Water-Based Formation Imaging and Resistivity Logging in Oil-Based Drilling Fluids-Today’s Reality

Abstract

Historically, acquiring borehole image logs and resistivity at the bit measurements of useable resolution have been difficult, if not impossible, when using oil-based drilling fluids. Conversely, in many applications, drilling objectives and other performance criteria prohibit the use of water-based fluids, which deliver logs of higher resolution. Azimuthal borehole images in oil-based mud systems are delivered by wireline conveyed ultrasonic measurements or azimuthal density measurements made while drilling. Higher resolution images have generally been out of reach in oil-based systems because the measurement technology used to deliver these services involves passing electric current through a conductive borehole fluid. Resistivity-at-bit devices used for geostopping and geosteering also rely on electric current transmission from the bit to the formation and are optimized for a conductive mud system. In many applications, drilling objectives and other performance criteria prohibit the use of water-based fluids, thus placing high-resolution images, resistivity-at-the-bit, and the high-value applications that these services can deliver out of reach of the reservoir evaluation and well placement teams. This paper describes the development and successful application of a conductive oil-based drilling fluid system that produces water-based logging quality and enhanced geosteering without sacrificing the performance advantages of invert-emulsion fluids. This unique system, employs an electrically conductive continuous phase that provides a high-performance fluid with a conductive mud, filter-cake and filtrate. The authors detail the development of the system, which included a yard test where fluids of varying conductivity were circulated through a test well with formation micro imager response determined and compared against measured conductivity. Further, the authors discuss the application of this new system on the Gullfaks South in the Norwegian sector of the North Sea, where the fluid was used to drill a highly deviated well through complex geological structures. In this application the fluid showed the drilling performance expected from an invert-emulsion oil-based fluid (i.e. high penetration rates, wellbore stability and low torque/drag). Results from formation micro image logs and resistivity-at-bit measurements taken using this fluid are presented and discussed. The authors also review lessons learned in the development and field application of this conductive fluid, and address current limitations and future developments. Introduction The use of finely detailed formation imaging logs is an increasingly critical component in evaluating the full potential of a field prior to initiation of the development phase. Over the years, a number of advances have improved the techniques and tools used to evaluate the potential productivity of a reservoir.1,2 Azimuthal image logging is aimed at acquiring detailed geological and petrophysical data of formations crossed by the wellbore. Acquisition and analysis of these images allow recognition and quantification of geological features, such as structural and sedimentary formation dip, sedimentary features, and the presence, orientation and type of fractures and faults. The level of detail available for subsequent processing, is driven primarily by the resolution and azimuthal coverage provided by borehole imaging devices.

Experimental Study of Electrical Properties of Oil-Based Mud in the Frequency Range From 1 to 100 MHz

“…For oil-in-water emulsion, it has been shown that the oil droplets were surrounded by electrolyte solution resulting in an ionic-double-layer polarization phenomenon at the fluid-tofluid interface (Lu and Yu 2002). Oil-based muds are less conductive, the nonconductive oil being the continuous phase of the emulsion (Laasted et al 2000). Above the percolation threshold of the water phase, the conductivity decreases linearly with decreasing water volume fraction.…”

“…Giustini et al (1996) analyzed the conductivity of water-in-oil microemulsions for a large range of water-volume fraction. Laasted et al (2000) analyzed the resistivity of oilbased mud in the range from 0.1 to 100 kHz. Below the percolation threshold, the conductivity of water-in-oil microemulsion decreases with decreasing surfactant concentration.…”