Geohysical well logging
Geophysical well logging (borehole logging,) is a set of borehole investigation methods that are based on special logging tools.
First developed in the beginning of the 20th century, well logging comprises today several tens of methods that involve measurements of natural or induced physical fields in the borehole.
Extreme conditions prevail in a majority of drilling wells: rocks that are penetrated several kilometers below the ground surface have temperatures in the order of hundreds degrees centigrade (controlled by geothermal gradient; in Poland, the temperature is equal to approx. 150oC at the depth of 5 km). Moreover, throughout drilling operations the borehole is normally filled with a mix of water and clay, called drilling mud. A several kilometres high mud column exerts a pressure that is several hundred MPa high at the bottom of the well. Delicate and sensitive detectors or other instruments require special protection, while the measurements have to be adjusted accordingly (considering, for example, that drilling mud infiltrating into the sandstones changes their characteristics, temperatures affect detector sensitivity, etc.).
In addition to mud logging and core sample tests, geophysical well logs provide a wide array of information on penetrated rocks. They are relatively inexpensive to perform and allow for continuous data logging over long intervals of formation rocks that are in their natural position. Nevertheless, logging data are slightly diffuse (especially if the thickness of formation beds is lower than tool resolution). Moreover, logging data should be correlated with the results of laboratory tests of core samples, especially in early stages of exploration.
Coring operations are time consuming and laboratory test results are not readily available. On the other hand, laboratory data are very accurate and the range of available tests is in practice infinite. Since cores are sampled in specific points only (over a few or several centimetres out of a several kilometres deep borehole), laboratory data have to be correlated with respective logging curves. For example, TOC determinations in core samples are correlated with resistivity and acoustic logs.
Wireline logging (WL) measurements consist of lowering the logging into usually open wellbore on a wireline. Measurements are normally conducted on the way out of the wellbore (borehole temperature logging measurements are made on the way down so as to not disturb the column of mud which has the temperature of the surrounding rocks). The signal is transmitted through multiple conductor wireline to the surface and recorded. The logging while drilling (LWD) measurement techniques have been developed since 1980’s. Logging tools are integrated into the rotating bottom assembly. Some basic data are transmitted to the surface using telemetry, the other data are recorded in the tool memory and then retrieved on the surface.
An appropriate correlation of all tests and measurements made allows for the determination of such important elements as, for example, full lithostratigraphic profile of the well, sedimentary rock depositional environment, the oil-water contact, percentage share of particular minerals, mechanical modulus, porosity, permeability or hydrocarbon content in prospective zones.
Basic, traditionally used in petroleum exploration logging methods are: resistivity logs (measured at various distances from wellbore axis and different resolutions), natural gamma log (frequently spectrometric, which enables estimation of thorium, uranium and potassium contents in the rock), neutron porosity log, bulk density log (often with photoelectric absorption index which provides lithology data) and sonic (acoustic wave velocity) log (besides porosity assessment, sonic logs are used in seismic data interpretation).
The parameters of key importance in shale gas exploration that are determinable by geophysical well logging include:
- organic matter content estimated from total organic carbon (TOC);
- mineral composition of shale rocks;
- porosity and pore size distribution;
- mechanical parameters of shale rocks;
- stress field in the rock mass.
The logs of a shale rich in organic matter display a higher level of radioactivity (uranium has an affinity for organic matter), a higher electrical resistivity and porosity (as indicated by lower velocity of acoustic waves, lower volumetric density or a higher neutron porosity). By appropriate correlation of resistivity and porosity sensitive logs, and having calibrated them against core samples, we are able to establish the profile of TOC percentage content in the well.
Interpreted logging data are the inputs to dynamic three-dimensional models of petroleum systems as parameters of the sub-divided rock intervals and (along with vertical seismic profiling) are used in time-depth conversion of seismic data.
New methods that have been developed since late 1980's provide useful information for shale oil and gas exploration:
- Neutron gamma spectrometry log – measures the energy of gamma quanta derived from interactions between neutrons and nuclei of atoms that build the rock. The neutrons are emitted by a source encapsulated in the tool. As a result of interaction with neutrons, each element emits gamma radiation with a specific spectrum that enables its identification. Concentration of a particular element in the rock can be derived from the intensity of radiation with a specific energy. Mineral composition of penetrated rocks, including organic carbon content estimates, is derived from data calculation. Some probes count the dispersed neutrons and the emitted gamma quanta in relation to time. This enables an accurate direct determination of organic carbon content without reference to core samples
- Nuclear magnetic resonance – magnetic spin of hydrogen atoms present in the rock is arranged along a magnetic field generated by the probe and then the time of the arrangement disappearance is measured. The distribution provides information on rock porosity and on the size of the pores in which water or hydrocarbons are present (the sources of hydrogen in rocks).
- Full wave sonic log – the amplitude of acoustic vibrations is recorded versus time (rather than the time of longitudinal acoustic wave appearance which is used for calculation of its velocity). Vibrations emitted by tool-mounted directional and radial sources propagate in the rock and then are recorded by receivers located at some distance from the sources. The measurements provide information on, among other things, porosity and permeability or – in combination with density log – on mechanical parameters (mechanical modulus) of the rocks. Vibrations emitted in different directions enable the determination of rock anisotropy arising from fractures that are associated with stresses present in the rock mass.
- Borehole wall imaging – electric or acoustic logging at a very high resolution (that allows for detecting centimetric non-uniformities) which make it possible to interpret, among other things, dip and thicknesses of laminae, fracture orientation and the so-called breakouts (vertical belts at the borehole wall at which rocks tend to get loose). Breakout orientation indicates the direction of the highest stress in the rock mass, which is a valuable information at planning the direction of production wells and the hydraulic fracturing procedures.
Layout of well logging curves from a Polish exploratory well
Central lithological column was created using geochemical log. Correlation between TOC and gas is noticeable.
- CALM – borehole diameter log,
- LLDC and LLSC – resistivity curves,
- GR – gamma ray log,
- PE – photoelectric absorption log,
- ROMA – density of rock matrix curve,
- NPHL – neutron porosity log,
- DT – sonic log (correlation between DT and NPHL indicates a good quality of the estimation of porosity),
- RHOT – bulk density curve theoretically calculated,
- RHOB – measured bulk density curve (worth of noting is a highly similar layout of the two density curves. This indicates that a proper rock composition
model has been adopted and testifies to good quality of the geochemical log)
author: Michał Roman