Pore Pressure and Methods of Analysis Section 3 PPFG Modeling Principles Parts 1, 2, and 3

Part 1.???Compaction, Pore Pressure, and Porosity Decline Rate

Pore pressure is the pressure of the fluid within the pore space. Effective stress is the grain-to-grain stress of the rock particles surrounding the pore space. The overburden stress is the combined force of gravity acting on the total mass above a specific depth of investigation.

“Normal” pressure is hydrostatic pressure.

All compaction models are impermeable rock models. The relationship between the pore pressure of an impermeable rock and the pore pressure of a permeable rock is assessed with the theory of centroid. Centroid calculations will be addressed later in this section.

Compaction as a result of increased burial causes the porosity to decrease. As the porosity decreases, the fluid contained in the pore space being decreased migrates out of the pore space. The pore space will decrease at a rate with increasing overburden stress with depth, given the fluid migrating out of the decreasing pore space is not restricted. When the fluid migrating is restricted, the pressure of the fluid increases and introduces an additional force resisting compaction. With the addition of this additional force, the porosity decline rate deviates from the porosity decline trend established at the shallower depth where the fluid migrating was not restricted.

Pore pressure compaction models identify changes in porosity trends of impermeable rock, and associate that porosity trend change with a change of pore pressure gradient.

Part 2.???The Equivalent Depth Method

A pore pressure model that uses the Terzaghi relationship between pore pressure, effective stress and overburden stress is an equivalent depth method model. It could be more appropriately named "equivalent deposition and vertical stress history model" because it is assumed the strata is from the same depositional source and has endured the same vertical stress history, thus being equivalent at a hypothetical depth. In reality there is always some difference from one geologic point to another even though they were deposited at the same time from the same depositional source. Additionally, clastic deposits from varying depositional sources exhibit similar characteristics associated with increased burial.

The normal compaction trend is the trend of any physical property (resistivity, acoustic impedance, density, etc.) through the normally pressured strata. Below the normally pressured strata the pore pressure gradient increases, and the porosity decline rate will be different than in the normally pressured strata above. Log data and interval velocity trends will also change in accordance with the change of the porosity decline trend.

The equivalent depth method is defined by a relationship between pore pressure, effective stress, and the overburden stress described by the Terzaghi relationship. The Terzaghi relationship assumes the overburden is supported by the sum of the pore pressure and the grain-to-grain contact, termed the effective stress, of the rock matrix. The accepted relationship is expressed by the Terzaghi Principle, which states the downward force of the overburden is equal to the sum of the upward forces of pore pressure and the effective stress.

Figure 16?????????????Terzaghi Principle

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An equivalent depth method model requires the use of a normal compaction trend. The equivalent depth method assumes that there is a depth section over which the pore pressure is hydrostatic, and the sediments are normally compacted because of the systematic increase in vertical stress with depth. When the log of a measured physical property (resistivity, acoustic travel time, density) is plotted as a function of depth, normal compaction trends are fitted to the data over the normally pressured interval, and extended through a minimum distance of at least the base of the depositional sequence. Where the data set deviates from the normal compaction trend below the hydrostatic pressure region, the difference between the data set and the normal compaction trend at a specific depth is used to calculate a pore pressure gradient not equal to hydrostatic pressure.

Because the value of the measured physical property is related to effective stress, which in turn is related to the overburden stress, an approximation of the pore pressure at any depth where the measured value is not on the normal compaction trend line can be calculated.

Often, because the physical properties are similar, the same normal compaction trend is adequate for the subsequent depositional sequence. More often, the normal compaction trend must terminate at the base of the depositional sequence where the hydrostatic pressure interval was used to define the normal compaction trend, and a new normal compaction trend must be established to model the deeper formations.

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Part 3.???Pore Pressure Model Principles

???????????????Most common pore pressure models use either resistivity, sonic travel time, or interval velocity data. This course references models using these data sets.

As depth increases, porosity declines due to compaction. As porosity declines, the percentage of water-filled porosity of the rock declines. Log response will be in accordance with the effect of decreasing water-filled porosity.

???????????????Where the pore pressure gradient is constant with increased depth, the porosity decline trend will be unchanged.

???????????????Where the pore pressure gradient increases with depth, compaction is resisted by the increased pore pressure gradient, and the porosity decline rate deviates from the porosity decline trend.

???????????????The log response trend will change as the trend of water-filled porosity changes. The change in the trend of water-filled porosity with depth is associated with a change of pore pressure gradient.

???????????????Pore pressure calculations are based solely on true vertical depth.

???????????????A hypothetical normal compaction trend assumed to be normally pressured is plotted with the resistivity, sonic, or interval velocity data.

???????????????Impermeable intervals are selected for analysis, and the resistivity or sonic travel time, value are used with the normal compaction trend value to calculate a pore pressure for each shale interval selected.

???????????????Permeable intervals are excluded from the model.