Pertrophysics 07Wireline Logging

Shell Nigeria Graduate Training Programme

Petrophysics

7 WIRELINE LOGGING INTRODUCTION In this session, wireline logging is introduced. The subsequent chapters presents the measurement principles, interpretation and evaluation techniques of the basic open hole logging tools. The fundamentals of interpretation of gamma ray, density, neutron, resistivity, wireline formation tester and the spontaneous potential logs are explained. The techniques for determination of lithology, net reservoir, porosity, fluid type/ distribution, water saturation and free water level are demonstrated. Working knowledge of these techniques is acquired from tutorials, which include basic evaluation of sets of logs. No pre-knowledge of wireline logging is assumed. After working through these chapters, the reader should be able to: v Define and determine basic reservoir rock properties v Describe the basic principles of the common open hole logging tools v Perform basic interpretation and evaluation on a standard set of open hole logs: reservoir boundaries, lithology, net reservoir, porosity, fluid type/ distribution, Archie equations, water resistivity, water saturation, fluid contacts and free water level from pressure-depth plot

OPEN HOLE WIRELINE LOGGING Alter a section of a well has been drilled, measuring sondes are lowered into the open hole at the end of an electrical cable. Whilst pulling the tools out of the well, various properties of the formations are measured continuously as a function of depth. These physical properties can be interpreted in terms of lithology, porosity, hydrocarbon saturation, etc.

DEPTH MEASUREMENT The depth is measured along hole (AHD) in meters below derrick floor (mbdf). When the bottom of the tool string touches the drill floor the depth measurement is set at zero. The distance between the various tool detectors and the bottom of the tool Martey, A.O

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string is automatically compensated by the computer in the surface logging unit. The length of cable in the hole is measured with an accuracy of around 0.1 %.

In vertical wells the AHD is equal to the true vertical depth (TVD). In deviated wells, a deviation survey is needed to calculate the TVD from the AHD. The TVD is often expressed in meters below a local datum, e.g. meters subsea (mss).

LOG HEADER Data, crucial for the evaluation, can be found in the log header: v Well name and location v Date v Drill floor elevation (DFE) Martey, A.O

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v Ground elevation (GE) v Bit size v Mud type and properties v Resistivities of the mud (Rm), mud filtrate (Rmf) and mud cake (Rmc) v Total depth (TD) v Bottom hole temperature (BHT) v Weight, size and depth of previous casings v Time of last mud circulation v List of all tools run in this section v Serial number of tools and logging unit used v Names of logging engineer and company representative

LOG TYPES Logs, which are used to quantify the hydrocarbon in place can be classified into three families: v Reservoir Thickness: Gamma Ray, Spontaneous Potential. These logs discriminate reservoir from non-reservoir. v Porosity: Density, Neutron, Sonic. These logs are used to calculate porosity, identify lithologies, differentiate oil from gas. v ResistivIty: Laterolog, Induction, Microresistivity. These logs, together with porosity logs, are used to calculate hydrocarbon saturation. Other types of wireline tools are: v Side Wall Sampler: Takes small rock samples, which are used for lithology and fluid type confirmation

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v Wireline Formation Tester: Measures formation pressures and can retrieve fluid samples v Dipmeter and Formation Micro-Scanner: Measure dip and azimuth of reservoir layers v Well shoot and Vertical Seismic Profile: Used to calibrate seismic

CORRECTIONS Large and irregular boreholes can adversely affect the accuracy of the measurements. The log corrections needed in these cases can be quantified using charts available in logging contractors’Log Interpretations Charts.

READING LOG RESPONSES All tools have a limited vertical resolution. Close to lithology boundaries the measurements will be affected by the adjacent beds. In very thin beds this could lead to tool responses which deviate from the true formation profile. Below a technique is presented, for reading log values of each "constant" formation bed. These values can be used to calculate the petrophysical parameters of each bed.

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In a quick look evaluation one can limit the amount of blocks by taking average readings over intervals with more or less “constant” log responses. The block boundaries must be at the same depth on all involved logs.

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Tutorial Two logs from a vertical well are plotted below. Block these logs and read off the values for each block, such that the results could be used for a quick look evaluation Hint: Around 11 blocks are needed. You are advised to keep your results as they will be required in later tutorials.

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