TECHNIQUE TO REHEAT CORES IMPROVES ANALYSIS

Patrick W. Bent
Meridian Oil Inc.
Houston

Steven R. Radford
Eastman Christensen
Salt Lake City

Lawrence B. Owen
TerraTek Inc.
Salt Lake City

A temperature-compensated gas measurement technique reheats coalbed methane cores to downhole temperature, thereby allowing more accurate gas measurements.

With the cores at bottom hole pressure and temperature, the true volume and nature of gas can be measured to help predict overall coal seam gas reserves with much greater accuracy.

In the U.S., more than 2,000 wells in 13 different geographical basins produce natural gas from coal seams, with most of the activity in the Black Warrior basin of Alabama and the San Juan basin of Colorado and New Mexico. Other areas of coalbed methane activity include the Appalachian, Arkoma, Piceance, Powder River, Raton, and Western Washington basins.

Geologists and reservoir engineers typically rely on drill cuttings and cores to assist in development of formulas to extrapolate true methane deliverability from each zone of a coalbed formation. The methodology is based on empirical data and analogy, with a certain amount of uncertainty.

A number of operators use cuttings desorption instead of conventional coring, particularly in open hole completions. The same gas measurement process employed during coring operations is used to estimate gas in place from drill cuttings, but with a somewhat greater degree of uncertainty. As with all gas content calculations, the lost gas component is a substantial and highly variable part of the equation.

Coalbed cores are typically obtained with conventional coring equipment with a variety of inner tubes and liners. Immediately after retrieval, cores are placed in gas-tight canisters and monitored through the gas desorption process. Empirical formulas are then used to estimate the volume of gas assumed lost during the trip out of the hole and during handling prior to sealing the cores in canisters. The accuracy of lost gas volume estimates and total adsorbed gas volumes can be greatly improved with temperature-compensated gas measurement technology.

During the past 10 years, pressure coring systems have been the only commercially available means capable of retrieving cores at true bottom hole pressure. After the pressure coring system cuts the cores, the mechanical core barrel is triggered to close on bottom. This contains the core between an upper seal and a lower ball valve which can capture and maintain pressures up to 10,000 psi. Thus, the core typically reaches the surface with the fluids and gases at near in situ conditions.

The pressure coring system can successfully cut and retrieve pressured cores, thereby providing excellent examples of in situ quality, methane-laden coal reserves. However, one persistent problem was that the core would cool before engineers could measure the quantity of gas contained within the core barrel.

As the core barrel returns to the surface, its temperature typically decreases. The trapped mud and gas reduce in volume during cooling. The change in volume can result in an inaccurate measure of true gas content and gas deliverability. Furthermore, the time required to desorb gas from the core in the barrel increases significantly as the core barrel temperature decreases.

Two common sources can cause artificially low core pressure as the pressure core barrel is monitored on surface:

• The drilling mud contracts as it cools because of its bulk coefficient of thermal expansion.

• The pressure of the gas decreases in a linear fashion as the temperature of the gas drops (as expressed by the ideal gas law, with all other factors held constant).

The cooling effects on the gas and mud can be reversed by increasing the temperature of the core barrel and its contents at the surface. The reheating causes the volume of mud and gas inside the sealed barrel to expand to bottom hole levels. Simulation of the in situ conditions allows for more accurate measurement.

HEATED PRESSURE BARREL

Meridian Oil Inc., TerraTek Geoscience Services Division, and Eastman Christensen have developed equipment and procedures to reheat a core to true bottom hole temperature before gas measurement is initiated.

The core barrel, core, and formation fluids can be accurately reheated to bottom hole temperature if the interior of the pressure core barrel closest to the core can be monitored. The system maintains the pressure integrity of the core chamber during the reheating process.

Simply heating the outside of the core barrel and estimating the internal temperature using heat transfer coefficients proved inaccurate. Additionally, under field conditions this process would be too cumbersome to maintain a constant core temperature during the entire desorption process, which often requires days to complete.

For temperature-compensated gas measurement, the pressure core barrel was modified by the addition of thermocouples within the pressure chamber and the installation of pressure-tight contacts which transmit the temperature signal across the pressure boundary. The surface equipment rapidly heats the pressure core barrel and maintains it at a specified downhole temperature using electrical heating tapes.

An automatic digital temperature controller regulates the heaters, and a digital monitor measures the internal and external surface temperature of the pressure core barrel. The output of the thermocouples within the pressure chamber is used to establish a set point for the temperature controller. Thermal insulating blankets cover the heat tapes and core barrel (Fig. 1).

Gas desorption measurements begin after stabilization of the core temperature. Once the core temperature reaches bottom hole conditions, the gas content and gas deliverability are measured by standard methods. This process eliminates the need to estimate lost gas volumes and results in more rapid and accurate measurement of adsorbed gas volumes in the core.

FIELD OPERATION

The temperature measurement and controller devices were tested on a core barrel with frac sand and water as the core and drilling fluid. This test provided a positive check of the electronic equipment and data on the speed of barrel reheating.

The relationship between the external core barrel skin temperature and the true internal barrel temperature was established from the trial tests. Optimized procedures were then developed to enable the controller to maintain a constant internal core barrel temperature throughout the initial gas desorption process.

The temperature-compensated coal gas measurement techniques were then run on actual coring operations on six of Meridian Oil's coalbed wells in the San Juan basin. The cores were cut with the pressure core barrel (set in 10, 15, or 20-ft long intervals).

After the core barrel was pulled out of the hole, core pressure was immediately monitored through a tap on the outer barrel (Fig. 2). Heating tapes were quickly wrapped around the pressure core barrel and plugged into a power source. The thermocouples within the core barrel modulated the temperature.

As the internal temperature of the core barrel increased, pressure also increased. Because the heating tapes are on the barrel exterior and the thermocouples are located in the center of the barrel, when the thermocouples reached the desired temperature, the outer parts of the barrel were much hotter. The equipment was monitored to prevent overheating of the core barrel.

The cores reached temperature equilibrium after approximately 1 1/2 hr. The gas was then extracted, measured, and sampled according to standard practices (Fig. 3). The gas desorption equipment separates the mud and measures cumulative gas volumes.

The results of Meridian Oil's six-well test program demonstrated a dramatic decrease in gas desorption time, The heated equipment quickly took the gas down to a level which would allow the pressure core barrel contents to be transferred to sealed canisters. This allowed the pressure core barrel to be redressed and made operational again in a relatively short period.

These temperature-compensated coal gas measurements gave higher values for gas content than conventional core and cuttings desorption methods used on the same six wells and on Meridian Oil's offset wells in the Fruitland Coal. These increases were significant enough to merit continued use of the technology on subsequent wells in Meridian Oil's Fruitland Coal development program in the San Juan basin.

BIBLIOGRAPHY

1. Kuuskraa, Vello A., Bradenburg, Charles F., "Coalbed methane sparks a new energy industry," OGJ, Oct. 9, 1989.

2. Mevor, M.J., Close, J.C., and McBane, R.A., "Formation Evaluation of Exploration Coalbed Methane Wells," paper CIM/SPE 90-101.

3. Eaton, N., Radford, S.R., and Segrest, C., "Pressure Coring: Better Cores for Better Analyses," presented at Society of Core Analysts Eurocas II in London, May 1991.

4. TerraTek Geoscience Services, "Coal Isotherm Testing," Department of Energy SHCT No. 1 , Resource Enterprises Inc., April 1991.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.