Abstract

This is a case study of a successful, field-wide implementation of horizontal, barefoot completions in a moderately competent formation - the Alpine Reservoir in Western North Slope, Alaska, Figure 1. A barefoot completion is a borehole without tubular liners and no cemented support - the least costly but riskiest completion strategy. (In this paper, barefoot and unsupported mean the same and are used interchangeably.) The Alpine experience provides a successful example where the benefits of unsupported boreholes outweigh the risks of borehole failure. To date, this aggressive yet simple completion technique has an aggregate length of more than 160,000 ft, all unsupported.

Combined with good drilling practices, the success of barefoot horizontal wells in Alpine is also due to the following petrophysical and geomechanical factors:

  1. Consistent reservoir quality within the layer

  2. Absence of shale-breaks in the producing zone

  3. Moderate strength in normal fault geotectonic setting

  4. Low variability of strength

  5. Linearly elastic behavior

  6. Non-severe, slight weakening when water-saturated

  7. Good permeability retained under post-elastic strains

Introduction

In general, barefoot horizontal completions are implemented only in very competent, hard formations that pose little risk for wellbore collapse and/or sand production, such as dolomites, hard limestones, hard sandstones, and shale-free siltsones. The advantages of barefoot completions are:

  1. Low completion cost

  2. Simple and fast implementation

  3. Potentially lower completion skins if undamaged

  4. High productivity per unit length (producers)

  5. High injectivity (injection wells)

  6. Higher critical drawdown pressure for sanding

However, the risks and disadvantages of unsupported, long, horizontal wells in Alpine are:

  1. Potential for collapse or sanding in weak zones

  2. Higher wellbore stresses compared to vertical wells

  3. Costly and limited options for zonal isolation

  4. Higher sensitivity to formation damage

  5. Limited options for stimulation

  6. Limited options for future remediation

  7. High friction factors for future coiled tubing workover

There are very few reservoirs that are developed exclusively with barefoot horizontal wells, owing to the abovementioned risks. One recent example is described in Australia (Allard, 1998). The Alpine experience is the first reservoir in Alaska developed exclusively with barefoot horizontal wells.

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