Abstract

In heterogeneous reservoirs rate of production of hydrocarbon vary significantly between the wells. Estimation of flow rates prior to drilling is very important in formulating development scheme. Any change in petrophysical properties of a reservoir is reflected in variation of heterogeneity. Quantification of variation of heterogeneity will help in predicting the reservoir performance and production of hydrocarbons. This paper outlines a method for deriving heterogeneity gradient using seismic data and its application in predicting the performance and porosity of unexploited part of the Narimanam field in Cauvery Basin, Southern India.

Introduction

3D seismic attribute are being quantified in petrophysical terms and used to extrapolate petrophysical properties into the interwell areas (Martin et al., 1999)1. Using seismic data the lithofacies and reservoir properties are estimated away from the well control (Schultz et al., 1999)2. A correlation between seismic attribute and porosity from wells has been established and resulting relationship is used to generate 3D porosity models (Yang et al., 1995)3.

Many scientists (Dix, 1955; Vetter, 1987;Byan et al., 1988)4,5,6 studied in detail the properties of different velocities. AL-CHALBI (1974)7 studied a relationship among root mean square, average and stacking velocities and indicated that the difference of these velocities varies as a function of vertical heterogeneity (i.e., degree of stratification). Mohan (1998)8 demonstrated the influence of lateral heterogeneity on seismic interpretation. Mohanty and Krishna (2000)9 derived variation of heterogeneity factor (i.e., heterogeneity gradient) and this is an effective tool for estimating change in petrophysical properties of a reservoir. Proper understanding of reservoir heterogeneity will always help in predicting reservoir performance even during primary stage of development of a field. This knowledge-based prediction will lead to improvement in reliable reserve estimation, which in turn is related to the economic viability of the field.

Reservoir heterogeneity refers to the variation of petrophysical properties that affect the rate of production of hydrocarbons. Reservoir sands vary in composition and in quantum of porosity. Heterogeneity gradient (dg) indicate lateral and vertical changes within the reservoir, which can be mapped. For homogeneous shale, heterogeneity gradient (dg) is nearly equal to zero. In the studied area, a fairly good correlation has been observed between heterogeneity gradient (dg) and porosity variation in different layers within the reservoirs.

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