Lithologies

The rate at which the drill bit penetrates the formation gives qualitative information about the lithology being drilled. For example, in a hard shale the rate of penetration ROP) will be slower than in a porous sandstone. 

The formation cuttings that are ohipped off by the bit travel upward with the mud and are caught and analysed at the surface. This provides information about the lithology and qualitative indications of the porosity. 

Whereas faults displace formerly connected lithologic units, fractures do not show appreciable displacement. They also represent planes of brittle failure and affect hard... 

If a conventional core has been cut, it will be retrieved from the barrel on the rig floor and crated. It is common to do a lithologic description at this stage. To avoid drying out of core samples and the escape of light hydrocarbons some sections will be immediately sealed in a coating of hot wax and foil. 

A vast variety of logging tools are In existence and Section 5.4 will cover only those which enable the evaluation of essential reservoir parameters, specifically net reservoir thickness, lithology, porosity and hydrocarbon saturation. 

If appropriate, correlation panels may contain additional information such as depositional environments, porosities and permeabilities, saturations, lithological descriptions and indications of which intervals have been cored. 

The formation bulk density (p ) can be read directly from the density log (see Figure 5.51) and the matrix density (p J and fluid density (p,) found in tables, assuming we have already identified lithology and fluid content from other measurements. The equation can be rearranged for porosity ((])) as follows ... 

The isochore thickness of the total interval ( gross thickness ) "egardless of lithology. 

To summarize, the total gamma ray measurements are used for real-time correlation, lithology identification, depth marker and kick-off point selection. 

By counting the gamma of low energy reaching the first counter a Pe curve sensitive to the nature of the formation can be recorded. A special counter protection fairly transparent to low-energy gamma ray (beryllium) is used. Table 4-129 shows the value of Pe for various lithologies. 

What are the boundaries of the main sand What curve is used for lithology ... 

Lithology. Gamma ray is used to differentiate between shales and clean formations. Pe is used to determine the nature of the clean formations sandstone, limestone, or dolomite. 

With the lithology matching the log scale, and assuming the formation fully invaded by mud filtrate, a neutron porosity and a density porosity can be determined. 

Porosity and Lithology Determination from Litho-Density Log and CNL Compensated Neutron Log... 

Figure 4-303. Porosity and lithology determination from the Litho-Density logs and the CNL Compensated neutron log. Courtesy Schlumberger.) ( trade mark of Schlumberger.)...

Rock Mechanical Properties. In the previous section (Figure 4-313), the wear of the bit teeth can be determined in shales by plotting the dimensionless bit torque (T, ) versus the dimensionless ROP (R,). By introducing a new parameter, namely the apparent formation strength, the bit effects can be separated from the lithology effects. 

The various volumes of rock matrix, shale, porosity, overpressure porosity and hydrocarbons are used to compute the various tool responses according to a model. The responses are compared to the measured values and a volume optimization is made to minimize the errors grouped in an incoherence function. The value of the incoherence function for the best fit determines the quality of the answer. Figure 4-335 is an example of formation pressure calculation as well as formation evaluation for lithology and fluid content. 

Figure 4-335. Computations of the formation pressure, lithology, and fluid content made with MWD formation-strength, LWD resistivity, gamma ray, neutron, and density measurements. (Courtesy Anadrill [113].,)...

Flowline temperature is also an overpressure indicator. In overpressured zones the formation temperature increases. The flow line temperature gradient (increase in temperature per 100 ft) can increase by 2° to 10° over the normal gradient. However, other effects (salt domes, lithology changes) may also cause gradient changes. 

Photoelectric Effect (Pe). Only one service company was offering a commercial Pe log. The readings of the LWD tool were very sensitive to washouts. For a qualitative lithology identification of the strata, the LWD Pe curve is satisfactory. 

Falconer, 1. G., Burgess, T. M., and Sheppard, M. C., Seaprating Bit and Lithology Effects from Drilling Mechanics data, paper IADC/SPE17191, lADC/SPE Drilling Conf., Dallas, TX, February 1988. 

Evaluated for formations or lithological-stratigraphic complexes on the basis of general geological premises and analogies... 

The presence of particles in the fluid medium complicates diffusion in a sediment due to the effects of porosity, represented by n, and tortuosity. Since tortuosity of natural sediments is seldom known it is more convenient to use the term "formation factor" or "lithological factor," denoted L, which takes into account everything but porosity. Tick s diffusion constant D is replaced by the whole sediment diffusion constant Ds, where < D. 

Equivalent igneous and metamorphic lithologies appear to weather about twice as rapidly in island-arc and younger volcanic... 

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