MAE108 Homework 2 Solutions - Spring 2015 April 19, 2015 Problem 2.7 Define • D1 = the event that the first weld is defective • D2 = the event that the second weld is defective • D3 = …Full description
ResGeo 202 Reservoir Geomechanics Spring 2016, OpenEdX Homework 2 – 2 – Estimating Estimating the Onset of Overpressure SOLUTIONS Click the link at the top of the page to download the solutions data Part 1: Estimating the onset of overpressure
a. Plot porosity vs. depth. b. Now calculate theoretical porosity assuming an exponential compaction trend tren d . c. Estimate the onset of overpressure – see see red stars as possible answers and range of accepted depths below.
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Part 2: Estimating the magnitude of overpressure
a. Compute pore pressure using the calculated porosity data. b. Estimate the magnitude of overpressure.
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Part 4: Answers to Homework 2 Questions
Question1: Answer the questions below on the concepts of overpressure in geologic reservoirs. Choose the best possible answer of the available choi ces. a. Which of the following is a mechanism of overpressure generation in geologic reservoirs? ( ) Hydrocarbon maturation ( ) Mineral diagenesis ( ) Tectonic compression (x) All of the above are possible mechanisms of overpressure generation. Explanation: All of the above are possible mechanisms of overpressure generation. Hydrocarbon generation from thermal maturation of organic matter in source rocks creates volumetric increases in pore fluids, and can thus lead to overpressure in geologic compartments or low permeability reservoirs. Mineral diagenesis is often associated with dehydration reactions, which can lead to overall volume increases of both the pore fluid and the rock matrix itself, particularly in the case of clay minerals. Tectonic compression can create overpressure by increasing the rate of compaction relative to the rate that fluids can flow out of geologic reservoirs. See Lecture 3, Slide 19 for details. b. Overpressure generation by disequilibrium compaction is most likely to occur in which case? ( ) The rate of fluid flow exceeds the rate of compaction and porosity loss (x) The rate of fluid flow is exceeded by the rate of compaction and porosity loss ( ) The rate of fluid flow is equal to the rate of compaction and porosity loss Explanation: The rate of fluid flow is exceeded by the rate of compaction and porosity loss. See Lecture 3, Slides 21, 22 for details. c. Which of the following is a problem with using compaction trends to estimate pore pressure in geologic reservoirs? (x) Observed hysteresis in compaction - effective stress trends ( ) Ambiguity in the results of porosity estimated by various wellbore logging methods ( ) Compaction of geologic sediments occurs through recoverable deformation only Explanation: Observed hysteresis in compaction - effective stress trends. The relationship between porosity and effective stress during loading makes it difficult to infer pore pressure from porosity during unloading. See Lecture 3, Slides 36, 37 for details.
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d. Pressure reversals (that is, pore pressure decreases with increasing depth) can result from: ( ) Drilling into a formation that was subjected to high tectonic compression ( ) Drilling into a formation that was produced by fast sedimentation (x) Drilling into a less hydrologically isolated reservoir (that is, a reservoir that is more drained during consolidation than the formations above) Explanation: Pressure dissipation rates are higher in reservoirs that are more permeable and less hydrologically isolated causing them to be less subject to overpressure than less permeable and/or hydrologically isolated reservoirs. Question 2: Estimating the onset of overpressure Use your plot comparing the theoretical and calculated porosities to estimate the depth of the onset of overpressure in the GOM data set in ft. Accepted range: 3668-6000 Question 3: Estimating the magnitude of overpressure Use your plot comparing the theoretical and calculated pore pressures to estimate the magnitude of overpressure at 11,000 ft in the GOM data set in psi. =1597 Question 4: Estimating the pore pressure gradient For calculation of mud weight, drillers are interested in the average gradient from the surface to the depth of interest, which is pore pressure divided by the depth. a. What is the average pore pressure gradient at 57 00 ft in psi/ft? What is the corresponding mud density in specific gravity (dimensionless; has no units) and pounds per gallon (ppg)?
Pore pressure gradient in psi/ft = 0.66
Corresponding mud density in specific gravity = 1.53
Corresponding mud density in ppg = 12.69
b. What is the average pore pressure gradient at 80 00 ft in psi/ft? What is the corresponding mud density in specific gravity and pounds per gallon (ppg)?
Pore pressure gradient in psi/ft = 0.34
Corresponding mud density units of specific gravity = 0.79
Corresponding mud density in ppg = 6.54
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c. Choose True or False to answer the following question: The smaller the “mud window” the harder it is to figure out drilling parameters such as mud weight. Ans.: True. It will be harder to utilize a mud weight that is larger than pore pressure (so the well would not flow while drilling) yet smaller than Shmin (to avoid frac’ing the well). Solution data is available as a .CSV file (comma delimited text file).
