Very Low Water Saturations Confirmed in Sandstones
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 113162, "Very Low Water Saturations Within the Sandstones of the Northern Barmer Basin, India," by T. O'Sullivan, SPE, Cairn India; R.J. Zittel, SPE, Cairn Energy; D. Beliveau, SPE, Cairn India; S. Wheaton, SPE, Tullow Oil; H.R. Warner Jr., SPE, and R. Woodhouse, SPE, Independent Consultants; and B. Ananthkrishnan, Cairn India, prepared for the 2008 Indian Oil and Gas Technical Conference and Exhibition, Mumbai, 4-6 March. The paper has not been peer reviewed.
The Mangala, Bhagyam, and Aishwariya (MBA) fields were discovered in 2004 in the northern Barmer basin of Rajasthan, in northwestern India. The data acquired from field wells enabled a precise estimation of oil initially in place. Techniques are presented that allow estimating the oil initially in place more precisely and revising upward by 12%.

Introduction
The MBA fields are part of a series of simple, tilted fault-block traps formed within the rifted, Tertiary Barmer basin. Wells encountered gross oil columns in excess of 500 ft within the Fatehgarh sandstones of Late Palaeocene age. The Fatehgarh sandstone consists of interbedded sands and shales and has been subdivided into the lower Fatehgarh sandstone dominated by well-connected sheet-flood and braided-channel sands and the upper Fatehgarh sandstone dominated by sinuous, meandering, fluvial-channel sands. The sands consist almost entirely of mature quartz grains, with virtually no diagenetic alteration, high primary porosities, and excellent permeabilities. Porosities range from 17 to 33% (average 26%) with in-situ permeabilities of 200 md to more than 20 darcies (average 5 darcies).
Oil from the MBA fields is waxy sweet crude with gravity ranging from 18 to 20°API close to the oil/water contact (OWC), where the crude has been slightly biodegraded, to 29°API higher in the oil column (average of 27°API). The crude has a low gas/oil ratio of approximately 180 scf/bbl, and the in-situ oil viscosity above the biodegraded zone ranges from 9 to 22 cp.
An appraisal program was carried out in 2004–05, with the drilling of 20 wells, acquisition of 3D-seismic surveys, and an extensive coring and testing program including multiwell-interference tests. Areal-interference tests between wells separated by up to 4,900 ft have confirmed excellent lateral continuity within the Fatehgarh reservoir. Test rates in excess of 5,000 BOPD with limited drawdown have been achieved from MBA-field wells, confirming the high deliverability of the reservoir as inferred from logs and demonstrated with core data.

Available Data

Log Data.
All MBA-field wells have full logging suites as well as image-log data. Eighteen wells have nuclear-magnetic-resonance (NMR) data. Fourteen wells were drilled with a water-based mud (WBM) containing appreciable amounts of potassium (&gt;20,000 ppm), either in the form of potassium chloride or as potassium sulfate. The remaining wells were drilled with a synthetic-oil-based mud (SOBM), including wells Mangala-7ST and Bhagyam-5, which were fully cored across the reservoir unit.