Abstract

Formation water sampling is important for characterizing hydrocarbon/water transition zones, understanding scaling and corrosion potential of the water, and determining compatibility between formation water and injection water. When sampling water with wireline formation testers at wells drilled with water-base muds (WBM), it is important to track mud filtrate contamination by distinguishing between formation water and mud filtrate in real time while sampling. Current techniques are mainly qualitative (e.g., resistivity sensors), use readings by stations (e.g., downhole pH measurements), or require colouring the mud (e.g., blue dye) and do not always allow a continuous quantitative monitoring.

We present a solution that allows continuous monitoring of the water sampling cleanup process as a way to better understand cleanup profiles. To achieve this goal, a fluorescent tracer was added to the mud system while drilling the zone of interest. By adding this tracer, we were able to continuously monitor the cleanup process by means of the downhole fluorescence sensor of a wireline formation tester (WFT) string. Prejob calibrations allowed us to interpret the fluorescence sensor's reading, considering that the formation water is free of fluorescence response and that any response will indirectly indicate the presence of filtrate in the flowing fluid.

The fluorescent tracer was found appropriate to this task because it is detectable at very low concentration levels during qualification tests performed at surface conditions. Additionally, there were no detected tracer absorption issues in the reservoir affecting the process.

Field examples are presented of downhole fluid sampling operations in heterogeneous offshore carbonate systems, which are compared with laboratory results that confirm the success of this real-time monitoring solution. It also helps to improve best practices for selection of formation sampling stations and formation testers as function of the reservoir heterogeneity, wellbore drilling parameters, and formation testers' capabilities.

Introduction

Analysis of formation water can provide crucial input to analyses during every stage in the life of a reservoir (Abdou et al. 2011). It provides information about the scaling and corrosion potential of the water, establishes the salinity of the water for petrophysical evaluation, and helps evaluate reservoir connectivity. It is a critical input to field development planning and economics.

Representative downhole water samples of the formation of interest should be free of any contamination by drilling fluids. Water-sample quality depends strongly on drilling-fluid type and on sampling technique and monitoring. Oil-base mud (OBM) usually allows acquisition of good water samples because the mud filtrate, being immiscible with water, does not contaminate the sample of formation water (Schroer et al. 2000). In contrast, water-base mud (WBM) is miscible with formation water, causing chemical reactions and mixing that can contaminate formation-water samples. For example, a WBM containing sulfates in contact with a formation water containing barium could cause precipitation of barium from the water sample. Analysis of the resulting water sample would underestimate barium content and thus underestimate scaling potential of the formation water (Raghuraman, O'Keefe et al. 2005). Good water-sampling techniques must allow for precise monitoring and control of the presence of WBM or water from the drilling fluid.

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