Petrophysics
Petrophysics is more than just "log analysis." Petrophysics is the description of the oil and/or gas distributions and production flow capacity of reservoirs, from interpretations of pore system and fluid interactions using all available data.
Petrophysicists use acoustic and density measurements of rocks to compute their mechanical properties and strength. They measure the compressional (P) wave velocity of sound through the rock and the shear (S) wave velocity and use these with the density of the rock to compute: The rocks compressive strength which is the compressive stress that causes a rock to fail. The rocks flexibility, the relationship between stress and deformation for a rock. Converted-wave analysis is also used to determine subsurface lithology and porosity. These measurements are useful to design programs to drill wells that produce oil and gas.
Coring and core analysis is a direct measurement of petrophysical properties. In the petroleum industry rock samples are retrieved from subsurface and measured by core labs of oil company or some commercial core measurement service companies. This process is time consuming and expensive, thus cannot be applied to all the wells drilled in a field.
Well Logging is used as a relatively inexpensive method to obtain petrophysical properties downhole. Measurement tools are conveyed downhole using either wireline or LWD method.
Petrophysicists use acoustic and density measurements of rocks to compute their mechanical properties and strength. They measure the compressional (P) wave velocity of sound through the rock and the shear (S) wave velocity and use these with the density of the rock to compute: The rocks compressive strength which is the compressive stress that causes a rock to fail. The rocks flexibility, the relationship between stress and deformation for a rock. Converted-wave analysis is also used to determine subsurface lithology and porosity. These measurements are useful to design programs to drill wells that produce oil and gas.
Coring and core analysis is a direct measurement of petrophysical properties. In the petroleum industry rock samples are retrieved from subsurface and measured by core labs of oil company or some commercial core measurement service companies. This process is time consuming and expensive, thus cannot be applied to all the wells drilled in a field.
Well Logging is used as a relatively inexpensive method to obtain petrophysical properties downhole. Measurement tools are conveyed downhole using either wireline or LWD method.
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An example of wireline logs is shown in Figure 1. The first “track”, shows the natural gamma radiation level of the rock. The gamma radiation level “log” shows increasing radiation to the right and decreasing radiation to the left. The rocks emitting less radiation have more yellow shading. The detector is very sensitive and the amount of radiation is very low. In clastic rock formations, rocks that have smaller amounts of radiation are more likely to be coarser grained and have more pore space, rocks with higher amounts of radiation are more likely to have finer grains and less pore space.
The second track over in the plot records the depth below the reference point which is usually the Kelly bush or rotary table in feet, so these rocks are 11,900 feet below the surface of earth. In the third track, the electrical resistivity of the rock is presented. The water in this rock is salty and the salt in the water causes the water to be electrically conductive such that lower resistivity is caused by increasing water saturation and decreasing hydrocarbon saturation. The fourth track, shows the computed water saturation, both as “total” water (including the water bound to the rock) in magenta and the “effective water” or water that is free to flow in black. Both quantities are given as a fraction of the total pore space. The fifth track shows the fraction of the total rock that is pore space, filled with fluids. The display of the pore space is divided into green for oil and blue for movable water. The black line shows the fraction of the pore space which contains either water or oil that can move, or be “produced.” In addition to the what is included in blackline, the magenta line includes the water that is permanently bound to the rock. The last track is a representation of the solid portion of the rock. The yellow pattern represents the fraction of the rock (excluding fluids) that is composed of coarser grained sandstone. The gray pattern represents the fraction of rock that is composed of finer grained “shale.” The sandstone is the part of the rock that contains the producible hydrocarbons and water. Any method that is used for Petrophysics requires various resources, manpower, equipment, material and consumables. The NoDoC Cost Model for this most important aspect includes a centeral database that guides users to do the estimate before participation in the process. |
