ResGeo 202 Reservoir GeomechanicsSpring 2016, OpenEdX Homework 2 – 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. c. Estimate the onset of overpressure – see red stars as possible answers and range of accepted depths below. 1 b. 2 . Compute pore pressure using the calculated porosity data. Estimate the magnitude of overpressure.Part 2: Estimating the magnitude of overpressure a. See Lecture 3. Slide 19 for details. 22 for details. a. 3 . Which of the following is a problem with using compaction trends to estimate pore pressure in geologic reservoirs? (x) Observed hysteresis in compaction . Slides 36. particularly in the case of clay minerals. and can thus lead to overpressure in geologic compartments or low permeability reservoirs. Choose the best possible answer of the available choices. Mineral diagenesis is often associated with dehydration reactions. 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.effective stress trends.Part 4: Answers to Homework 2 Questions Question1: Answer the questions below on the concepts of overpressure in geologic reservoirs.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 . b. Hydrocarbon generation from thermal maturation of organic matter in source rocks creates volumetric increases in pore fluids. 37 for details. 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. Explanation: All of the above are possible mechanisms of overpressure generation. c. which can lead to overall volume increases of both the pore fluid and the rock matrix itself. See Lecture 3. The relationship between porosity and effective stress during loading makes it difficult to infer pore pressure from porosity during unloading. See Lecture 3. Slides 21. Tectonic compression can create overpressure by increasing the rate of compaction relative to the rate that fluids can flow out of geologic reservoirs. has no units) and pounds per gallon (ppg)? Pore pressure gradient in psi/ft = 0.66 Corresponding mud density in specific gravity = 1. What is the average pore pressure gradient at 5700 ft in psi/ft? What is the corresponding mud density in specific gravity (dimensionless. a.79 Corresponding mud density in ppg = 6. 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.d. =1597 Question 4: Estimating the pore pressure gradient For calculation of mud weight.69 b. drillers are interested in the average gradient from the surface to the depth of interest. which is pore pressure divided by the depth. Pressure reversals (that is.53 Corresponding mud density in ppg = 12. 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.54 4 . 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.34 Corresponding mud density units of specific gravity = 0. What is the average pore pressure gradient at 8000 ft in psi/ft? What is the corresponding mud density in specific gravity and pounds per gallon (ppg)? Pore pressure gradient in psi/ft = 0.000 ft in the GOM data set in psi. Solution data is available as a . 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).c. Ans.CSV file (comma delimited text file). 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.: True. 5 .