Abstract

This paper presents solutions for detecting fluid contacts based on downhole logging, flow testing, and well logging in gas condensate reservoirs with oil rims. The horizon under study is BU16 1-4 which is a heterogeneous reservoir made of shelf sediments. The reservoir comprises a number of lenticular bodies containing oil, gas, and condensate. In some of the lenses, oil rims and underlying aquifers are present. Problems of determining reservoir fluid content and fluid contact depths are attributed to: - Lithological variation due to reservoir geology - Uncertainty of formation water salinity due to poor quality of the available downhole water samples - Influence of secondary rock changes on well logging results. These issues will be discussed later in details. The reservoir under study is characterized by lithological variation of rocks. Facies have different porosities and permeabilities depending on pore size and configuration, as well as on grain size and packing. Procedure for facies identification from core data and well logging data has been developed. Individual porosity-permeability relationships have been obtained from SCAL (special core analysis) to update their distribution. The paper also reviews the problem of uncertainty in formation water salinity. The available water samples have been analyzed to exclude zones with flush fluid. Only one out of all the available BU16 1-4 water samples proved to be of good quality, with salinity of 4.7 g/l. This data was used when developing a petrophysical model to estimate oil and gas saturation by a resistivity method. A factor that complicated development of a reservoir saturation model was the effect of secondary rock changes, including zeolitization which amounts to 7%. Zeolitization significantly affects well logging results. Reservoirs containing zeolite are often mistaken for gas-bearing formations, which in turn complicates determination of fluid content and fluid contact depths. The paper presents an attempt to identify reservoir intervals prone to zeolitization based on well logging results. The paper considers a problem of determining fluid contact depths in gas condensate reservoirs with an oil rim by the example of BU161-4 horizon of the VostochnoUrengoiskoye+Severo-Yesetinskoye field. A suite of methods has been proposed to determine fluid content and fluid contact depths that recognize complex lithological structure and formation water salinity uncertainty. This allowed updating WOC and GOC, as well as developing a program of further reservoir study.

Key words:

horizon, reservoir, oil rim, fluid contact, Lithotypes, fluid content

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