DRILLING ENGINEERING

2. DRILLINGENGINEERING Vahid Salimi

3. Textbook Applied Drilling Engineering, by :Adam T. Bourgoyne Jr., Martin E. Chenevert, Keith K. Millheim F.S. Young Jr.,.

4. Contents: pore pressure and fracture pressure drilling hydraulics casing design under balanced drilling directional drilling

5. Chapter 1 pore pressure and fracture pressure

6. Hydrostatic Pressure • Hydrostatic pressure is defined as the pressure exerted by a column of fluid. • The pressure is a function of the average fluid density and the vertical height or depth of the fluid column. • Mathematically, hydrostatic pressure is expressed as: HP (psi) = 0.052 x ρf (ppg) x D (ft) where: HP = hydrostatic pressure ρf = average fluid density D = true vertical depth or height of the column

7. Hydrostatic Pressure(cont’d) • Hydrostatic pressures can easily be converted to equivalent mud weights and pressure gradients. • Hydrostatic pressure gradient is given by: HG = HP / D … (psi/ft)

8. Example • Calculate the hydrostatic pressure for the following wells: • a. mud weight = 9 ppg, hole depth = 10100 ft MD (measured depth), 9900 ft TVD (true vertical depth) • b. mud gradient = 0.468 psi / ft, hole depth = 10100 ft MD (measured depth), 9900 ft TVD (true vertical depth) solution a. HP (psi) = 0.052 x ρf (ppg) x D (ft) = 0.052 x 9 x 9900 = 4632 psi b. Hydrostatic pressure = fluid gradient (psi / ft) x depth (ft)..........psi = 0.468 (psi /ft) x 9900(ft) = 4633 psi

9. Mud Weight (MW) should be kept heavy enough so that hydrostatic head of mud column is higher than formation pressure at any depth. Usually 150 psi Need to know formation pressure in order to determine MW Pf + 150 = 0.052 MWD Pf Formation Pressure, psi MW Mud Weight, ppg D True Vertical Depth, ft 150 Safety, psi

10. Example • You are drilling with 7.9 ppg oil base mud. If the formation pressure is predicted 5,000 psi at 9,000 fttrue vertical depth, what is the required MW in order to have 150 psi overpressure ? 5,000 + 150 = 0.052 MW 9,000 MW = 11 ppg

11. OVERBURDEN PRESSURE • The overburden pressure is defined as the pressure exerted by the total weight of overlying formations above the point of interest. • The total weight is the combined weight of both the formation solids (rock matrix) and formation fluids in the pore space. • The overburden pressure can therefore be expressed as the hydrostatic pressure exerted by all materials overlying the depth of interest: σov = 0.052 x ρb x D where σov = overburden pressure (psi) ρb = formation bulk density (ppg) D = true vertical depth (ft)

12. OVERBURDEN PRESSURE(cont’d) • Overburden gradient under field conditions of varying lithological and pore fluid density is given by: σovg= 0.433[(1 – φ)ρma +(φxρf)] where σovg= overburden gradient, psi/ft φ= porosity expressed as a fraction ρf= formation fluid density ρma= matrix density

13. matrix and fluid densities SubstanceDensity (gm/cc) Sandstone 2.65 Limestone 2.71 Dolomite 2.87 Anhydrite 2.98 Halite 2.03 Gypsum 2.35 Clay 2.7 - 2.8 Freshwater 1.0 Seawater 1.03 - 1.06 Oil 0.6 - 0.7 Gas 0.15 • To convert densities from gm/cc to gradients in psi/ft use: Gradient (psi/ft) = 0.433 x (gm /cc) • To convert from psi/ft to ppg, use: Density (ppg) = gradient (psi/ft) / 0.052

14. Pore pressure • The magnitude of pressure in the pore of formation known as the pore pressure • Pore pressure = formation pressure =formation fluid pressure =reservoir pressure =pressure in fluid contained in the pore spaces of the rock

15. Solution p = 0.465 psi/ft * 9,000 ft = 4,185 psig Example • Determine the pore pressure of a normally pressured formation in the Gulf of Mexico at 9,000’ depth.

16. Homework: Pore Pressure Profiles • The following pore pressure information has been supplied for the well you are about to drill. • a. Plot the following pore pressure/depth information on a P-Z diagram :

17. b. Calculate the pore pressure gradients in the formations from surface; to 8000ft; to 8500ft; and to 9500ft. Plot the overburden gradient (1 psi/ft) on the above plot. • Determine the mud weight required to drill the hole section: down to 8000ft; down to 8500ft; and down to 9500ft. • Assume that 200 psi overbalance on the formation pore pressure is required.

18. c. If the mudweight used to drill down to 8000ft were used to drill into the formation pressures at 8500ft what would be the over/underbalance on the formation pore pressure at this depth?

19. d. Assuming that the correct mudweight is used for drilling at 8500ft but that the fluid level in the annulus dropped to 500 ft below drillfloor, due to inadequate hole fill up during tripping. What would be the effect on bottom hole pressure at 8500ft ?

20. e. What type of fluid is contained in the formations below 8500ft.

