Saturday 18 June 2011

INTERPRETATION DESORPTION SAMPLE OF COALBED METHANE

INTERPRETATION OF RAVEN RIDGE RESOURCES DESORPTION REPORTS



Raven Ridge Resources desorption reports provides the results and a complete desorption history for each well. Each report includes “Desorption Sample Summaries”, for each desorption sample collected from the well. The term “Desorption Sample Summary” refers to the raw and calculated data presented for a single desorption canister. The following is an interpretation of the information contained on the summary page of the “Desorption Sample Summary”.


RAW MEASURED DATA

Total Residual Gas (STP) is the total volume of gas released during crushing of a completely desorbed sample described below.

Total Desorbed Gas is the total volume of gas that was desorbed from the sample in the canister.

Raw Sample Weight is the weight of the desorption sample, without residual moisture and drilling mud, and before float/sink tests and proximate analysis is performed.

DAF Sample Weight is the raw sample weight less the moisture and ash as determined by the proximate analysis.

Float% (performed on cuttings sample) refers to percentage of the desorption sample that floats in a float-sink (gravimetric separation) test. The sample is ground to 8 mesh size and placed in a liquid with a specific gravity of 1.75 g/cc. The denser, non-coal portion of the sample will sink, while the lighter, clean coal portion of the sample floats.

Clean Coal Weight is the decimal fraction of the sample which floats (i.e., clean coal) multiplied by the raw sample weight. Typically used for desorption reports using cuttings samples.

Raw Conditions refers to calculating the gas content by dividing total desorbed gas by raw sample weight at ambient temperature and pressure conditions. Because most intervals contain shale and other material in addition to clean coal, we present here an estimate of the volume of gas present per ton of all material in the sampled interval.

Raw STP Conditions refers calculating the gas content by dividing total desorbed gas by raw sample weight at standard temperature and pressure, or raw STP conditions. As with “raw Surface Conditions” this is also an estimate of the volume of gas present per ton of all material in the sampled interval.

Canister Headspace is the void space in the desorption canister that is not filled with coal or fluid. The expansion and contraction of the gas in the headspace influences the volume of gas measured during the desorption process. Therefore, it is necessary factor in the headspace when calculating the total volume of gas desorbed.



PROXIMATE ANALYSIS

The proximate analysis provides a measurement of the relative amount of volatile and non-volatile organic compounds in the clean coal (float portion) as well as the percentage of water and non-combustible mineral materials. The proximate analysis reports the percentages of moisture, ash, volatile matter, and fixed carbon on an “as received basis” (before drying). Also included is the percent of sulfur and the heating value for each coal desorption sample. The analysis is performed using AST procedures as follows:

Moisture content is determined by weighing the coal desorption sample before and after it is dried under standardized conditions. The coal is first dried of visible surface moisture (drilling fluids, etc.) and weighed. The coal is then dried eliminating all inherent moisture (water held inside the capillaries, organic compounds, clays and minerals). The coal sample now is weighed a second time. The difference in the two weights represents the moisture content of the coal sample. It is then reported in percent.

Ash is the non-combustible inorganic residue that remains when coal is burned. It represents the bulk of the mineral matter in the coal, after volatile components such as CO2, SO2 and H2O have been driven off. It consists of almost entirely of the decomposed residues of silicates, carbonates, sulphides and other minerals.

Volatile matter content is determined by weighing the sample before and after it is heated to the point where its readily vaporized (volatile) constituents are driven off. Volatile matter represents the components of the coal, except for the moisture content, that are liberated at high temperature in the absence of air.

Fixed carbon is the combustible material remaining after moisture and volatile components have been removed. If corrections are applied for ash and mineral matter, it may be used for as an index of coal rank and a parameter in coal classification. Fixed carbon is not determined directly, but is the difference, in an air dried coal, between the sum of the other components (moisture, volatile matter, ash) and 100%.

Heating Value of the coal represents caloric value of the coal sample measured in (Cal/g).

% Sulfur refers to the total sulfur found in the coal desorption sample. Sulfur is primarily a component of the ash, with a small amount in the carbon. It occurs in the form of organic sulfur, sulfide minerals, such as pyrite, and sulphate minerals.

Apparent Specific Gravity refers the specific gravity of the clean coal. It is the ratio of the weight of clean coal to the weight of an equal volume of water.


COAL GAS CONTENT

The gas content of the desorption sample is reported on a clean coal basis and a fixed carbon basis for the Smith and Williams and Raven Ridge’s modified Decline Curve method. Gas content reported on a clean coal basis is the most widely used when comparing gas contents to one another. It includes the desorbed gas from all coal material, while the gas content calculated on a fixed carbon basis includes only the weight of the carbon from the coal sample. Because the carbon is always a fraction of the total weight of a coal sample, the gas content will always be higher for the fixed carbon basis.

Lost Gas is the gas desorbed from the coal sample from the time the coal seam was penetrated until the sample is sealed in the canister.

