Reservoir conditions mechanic oil field processing

mechanic oil field processing

1. RESERVOIR CONDITIONS Supervised by : Prof. Dr. Shouhdi E. Shalaby Presented by : Ahmed Amer Mohamed Rashad Ahmed Mohamed Elmaadawy Abdelaziz

2. Introduction Reservoir conditions : are pressure and temperature, and each of them is a form of stored and available energy. The history of any petroleum pool divided into two stage: 1.Static : “reservoir rock, reservoir fluids, traps”. • stabilized during a long period . • Changes rate are very slowly 2.Dynamic : • It starts once pool is discovered and withdrawal of fluids begins. • Changes rate are very rapidly .

3. Importance of reservoir conditions study: Affect the volumes of the rocks and its fluid content. Affect viscosity and buoyancy of oil and gas (generally increase with depth). The gradients indicate that energy has changed from potential to kinetic (there is a fluid movement through the porous medium ) may be a migration or from a pool into a well bore . Study of changes in reservoir pressure that accompany production help to estimate (character and amount of reserves from decline curves , the highest rate of production, and the efficiency of the operation).

4. Contents : Reservoir Temperature Temperature Measurement Geothermal Gradient Uses of Thermal Measurements Sources of Heat Energy Effects of Heat Effects of Heat from Intrusive and Extrusive Rocks Reservoir Pressure Introduction Pressure Measurement Kinds of Reservoir Pressure Hydrostatic Pressure Gradient Hydrodynamic Pressure Gradient Sources of Reservoir Pressure A. The major sources B. The lesser sources Variations from Expected Pressures

5. Reservoir pressure Reservoir pressure : is the ( original or virgin ) pressure of the fluids that confined in the pores of the reservoir rock, pressure that existed before the static pressure equilibrium of the formation had been disturbed by any production , but this is impossible to be directly measured . Original reservoir pressure can be expected only from the first well drilled into the reservoir The changes in pressure that occur during production will be considered more fully in reservoir mechanics . It also called: fluid pressure, formation pressure, static bottom-hole pressure , water pressure , closed-in pressure, or rock pressure . The difference between the shut-in pressure and the original reservoir pressure is a measure of the decline in reservoir pressure .

6. Some important definition to be not confused Flowing pressure: “bottom-hole flowing pressure” Is measured while the well is producing. differential pressure : the difference between flowing pressure and static pressure. Casing or Surface pressure : is the static pressure exerted within the well casing at the top of the hole when the well has been shut in and the pressure allowed to build up as much as it can. Tubing pressure : is the pressure at the top of the hole within the tubing, It may be (static= Csg pressure) or flowing. Back Pressure: is the pressure against which the well is producing, is the resistance to flowing pressure. It is the psig at the surface + the pressure exerted by the fluid column within well bore + friction in the tubing.

7. Why we call it as a water pressure ? The original pressures are associated with the history of the water content of the rocks as: The amount of oil and gas in the rocks is very small compared with the amount of water present. And most oil and gas pools may be said to occur in aquifers . Water acts not only as the medium through which the oil and gas must pass to accumulate into pools, but also as the main transmission agency of the reservoir pressures from one area to another.

8. Pressure measurement: 1. In holes drilled with cable tools : N early all water-bearing formations have enough pressure to support a column of water to some height in the hole. Using bailing to get the pressure : the rate is recorded as the number of bailerfuls per hour ( No/B/ Hr ) required to remove the water as fast as it enters the hole.

9. Pressure measurement: 2 . In rotary-drilled holes filled with drilling mud: Several measuring instruments are generally self-recording and self-contained sensitive pressure gauges, sometimes called pressure bombs , which are lowered into the hole on the testing tool until they opposite the face of the reservoir, a packer is set above the tool to keep the drilling mud of the fluid-bearing formation. Like RFT.

10. Kinds of Reservoir Pressure (1) Hydrostatic Pressure which is the static weight of a column of water increasing vertically downward, and its rate of increase known as the hydrostatic gradient . (2) Hydrodynamic Pressure directed laterally along the bedding from a higher to a lower pressure head, and its rate of change, known as the hydrodynamic gradient. The total pressure exerted by the water at any point in the reservoir is the sum of these two pressures, and the total pressure gradient exerted at any point is the vector sum of these two gradients.

