المحاضرة الثانية (1).pptx المحاضرة الثانية (1).pptx

المحاضرة الثانية (1).pptx

1. Measuring Porosity • There is no direct way to measure (φ). •The values are obtained indirectly by interpretations and by combining different measurements. •We will review 3 main measurements here that is used to quantify (φ): a. Acoustic b. Bulk Density c. Neutron Porosity

2. Porosity : is a measurement of the capacity of rock to contain fluids. Porosity = Bulk Volume - Matrix Volume / Bulk Volume Porosity Porosity Sandstones The porosity of a sandstone depends on the packing arrangement of its grains. The system can be examined using spheres.

3. rock porosity determined by: the ability of matrix particles to fit together the matrix characteristics of grain size sorting cementation angularity (roundness) overlying pressure have a great influence on the amount of porosity present in any given rock. Two fundamental attributes influence porosity: The manner in which the grains are packed The degree to which the grains are sorted

4. In a Rhombohedral packing, the pore space accounts for 26% of the total volume. With a Cubic packing arrangement, the pore space fills 47% of the total volume. In practice, the theoretical value is rarely reached because: a) the grains are not perfectly round, and b) the grains are not of uniform size.

5. Porosity and Grain Size A rock can be made up of small grains or large grains but have the same porosity. Porosity depends on grain packing, not thegrain size.

6. Carbonate Porosity Intergranular porosity is called "primary porosity". Porosity created after deposition is called "secondary porosity". The latter is in two forms: Fractures: Fractures are caused when a rigid rock is strained beyond its elastic limit - it cracks. The forces causing it to break are in a constant direction, hence all the fractures are also aligned. Fractures are an important source of permeability in low porosity carbonate reservoirs. Vugs .: Vugs are defined as non-connected pore space. They do not contribute to the producible fluid total. Vugs are caused by the dissolution of soluble material such as shell fragments after the rock has been formed. They usually have irregular shapes.

7. Permeability permeability (K) : is the ability of a rock to transmit fluids. The ability of a rock to transmit a single fluid when it is completely saturated with that fluid is called absolute permeability . Effective permeability refers to the ability of the rock to transmit one fluid in the presence of another fluid when the two fluids are immiscible. Relative permeability is the ratio between effective permeability of a fluid at partial saturation, and the permeability at 100% saturation.

8. Petrophysics Outline Permeability is : measure of the ease with fluid flows through the connecting pore space of a reservoir rock. The natural plumping system conducts the fluid toward the borehole and is important in predicting the rate of production from a reservoir. Permeability depends on: Size of pore opening Degree and size of pore connectivity Degree and type of cementing material in the pore. Henri d"Arcy (1856) (Unit of measurement is darcy . Typically expressed in MD).

9. Petrophysics Outline When more than one immiscible fluid occupies the pore space, its distribution is controlled by wettability and capillary forces.

10. Petrophysics Outline For oil to enter a structure water has to be displaced. The oil never completely displaces all the water from the pore space. There is always some water present and where the water displacement is greatest, residual water saturation will exist which is a function of the rock. The amount of oil that can be recovered economically by both primary production and water flooding varies enormously from less than 10% to more than 80% of the initial oil in place. To understand the process responsible for these large variations one has to understand the mechanism controlling the movement and the distribution of fluid within a rock, on both the pore and reservoir scale Contributing factors include the nature of the rock pore system, the shape and the connectivity of the pores and throats, surface area, surface roughness and electrical charge of the pore walls, the phase behavior and the properties of the fluid under reservoir conditions (viscosity, interfacial tension, density and wettability ).

11. Petrophysics Outline Capillary Pressure is defined as the pressure required to desaturate a capillary tube. Rocks can be modeled as a bundle of capillary tubes, with the capillary pressure curve modeling the response of the entire pore system.

12. Petrophysics Outline Capillary pressure: Laboratory measurement of capillary pressure, the pressure required to displace a single fluid phase in a multi phase fluid system, provides useful information. Laboratory capillary pressure data can be related to reservoir conditions to allow us to predict water saturation distributions (initial and residual water saturation), oil/water contact and water flood response. When a capillary tube is inserted into a container containing water, water rises inside the capillary tube due to surface tension. Capillary pressure is defined as the pressure that has to be applied to push the water level down to the same level in the container. Rocks can be thought of as a bunch of capillary tubes. The graph shows the pressure required to drive the water out of a bundle of capillary tubes. Curve “A” represents a bundle of capillary tubes with large diameters. Curve “B” represents a bundle of tubes with small diameters. In a simple capillary tube the capillary pressure increases as the capillary radius decreases. In a rock-decreasing pore throat leads to an increase in capillary pressure. Sorting and grain size is related to pore-throat. As sorting and grain size decreases the pore throat size decreases leading to increase in capillary pressure

13. Resistivity R = r . L /AR = Resistance (ohms) r = Resistivity ( property of the material, Ohm.m /m ) L = Length (m) A = Cross sectional Area (m ) Resistance decreases with increasing Cross-Sectional Area Resistance increases with increasing Length

