Abstract

With conventional reserves in the United States in a decline, more and more operators are turning to unconventional reservoirs such as shale gas to meet the energy needs of the country. Unconventional reservoirs provide additional economic reserves in the United States through advancements in technologies and new applications for existing technologies. There are a number of shales in the Appalachian Basin that contain trillions of cubic feet of recoverable gas reserves. The Huron Shale, a Devonian aged formation, has been developed since the early 20th century. The Huron Shale is a very low pressure formation and wells are drilled with air to minimize formation damage from drilling fluid. To achieve optimum gas flow, the wells must be stimulated. The most common methods used are hydraulic fracturing with N2 gas only or N2 foam with proppant. There are many studies in the literature with no clear indication of which method is best. The benefits of N2 only are presented in this paper.

Areas of the Huron shale that once showed to be non-productive with vertical wellbores are now being developed through advancements in horizontal drilling and cement-less multi-stage fracture treatments. One of the critical components of a successful multi-stage fracture treatment in a horizontal wellbore is effective annular isolation. Effective annular isolation ensures that fracture placement is known and that the entire length of the lateral is treated. This paper will describe one approach in completing and performing multi-stage fracture stimulations in an air drilled hole using inflatable casing annulus packers and ball activated frac sleeves.

Introduction

The development of unconventional reservoirs has been the most rapidly expanding area of activity in the United States and parts of Canada for the past 10 years. Although some unconventional reservoirs have been developed since the early part of the 20th century, advancements in drilling and completion technology has turned areas that were once considered non-productive into valuable assets. Two of the key factors that created this trend are horizontal drilling and multi-stage fracturing treatments.

Unconventional reservoirs, or continuous deposits, are defined as petroleum accumulations that are pervasive throughout a large area and are not significantly affected by hydrodynamic influences 1. Some characteristics of these reservoirs include abnormal pressures, close association with source rocks, and low recovery factors. Typical examples are coal bed methane, low permeability reservoirs, shale gas/oil, basin-centered gas, and gas hydrates.

In recent years, considerable attention has been drawn towards the increase in activity in the Williston Basin area of Wyoming and North Dakota. Horizontal drilling and multi-stage fracture treatments have made a significant impact on the oil production from that basin. However, a much broader increase in activity across the United States has been in the development of shale gas resources, largely in part to natural gas playing a key role in meeting the United States energy demands. Obviously, additional drivers to this increase in activity are the advancements in drilling and completion technologies, as well as commodity price. Natural gas, coal and oil supply about 85% of the nation's energy, with natural gas supplying about 22% of the total 2. Much of this natural gas will come from shale gas resources. In fact, the successful Barnett Shale in Texas produces 6% of the total natural gas produced in the lower 48 states. As the Barnett Shale play ages, new plays such as the Fayetteville Shale in Arkansas and the Haynesville Shale in Louisiana are being rapidly developed. One of the newer areas of increased activity is in the Appalachian basin.

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