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OS5C-4：THE EFFECT OF DRILLING MUD INVASION ON RESISTIVITY LOGGING IN GAS HYDRATE-BEARING SEDIMENTS: A 2D NUMERICAL SIMULATION
发布时间:2014-07-28
Fulong NING1, Jiaxin SUN1, Ling ZHANG1, Mingming ZHENG, Nengyou WU2
1. Faculty of Engineering, China University of Geosciences, CHINA; 2. Centre for Gas Hydrate Research, Institute of Energy Conversion, Chinese Academy of Sciences, CHINA
Integrating 3D seismic survey and well logging can achieve more accurate quantification of natural gas hydrates as a potential energy and environmental impact. However, some factors can influence the accurate interpretation and evaluation of well logging results. Except drilling mud washouts, the invasions of drilling mud probably also seriously distort the results of well logging. In this work, we performed 2D numerical simulations to study the dynamic behavior and general rules of mud invasion into oceanic gas hydrate bearing sediments (GHBS) that have different hydrate saturations by taking hydrate reservoirs in the borehole SH7 of the first China expedition to drill gas hydrate project as a case. The simulation results show that the temperature and pressure near the borehole increase dramatically. The higher hydrate saturation of GHBS, the larger degree of hydrate dissociation, and meanwhile pore water salinity reduction becomes more significant and more gas produces by decomposing, but the invasion depth of drilling mud and the area of pore water salinity decreasing are smaller. Moreover, the longer invasion time is, the more hydrate around the borehole dissociation is. In the early stages of drilling fluid invasion, regardless of the level of hydrate saturation, it is difficult to form the “secondary hydrates”. While it is completely opposite in the later periods, and obvious pore pressure fluctuations as well as “temperature inversion” are observed around the borehole because of secondary hydrates formation. For low-saturation hydrate sediments, free gas by decomposing firstly occurs a increase and then a decrease until it disappears as a result of dissolving. In addition, there occurs a gasless gap close to the higher hydrate saturation sediment, in the low-saturation hydrate sediment. Due to the “interface effect” as well as the “secondary hydrate”, some low salinity and high salinity regions can be observed around borehole respectively. The hydrate dissocaiton and the secondary hydrate formation can change electrical resistivity of GHBS around the borehole, epecailly shallow resistivity is affected more greatly compared with deep resistivity in dual lateral logging. The higher hydrate saturation of GHBS is, the greater difference between the deep and shallow logging results. After drilling to GHBS, wire logging should be carried out as soon as possible to avoid resistivity logging distortion caused by drilling fluid invasion.
1. Faculty of Engineering, China University of Geosciences, CHINA; 2. Centre for Gas Hydrate Research, Institute of Energy Conversion, Chinese Academy of Sciences, CHINA
Integrating 3D seismic survey and well logging can achieve more accurate quantification of natural gas hydrates as a potential energy and environmental impact. However, some factors can influence the accurate interpretation and evaluation of well logging results. Except drilling mud washouts, the invasions of drilling mud probably also seriously distort the results of well logging. In this work, we performed 2D numerical simulations to study the dynamic behavior and general rules of mud invasion into oceanic gas hydrate bearing sediments (GHBS) that have different hydrate saturations by taking hydrate reservoirs in the borehole SH7 of the first China expedition to drill gas hydrate project as a case. The simulation results show that the temperature and pressure near the borehole increase dramatically. The higher hydrate saturation of GHBS, the larger degree of hydrate dissociation, and meanwhile pore water salinity reduction becomes more significant and more gas produces by decomposing, but the invasion depth of drilling mud and the area of pore water salinity decreasing are smaller. Moreover, the longer invasion time is, the more hydrate around the borehole dissociation is. In the early stages of drilling fluid invasion, regardless of the level of hydrate saturation, it is difficult to form the “secondary hydrates”. While it is completely opposite in the later periods, and obvious pore pressure fluctuations as well as “temperature inversion” are observed around the borehole because of secondary hydrates formation. For low-saturation hydrate sediments, free gas by decomposing firstly occurs a increase and then a decrease until it disappears as a result of dissolving. In addition, there occurs a gasless gap close to the higher hydrate saturation sediment, in the low-saturation hydrate sediment. Due to the “interface effect” as well as the “secondary hydrate”, some low salinity and high salinity regions can be observed around borehole respectively. The hydrate dissocaiton and the secondary hydrate formation can change electrical resistivity of GHBS around the borehole, epecailly shallow resistivity is affected more greatly compared with deep resistivity in dual lateral logging. The higher hydrate saturation of GHBS is, the greater difference between the deep and shallow logging results. After drilling to GHBS, wire logging should be carried out as soon as possible to avoid resistivity logging distortion caused by drilling fluid invasion.
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