21. Normal Pore Pressure • Pressure of a column of water extending from the formation to the surface • The magnitude of normal pore pressure varies with the concentration of dissolved salts, type of fluid, gases present and temperature gradient. =0.433 psi/ft for fresh water =0.465 psi/ft for seawater

22. Subnormal Formation Pressure • Subnormal pore pressure is defined as any formation pressure that is less than the corresponding fluid hydrostatic pressure at a given depth. • Subnormal formation pressure can cause lost circulation of water as the drilling fluid.

23. ABNORMAL PORE PRESSURE Abnormal pore pressure is defined as any pore pressure that is greater than the hydrostatic pressure of the formation water occupying the pore space. Abnormal pressure is sometimes called overpressure or geopressure. Abnormal pressure can be thought of as being made up of a normal hydrostatic component plus an extra amount of pressure. This excess pressure is the reason why surface control equipment (e.g. BOPs) are required when drilling oil and gas wells.

24. ABNORMAL PORE PRESSURE(cont’d) • Abnormal formation pressure can cause a kick with water as the drilling fluid.

25. Normal and Abnormal Pore Pressure Normal Pressure Gradients West Texas: 0.433 psi/ft Gulf Coast: 0.465 psi/ft Abnormal Pressure Gradients Depth, ft 10,000’ Pore Pressure, psig

26. Pore Pressure vs. Depth 0 5,000 10,000 15,000 20,000 0.433 psi/ft 8.33 lb/gal 0.465 psi/ft 9.0 lb/gal Depth, ft Normal Abormal • 9 10 11 12 13 14 15 16 • Pore Pressure Equivalent, lb/gal { Density of mud required to control this pore pressure }

27. Fracture Gradient Pore Pressure Gradient

29. Transition zone The upper portion of the region of abnormal pressure is called the transition zone

30. Causes Of Abnormal Pore Pressure • Compaction Effects • Diagenetic Effects • Differentional Density Effects • Fluid Migration Effects

31. Diagenetic Effects • With increasing pressure and temperature, sediments undergo a process of chemical and physical changes collectively known as diagenesis. • Diagenesis is the alteration of sediments and their constituent minerals during post depositional compaction. • Diagenetic processes include the formation of new minerals, recrystallisation and lithification. • Diagenesis may result in volume changes and water generation which if occurring in a seabed environment may lead to both abnormal or sub-normal pore pressure.

32. Clay Diagenesis • Clay Diagenesis (Conversion of Smectite to Illite) • If the water released in this process cannot escape during compaction, then the pore fluid will support an increased portion of the overburden and will thus be abnormally pressured. • Diagenesis of Sulphate Formations • Anhydrite (CaSO4) is diagenetically formed from the dehydration of gypsum (CaSO4.2H2O). • During the process large volumes of water are released and this is accompanied by a 30-40% reduction in formation volume

33. HIGH PRESSURE NORMAL PRESSURE 661. Drilling Engineering

36. Homework

37. When crossing faults it is possible to go from normal pressure to abnormally high pressure in a short interval. 661. Drilling Engineering

38. Well “A” found only Normal Pressure ... 661. Drilling Engineering

39. Methods of estimating pore pressure Direct measurement It is possible only when the formation has been drilled It is expensive Indirect measurement The main parameter is the variation of porosity with depth (porosity dependent parameter) If pore pressure is normal, porosity-dependent parameter (x) have an easily recognized trend because of the decreased porosity with increased depth of burial and compaction. A departure from the normal pressure trend signals a probable transition zone. Detection of the depth at which this departure occurs is critical because casing must be set in the well before excessively pressured permeable zones can be drilled safely.

40. Prediction and Detection of Abnormal Pressure Zones 1. Before drilling Shallow seismic surveys Deep seismic surveys Comparison with nearby wells 661. Drilling Engineering

41. Prediction and Detection of Abnormal Pressure Zones 2. While drilling Drilling rate, gas in mud, etc. etc. D- Exponent DC - Exponent MWD - LWD Density of shale (cuttings) 661. Drilling Engineering

42. Prediction and Detection of Abnormal Pressure Zones 3. After drilling Resistivity log Conductivity log Sonic log Density log 661. Drilling Engineering

43. Compaction Theory of Abnormal Pressure During deposition, sediments are compacted by the overburden load and are subjected to greater temperatures with increasing burial depth. Porosity is reduced as water is forced out. Hydrostatic equilibrium within the compacted layers is retained as long as the expelled water is free to escape If water cannot escape, abnormal pressures occur

46. Compaction Theory • In Porous formation the overburden stress is supported by rock matrix stress and pore pressure • Bulk Density = ρm (1-Ф) + ρfФ • The average porosity in sediments ,generally decreases with increasing depth - due to the increasing overburden • This results in an increasing bulk density with increasing depth, and increasing rock strength • Average Porosity Ф = ρm - ρb / ρm – ρf • Plot Ф Vs. Depth on similog graph.

49. Example • Calculate the overburden stress at a depth of 7,200 ft in the Santa Barbara Channel. Assume φo = 0.37 ρma = 2.6 gm/cc kφ = 0.0001609 ft-1 ρf = 1.044 gm/cc

50. Solution