NUCLEAR WELL LOG PROPERTIES OF NATURAL GAS HYDRATE RESERVOIRS

Characterizing gas hydrate in a reservoir typically involves a full suite of geophysical well logs. The effort to understand and further characterize permafrost and ocean floor reservoirs is critical in quantifying gas hydrates as a geohazard and potential future resource. The most common method involves using resistivity measurements to quantify the decrease in electrically conductive water when replaced with gas hydrate. Compressional velocity measurements are also used because the gas hydrates strengthen the moduli of the sediment significantly. At many gas hydrate sites, nuclear well logs, which include the photoelectric effect, formation sigma, carbon/oxygen ratio and neutron porosity are also collected but often not used. In fact, the nuclear response of a gas hydrate reservoir is not known. In this research we will focus on the nuclear log response in gas hydrate reservoirs at the Mallik Field at the Mackenzie Delta, Northwest Territories, Canada and the Gas Hydrate Joint Industry Project Leg 2 sites in the northern Gulf of Mexico.

Nuclear logs may add increased robustness to the investigation into the properties of gas hydrates and some types of logs may offer an opportunity to distinguish between gas hydrate and ice permafrost. For example, a true formation sigma log measures the thermal neutron capture cross section of a formation and pore constituents. Chlorine has a high absorption potential, and is used to determine the amount of saline water within pore spaces. Gas hydrate offers a difference in elemental composition than water-saturated intervals. In permafrost areas, the carbon/oxygen ratio may vary between gas hydrate and permafrost, due to the increase of carbon in gas hydrate accumulations.

At the Mallik site, we observe a hydrate-bearing sand (1085-1107 m) above a water-bearing sand (1107-1140 m), which was confirmed through core samples and mud gas analysis We observe a decrease in the photoelectric absorption in the water sand of ~0.5 barnes/e^-, as well as an increase in the formation sigma readings of ~5 capture units in the water-bearing sand. This is further correlated with the carbon/oxygen ratio showing a decrease of 20% in the water sand. In future research, we will quantify the effect of gas hydrate on the nuclear logs at the Mallik well and compare it to wells in the Gulf of Mexico.