Petrophysical Evaluation of Gas Hydrates : Estimation of Hydrate Saturation, Krishna Godavari Basin, India

摘要

The Natural Gas Hydrate Program Expedition (NGHP) has been formed to explore and develop the gas hydrate resources of Indian subcontinent in three stages. The first stage was to identify the presence of gas hydrate deposits in Indian Offshore Basins and the second stage was to identify gas hydrate in sand rich geological setting within gas hydrate stability zone and suitable locations for production testing. During the second stage, 42 gas hydrate wells at 25 sites were completed in deep water areas of Krishna Godavari and Mahanadi offshore in Eastern Coast of India. This paper provides an insight into formation evaluation techniques as effective tools of evaluating gas hydrate saturation. During the second stage of the expedition; LWD, wireline well log data and pressure cores were acquired and they have been used for estimation and validation of gas hydrate saturations. The gas hydrate saturation have been estimated by three methods viz., standard deterministic Archie method, probabilistic method using ELANPlus model and Density Magnetic Resonance (DMR) technique. This paper also emphasizes the estimation of gas hydrate saturation considering gas hydrate as a part of matrix in the ELANPlus model and validation with pressure core results. In Petrophysical model, hydrate can be considered either as a pore-filling hydrocarbon (fluid) or as a matrix mineral (rock constituent). When it is assigned as a pore-filling hydrocarbon, the model calculates the matrix volumes & porosity and calculates the water saturation in the same way as the basic interpretation model. When the hydrate is considered as a constituent of the matrix in the model, hydrate volume is calculated and the hydrate saturation is then arrived at by dividing this volume by the total porosity. Hydrate is invisible to Nuclear Magnetic Resonance (NMR) measurements. In DMR technique, the deficit in NMR porosity as compared to Density porosity is used to estimate the hydrate volume accurately. Gas hydrate saturation estimation in one of the well drilled during the second stage of the expedition from Krishna Godavari basin is discussed in detail in this paper. Gas hydrate saturation estimated from DMR method is found to be close agreement with other methods such as deterministic Archie and ELANPlus based probabilistic methods. The gas hydrate saturation is found to be in the range of 20-60% in the considered well. The results have been validated with gas hydrate saturation obtained from pressure cores retrieved from the nearby core hole.The present approach is to estimate gas hydrate saturation with different methods and validation with pressure core data to minimize uncertainty in estimation of petrophysical parameters for such type of unconventional reservoirs.