11. Hydrostatic Pressure Gradient The average increase in pressure for each 100 feet of depth 45 psi per 100 feet , which is about the weight of a column of brine carrying 55,000 ppm of dissolved salts. Normal pressure is the pressure that is equal to the weight of equivalent column of reservoir water extending from the surface to the subsurface formation being considered. Normal pore pressure is not a constant. The magnitude of normal pore pressure varies with the concentration of dissolved salts. As the concentration of dissolved salts increases the magnitude of normal pore pressure increases.

12. In the Smackover limestone (Jurassic) of southern Arkansas , the hydrostatic pressure gradients were measured in ten pools of varying depths and were found to increase an average of 52 psi per 100 feet of depth below the surface. Such pressures would be called normal for depth in the Smackover limestone, for the density of the brine found in that formation is 1.22 , which corresponds to 52 psi per 100 feet of depth.

13. Another example: The pressures in a group of pools in Iran are shown in the following; in some of these pools the gradient is above the normal gradient, and in some it is below.

14. Aquifers A rock formation capable of holding and transmitting water Types of aquifer: 1- Unconfined aquifer (Water table aquifer ): Water table well 2- Confined aquifer (Artesian aquifer) It’s an aquifer that is overlain by impervious rock layer( aquitard ) which causes positive pressure, it has no water table but has potentiometric surface.

15. Hydrodynamic Pressure Gradient Hydrodynamic condition occurs when there is a flow of water from the area of higher head ( recharge ) to the area of lower potential ( discharge )

16. Pressure vs Fluid potential ɸ = gH = gZ + = H = Z + ɸ = fluid potential, g = the gravitational constant, Z = elevation of the point of measurement above or below the datum plane, P = the static fluid pressure, and ρ = the density of the reference fluid-usually water, H is the total head or potentiometric surface Conditions of flow Elevation + Pressure  

17. Potentiometric surface It’s an imaginary surface that defines the level to which water in a confined aquifer would rise.

18. Water table well Flowing artesian well Non flowing artesian well Hydrostatic conditions prevail when there is no movement of water in the formation where the potentiometric surface is horizontal.

19. Two general sources for fluid Potential Gradients: 1- Man-made fluid potential gradients in and surrounding a producing well or pool. 2- The natural fluid potential gradients of the region. Hydrodynamic pressure gradient present in sedimentary basin in two forms : 1-The occurrence of difference in fluid potentials within an aquifer, which causes flow of water in the aquifer along bedding planes 2-The existence of differences in fluid potentials between aquifers in the geologic section.

20. Sources of r eservoir p ressure and their effect on pressure variation Help to determine the original reservoir pressure , and it is often difficult or impossible to determine how much has been contributed by each one . The effect of some of the them is to contribute to the permanent pressure system , whereas the effect of others is only temporary . There are main two categories: (major and lesser sources)

21. A. The major sources : Pressure due to a column of water. Pressure due to overburden rocks, (enclosing rock) or (compaction phenomenon). Osmosis phenomenon .

22. A. The major sources : A.1 . Pressure due to a column of water : The interconnected pores of nearly every potential reservoir rock are filled with water, the weight of which exerts pressure. This is a major source of the permanent pressure system . When the water is at rest, it exerts hydrostatic pressure, which: Acts at right angles to the boundary surface. Is the same in all directions at any point within the fluid. Is the same at all points of equal elevation. The average weight of oil-field water is about 0.45 psi/ft. Some deviations can be found due to varying water salinity .

23. A. The major sources : A.2. Compaction phenomenon : It is the pressure exerted by the overburden rocks against the fluids. E ither by: The weight of the rocks ( rock load ) called ( geostatic pressure , overburden pressure, earth pressure, lithostatic pressure or rock pressure ) The forces transmitted through the rock as a result of diastrophism and rock deformation called ( geodynamic pressure ). The average overburden pressure gradient is about 1psi/ ft of height.

24. Compaction phenomenon as a reason for abnormal pressure The touching mineral particles act as struts and keep the two pressures separate within the rock pores (it is mean that in consolidated formation the rock carry itself and not affect the pore space), where the struts fail, as the lithostatic pressure becomes great enough to squeeze the rock volume into a smaller space and reduce the pore space , then the rock pressure is transmitted to the fluid pressure .