14. The Drilling Process and Permeable Beds Rxo Rt

15. Resistivity Introduction The resistivity of formation rock being evaluated is a key parameter in determining hydrocarbon saturation. Electricity can pass through a formation only because of the conductive water it contains. With a few rare exceptions, such as metallicsulfide and graphite, dry rock is a good electrical insulator. Subsurface formations generally have finite, measurable Resistivities because of the water in the rock pore space, bound by grain size or capillary pressure, or absorbed in the native interstitial clay. We divide substances into two general categories, conductors or insulators. Conductorsare substances that pass electrical current e.g. water, shales , mud. Insulatorsare substances that do not allow electrical current to flow (because of their electron structure and distribution) e.g.hydrocarbons , or rock matrix. The measured resistivity of a formation depends on the: Resistivity of the formation water (RW) Amount of water present. (Ø and SW) Pore structure geometry (F)

16. Petrophysics Outline Resistivity of a rock containing water and hydrocarbon is a function of the amount of water present and the geometry of the pore space. Most widely used saturation equation is Archie’s Equation. Archie’s equation was developed from two empirical relationships.

17. Water Resistivity Dry rocks are generally very good insulators. The resistivity of the rock is a function of the geometry of the water in the rock and its resistivity. When two metal electrodes are inserted into a salt solution there exists a voltage across the electrode. Because of this voltage the ions start to move, the – ve ion towards the + ve electrode and + ve ion towards the – ve electrode. The force on the ion depends on the charge and the voltage. The velocity of the ion depends on the size of the charge and the viscosity of the solution.

18. Viscosity of the water is controlled by the extend of hydrogen bonding of the water molecule which in turn is a function of temperature. Conductivity of a salt solution increases with temperature or the resistivity decreases with temperature. This nomograph shows electrical resistivity as a function of temperature and ion concentration . Water Resistivity Nomograph for NaCl Solutions

19. RW–Formation Water Resistivity Waters ability to conduct electricity is a function several factors: Water Salinity As salinity increases, more ions are available to conduct electricity so Rw (water resistivity) decreases . Water Temperature As water temperature is raised, ionic mobility increases and resistivity decreases. Chart Gen-9 in the Log Interpretation

20. Resistivity of NaCl Solutions This chart is used to compute salinitiesfrom Resistivities of solution e.g. mud, and vice versa. It is also used to find the Resistivities at a given temperature. Example: Rw ? When Salinity = 80000ppm Temperature= 80 deg. C What isRwwhen the temp = 180 deg CRw = Rw = 0.05 at 120 deg C:What is the water salinity:

21. Ro is the resistivity a formation whose pore space contains only water Rw is the resistivity of the water saturating the rock F is a constant for the given rock under examination and is called Formation Resistivity Factor, or more commonly Formation Factor Formation factor can be related to formation porosity by the general formula Where m = cementation factor Formation Resistivity Measurements

22. Slope = F F = 10% F = 20% F = 30% Rw Ro Ro = F *Rw F = a / f m F = 100% F = 100% Formation factor related to Rw and Ro Rw increases the resistivity increases for a given porosity. porosity increases resistivity decreases for a given Rw .

23. Saturation Evaluation

24. Resistivity Test

25. Symbol Character Derived from Φ Porosity Porosity logs (sonic, neutron, density), cross-plots, etc. 0.62 Φ 2.15 F (formation factor) Calculated using empirical formulae (e.g. Humble formula) and porosity as above Rw Formation water resistivity SP or laboratory measurements of resistivities of formation water samples Ro Rock resistivity saturated 100% with water Ro = F x Rw (can only be calculated, cannot be measured with logs) Rt True formation resistivity Induction Logs and Laterologs ( deep resistivity ) Sw Water saturation of pores Sw hydrocarbo = Rt /Ro Sw 100%

26. Given a rock of 25 p.u . saturated with water with a salinity 50,000 ppm NaCl (a=1, m=2, n=2): what is the resistivity of this formation at 150 degrees F ? what is the resistivity if half of the water is replaced by oil? Petrophysics Problem set

27. Invasion

28. Invasion Invasion Process 1.Mud in the borehole will slowly invade the Virgin Zone 2.The solids in the mud will accumulate at the borehole wall (too large to enter the pores). Some small particles may enter. 3.The mud liquid (filtrate) will enter the pores, expels the formation water and some of the HC 4.Progressively, more filtrate will invade the pores 5.This invasion process will stop when enough solids accumulate at the borehole walls create an impervious barrier. We will then have Mud Mud cake Invaded Zone Transition Zone Original Virgin Zone

29. Invasion

30. Borehole Terminology

31. Borehole Environment Problem Given a well drilled to 10,000 ft and a max recorded temperature of 223 o F, and a mean surface temperature of 70 o F. What is the temperature at 8000 ft? If the well was drilled in 4000’ feet of water with a temperature at the seafloor of 35 o F. What is the temperature of a formation at 8000 ft ? If the mud weight was 12 lbs/gal what is the pressure at that depth ?

32. Calculating Geothermal Temperature Example 1 0’ 10,000’ 70°F 223°F ?°F 8,000’ Example 2 0’ 4000’ 10,000’ 223°F 35°F 8,000’ ?°F Depth Temp. 1. 0’ 70°F 2. 10,000’ 223°F 3. 8,000’ ? Depth Temp. 1. 4000’ 35°F 2. 10,000’ 223°F 3. 8,000’ ?