Abstract

Outcrop analogues are very helpful in generation of the reservoir depositional model but are restricted in their application to formation evaluation. They represent a missed opportunity, in particular in the interpretation of high angle and horizontal (HA/HZ) well log response. In our vision they give us access to the depositional controls on vertical and lateral petrophysical rock properties variations as well as actual geometry of the geological bodies; both matters are critical for confident formation evaluation in HA/HZ well setting.

The primary objective of this study is the integration of geological answers and petrophysical information to construct forward models of our high technology LWD datasets. We assign a major significance to the visual comparison of the rock picture and a simulated tool response, supported by a detailed petrophysical analysis.

We initially used the Ainsa 1 Pyrenean deepwater turbidite outcrop with the petrophysical properties of analogous offshore West Africa reservoirs. Across-channel geological complexity (thin layering and low NTG in marginal part; pinch-outs, amalgamations and rock property variation in the axial part) is valuable to demonstrate improved strategies of interpretation solutions in channel to lobe turbidite settings. Steps to forward model the LWD data include many of today's reservoir characterization procedures: sedimentological description, lithofacies to petrofacies associations, core and field scale 3D petrophysical properties simulation, upscaling, true resistivity matrix generation and resistivity anisotropy evaluation. Forward modeling of tool response accounts for different measurement natures, geometries and DOI's (from meters in resistivity to cm in radioactivity, images and magnetic resonance).

The range of the results acquired shows that basic LWD suites often do not provide an accurate result in a heterogeneous environment. For example, in our case water saturation is overestimated by around 30% total through improper use of the classical data. Our study highlights that reservoir parameterization in the presence of all scales of heterogeneities, sand mixtures and thin laminations is enhanced through proper application of gamma ray, resistivity, density, neutron and NMR for the pore volumetrics and imaging for the geobody shaping.

Using the approach to a contrasted West Siberian field case with inherent low resistivity contrast and invasion of WBM demonstrates further interpretational challenges. The work ultimately permits a more confident selection of logging suites and subsequent improvement in application of the acquired data to formation evaluation in high angle and horizontal well situations.

Introduction

Well log data has a rich and diverse history of successful application in hydrocarbon exploration and production activities. Historically vertical well drilling of minimally dipping formations has prevailed and naturally wireline logging tool design was optimised for this condition. The objective of tool design was based on balancing the ideal of reading virgin formation properties (beyond the ‘damaged’ zone) yet maintaining sufficient vertical resolution to adequately define the geological and petrophysical properties of the rock levels under investigation. With sufficient resolution the vertical well case is rich in information for the vertical dimension as it passes through the geology yet the dataset is more limited in characterising the horizontal change in formation properties away from the wellbore. This is largely due to the restricted depth of investigation of the logging devices as well as the lower variability of the geological depositional processes in the near wellbore horizontal area.

This content is only available via PDF.
You can access this article if you purchase or spend a download.