Assignment Rock Mechanical Properties
You have been asked by your manager to derive a geophysical model of rock strength in a reservoir of interest. What are some of the parameters that you should consider? What are the effects of each of those properties on rock strength?
Solution
Rock strength is not measured directly but is modeled using geophysical measurements. Model parameters are derived from measurements of rock properties that are correlated with rock strength such as porosity, lithology, clay content, and Young’s modulus. A measure of a material’s strength is also called the unconfined compressive strength (UCS) which is the maximum axial compressive stress that a sample can withstand. The relationship of these parameters, both intrinsically (Table 1) and extrinsically (Table 2), and their individual effects on rock strength are outlined in the two tables below:
| Intrinsic Rock Property | Impact on strength |
|---|---|
| Porosity (+) | – |
| Grain size (+) | – |
| Mineral Cements (+) | + |
| Young’s Modulus (+) | + |
| Volume of clay minerals (+) | – |
| Volume of quartz (+) | + |
| Extrinsic Rock Property | Impact on strength |
|---|---|
| Confining pressure (+) | + |
| Pore pressure (+) | – |
| Loading rate (+) | + |
| Temperature (+) | – |
Assessment Rock Mechanical Properties
1. What type of properties are determined from measurements of elastic wave velocity and bulk density?
A .Static elastic properties
B .Dynamic elastic properties
C .Both static and dynamic elastic properties
2. Laboratory measurements show that unconfined compressive strength (UCS) is inversely proportional to ___________ .
3. When compressional slowness is ___________ fluid slowness, compressional waves are not detectable using a dipole tool.
4. Measurements made with wireline or logging while drilling (LWD) tools provide the most accurate data with the highest ___________ resolution.
5. Seismic waves respond to average elastic properties over a distance equal to the wavelength, which is approximately _______ meters.
6. Which of the following is an extrinsic rock property?
A .Mineralogy
B .Pore pressure
C .Density
D .Temperature
7. Which parameter defines the dependence of compressive strength on confining pressure?
A .Tensile strength
B .Unconfined compressive strength
C .Friction angle
D .Coulomb failure
8. Which of the following is an intrinsic rock property?
A .Pore pressure
B .Density
C .Temperature
D .Mineralogy
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