Volume correction factor is determined from a set of STR (surface time ratio) curves and the calculated LTR (lost time ratio); see Smith and Williams (1981).

Clean Coal Basis is calculated by multiplying the volume of desorbed gas by the volume correction factor and dividing the result by the clean coal weight. The clean coal weight refers to the weight of the coal material in the float portion of the cuttings sample. It is reported only in desorption reports using cutting samples.

Fixed Carbon Basis is calculated by multiplying the gas content of the clean coal by the decimal fraction of fixed carbon. Presently reported only on cuttings samples desorption reports.

Residual Gas is the gas remaining in the desorption sample after it has stopped desorbing gas for at least two consecutive days. It is calculated from crushing a split of the desorption sample at -200 mesh. Residual gas is optional and only performed upon request.


MODIFIED DECLINE CURVE METHOD

Raven Ridge calculates and displays the gas content for each desorption sample in two formats using the Smith and Williams and Raven Ridge modified Decline Curve methods. Raven Ridge Resources has modified the decline curve method of Chase (1979) to more accurately account for lost gas. Raven Ridge’s modified decline curve method utilizes all available desorption data, instead of only a few early points to extrapolate back to zero time (initial penetration of the coalbed). This method is used for drill cuttings as well as core.

Specifically, a graph of elapsed desorption time vs. cumulative desorbed gas usually produces data described by an equation in the form 7=a*ln(x) = b, where x is the time from seam penetration to container sealing (Ts). The equation is solved for y, which is lost gas. Occasionally, variations in sample type and/or coal characteristics produce data best described by linear or exponential curves. Lost gas is then calculated according to the equation whose curve best fits the data.

Results are presented in the same format as for the Smith and Williams Method, but are derived somewhat differently:

• Clean Coal Basis is the total volume of gas desorbed per unit weight of coal (expressed in cc/g), including lost gas. Reported on cuttings samples desorption reports only.

• Fixed Carbon Basis is calculated by multiplying the clean coal basis value by the decimal fraction of fixed carbon. (This is the same calculation used in the Smith and Williams’s presentation). Presently reported only on cuttings samples desorption reports.
• The Volume Correction Factor is the volume of gas desorbed per unit weight of clean coal including lost gas divided by the volume of gas desorbed per unit weight of clean coal excluding lost gas.


SMITH AND WILLIAMS METHOD

Smith and Williams (1981) developed a variation of the direct method to more accurately determine the gas content of coal cuttings from rotary drilled boreholes. According to the Smith and Williams Method of calculating gas content, lost gas is determined by multiplying the volume of desorbed gas by the volume correction factor, then subtracting the volume of desorbed gas by the result. The total volume of gas desorbed per unit weight of coal (including lost gas) is then presented in two formats:


CALCULATION OF GAS VOLUMES AND DESORPTION RATES FROM RAW DATA*

The lower portion of each Raven Ridge Resources desorption report contains raw data including the date and time of each desorption measurement, and the ambient pressure, temperature, and volume of gas measured at each of those times. From this data, parameters such as the volume fraction of gas desorbed over a specified time period and the rate of desorption are calculated and also presented.

Time (h) Elapsed is the time, in hours, from when the canister was sealed to the time a specified desorption measurement was taken. The number of days elapsed is also presented, as Time (d) Elapsed.

P amb is atmospheric pressure read directly from a barometer.

T amb is the temperature in Celsius read from a thermometer.

Meas V is the measured volume of gas at ambient conditions.

V @ STP is the measured volume of gas at STP (15 deg. C., 1 atm.).

VOL. is expressed as gas content measured in standard cubic centimeters per gram.

CUM. VOL. is the cumulative volume of gas desorbed from the time the canister was sealed until the specified measurement was taken.

VOL. FRAC. DESORBED is the cumulative volume of gas desorbed at a specific time divided by the total volume of gas desorbed upon completion of the entire desorption process.

RATE is the volume of gas desorbed since the previous measurement divided by the time elapsed since the previous measurement.

*NOTE: If attempting to duplicate values calculated in a Raven Ridge Resources desorption report, such as Rate, remember that many values presented in the report, such as Time Elapsed are rounded so that some significant digits may not appear in the printed report. The actual calculations, however, were performed using all significant digits. The calculated values presented in the report are thus more accurate than values calculated using only the truncated numbers which appear in the report.


REFERENCES

Chase, R. W., 1979, A comparison of methods used for determining natural gas content of coalbeds from exploratory cores. METC/ CR-79/18, U.S. Dept. of Energy, p 22.

Smith, D. M., and Williams, F. L., 1981, A new technique for determining the methane content of coal. Proceedings of the 10th Intersociety Energy Conversion Engineering Conference, Atlanta, GA, p. 1267-1272.

Ward, Colin R., 1984, Coal Geology and Coal Technology. School of Applied Geology University of South Wales, Australia