METHOD CALCULATES GAS CONTENT PER FOOT OF COALBED METHANE PRESSURE CORE

Lawrence B. Owen, John Sharer,
Terra Tek Inc.
Salt Lake City, Utah

A simple, rational method has been developed to determine the gas content of recovered pressure cores from coalbed methane wells on a per-foot basis.

With this approach, pressure-core gas yields can be directly compared with gas yield data obtained from cuttings or conventional wire line and sidewall cores.

This procedure can be conveniently implemented using a personal computer spreadsheet.

COALBED METHANE

The coalbed methane resource is emerging as an important exploration and production target in the U.S. and overseas. A conventional approach to the assessment of coalbed methane is carried out by performing a combination of field and laboratory measurements of total desorbed gas volumes on recovered cores or cuttings.

However, one difficulty with this approach to the measurement of desorbed gas is the uncertainty in estimates of the so-called "lost gas volume."

Gas loss from core and cuttings depends on the drilling medium, temperature, and well bore pressure.

A common assumption, when drilling or coring coal with a mud, is that the desorption process begins during core recovery at the point corresponding to one half the depth of the well. In the case of drilling or coring coal with compressed air, the gas desorption process is assumed to begin at the time the drill bit penetrates the coal seam.

The loss of gas continues from the onset of desorption until samples are retrieved at the surface and sealed in pressure-tight canisters.

While total volume of lost gas can be estimated using well-known procedures,1-3 the impact of errors in the estimated lost gas volumes can be significant.

In many cases, lost gas volumes are about 30% of the total measured gas volumes. However, frequently, lost gas volumes can reach 50% or more of the total measured gas, especially in those cases when long trip times are required to recover cores from deep seams,

Pressure coring eliminates the need for lost gas corrections, and thus is capable of providing a more accurate estimate of true gas volumes. Although more expensive than conventional coring, this approach is very useful in initial exploration stages of new reservoirs as a means of evaluating the accuracy of lost gas estimates.

Pressure coring can recover up to 10 ft of coalbed from a single run. During the subsequent gas desorption measurements, total gas volume produced by the recovered core is obtained, but it is not possible to directly assign gas contents to individual portions of the recovered core.

That is, gas content is expressed as a total volume for the entire section of recovered core,

It is well known, however, that gas content is not uniform in thick coal seams such as the Fruitland coal of the San Juan basin because of variations in coal composition, ash, and moisture content.

ANALYTICAL APPROACH

The overall process for deriving total desorbed gas volume from a pressure core requires two discrete steps.

First, the pressure core must be fully desorbed in the field in a reasonable time period so that the coring tool can be released as quickly as possible.

Following desorption of the core in the pressure coring tool, its inner liner is removed and samples are transferred to gas-tight canisters in 1-ft sections.

Gas loss during the core transfer step is minimized by desorbing the core while still in the pressure coring tool as completely as possible.

Normally, the core is desorbed until the gas production completely stops as indicated by the absence of gas bubbles in a gas saturator unit located immediately upstream of the wet test meter used to record cumulative desorbed gas volume.

If the pressure core is desorbed at reservoir temperature, cooling the tool to ambient temperature before core removal helps to further retard gas desorption and reduce losses during the core transfer process.

The canister samples are then desorbed until their gas yields fall to a low value (typically, less than 1-2 ml/day). The total desorbed gas volume is then calculated as the sum of the pressure core-derived gas volume and canister derived gas volumes.

Pressure core retains samples at in situ pressure during the time required to retrieve the core barrel to the surface. The gas content of the recovered core is best measured after reheating the core barrel to reservoir temperature. 4 5 This procedure permits more rapid desorption of gas from the core and is useful for determining if the core barrel lost gas during the recovery operation.

Reheating permits direct confirmation of retention of downhole reservoir pressure by the coring tool. If the pressure core is reheated to reservoir temperature prior to desorption, the core can be desorbed usually in less than 8 hr to a level where core can be transferred to canisters with minimal loss of gas.

The subsequent canister desorptions, if also carried out at reservoir temperature, will typically require an additional 1-3 weeks in most cases. Appalachian coals are a notable exception. Matrix permeability in these coals is so low that long-term desorption can require months.

It is not possible to directly determine the contribution of each 1-ft section of core to the total measured gas volume. However, a rational basis for making such an assignment has been developed using the measured gas yields of each individual canister sample. That is, the desorbed gas volume of a canister sample should be proportional to its contribution to the total desorbed gas from the pressure core.

GAS YIELDS

Assignment of gas yields, on a per foot basis, is accomplished as shown in Table 1.

Following canister desorption, sample weights are measured, and ash and moisture contents for each sample are obtained by proximate analysis. The sample weights, expressed on a DAF (dry ash free) basis, are calculated with Equation 1 (see Equation and Nomenclature boxes).

With Equation 2, the contribution of each 1-ft canister sample to the measured pressure core gas volume is calculated.

In Equation 3, the total desorbed gas volume, which includes the gas derived from the pressure core and the gas derived from the canisters is calculated as the sum of the canister-derived gas and the measured volume of gas desorbed from the pressure core.

The incremental contribution of any 1-ft canister sample to the total desorbed gas volume is given by Equation 4.

Usual industry practice is to express gas content of coalbed methane samples on an as received and DAF basis in units of scf/ton. This calculation requires a simple unit conversion in Equations 5 and 6.

The per-foot gas yields corresponding to each 1 ft section of pressure core placed in canisters for long-term desorption can now be calculated with Equation 7.

The gas yields can be expressed on an as received or DAF basis by substituting the appropriate sample weights (Ws or Wdaf) and total gas yields (Ym or Ydaf) in Equation 7.

Table 1, which summarizes these calculations, is internally self-consistent. This approach ensures that the average of the calculated gas yields, on a per-foot basis, are exactly reconciled with the gas yields derived as the quotients of the total desorbed gas volume (pressure core plus canisters) and the corresponding total Sample weights (as received or DAF basis).

REFERENCES

Diamond, W.P., and Levine, J.R., "Direct Method Determination of the Gas Content of Coal," USMB RI 8515, 1981, p. 36.
Smith, D.M., and Williams, F.L., "Diffusion Models for Gas Production From Coals," Fuel, Vol. 63, 1984, pp. 261-65.
Ulery, J.P., and Hyman, D.M., "Be Modified Direct Method of Gas Content Determination: Applications and Results," Proceedings Coalbed Methane Symposium, Tuscaloosa, Ala., May 1316, 1991, pp. 489-500.
Bent, P.M., Radford, S.R., Eaton, N.G., and Owen, L.B.. "Reheat Cores to Measure Gas Better," Petroleum Engineer International, No. 10, Vol. 63, October 1991, pp. 46-55.
Bent, P.M., Radford, S.R., and Owen, L.B., "Technique to Reheat Cores Improves Analysis," OGJ, No. 51, Vol. 89. Dec. 23, 1991, pp. 90-92.