21. Intro to Formation Logging Flashcards
What are the benefits and disadvantages of core analysis?
Advantages-allow direct measurement of reservoir properties; used to correlate indirect measurements, such as WL/LWD log; used to test compatibility of injection fluids; used to predict borehole stability; used to estimate probability of formation failure and sand production; Disadvantages-if reservoir is highly heterogenous, many core samples needed
Passive Measurements
Gamma ray, Spontaneous potential, Caliper
Active Measurements
Resistivity, Density, Sonic Log, Neutron
What does Gamma ray emit and measure?
Gamma ray tool is passive, does NOT emit anything, but measures gamma rays from natural radioactive decay in wellbore
Vsh= Shale Index=Volume of Shale
(Grlog-Grclean)/(Grshale-Grclean)
What does SP measure?
Measures a natural electrical current produced when a well between sand and shale
What are the necessary conditions in order to have deflection on SP curve?
Shale must be present next to a permeable zone
What is the use of a caliper log?
Measure actual bore hole diameter which in turn can determine cement volume need, hole condition, direction of stresses, and possibly lithology b/c shale has more of a tendency to have borehole washout
What kind of information do we get from Pef reading?
Amount of incident gamma ray completely absorbed by electron of material. Photoelectric absoption index should ideally give lithology but Shale range is very large
3 porosity logs
Density, Neutron, and Sonic (indirectly get porosity)
How does Density log works?
Measures density of rock matrix and total fluid present; uses ratioactive source to generate gamma rays; gamma rays collide with electrons in formation losing energy; detector measures reduced intensity of gamma rays; Density is inversly proportional to Porosity
Bulk Density vs Density
Bulk Density is function of the density of minerals forming the rock matrix and the enclosed volume of free fluids; density is mass per unit volume
How does Neutron log work?
Porosity is a function of hydrogen contents within the fluids of the formation; logging tools eemits high eneergy neutrons into formation; collide with nuclei of formation’s atom, lose energy/velocity with each collision until eventually slowed enough to be captured by nuclei; most energy lost colliding with hydrogen atom nucleus;
How can gas zone be identified on the log?
Crossover between Density Porosity and Neutron Porosity scale indicates gas presence
How sonic logs work?
Porosity calculated from the fastes path for sonic waves through solid rock
