QUESTION: sir what are step profile,transition and annulus profiles in resistivity? how they are affected by distance?
ANSWER: Dear Muhammad Musa,
Now I am start enjoying answering your questions and wish that your enthusiasm of knowledge and curiosity should bring success to your career.
I am attaching a diagram of a well profile across well bore and formation.
Annulus Zone is the transition zone between invaded and uninvaded zone. This means that fluid nature in this zone is a blend of from the fluids of formations and drilling mud. The depth or radius of the zone is dependent of the nature of formation permeability and also about the hydrostatic pressure exerted on the formation by drilling mud.
If the hydrostatic pressure is higher than formation pressure in zone of high permeability the annulus is expected to displace formation fluids further inward. if the hydrostatic pressure is nearly balance it is expected that Deep Resistivity will be able to read formation fluid which sits in the virgin zone. In order to cross check for quality control of the logs curve readings, the contrast of deep resistivity and shallow resistivity curves should be very visible.
If both resistivity curves fall on each other that imply that the fluids of transition zone and uninvaded zone should be the same (chemistry of the fluids) lead to assume that either drilling fluids/filtrate would have been penetrated deep inside formation displacing all the original formation fluids (due to higher hydrostatic pressure and zone's higher permeable nature) and uninvaded zone is completely filled by drilling fluids or there is no significant permeability that allow filtrate to penetrate. You have to take the help of SP curve to define if the zone is permeable or not, and also refer porosity logs (sonic, density and neutron)for calculating porosity units.
The contrast or difference of shallow and deep resistivity indicate two different fluids in uninvaded zne and also in the transition zone or annulus.
Distance of fluid affect on resistivity profile could be as deep as 5 to 8 feet.
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QUESTION: Sir in case if formation is not permeable or have low permeability and hydro static pressure is more enough to invade or fracture the formation and we are mis-guided.is it possible?how which log will help us to understand this invasion? Sir sometimes i confused with very basic question so please don't mind......
Dear Muhammad Musa
There is no right and wrong answer but you have to see complete dataset of logs.
Keep in mind that all wireline logs read and measure several physical properties of the wellbore, formation and formation fluids. There are factors affecting adversely the log data and some time the reliability of the data cannot be precisely taken as authentic and often needs to apply corrections to regain the true data for quantitative analysis.
Density data is seriously affected if the borehole exceeds in size due to washout and if the formation is too permeable thick mud cake also affects the data of resistivity and also the density. On the other hand washout (more than bit size hole) and thicker mudcake (less than bit size hole) also characterize the type of softer or porous lithology and permeable formation respectively. So at one hand the data gets poorer but at the same time indirectly provides the reason of poor data which in turn is a useful insight.
If hydrostatic pressure is greater than formation pressure the formation may break and mud loss should be started while drilling. This clearly means that when you interpret logs you get to consult the mudlog and drilling logs at all time and take note of all those drilling and engineering information which may affect the petrophysical interpretation such as mud loss, mud flow, gas shows, water cut in mud, gas cut mud, tight hole, tool sticking etc, etc. As an industry standard practice wireline logging engineer usually takes notes and records at the comments section of the logs either at the header of the log or bottom or on both places.
Resistivity and SP are the two most important logs which deal the electrical and chemical properties of the formation and formation fluids respectively and they will guide you to characterize if the formation has higher permeability or if the formation fluids varying than before.
You need to develop your eyes to visualize all logs at one time and should be able to rationalize why one log changes when others donít or vice verca.
Neutron and density are affected by presence of gas in the formation whereas the presence of fracture system with the formation may affect sonic. Too thick mudcake especially when the mud density is high sonic and density may be affected too.
I am trying to give you what I call to understand a bigger picture of the petrophysics and also to include those factors that may affect interpretation if not properly taken into account.