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OVERVIEW OF PRODUCTION TEST RESULTS FROM THE JAPEX/JNOC/GSC MALLIK 5L-38 GAS HYDRATE RESEARCH WELL
S. Hancock1, T. Collett2, S.
Dallimore3, T. Satoh4, T. Inoue5, B. Weatherill1,
G. Moridis6, E. Huenges7, J. Henninges7, and D.
Carle1
1 APA Petroleum Engineering Inc., 1400, 800 Fifth Ave. SW, Calgary, Alberta, Canada, T2P 3T6
2 U.S. Geological Survey, Box 25046, MS-939, Denver, Colorado, 80225, U.S.A.
3 Geological Survey of Canada, P.O. Box 6000, Sidney, British Columbia, Canada, V8L 4B2
4 Japan Petroleum Exploration Co. Ltd., 2-2-20 Higashi-Shinagawa, Tokyo 140-0002, Japan
5 JOGMEC, Japan Oil, Gas and Metals Corporation, 2-2 Hamada, Mihama-ku, Chiba 261-0025, Japan
6 Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, U.S.A
7 GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
Natural gas hydrate accumulations occur at relatively shallow depths over wide spread areas in the Mackenzie Delta region of northern Canada. In 1972, two exploration wells drilled by Imperial Oil Limited, Mallik L-38 and Ivik J-26, encountered significant gas hydrate accumulations. As part of the evaluation program, a number of cased hole closed chamber drill stem tests were conducted in the gas hydrate and associated free gas intervals. While some gas was initially produced from the gas hydrate intervals (probably dissociated during the drilling process), production during the extended flow periods was generally too small to measure. A re-analysis of the bottomhole data using modern pressure transient analysis techniques has indicated that gas apparently continued to evolve into the wellbore throughout the shut-in periods, which indicates that hydrate dissociation may have occurred in response to the pressure drawdown created by the closed chamber tests. This important finding formed the basis for conducting a similar pressure drawdown experiments at the Mallik 5L-38 well.
The main objective for the Mallik 5L-38 test program was to collect field data in order to determine the in-situ kinetic and thermodynamic properties of a naturally occurring hydrate The Mallik location has only a limited operating season, and with consideration for testing time and budget, a combination of pressure drawdown tests using Schlumberger’s MDT©(Modular Dynamic Tester) cased hole wireline tool, and a separate thermal stimulation flow test were selected as the best methods to meet this objective.
During the pressure drawdown tests, three separate hydrate intervals, in addition to free gas and water zones, were successfully tested using the MDT tool. The original premise for the gas hydrate MDT tests was to reduce reservoir pressure below the hydrate stability point, and to shut-in and observe the pressure build-up (similar to the previous closed chamber tests from 1972). It was anticipated that the rate of gas production would be too small to measure; therefore, the rate of hydrate dissociation would have to be inferred from changing pressure versus time data. Two important phenomena were observed during the gas hydrate tests: after an initial clean-up flow of water, gas with trace amounts of water was continuously produced; and upon shut-in, the pressure transient response appeared to be that of porous media with both flow contribution and pressure effects well beyond the surface area of the hydrate open to the wellbore. The MDT test procedures for the hydrate intervals were then modified to include multiple flow and build-up periods, as well as injection (mini-frac) and pressure fall-off periods. Pressurized gas samples from the free gas and gas hydrates were obtained, as well as water samples from free water zone, were also obtained.
The thermal stimulation test conducted at the Mallik 5L-38 well was not a production test in the normal oilfield understanding, in that it was not designed to evaluate a potential production method or to prove the commerciality of the hydrate deposit. The objective of the test was to observe the dissociation of a well defined and constrained hydrate interval at temperatures above the hydrate stability point, while maintaining constant pressure. The results of this test would then be used to calibrate numerical simulation models to determine the in-situ kinetic and thermodynamic properties of the hydrate. In addition to the surface and downhole instrumentation and data collection programs associated with a typical production test, the Mallik 5L-38 program included a number of advanced monitoring and investigation tools and services, including mass flow meters to measure the anticipated low and unsteady-state gas production volumes; a fibre-optic distributed temperature sensing system installed on the outside of the production casing, which provided temperature profiles from surface to below the hydrate thermal test interval; a chemical tracer in the thermal circulation fluid to detect dilution; continuous on-line gas chromatograph readings; a gas sampling program for conventional and isotope analyses; cross-hole tomography and other seismic investigations; and a post-test cased hole logging program with Schlumberger’s RST© (Reservoir Saturation Tool) and CHFR© (Cased Hole Formation Resisitivity) tools, to determine the radius of hydrate dissociation. The thermal stimulation test was successful in that the bottomhole temperature was increased and held constant in excess of 50°C, the dissociated gas was produced, sampled, and flared at surface, and significant amounts of real-time downhole temperature and pressure data, as well as other scientific measurements, were obtained.