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Application of Formation Pressure in Evaluation and Risking of Prospect

Abstract

Formation pressure measurement is of immense value in quantitative evaluation and risking of prospects. This paper demonstrates the use of pressure data to understand impact of over-pressurization and structural compartmentalization on hydrocarbon pool extent, structural fill-spill and saturation height modeling of Eocene Upper Thumbli Formation in Raageshwari-Tukaram area of Barmer Basin, NW India. Integrated analysis of pressure data from the wells defined a clear pattern of increasing overpressure (100 – 200 psi) from north to south along the structural trend. The Eocene Thumbli reservoir is intensely faulted and the pressure data indicates fault compartmentalization where each fault block is hydro-dynamically independent with separate aquifers. The uncertainty in Free Water Level (FWL) in each fault block was estimated from the minimum and maximum range of overpressure to calculate the hydrocarbon pool extent. Sensitivity analysis of different degree of overpressure helped to define the regional FWL. In absence of capillary pressure data from core, log data was used to prepare saturation height model for different FWL. Variation in rock quality was captured in the modeled saturation height function that matched with Dean-Stark saturation for that rock type. For a given rock type, the position of the well with respect to the structural crest led to over-estimation or under-estimation of in-place volume for the entire structure. Relationship of formation pressure to hydrostatic vis-à-vis interpreted structural spill from depth structure map was used to de-risk prospects below the FWL in this region. This methodology helped in determining the thickness of the hydrocarbon column, understanding the uncertainty in FWL, calculation of inplace volume and optimizing the position of appraisal wells.