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Abstract
Since 2001, Williams Production Mid-Continent Company has drilled over 200
horizontal wells in the Hartshorne Coal
in the Arkoma Basin in southeastern Oklahoma. The Arkoma basin is an elongate
sedimentary basin extending
from east-central Oklahoma into Arkansas (Figure 1). The primary zones
producing CBM gas in the Arkoma Basin are
the Hartshorne sand, and, the Upper Hartshorne Coals, which coalesce into one
bed towards the north, and, are the
most actively explored gas producing reservoir in the western Arkoma basin. The
Hartshorne Coal is a member of the Pennsylvanian Hartshorne Formation, which is
a part of the Krebs Group of the Desmoinesian Series (Figure 2). The coals
range from high-volatile bituminous to low-volatile bituminous in rank. The
Hartshorne Coal encompasses a large area in Haskell, Pittsburg, LeFlore, and,
McIntosh counties, and, ranges from 3 ft to 7 ft in thickness. Typical average
horizontal
drilled length has ranged from 2000 ft to 2500 ft, with the longest well being
over 3000 ft. The wells are usually
completed with a slotted liner. Unlike CBM wells in other areas of the United
States, Hartshorne coal wells are relatively “dry”, and, typically produce gas
from the first month, with very little water production. This paper presents
examples of using analytical techniques for production forecasting, and,
reserves estimation for horizontal CBM wells in the Arkoma basin. The
production analysis results were integrated with core data, and, material
balance calculations, and, indicate
recovery factors ranging from 50% to 80% for the Hartshorne CBM wells. The
paper also provides a
comparison of the performance of horizontal and vertical CBM wells in the
basin. In addition, pressure buildup tests were conducted on over 40 horizontal
wells, to estimate the reservoir pressure, and, to calculate permeability and
skin factor. The paper presents two examples of pressure buildup analysis for
Arkoma horizontal CBM wells. The PBU analysis indicated effective permeability
in coal of the order of 30 md, which is attributed due to the significant
cleating of the Hartshorne coal in the area. The paper includes guidelines for
drilling horizontal wells for exploitation of Hartshorne CBM.
1. Introduction
CBM activity in the US has been growing steadily nearly every year since 1989.
According to the US department of
energy, the contiguous United States is estimated to have CBNG in-place
resources of 700 trillion cubic feet (Tcf), of
which 100 Tcf may be economically recoverable1. In Oklahoma, in the last decade
alone, over 3000 CBM wells
have been drilled, primarily in the Arkoma and the Cherokee basins. The CBM
production in Oklahoma has been growing at an exponential rate for the last
several years, and, the cumulative gas production through 2005 was 154 Bcf 2.
In general, the Hartshorne coal in the Arkoma are thicker and deeper than those
in the northeast Oklahoma shelf. Statistically, they also have higher initial
producing rates and significantly lower water production. The low water
producing rates is one of the main things that makes the Hartshorne coal unique
from most coals in the U.S. Even in the horizontal completions where
significantly more pay is exposed, the average water producing rate is 10
barrels of water per day. There is obviously no “dewatering period” with
Hartshorne coals that a lot of other coals exhibit.
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