25. Struts fail when the balance between rate of sedimentation and the rate of expulsion is disturbed due to: An increase in the rate of sedimentation. A decrease in permeability due to solids blocking the passages. The deposition of a permeability barrier such as limestone or evaporite stringers. So abnormal pore pressure will result

26. A.2 . Compaction phenomenon :

27. A. The major sources : A.3. Osmosis phenomenon:

28. Osmosis phenomenon as a reason for pressure variation Osmosis may contribute to the development of abnormal formation pressure as it increases the pressure in the high salinity side and vice versa.

29. B. The lesser sources : 1. Temperature change 2. Chemical and biochemical Reactions 3. Secondary Precipitation or Cementation 4. Atmospheric and Oceanic Disturbances 5. Potentiometric Surface 6 . Tectonic movements

30. B . The lesser sources : B.1. Temperature change: Increase in temperature causes the oil, gas, and water to expand more than the rocks. I f it is a sealed reservoir rock with no additional pore space available, it causes a permanent increase in the fluid pressure. It also a reason for movement of fluids as it create a pressure gradient due to t he different in temperature from area to another. Temperature changes are probably much more important in their effects on the viscosity of the fluids than on their volume, because of the low coefficients of expansion of subsurface fluids especially water and oil .

31. Temperature change as a reason for pressure variation By the availability of the sealing condition then: Increase in temperature causes increase in pore pressure. Decrease in temperature causes decrease in pore pressure .

32. B . The lesser sources : B.2. Chemical and biochemical Reactions: Breakdown of the large hydrocarbon molecules into simpler compounds tends to increase the volume of H.C and if the volume of the system is relatively fixed, the fluid pressure will increase . This breakdown can occur due to ( Catalytic reactions, bacterial reactions, radioactive decay or temperature change ). The decomposition of organic matter through bacterial actions form methane gas pocket under pressure. In unsealed or unconfined rock, it would be a temporary increase in fluid pressure and quickly disappeared.

33. B . The lesser sources : B.3. Secondary precipitation or cementation : In many reservoir rocks the original minerals have been partly reconstituted and recrystallized and secondary minerals have been formed. Where there has been more deposition than solution , a decrease in porosity has resulted, which in a sealed reservoir means an increase in pressure. Where solution is greater than deposition , an increase in pore space results, and there is a decrease in pressure.

34. B . The lesser sources : B.4. Atmospheric and Oceanic Disturbances: Tidal and other disturbances of oceans undoubtedly cause minor and temporary elastic pressure effects in the underlying rocks. Shifting of water into the ice caps and back into the ocean at various times during geologic history shifted the load over large areas of the earth.

35. B.4. Atmospheric and Oceanic Disturbances: The great tsunamis or seismic sea waves, caused by submarine earthquakes, caused a lot of temporary changes in pressure due to change of the ocean load which compress the underlying shallow aquifers . Even the usual changes in atmospheric pressures from night to day , and from high to low related to storm movements (also have an effect on the ocean load ).

36. B . The lesser sources : B.5 . Potentiometric Surface We can define normal pore pressure as if we drill a well the fluid will rise to the ground surface at the wellsite. Abnormal pore pressure will be experienced if the potentiometric surface of the aquifer is higher than the ground surface and vice versa.

37. B . The lesser sources : B.6. Tectonic movements : Tectonic activity can result in the development of pore pressure variation from normal as a result of a variety of mechanisms including: earthquakes, folding , faulting, and salt domes. But also the sealing condition should be available for permanent effect. B.6.(a) Earthquakes: Compressive waves of earthquakes have been observed to cause elastic compression in shallow aquifers, caused many sudden changes in fluid pressure throughout past geologic time. EX . The Tehachapi-Bakersfield (California) earthquake of 1953, caused the production in the near-by Mountain View pool to double over period of several weeks .

38. B.6. Tectonic movements: B.6.(b) Folding: folding compacts the formation laterally, so If the formation water cannot escape, abnormal pressure will result . B.6.(c) Salt dome: It is a reason for abnormal pressure as: The movement of the salt creates additional tectonic stresses (folding & faulting) within the surrounding sedimentary rock. At the same time providing a lateral seal limiting pore water expulsion .

39. B.6. Tectonic movements : B.6.(d) Faulting: Can create abnormal pressure due to: In normal sealing fault the fault plane act as a seal against a permeable formation, preventing further pore fluid expulsion with the addition compaction which would be formed on the zone, making it over pressured. If the fault is non sealing , it may transmit fluids from a deeper permeable formation to a shallower zone, causing abnormal pressures in the shallow zone .

