What is the role of the Petrophysicists in Petroleum Exploitation? What are the
ID: 108682 • Letter: W
Question
What is the role of the Petrophysicists in Petroleum Exploitation? What are the rock and fluid properties that influence Formation Evaluation? What is geophysical anomaly? What are the main advantages of using drilling mud? Differentiate between mud cake and mud filtrate. Explain Drift Correction Forward gravity modelling is? The differences between gravity and magnetic method are? What is magnetic hysteresis? Magnetic declination is Discuss magnetic total intensity What is geomagnetic reversal List the basic approaches for magnetic data interpretation. The limitations of the Seismic methods are? What are the disadvantages of the Seismic refraction method?Explanation / Answer
Role of petrophysicists in petroleum exploration
A major role of Petrophysicists to help reservoir engineers and geoscientists for understanding the rock properties of the reservoir, particularly how pores in the subsurface are interconnected, controlling the accumulation and migration of hydrocarbons. Some of the key properties studied in petrophysics are lithology, porosity, water saturation, permeability and density. A key aspect of petrophysics is measuring and evaluating these rock properties by acquiring well log measurements - in which a string of measurement tools are inserted in the borehole, core measurements - in which rock samples are retrieved from subsurface, and seismic measurements. These studies are then combined with geological and geophysical studies and reservoir engineering to give a complete picture of the reservoir.
Rock and Fluid properties that influence on the Formation Evaluation
The identification of a bed’s lithology is fundamental to all reservoir characterization because the physical and chemical properties of the rock that holds hydrocarbons and/or water affect the response of every tool used to measure formation properties. Understanding reservoir lithology is the foundation from which all other petrophysical calculations are made. To make accurate petrophysical calculations of porosity, water saturation (Sw), and permeability, the various lithologies of the reservoir interval must be identified and their implications understood
Geophysical Anomaly
Area where geophysical properties (e.g. radiometric, magnetic, electromagnetic, gravity) differ from surrounding areas and which may be the result of mineralization or Accumulation of hydrocarbon .
Advantages of Using drilling mud
It is good for drilling into shale formation because it does not react with formation clay causing shale instability.
• It typically creates thin mud cake. This is really good because you can reduce risk for pipe stuck situation.
• It can be treated and reused. Using this mud for long run can reduce overall drilling mud cost.
• Oil base as external phase is good lubricant so it greatly reduce drilling torque.
• It is good to use in some areas where you face with hydrate problem such as deep water drilling.
• Typically, when drilling with oil base mud, gauge hole can be easily achieved.
Difference between mud cake and mud filtrate
Invasion is a process whereby drilling mud fluid is forced into the rock due to differential pressure. The drilling mud is made up of solid particles and ions dissolved in water. This water displaces the native formation water to some degree, and mixes with formation water that is not displaced. The distance to which some displacement and/or mixing occurs is called the invasion diameter, and the zone so disturbed is termed the invaded zone. The zone nearest the borehole, or flushed zone, is the portion of rock where the maximum amount of displacement and mixing has occurred. The balance of the invaded zone is sometimes named the transition zone, where the transition between maximum flushing and no invasion occurs. This is confusing, as there may be a vertical transition zone between oil and free water in a reservoir, so we avoid the use of transition zone in the invasion profile description.
The invasion process leaves behind the solid particles of the mud, which collect on the borehole wall. The resulting material is called mud cake, and may be 3 to 4 inches thick or very thin and difficult to detect. The mud cake thickness by definition is one half the difference between the bit size and the borehole diameter. If the hole is enlarged by erosion beyond the bit size during drilling, the mud cake thickness may be impossible to determine.
Therefore the portion of mud which goes in to the invaded zone and also at transition zone is called as mud filtrate
Drift Corrections
Observed gravity changes as a function of time at a given location, you could measure this with the gravimeter in the lab. The changes are due to tidal effects, instrument drift, and in some cases real changes (e.g. motions on faults, swelling magma chambers). Tidal effects have a period of roughly 12 hours and our Worden gravimeter drifts at hundreths of a milligal per hour.
To accomodate mechanical and tidal drift problems so that they do not significantly affect our gravity determinations you will return to the base station in my lab every three hours or so during your gravity survey. You will discover that you get a slightly different meter reading at the base station every time you return. If you assume that real changes in gravity are minimal over the two to three hour interval of observation you can further assume that the error in observation over the interval is linear (of the form: error = m*time + b). You then make a simple linear correction to all your observations so that the drift-corrected observations at the base station are all equal; intervening measurements get the appropriate corrections..
For an example relevant to the Worden gravimeter, assume that over two hours the meter reading at the base station changes from 2170.0 to 2171.2 giving a difference of 1.2 scale divisions over 120 minutes; the slope of your drift-correction is then 0.01 scale division/minute. Thus if you have an observation 15.5 minutes after the first occupation of the base station you have to remove 0.01*15.5 = .155 scale divisions from that observation. For the second observation at the base station you remove 0.01*120 = 1.2 scale divisions to get 2171.2-1.2 = 2170.0, the initial reading.
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