Production Logging and Flow Diagnosis in Heterogenous Reservoirs
Authors: Nnebocha Ezinne Amanda , Okoroafor Rita Esuru , Singh Kamaljeet 
Published: April 2013

Production logging measures downhole flow properties under dynamic conditions with an objective to gain insight into the downhole flow dynamics and quantify zonal flow contributions. Production logs are acquired during "stabilized flowing and shut-in conditions?? and the observed flow behavior is assumed to be representative of the reservoir flow behavior. In a homogenous reservoir this assumption may not be far from reality. Heterogeneous reservoirs, however, pose another challenge. Flow dynamics observed in heterogeneous reservoirs can be time dependent; therefore the time frame of data acquisition is critical to the conclusions that can be drawn from the interpretation. Presented herein is a case study of production logs acquired in a seemingly homogenous reservoir, the interesting dynamics observed from the data, and the methodology used to unravel the reservoir behavior. The production logs acquired during shut-in conditions revealed downhole cross flow between the reservoir layers. However a change in the direction of cross-flow between two shut-in acquisitions, acquired within 10hours of each other, revealed a much more complex reservoir than originally anticipated. Further analysis of the data and integration with other reservoir evaluation techniques revealed the heterogeneity of the reservoir with the existence of three radial compartments exhibiting different reservoir pressures, mobility and productivity index. The flow behavior observed from the production logs is unique and provides an insight to downhole flow dynamics in heterogeneous reservoirs. The methodology used in unraveling the reservoir behavior, and the integration of the production logs with other reservoir evaluation techniques, aided in improving reservoir characterization and ultimately reservoir management.

Introduction

Production logs provide measurements of downhole flow properties to promote an understanding of the downhole flow repartition of several zones, which then provides more information on the contribution the different reservoir layers make to the total production. In a homogenous reservoir, with constant properties, the reservoir response does not change with time and the flow behavior recorded at any time can be assumed to be representative of the reservoir behavior. However, in a heterogeneous reservoir, the nature of the reservoir may cause its flow behavior to change with time. A typical example will be a dual porosity reservoir system: in a dual porosity system, the reservoir is described to be composed of two systems - a block system and a matrix system. The matrix system has high permeability but low storativity, meaning that the fluid will flow faster through the matrix but it does not have enough volume to support the production (Fig 1). The block system on the other hand has high storativity but low permeability, meaning that fluid cannot flow directly from the block system into the wellbore. The fluid will flow from the block system into the matrix and then into the wellbore (Fig 2). When this type of reservoir is put on production, the initial flow will be from the matrix and the time it will take for the pressure to equalize between the matrix and the block will depend on the storability and permeability of the reservoir system. When production logs are acquired, the well is put on production and the production is allowed to stabilize before the well is logged. Without sufficient knowledge on the reservoir system or understanding of the heterogeneity inherent in the reservoir, if production logs are run in this type of system, it is possible to acquire the logs during the matrix flow period. Any inference made from this analysis will not be representative of normal flowing conditions, i.e. when the well has had sufficient time for pressure equalization between the block and matrix system. Therefore the time frame during which production logging data is acquired becomes critical in understanding the behavior of the reservoir.