40. B.6. Tectonic movements : B.6.( d) Faulting : 3. If it uplifts an sealed pressurised zone to a shallower depth.

41. Reservoir temperature Introduction: The temperature underground normally increases with depth below the surface, and rate of increase with depth is called geothermal gradient. The gradient within the first ( 50—400 feet ) is approximately constant , where temperature is affected by: Atmospheric temperature changes. The circulation of ground water. While the temperature gradient is generally constant within any one hole , it may vary greatly from area to area , even in the same stratigraphic sequence of rocks , due to a set of factors included the presence of the source of heat, its amplitude and its distance to the strata. Unlike reservoir pressures which generally decline with oil and gas production , the reservoir temperatures remain fairly constant.

42. Geothermal Gradient: The geothermal gradient is obtained by dividing the difference between the temperature of the formation and the mean annual temperature by the depth of the formation. Then the bottom hole temperature = depth * geothermal gradient + T s.c (60 o F ) There are several ways to describe it the most common are: Give the number of ( o F / 100 Ft). Give the reciprocal (Ft / o F ). Average geothermal gradient is ( 1.5 - 2 o F /100 Ft ). The general range is ( 20:180 Ft / o F ). The abnormal gradients are around ( 20:40 Ft / o F ). The subnormal gradients are around ( 120:180 Ft / o F ).

43. An isothermal surface: Is an imaginary surface in the ground at which the temperature is everywhere the same. the geothermal gradient near Oklahoma City is around 10 o F /100 ft of depth. whereas it is 10 o F /36 ft of depth near Tulsa. As all the rocks near Tulsa are older and nearer the pre-Cambrian granites and metamorphic rocks of the basement than those at Oklahoma City.

44. An isogeothermal contours: Contours that show the position above or below sea level of planes of equal temperature , and a single such contour is called an isogeotherm .

45. An isogradient contours: Contours that show the gradient per 100 ft of depth. Help me to estimate the subsurface temperature at any point by multiplying the gradient shown on the map by the depth in hundreds of feet and adding the average surface temperature

46. The geothermal gradient depends on: the thermal conductivity of rocks: An EX. Explain this factor: The change in the isothermal gradient from 1 o F /51 ft. to 1 o F /68 ft , occurring at the contact between shale (higher conductivity) above and sand (lower conductivity) below

47. 2. The rate of heat production at depth (amount of heat sources): When the data are accurate and the gradient is plotted on a depth-temperature chart, it is found that the gradient rarely follows a straight line, but often the temperature gradient gradually increases with depth .

48. *Bottom hole temperature (BHT) data from 12 oil wells in Iraqi Kurdistan were used to obtain the thermal trend of Iraqi Kurdistan. And the results verify this. *some examples from this 12 wells are below:

49. Uses of Thermal Measurements Detecting top of cement Locating the point where gas enters the well due to cooling which accompanies expansion of gas. Detect gas leaks

50. Sources of Heat Energy Main Sources: Outward flow of heat from the central core of earth. Presence of igneous magmas that are cooling. Disintegration of radioactive elements. Heat of subcrustal thermal convection current.

51. Lesser: Frictional heat Formed during Diastrophism when individual grains rub against one another. Exothermal Chemical Reaction take place within permeable reservoir rocks.

52. Effects of Heat 1- Viscosity I ncrease in temperature increases the viscosity of gas and decreases the viscosity of oil 2- Volume of gas, oil, and rock.

53. Effects of Heat from Intrusive and Extrusive Rocks Dykes are igneous rocks that intrude vertically (or across), while sills are the same type of rocks that cut horizontally (or along) in another land or rock form.

54. (1) Source Rock: Act on the source rock, essentially accelerating hydrocarbon maturation. (2) Migration: Positive effect: Intrusions may channel fluid flow (e.g., hydrocarbons) towards traps. Negative effect: Intrusions may channel hydrocarbons away from traps.

55. (3) Reservoir: Positive effect: Fractured intrusions may act as an unconventional reservoir, with hydrocarbons primarily confined to the fracture network. Negative effects: Contact metamorphism will locally lead to porosity and permeability reduction.

56. (3) May crack oil to gas.

57. (4) Traps May form new as the intrusive dike or sill or the metamorphosed zone surrounding the intrusion becoming an impermeable boundary. May form fold above the intrusion

58. Thank you