Mallik 2L-38 Gas Hydrate Research Research Articles

Report Outlines Knowledge Gained in Gas Hydrate Production Testing

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 29516, “Gas Hydrate Production Testing: Knowledge Gained,” by Timothy Scott Collett, SPE, US Geological Survey, prepared for the 2019 Offshore Technology Conference, Houston, 6-9 May. The paper has not been peer reviewed. Copyright 2019 Offshore Technology Conference. Reproduced by permission. Gas hydrates are an important potential source of unconventional natural gas. Significant progress has been made with regard to understanding geologic and engineering limitations of the ultimate energy potential of gas hydrate; however, more work is required. The complete paper reviews the results of gas hydrate engineering and production testing studies associated with northern Canada and Alaska. The results of the marine gas hydrate producing testing efforts in Japan’s Nankai Trough and in the South China Sea are also summarized. Mallik (Canada) The Mallik gas hydrate research site in northern Canada has been the focus of three important gas hydrate field tests (in 1998, 2002, and 2007-08). The Mallik 2L-38 gas hydrate research well (part of the 1998 testing project) was drilled to evaluate the geologic controls on the occurrence of gas hydrate and to acquire specialized core and well data needed to characterize reservoir properties. During the 2002 project, gas hydrate was produced for the first time by both depressurizing and heating the reservoir. Depressurization alone appeared to be the most-feasible method for producing gas hydrates. However, because of the limited nature and duration of the 2002 tests, it was determined that a longer duration test would be required, leading to the 2007-08 research program. A 12-m-thick sand-rich hydrate-bearing reservoir was tested at a down-hole flowing pressure of approximately 7.3 MPa. The fact that gas hydrates can be produced by depressurization techniques was demonstrated. The winter 2006 operations included a 6-day depressurization flow, which was able to establish a sustained and stable gas flow rate averaging approximately 3000 m3/d. The total volume of gas and water produced over the duration of the test was approximately 13 000 and 100 m3, respectively. Alaska North Slope (US) The occurrence of gas hydrate on the Alaska North Slope is associated closely with well-characterized petroleum systems. The Mount Elbert gas hydrate test well, in 2007, showed a mobile water phase within hydrate-bearing reservoirs even at very high gas hydrate saturations. The recognition of the presence of a mobile water phase within gas hydrate reservoirs was an important development in that it provided the means, or pathway, to draw the pressure down on hydrate-bearing reservoirs. The PBU L-Pad area was identified as the optimal site for the subsequent Iġnik Sikumi gas hydrate production test.

Relationship of gas hydrate concentration to porosity and reflection amplitude in a research well, Mackenzie Delta, Canada

Well logs acquired at the Mallik 2L-38 gas hydrate research well, Mackenzie Delta, Canada, reveal a distinct trend showing that the resistivity of gas-hydrate-bearing sediments increases with increases in density porosities. This trend, opposite to the general trend of decrease in resistivity with porosity, implies that gas hydrates are more concentrated in the higher porosity. Using the Mallik 2L-38 well data, a proportional gas hydrate concentration (PGHC) model, which states that the gas hydrate concentration in the sediment's pore space is linearly proportional to porosity, is proposed for the general habitat of gas hydrate in sediments. Anomalous data (less than 6% of the total data) outside the dominant observed trend can be explained by local geological characteristics. The anomalous data analysis indicates that highly concentrated gas-hydrate-bearing layers would be expected where sediments have high proportions of gravel and coarse sand. Using the parameters in the PGHC model determined from resistivity–porosity logs, it is possible to qualitatively predict the degree of reflection amplitude variations in seismic profiles. Moderate-to-strong reflections are expected for the Mallik 2L-38 well.

Elastic properties of gas hydrate‐bearing sediments

Downhole‐measured compressional- and shear‐wave velocities acquired in the Mallik 2L-38 gas hydrate research well, northwestern Canada, reveal that the dominant effect of gas hydrate on the elastic properties of gas hydrate‐bearing sediments is as a pore‐filling constituent. As opposed to high elastic velocities predicted from a cementation theory, whereby a small amount of gas hydrate in the pore space significantly increases the elastic velocities, the velocity increase from gas hydrate saturation in the sediment pore space is small. Both the effective medium theory and a weighted equation predict a slight increase of velocities from gas hydrate concentration, similar to the field‐observed velocities; however, the weighted equation more accurately describes the compressional- and shear‐wave velocities of gas hydrate‐bearing sediments. A decrease of Poisson’s ratio with an increase in the gas hydrate concentration is similar to a decrease of Poisson’s ratio with a decrease in the sediment porosity. Poisson’s ratios greater than 0.33 for gas hydrate‐bearing sediments imply the unconsolidated nature of gas hydrate‐bearing sediments at this well site. The seismic characteristics of gas hydrate‐bearing sediments at this site can be used to compare and evaluate other gas hydrate‐bearing sediments in the Arctic.