Abandoned Coal Mine Methane Services
Raven
Ridge Resources, Incorporated (Raven Ridge) has extensive experience in
numerically modeling and forecasting methane recovery from
active and abandoned coal mines using a three dimensional computational
fluid dynamics (CFD) simulator, GEM, from Computer Modeling Group Ltd
(CMG) ( http://www.cmgl.ca/ ), a market
leader in oil and gas reservoir simulation technology. The simulation
tracks material balance and uses the storage and flow characteristics of
methane in coal, as well as other reservoir rocks, to provide a
production forecast for a given recovery technique. Modeling abandoned
coal mine methane (CMM) is different from conventional gas, or even
coalbed methane (CBM) reservoir simulation in several respects:
-
The void
volume resulting from the mining must be taken into account both as
a gas storage area and as a conduit to the extraction point (well or
vent shaft);
-
The
permeability to gas of the mined coal and strata above and below the
mined seam is enhanced because of the fracturing related to stress
relief caused by mining; and,
-
The modeled
pressure regime is low, sometimes below atmospheric if suction pumps
are used to extract the gas.
-
As with all
reservoir simulations, certain data must be available and some
assumptions must be made to generate a forecast:
-
The initial
gas content of the coal needs to be established;
-
The
adsorption isotherms that define the gas storage behavior of the
coals that contribute to the methane production should be measured;
-
Some
estimate of the flow characteristics, or permeability, of the coals
and other strata that might transmit gas needs to be established;
and,
-
The volume
of void space in mined-out areas in contact with the well or shaft
needs to be known.
Raven Ridge
offers the following services relative to abandoned mine methane
recovery projects:
-
Data
Collection and Analysis
-
Resource
Determination
-
Project
Specific Production Forecasting
-
Methane
Reserves Determination and Categorization
-
Project
Economic Analysis and Business Planning
These items are
discussed in more detail below. The quality and quantity of input data
for the forecasting model has a significant impact on the degree of
uncertainty of the forecasts and hence the financial risk associated
with a project. Raven Ridge is familiar with risk analysis and will
provide it if desired. Please contact us for a project specific cost
estimate.
Data
Collection and Analysis
Test
and Monitor Existing Ventilation Shafts and Gob Vent Boreholes
A Raven Ridge
technician can monitor the total volumetric rate and gas quality of
existing vent shafts or gob vent boreholes over time to determine the
yearly average rate of methane emissions. This will help to establish a
baseline emission rate for possible future greenhouse gas (GHG) emission
reduction trading. Monitoring and analysis of the pressure in the shaft
or well after shut-in will also provide evidence for characterizing the
mine void contacted by the shaft or well and the gas content and
permeability of the coal near the mine void.
Manage
Core Well Program to Determine Existing Gas Content of Remaining Coal
Our Operations
Manager can design and implement a core well program to retrieve coal
samples at strategic locations relative to the abandoned mine workings
that will provide the best gas content characterization over the project
area. This is especially important for future (CFD) modeling.
Conventional plug testing of other potential gas flow units should also
be done at this time.
Determine
Gas Content by Canister Testing
The coal
retrieved by the core well program must be sampled on the well location
and placed in cylinders to determine the methane content of the coal.
Raven Ridge can supply the required number of quality control-tested
degas cylinders and the expertise to determine the gas content from
these samples. Standard coal ultimate analysis is also performed on all
samples by an outside laboratory. Gas content samples should be taken at
each core hole for all coal seams expected to contribute to the gas
production.
Determine
the Gas Storage Function with Adsorption Experiments
Fresh samples
from the desorption experiments will be used at the Raven Ridge
laboratory to determine the gas storage versus pressure function, most
commonly expressed in terms of the Langmuir Adsorption Isotherm
Equation. One adsorption isotherm should be determined for each coal
seam expected to contribute to the gas production.
