Abstract

Developments in recent years allow chemical experiments that have traditionally been performed in the laboratory to be carried out on small chip-type instruments. This miniaturization of flow and chemical reactions in microchannels is called microfluidics. Only small quantities (i.e. microlitres) of reagent are necessary to perform these tests fast, reliably and cost effectively.

This paper outlines the basics of microfluidics, describing what it is and how it works as a tool for chemical analysis applied to drilling- and production chemistry. The focus is for applications as a measurement tool for the concentrations of ions, scaling ions and scale inhibitors. Results of measurements of chloride concentrations, zinc content, total ion content and calcium concentrations and scaling are discussed. The advantage of a microfluidic test of scaling is discussed using a field example, where unexpected reactions between unspent acids and completion brine were observed in the well and also in microfluidic devices.

Introduction

A lot of university and industrial research of the last several years has focused on the development of micro electrical mechanical systems (MEMS). MEMS is the miniaturization and integration of macroscopic mechanical processes. For example, complicated mechanical processes like valving, pumps, and cantilevers can be reproduced on a microscale and assembled to perform tasks like accelerometers for airbag deployment.1

Closely related to MEMS technology is "Microfluidics," where a controlled chemical analysis is performed on a network of micrometer-sized fluid channels.2,3 The micro-sized channels can be etched on glass substrates, or can be injection molded in plastic, or can be embossed into a soft material. The channels can be a few tens of micrometers deep and wide, where the length and arrangement of the channels on the chip are application specific.

Microfluidic sensors and devices have been developed primarily for biomedical applications. Analyses such as DNA, RNA and protein characterizations have been carried out commercially on small chips.4 Another example of developments in microfluidics is a hand-held, integrated device for analyzing liquid and gas mixtures (for example for the analysis of explosives), under development at Sandia National Laboratories.5

Fluid analysis in drilling, completion and production is an integral part of quality assurance in drilling and production engineering. The current chemical test methods on the rig site are based on titration. For example, the API recommended rig-site method for chloride determination involves two titration steps: first with sulfuric acid solution, then with silver nitrate solution. Similarly, the determination of water hardness (Ca2+ content) involves a titration of a prepared solution with a so-called "total hardness titrating solution." Other titration tests performed on the rig are total ion content, potassium and silicate concentration, and PHPA or other polymer encapsulator concentration.

A more complex analysis for these species can be employed in the laboratory, using ion chromatography for the common anions (chloride included), inductively coupled plasma or atomic absorption analysis for the metals (potassium, calcium, and silicon), and the sum of all of the ions for total ion content (including all of the common anions, metals, ammonium ion, the carbonate group, and, where appropriate, the organic acids). Laboratory analysis yields accurate results, but these results may not be timely due to the time required to transfer samples from the rig to shore and thence to the lab. Real time decisions based on this remote chemical analysis are not possible.

It is envisioned that a rig-site bench top tool can be developed based on microfluidic technology for use in drilling and completion fluids. Such a fluid chemical analyzer could also measure drilling fluid performance and scale-forming ions in completion brines regularly and thus measure the tendency for scale deposition during production, thereby providing a basis for injection of the required inhibitor concentration. The advantages of a microfluidic system on the rig site are that the analysis is fast and flexible while promising a much higher reproducibility of the data that currently produced rig data.

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