Abstract

Thinly bedded reservoirs are increasingly a target of offshore exploration in the Malay Basin. These reservoirs exhibit heterolithic interbedding with vertical heterogeneity and a wide range of layer flow properties. This paper describes methods of real-time and high-resolution formation evaluation and formation testing used to characterize such reservoirs.

The formation evaluation of thinly bedded reservoirs has several objectives:

  • identify the layers that may contain hydrocarbons

  • verify productivity and fluid types with formation testing and sampling

  • calculate net pay thickness and uncertainty range.

The evaluation is complex because of bed geometry and lithology. The reservoir beds are often thinner than the resolution of the formation evaluation logs. They exhibit a silty lithology and fine grain texture, and require high quality borehole resistivity images to characterize their geometry. The exploration well offers the best chance to evaluate the prospect, but operational and economical constraints do not permit to core or test every potential reservoir. Therefore, a real-time evaluation must identify with certainty the prospects to test.

The sequence of real-time formation evaluation described here consists of the following steps.

  1. Wellsite petrophysical analysis of:

  • porosity

  • volumes of clay, minerals and fluids

  • permeability based on the combination of GR spectroscopy elemental yields, nuclear and sonic porosity, and array resistivity logs.

  1. Identification of thin beds, and computation of sand-count from an electrical borehole resistivity imager.

  2. Validation of the wellsite petrophysical analysis and the borehole image within two hours of recording the logs, and design of the wireline testing and sampling program.

A modular wireline formation tester is used to measure accurate in-situ pressures in potential reservoir rock and to establish fluid gradients and identify contacts. Additionally, local formation permeability is estimated from analyses of pretests, and is used to verify the productivity of thin beds identified by formation evaluation techniques.

Introduction

At the start of an exploration campaign, the operator wanted to ensure that the hydrocarbon column extent and the productivity of thin bed reservoirs would be accurately evaluated. The productivity of thin beds is best tested with a formation tester with inflatable packers and formation fluid pump-out. Therefore, the petrophysical and geological logs evaluations should deliver a list of "mini-DST" depths, and achieve the dual objectives of probing the productivity of thin sands while avoiding tests in zones without flow potential. The petrophysics and borehole geology analyses therefore had to be completed in near real-time, before the wireline formation tests started.

The petrophysical solution was to combine GR Spectroscopy with formation density and resistivity, to provide a rapid and robust evaluation of clay volume, porosity, hydrocarbon saturation and permeability. Produced and delivered on an interactive PC platform, this evaluation was complemented by the equally near-real-time analysis of high-resolution borehole images, to identify the thin bed reservoirs with the best productive potential.

The following sections describe the workflow and the details of the analysis.

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