Determine
the Mine Void Volume
Mine void volume
can be determined in several ways:
-
Mined
tonnage kept by the mine operators
-
Mined
tonnage kept by regulatory agencies
-
Analysis
of maps of the abandoned workings
-
Analysis
of ventilation shaft flow data and pressure response after shut-in
-
A
combination of the above techniques
Our Staff Mining
Engineer will be able to determine the minded volume from the existing
maps. He will also be able to identify volumes isolated from the main
workings by in-mine seals. These areas may have to be exploited
individually.
Determine
the Extent of Flooding
Areas flooded by
water will not yield methane and need to be removed from calculations.
These areas need to be determined either by drilling and/or hydrologic
studies. Documentation by the mine operators will often be available
relative to the location and rates of water entries.
Methane
Resource Determination
Determine
the Volume of Methane Stored in Void Space
The data
gathered above will enable our engineering staff to determine the volume
of methane stored within the mined-out void space of the abandoned mine.
Determine
the Volume of Methane Stored in the Remaining Coal
Raven Ridge
geologists and engineers will be able to determine the amount of coal
remaining in the mined seam(s) as well as the volume of unmined coal
likely to be in good pressure communication with the abandoned workings.
This is done by reviewing data from the mine operator (exploration data)
and other geologic documentation and by reviewing mining methods to
determine the possible effect on bounding unmined coals of roof and
floor relaxation and fracturing. This information together with the data
gathered on the gas content of the coals will provide a good estimate of
the original-methane-in-place before establishing a gas recovery
project.
Project
Specific Production Forecasting
Provide
an Initial Estimate of Production Rate and Project Life Using CFD
Modeling
A CFD model can
be built based on the information gathered in the above procedures. The
numerical model can be complex or relatively simple depending on the
degree of detail desired. For initial estimates of recovery, a
conceptual model that incorporates the coal volume, void volume, current
gas content and estimates of coal permeability can be built to estimate
the recovery through time given various development scenarios. Because
of the large range of uncertainty in these parameter values Raven Ridge
recommends that a sensitivity analysis be performed on these variables
to determine their effect on the production forecasts of the desired
scenarios.
Provide
a Refined Forecast of Production Based on Matching Actual Project
Performance with CFD Model
Projects that
have historical methane production data can use this data to refine the
conceptual model by calibrating the production rate and pressure
predicted by the model to actual data. This information can help to
reduce the uncertainty of the void and coal permeability as well as the
size of the void and quantity of coal actually being drained. A more
detailed model can also be developed to help understand the effects of
known in-mine seals on production.
Methane
Reserves Determination and Categorization
Determine
Reserve Volumes Using CFD Modeling Together With Economic Analysis
Production
forecasts must be combined with project capital and expense forecasts to
determine the economic life of a project and hence its reserves.
Categorize
Reserves into Proved, Probable and Possible Reserves Categories
Raven Ridge will
categorize reserves based on U.S. Security and Exchange Commission and
Society of Petroleum Engineers reserve definitions as we believe they
apply to abandon mine methane projects. The probabilistic approach of
determining reserves is recommended for initial reserve estimates for
projects with only a little vent or production data. This approach
assigns reserves that have a 90% chance or greater of actually being
produced as proved reserves. Reserves that have a greater than 50% but
less than 90% probability of being recovered would fall under the
Probable reserves category. Possible reserves volumes should have a
greater than 10% but less than 50% probability of being produced. The
quality and quantity of data is key to categorizing reserves.
Project
Economic Analysis and Business Planning
Assist
the Project Developer to Determine the Best Use Option for the Produced
CMM Gas
Raven Ridge can
help a project developer to determine the economically optimum use of
the resource. These uses can be pipeline injection (possibly with
containment removal) or as fuel for use as a heat source or in internal
combustion or turbine engines for the generation of electrical power.
Assist
the Project Developer in Scheduling Projects Based on the Abandoned Mine
Portfolio
Raven Ridge can
provide economic forecasts for portfolio development through time given
the abandoned mine portfolio. This would include capital and
expense as well as cash flow analysis. This type of analysis can
be generated for various project scenarios and combined with uncertainty
and risk analysis to provide management with a better understanding of
the options available and the range of probable outcomes for those
options.
|
