well-logging as a tool for hydrocarbon exploration. Case study: electrical resistivity log

ABSTRACT

Well logs are used both qualitatively and quantitatively for rock characterization in petroleum exploration. Most logs (except radioactivity logs) are dependent on direct contact with the rock via the walls of the well, and have to be run after successive intervals of the drilling, before each stage of the steel casing is installed in the well. Modern logging tools make several types of records at the same time, and the instruments are built into a long steel pipe which is only about 10 cm in diameter. Gamma ray logs are used for lithologic discrimination; resistivity logs are employed to identify formation fluid based on electrical response of reservoir formations, while combined density and neutron logs can be used to estimate reservoir porosity, as well as ascertain hydrocarbon type where present.  Hydrocarbon environments can be studied using a single logging or a combination of two at a time such as the sonic-resistivity logcombination. Some of these logging methods are said to be specific to some particular environment and under different conditions does not just have advantages and limitations over other logging methods but also over other exploration methods too, such as seismic reflection/refraction.

The following types of log include; Electric logs, Radioactivity logs, Acoustic (sonic) logs and Dipmeter logs.

Keywords: Logging, Schlumberger, reservoir, acoustic, resistivity, porosity and permeability.

Further reading: Geology project topics with materials

CHAPTER ONE

INTRODUCTION

Background Knowledge

1.1.1 Definition of well-log

Well logging is a means of measuring the physical, acoustic and electrical characteristics of rocks perforated by a well. It can also be defined as a qualitative and quantitative analysis of underground formations and their interstitial fluids. The purpose of logging is to correlate physical properties of rocks and fluids encountered in different wells in the search for fluids and to define lithologic bodies. Logging has the advantage that it measures in situ rock properties which cannot be measured in a laboratory from either core samples or cuttings.

1.1.2Historical Development of well-log

Logging started with simple electric logs measuring the electrical conductivity of rocks, but it is now an advanced and sophisticated method used routinely in different phases of hydrocarbon exploration, field development and monitoring. In 1926, Conrad and Marcel Schlumberger founded Schlumberger limited and are said to be the inventors of electric well-logging.By 1930, the resistivity log was recorded commercially in several countries, and in 1931 the self‐potential (SP) measurement was added. An electromagnetic (EM) teleclinometer was introduced in 1932, a continuous temperature log in 1933, and an anisotropy dipmeter ca. 1935.

The significance of the SP was an ongoing study until about 1962. The bases for the quantitative interpretation of resistivity for saturation were formulated by 1941. The years from about 1940 into the 1960s saw the development of other basic logs-dipmeter, gamma ray, neutron, induction, microresistivity, focused resistivity, density, and acoustic velocity—as well as other miscellaneous logs. Suites of resistivity logs were evolved to provide more accurate analysis. Much effort was expended on the interpretation of shaly sands. Neutron, density, and acoustic logs yielded information on porosity, lithology, gas saturation, and/or shaliness. Availability of porosity from these logs facilitated resistivity‐log interpretation. In the late 1950s, interest developed in the use of computers for log interpretation. By about 1961, dipmeter logs were being recorded on digital magnetic tape suitable for computer input, and other logs were available by about 1965. Digital recording has also facilitated transmission of logs via radio and telephone. Recently, digital systems for wellsite computations have been introduced. F. F. Segesman, (1980), “Well‐logging methods”.

1.1.3 Successes registered with well-logging

• In the 90s NMR log was a scientific success but an engineering failure. It is a technology which is now applied in the oil and gas, water and metal exploration industry, because it can identify hydrocarbon and the hydrocarbon types, and also estimates the hydrocarbon saturation.

• At first loggers had to run their tools somehow attached to the drill pipe if the well was not vertical. Modern techniques now permit continues information at the surface, this is known as logging while drilling (LWD) or measurement-while-drilling (MWD). LWD logs use mud pulse technology to transmit data from the tools on the bottom of the drill string to the processors at the surface.

• Modern interpretation methods include 2D inversion (after the curves are digitized) and iterative forward modeling for when they are not digitized.

1.1.4. Other logging methods

• Radioactive logs (Gamma logs)

• Acoustic (sonic) logs

• Dipmeter logs

• Density logs

• Temperature logs

• Caliper logs

• NMR Logs (Nuclear Magnetic Resonance)

• Image logs

• Resistivity logs

• Spontaneous self-potential logs (SP logs)

1.2 Aims and objectives

Aim

The aim of this work is to discuss the various well logging in terms of its traditional application to formation evaluation and to describe the wide variety of physical measurements which address the relevant petrophysical parameters.

Objectives

• To make an inventory of the different logging tools and methods of most relevance in common use.
• To highlight the differences, strengths and weaknesses of existing logging tools in common use.
• To make a comprehensive list and evaluation of electrical resistivity and conductivity as a logging tool.