Abstract

When dealing with horizontal wells in gas reservoirs with low bottomhole pressure, it is important to avoid traps -- that is, S-shaped portions of the wellbore. These wellbore traps look and act similarly to the traps under a sink or toilet, which hold water in the dip to prevent low-pressure, odorous gases in the sewer from rising into a house.

When analyzing survey data from a horizontal well, it is sometimes difficult to identify traps just by looking at the numbers. There are a few different techniques that can be employed to solve this problem. One technique is to import the survey data into a spreadsheet and solve for the slope of the well. Once the slope is found, the traps can be determined from multiple changes of slope orientation in row. Another, more visually friendly method of identifying traps is the use of 3-D graphs. By importing the survey data into a 3-D graph, the traps can easily be identified by the "S" shaped curves in the wellbore.

Once located, traps can be bypassed; depending on the length and severity of the trap, a line can be run through the trap to provide an exit for the trapped gas. Periodic treatments of N2 can be used to blow trapped water out of traps, allowing gas to migrate to surface.

This paper will discuss the concept of traps in horizontal gas wells and different methods of identifying traps, including spreadsheets and 3-D graphs. The paper will conclude with useful means of bypassing existing traps and preventing traps in future horizontal drilling.

Introduction

The popularity of horizontal wells is on the rise in today's Oil and Gas industry. With the constantly advancing technology of horizontal drilling, more and more companies are drilling horizontal wells. Although drilling companies are getting very good at drilling horizontal wells, there is still the occasional dip. In low-pressure natural gas reservoirs these dips can spell disaster. If the formation produces any type of fluid, the dip can fill up with this fluid, creating a hydrostatic pressure block or a "trap". A horizontal well "trap" is similar to the trap under a sink or toilet. The water in a sink trap prevents the odorous gases from the sewer or septic tank from coming back up the pipe and into your house. If the formation pressure isn't sufficient enough to over-come this pressure block, that part of the horizontal well could be sealed off from production.

The best solution to this problem is to avoid gas traps altogether. To accomplish this, dips during drilling cannot exceed the width of the hole. This means if you are drilling a 4 1/2" hole, your dip cannot exceed 4 1/2" from top to bottom. While this is not always the easiest thing to do, it is the best solution. If you can't avoid "traps" the next step is identifying them and finding ways to bypass them.

Identifying Traps

The first step in identifying a trap is to know the reservoir. Low-pressure gas wells such as Northern Michigan's Antrim shale are more prone to traps. The Antrim shale is a shallow, highly fractured shale formation. Production from the Antrim shale is mostly natural gas and light brine. At one time bottomhole pressure in the Antrim ranged from 150–750psi but due to the Antrim being a highly developed formation, some parts have a lower bottomhole pressure. Many wells don't have enough bottomhole pressure to unload the water they produce. As a result, different types of artificial lift systems, such as downhole pumps or gas lift systems, are used. Current horizontal wells drilled in the Antrim shale range from 4 1/2" to 7 7/8" and can be either open hole or cased.

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