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Abstract
During the last five years, many shale gas plays have emerged, such as the
Woodford, Haynesville, Fayetteville, Marcellus, Eagle Ford, etc. However, the
Barnett Shale is the most mature of the currently producing gas shale plays in
the US. Understanding the production mechanism and the production decline
characteristics of the Barnett Shale will contribute to a better understanding
of how newer shale plays will behave in a longer term. A production decline
curve analysis (DCA) in the Barnett Shale is presented that incorporates public
production data from over 8,700 horizontal wells. This analysis is performed by
grouping wells based on years on production. Production data are normalized to
zero-time, and regression history-matching is performed to estimate decline
curve parameters. Long-term production is forecasted for 30 years. Production
decline results strongly suggest hyperbolic decline. Particularly, the b value
ranges from 1.3 to 1.6. First-year decline rate ranges from 56% to 74%.
Thirty-year cumulative production ranges from 2.0 to 3.0 Bscf. The Best
12-month average gas rate and first 24-month cumulative gas production show a
consistent trend and correlate very well with the predicted 30-year cumulative
gas production. In spite of the advancement in completion and drilling
practices, the gas recovery from newer wells is slightly worse than that of the
older wells. This may be attributed to the poorer reservoir quality outside the
core area or interference between wells due to down-spacing. Wells producing
more than 5 years are often re-stimulated using hydraulic fracturing
techniques. It is still too early to tell the impact of re-stimulation on
ultimate gas recovery. However, the re-stimulated wells all show an increase in
the short-term well productivity.
Introduction
By October 2009, about 13,000 wells were completed in the Barnett Shale (IHS
2009). The Barnett has cumulated about 6.8 Tcf of gas since June 1982 (IHS
2009). Total gas production from the Barnett Shale contributes about 70% of all
shale gas production in the US. Horizontal well drilling has been increased
dramatically since early 2003. Horizontal wells are typically re-stimulated
after five years of production (Siebrits et al. 2000, Moore and Ramakrishnan
2006, Potapenko et al. 2009, Valko 2009). Fig. 1 shows the new well count
(H-horizontal well; V-vertical well; D-deviated well) in the Barnett Shale over
time. There have been many presentations and publications regarding well
production potential and decline trend in the Barnett (Ilk et al. 2008, Lewis
and Hughes 2008, Mattar et al. 2008, Valko 2009, Baihly et al. 2010, Valko and
Lee 2010). There have also been debates on average estimated ultimate recovery
(EUR) from horizontal wells, hyperbolic vs. exponential decline trend, range of
the b-factor, average well life, etc. Some of the comments have really caused
some concerns about the future potential of the Barnett Shale and other gas
shale plays in the US (Berman 2009). This, in turn, can affect many investment
and development decisions that have been based on the assumptions of
significant growth potential of the play. Instead of letting the debate and
worry continue, we performed a production decline analysis using production
data from all the horizontal wells drilled in the Barnett shale. Fig. 2 shows
existing well locations as of October 2009. There were a total 9,267 horizontal
wells (red dots) drilled in the Barnett. Of these horizontal wells, 8,707 wells
have production data and 8,462 wells are still active.
The objectives of this study are to find the answers to the following questions
for the industry regarding the Barnett Shale.
1. What is the average horizontal well’s EUR from a conventional DCA?
2. What is the true production decline trend (exponential or hyperbolic)?
3. What is the average b-factor for DCA? Can the b-factor ever be greater than
1.0? If yes, what does it mean?
4. Is there still room in the Barnett for further down-spacing?
5. Does re-stimulation really help increase EUR?
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