Very Low Water Saturations within the Sandstones of the Northern Barmer Basin, India

摘要

The Mangala, Bhagyam and Aishwariya Fields were discovered in early 2004 in the northern Barmer Basin of Rajasthan, in northwestern India. The data acquired in the field wells (including almost two kilometers of core) enabled a precise estimation of field stock tank oil initially in place (STOIIP). This paper summarizes the techniques that allowed the estimate of STOIIP to be more precisely defined and to be revised upward by 12%, a substantial increase when dealing with a billion barrel field (Mangala Field). Initial evaluation of the log data indicated a sequence of clean, quartzose sandstones with porosity greater than 25%. High porosities together with resistivity in the oil column over 5,000ohm-m, suggested that water saturations (Sw) were ~15% or even less. Based on the initial data and conventional techniques, the initial STOIIP estimates were made for the three fields. An extensive core analysis programme was begun in appraisal wells, with the objective of improved definition of the actual reservoir STOIIP's. Two appraisal wells were cored with synthetic oil based mud, and Dean-Stark Sw analyses were done. In addition to routine core analyses and the Dean-Stark Sw data, a sizeable set of other special core analyses is also available. This includes extensive capillary pressure data, laboratory NMR, and core electrical properties measurements. The petrophysical dataset verifies the existence of Sw's that are typically less than 5%PV, and often near 1%PV, in a very high-permeability and high-porosity reservoir containing little clay. The reservoir contains a medium gravity, highly paraffinic oil, and is moderately oil-wet. The various laboratory datasets challenged some of the traditional assumptions concerning the use of Archie constants in such reservoirs for Sw calculations. The upward revision of STOIIP is significant, and can be principally attributed to the more accurate estimation of reservoir fluid saturations. As this work demonstrates that very low Sw values exist in the Barmer Basin, the Mangala, Bhagyam and Aishwariya fields can provide a model for the appropriate economic evaluation of similar reservoirs. The laboratory results challenge some of the traditional thinking about the petrophysical properties of reservoirs such as these. It is indeed possible, that high quality reservoirs can have initial water saturations lower than 5% of pore volume on average, and with some zones less than 1%. Conventional log tools and analysis methods will not reveal these low levels without integration with core data and appropriately designed core analysis programmes. Also, and perhaps more importantly, this work clearly demonstrates the economic worth of extensive laboratory measurements and analyses on high- volume, high-value reservoirs such as those of the Mangala, Bhagyam and Aishwariya Fields.