Summary
Four distinct sequential phases form a recommended process for
coalbed-methane (CBM)-prospect assessment: initial screening, reconnaissance,
pilot testing, and final appraisal. Stepping through these four phases provides
a program of progressively ramping work and cost, while creating a series of
discrete decision points at which analysis of results and risks can be
assessed. While discussing each of these phases in some degree, this paper
focuses on the third, the critically important pilot-testing phase.
This phased CBM-prospect assessment process allows us to
- Gain local knowledge early at low cost
- Progressively acquire and compile appropriate data to assess the geological
situation and reservoir conditions systematically
- Identify and attempt to fill the most important knowledge gaps that
represent the greatest uncertainties and risks to the prospect
- Increasingly understand the distributions of key parameters that control
reserves, deliverability, and value
- Stage expenditures and provide multiple decision points through the
process
- Ultimately, produce a project with very low development risk
In the CBM-prospect assessment process, the pilot test serves the same
function as a conventional exploration well. If it proves successful, then the
prospect can be considered a discovery and can be appraised for development. By
drilling, completing, and producing a cluster of wells in a CBM pilot test,
short of proceeding directly to a partial development, we are able to locally
dewater and depressurize the coal seam to be tested and, thereby, desorb and
deliver measurable volumes of gas. If correctly implemented, the pilot test
allows us to assess the local variability of key reservoir parameters, collect
the information necessary to simulate the reservoir’s producibility, and,
thereby, estimate potential project reserves to a reasonable degree of
accuracy.
This paper contains roughly 30 specific recommendations and the fundamental
rationale behind each recommendation to help ensure that a CBM pilot will
fulfill its primary objectives of (1) demonstrating whether the subject coal
reservoir will desorb and produce consequential gas and (2) gathering the data
critical to evaluate and risk the prospect at the next--often most
critical--decision point. Importantly, these objectives must be met in a timely
manner. To do this, the specifications for the pilot are often not those that
will be used for an optimized well or field-development pattern in terms of
costs or production. This is intentional. The goals of piloting are different
from the goals of development. So, the recommended designs are different. The
pilot design recommendations focus on collecting superior data that will
quantify key parameters for interpretation and simulation of the reservoir,
retaining flexibility in the face of the level of uncertainty remaining after
the reconnaissance phase, and arriving at a definitive answer on the coal
reservoir’s viability in an acceptable time frame.
Detailed data-analysis methods for CBM are not discussed here--these are
well covered in the literature. Rather, we focus on the importance, use, and
potential pitfalls of data collected at the various phases of the assessment
process. Examples are used to highlight the purpose and importance of various
aspects of the data gathering and analysis. A general history-matching
process--valid at the pilot-stage analysis and beyond--is presented as a
guide.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
7 January 2008
- Meeting paper published:
10 February 2008
- Revised manuscript received:
16 February 2009
- Manuscript approved:
27 February 2009
- Published online:
28 October 2009
- Version of record:
28 October 2009
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