Abstract

This paper describes a review conducted on data from selected wells in a mature field with the objective of establishing techniques for the evaluation of laminated sand sequences and assessing the impact of including such sequences in the reserves of the field. The field is set in a deltaic environment and has been under development for twenty years with some three hundred wells having already been drilled. Recent core data have revealed that quite some potential reserves may be contained in thin sand / laminated sand sequences. Conventional log data have insufficient vertical resolution to resolve individual sands and consequently the contribution to the reserves of these intervals has been difficult to assess. Recent advances in logging technology and in data processing have improved this situation. This review concentrates on assessing the effectiveness of these new techniques. The integration of core and production data with imaging techniques are presented. The results show that such intervals can be evaluated although ultimately the contribution to STOIIP is also influenced by other factors not directly measurable in the borehole.

Introduction

The Champion field in offshore Brunei has been on production for some twenty years. The main production originates from well developed sand bodies and the petrophysical interpretation has been geared towards the identification of these sands. A 50 % shale volume cut-off was applied and a dispersed shale model used to derive the hydrocarbon saturation [l]. This approach was appropriate for these sands and has been used over a number of years. Recent core, high resolution log and production data suggests that quite some reserves are contained in laminated sand-shale sequences. These have a laminated shale volume of around and above 50 % and are not included in the reserves through the application of the cut-off. As the field matures there is a requirement to develop techniques to include such laminated intervals in the petrophysical evaluation. Two approaches are possible. Increasingly more detailed analysis of the formation can be made using core, high resolution and imaging log data. This approach is invaluable for the initial identification of laminated intervals, but quantified use of these data is difficult at present. Resolution down to 3cm can be obtained but the measurements are made on that part of the formation most affected by invasion and are influenced by invasion effects. The second approach is to consider conventional log as an averaged response to the combined sand-shale laminae. Then to create an analogue model of the formation which honours the log data [6]. It is this second approach which is presented here. Core data are used to create a fine resolution resistivity analogue of the laminated interval. The validity of the model is demonstrated by forward resistivity modelling and comparison with measured resistivities. Saturations are then calculated based on the resistivity analogue using Archie.

ILLUSTRATION OF THE PROBLEM

The evaluation problem is illustrated in Figure 1. which compares the normal petrophysical evaluation with core data from a recently drilled well, CP-XX1. Track 1 displays the gamma-ray (GR) log along with the core shale volume (COR-VSH). COR - VSH was measured directly from the core. Track 2 shows the laterolog resistivity (LL9D). The dashed lines in Tracks 3 and 4 display the standard evaluation of porosity (LOG-POR) and saturation (LOG-SH), respectively. The thick solid line indicates the intervals included as net reservoir (NET-POR and NET-SH) using the 50% shale volume cut-off.

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