Abstract

Real time in-situ high precision monitoring of water pH was recently performed during open hole sampling in two on-shore Abu Dhabi wells, A and B, drilled in limestone reservoirs. Sampling in well A, which was drilled with oil based mud (OBM) was to search for distinctive differences in connate water characteristics of three different reservoirs. The in-situ pH measurement interpretation agreed with those from subsequent surface analysis of (downhole) samples. The pH of one zone was identified to be significantly different from the two others. The pH values were integrated with other characteristics such as ionic and isotopic concentrations to provide a complete description of the waters. Sampling in well B, which was drilled with water based mud (WBM), was to determine the expected flow fractions of mobile formation oil and water across oil/water transition zones. The in-situ pH measurements were pivotal to this objective by providing a cost effective and reliable real time diagnostic of the level of contamination mud filtrate in formation water. Guidelines are given on how to design jobs to measure pH in-situ, and on the how to use of the data. Measurements of water characteristics, such as pH, has a wide range of applications, from determination of reservoir compartmentalization, to prediction of corrosion and scaling to improved sampling techniques. These are discussed.

Introduction

A method and apparatus to measure pH of water (the "pH measurement") whilst it is being pumped through the flow line of a formation tester (FT) was tested in 2 wells A and B, which were drilled in Middle East carbonate formations. Since then the same technique has been applied successfully in several more Middle East wells. The pH measurement consists of the following steps:

  • injecting a pH sensitive colored dye into the water inside the flow line of the formation tester,

  • ensuring that the dye reacts (and hence changes color) with the hydrogen ion in the water – when both oil and water flow a downhole separator must be used to extract the water,

  • determining the pH by using a spectroscope to measure the intensity of colors along the wavelengths where the dye has changed color.

This method is described in detail in (1). Figure 1 on page 2also illustrates the principle.

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