Abstract

All rocks are sensitive to changes in stress. In the Dulang Field sandstones, stress sensitivity is affected by differences in texture, mineralogy and clay type. In this study, we present the results of a study that shows how stress sensitivity is related to the make up of a rock and how itaffects the productivity. The sensitivity of permeability - and therefore transmissivity - to stress in the Dulang Field strongly correlates with the general classification of reservoir quality with better quality rocks showing lesser sensitivity to stress. However, there are some anomalies in the stress sensitivity of porosity. While the sensitivity of permeability to stress increases with decreasing rock quality, the same does not hold for the sensitivity of porosity to stress. These deviations are explained by changes in cementation, clay type, clay morphology and clay locations in the pore structure.

Understanding why stress sensitivity changes in the Dulang Field is essential for its future development. Knowing the reasons behind reservoir behaviour allows better prediction and control of future production. Some zones will compact differently from others as the field produces. Differences in compaction are due to the different make up of the reservoir rocks. Heterogeneity in compaction affects the productivity irregularly, and therefore, the economics of the field. By knowing which zones are likely to compact first, production rates and scheduling can be adjusted to reduce the stress-induced damage. The findings of this study can be used to make more realistic prediction of future production rates via numerical simulators, thus providing a tool for improving field management and company profits. This is important not only for the Dulang Field, but also for any field where changes in stress are likely to affect production due to heterogeneous compaction of the reservoir.

Introduction

Reservoir compaction during fluid withdrawal is simply a response to change in in situ stress conditions. Owing to the production of fluids from a reservoir, pore pressure declines while overburden stresses remain constant. The relative increase in stress caused by production alters all physical properties of rocks. The degree of alteration in rock properties is heterogeneous owing to the heterogeneous make up of the rocks. The varying degree in compaction manifests itself as different percentages of reductions in porosity and, most importantly, lessening of transmissivity. The direct effect of reduction in transmissivity is a drastic decline in fluid flow through the rock. This study generates a stress-sensitivity profile that shows vertical variation in the proneness of the reservoir to compaction. This provides a tool for adjusting and scheduling production rates to prevent stress-induced damage.

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