orientation relative to the maximum horizontal stress (orientation angle), need to be addressed.

  1. Fracture-wellbore communication area. Two extreme cases, longitudinal and orthogonal fractures, provide maximum and minimum communication area between the wellbore and propagating fractures.

  2. Fracture geometry near the wellbore. Important factor that may cause early screenouts. Several different fracture geometries can result when a horizontal well is fractured, among which are multiple fractures, T-shaped fractures, and complex fractures.

  3. Fracture tortuosity near the wellbore.

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