some useful exam and interview questions about Well Logging
Well logging methods?
– GR log
– Electric logs
. -Porosity logs
How do we measure the Mud cake thickness?
Using caliper log.
The most common tools used to determine hydrocarbon are:
• Sonic, gamma rays.
• Resistivity and Neutron density.
• Dipmeter, borehole geometry tool.
10 Petro physical Computation is performed on a ( Any data.
b) Raw data.
c) Data after EnvironmentalCorrection
1when SP cannot measure
Because a conductive fluid is needed in the bore hole for the SP log to operate, it can’t measure, when a non-conductive (oil based) drilling muds are present.
The SP curve is usually ‘flat’ opposite shale formations because
there is no ion exchange due to the low permeability, low
porosity properties (tight)thus creating a baseline. Tight rocks
other than shale (e.g. tight sandstones, tight carbonates) will
also result in poor or no response on the SP curve because of
no ion exchange.
2 what are the favorable conditions to form oil
♦ Mature source rock rich with organic matter
♦ Reservoir rock with optimal characters of porosity, permeability, and lateral extension.
♦ Good seal or cap rock
♦ Trap system
4 What type of log do we use for porosity calculation?
a) Density / Neutron.
5What type of log do we use for Lithologyidentification?
a) Micro spherical.
6 How can we differentiate between heavy and light oilresponse on resistivity curves?
a) Deep&Shallow readings are thesame.
b) Separation between Deep&Shallow readings in good permeablereservoirs.
7 Caliper loghelps to determine a) Oil zones.
8 Dip meter tool measures a) Oil bearingreservoirs.
9 We measureshaliness (VSH) from a) VSP.
10 Resistivity is inversely proportional to :
b) Oil presence.
Sonic tool is used to determine the primary porosity (√)
-The Sonic tool tends to ignore the effect of fractures or vugs, which result in secondary porosity. The Sonic tool therefore measures primary porosity only which is less than true total porosity if fractures or vugs exist.
11 what do you know about
a- Sp log
♦ SP log was one of the earliest electric logs used in oil industry.
♦ SP log is used to identify impermeable zones such as shale, and permeable zones such as sand, as well as several other uses.
♦ The SP log is a record of DC voltage differences between the naturally occurring potential of a movable electrode in the well bore and the potential of the fixed electrode located at the surface.
♦ It measured in millivolts.
♦ Electric currents arising primarily from electrochemical factors within the bore hole create the SP log response.
♦ These electro chemical factors are brought about by differences in salinities between mud filtrate (Rmf) and the formation water resistivity (Rw), within permeable beds.
♦ Because a conductive fluid is needed in the bore hole for the SP log to operate, it can’t be used in non-conductive (oil based) drilling muds.
♦ SP log is used to:
– detect permeable beds
– detect boundaries of permeable beds
– determine formation water resistivity RW
– determine the volume of shale in permeable beds.
♦ Gamma ray logs measure natural radioactivity in formations and because of this measurement, they can be used for identifying lithologies and correlating zones.
♦ Shale free sand stones and carbonates have low concentration of radioactive material. And give low gamma ray readings. As shale content increase, the gamma ray log response increase because of the concentration of radioactive material in shale.
♦ May be used in calculation of volume of shale:
♦ The sonic log is a porosity log that measures interval transit time (Δt) of a compressional sound wave traveling through one foot of formation.
♦ The sonic log device consists of one or more sound transmitters and two or more receivers.
♦ Interval transit time (Δt) in microseconds per foot is the reciprocal of the velocity of a compressional sound wave in feet per second.
♦The interval transit time is dependent up on both lithology and porosity. There for, a formation’s matrix velocity must be known to derive sonic porosity either by chart or by formula
Phisonic = ((Δt)log-(Δt)ma) / ((Δt)f -(Δt) ma)
♦ Neutron log is a porosity log that Measures the hydrogen ions concentration in a formation
♦ In clean formations where the porosity is filled with water or oil, the neutron log measures the liquid-filled porosity.
♦ Neutrons are created from a chemical source in the neutron logging tool. The chemical source may be a mixture of americium and beryllium which will continuously emit neutrons. The neutrons collide with nuclei of formation’s material result in losing some of its energy. Because the hydrogen atom is almost equal in mass to the neutron, maximum energy loss occurs when neutron collide with hydrogen atom. So, the maximum amount of energy loss is a function of hydrogen concentration because
hydrogen in a porous formation, so the energy loss can be related to the formation’s porosity.
♦ The formation density log is a porosity log that measures electron density of formation.
♦ It can assist the geologist to :
1. identify the evaporate minerals
2. Detect gas bearing zone
3. Determine HC density
4. Evaluate shaly-sand reservoir and complex lithology.
♦ The density logging device consists of a medium energy gamma ray source that emits gamma rays into a formation. The gamma ray source is either Cobalt-60 or Cesium-137.
♦ Gamma ray collides with electrons in the formation the collisions result in a loss of energy from the gamma ray particle.
♦ Scattered gamma rays – which reach the detector located a fixed distance from the gamma ray source – are counted as an indicator of formation density.
♦ The of Compton scattering collisions is a direct function of the of electrons in a formation (electron density).
♦ Resistivity logs are electric logs which are used to:
– Determine HC versus water bearing zones.
♦ Because the rocks matrix or grains are non-conductive the ability of the rock to transit a current is almost entirely a function of water in the bores.
♦ Hydrocarbons, like the rock’s matrix, are non-conductive; therefore, as the hydrocarbon saturation of the bores increase, the rock’s resistivity also increases.
♦ A geologist, by knowing a formation’s water resistivity (RW), its porosity (PHI), and a value for cementation exponent (m), can determine a formation water saturation (SW) from the Archie equation :
♦ The two basics types of logs which are used to measure the formation resistivity are;
– Induction log
– Electrode log
♦ A caliper log is a well logging tool that provides a continuous measurement of the size and shape of a borehole along its depthThe measurements that are recorded can be an important indicator of cave ins or shale swelling in the borehole.
h- Dip meter
♦ A dip log produced by reading of the direction and angle of formation dip as analyzed from impulses from a dipmeter consisting of three electrodes 120° apart in a plane perpendicular to the borehole.