May 19, 2014 - Nom Geo
Finding pore pressure zones using drilling parameters – The D’exponent.
This article is for a petrophysical study of geological formation pore pressure d exponent analysis using drilling data. If you find this interesting or in need of correction please leave a comment.
The d exponent uses drilling performance to look at variations in drilling performance to infer changes in formation pore pressure.
Since the drilling performance is influenced by pore pressure these drilling parameters an empirical method for estimating variations in formation pore pressures is available.
(after Jorden and Shirley 1966).
Using the formula below we will be finding pore pressure zones using drilling parameters as recorded by the drilling rig.
Where: R = penetration rate (ft/hr), N = rotary speed (rpm), Db = drill bit diameter (inches)
and W = weight on bit (k-lbf).
– This is an empirical correlation. This means that this is a “best fit” method that has been found to work well in a specific context within an actual instance of drilling a well. While there are difficulties with using this method for strictly scientific study it still provides a practical and quick identification of different pore pressure zones.
The modified d exponent, d(mod) (after Rhem and McClendon, 1973) is defined as:
Where; pf = formation fluid density [lb/gal] and pm = mud density at bit [lb/gal].
There are complications when trying to determine an accurate value which will be touched on below, however for the identification of abnormally pressured zones this method is the recommended starting point.
Using a sample data set, a section from chart of the modified D exponent curve for a shallow basin borehole might look as follows;
The borehole is in a shallow sedimentary, permo-triassic setting and shows shallow marine and fluvo-lacustrine successions. The zone at about 306-309 m is a coal. The D mod curve indicates a slight change in pore pressure within the coal seam. The left-most curve shows the calculated formation pore pressure curve.
Note that there are gamma and density curves included to allow coal, clean sands and high clay content horizons such as shales and claystones to be identified.
The fluid pressure gradient is then calculated for abnormally pressured zones using a method such as the Zamora (1972) method. However there is also the Rhem and McClendon method. The expressions for each are summarized in the table below.
The challenge posed by these results is understanding why the pore pressure in the coal was found to be higher than the surrounding formation. This could be due to several factors and without speculating too much on what these might be some examples are;
- Higher hydrostatic head due to shallow zone de-watering (natural or mine induced) of regional seam outcrops zones.
- Higher specific formation water gravity. This last cause could occur where the water has a higher concentration of dissolved ions, or in other words, a higher salinity and hence is a heavier water.
- Other ; Shallowing due to tectonic upthrow or de-roofing by erosion.This effect could be counterbalanced by the natural degassing of the seam over geologic time and prior to a deepening event which additionally causes some matrix shrinkage and a reduction in pore pressure and so is an unlikely scenario.
The pore pressure result indicated by the d exponent does suggest that the coal seam fluids are operating as a discreet aquider which is separate from the surrounding formations.
In order to get the d’mod to accurately define the pore pressure in the coal seam I wondered if the weight on bit had a higher influence on the rate of drilling thru coal than thru the surrounding formation. It does suggest that an empirical revision to the original expression is required for coal.
For now the method seems well suited to finding contrasting pore pressure but is poorly suited to calculating a pore pressure value for a shallow borehole.
As a further example there is another zone which could be the location of a minor aquifer. In this example the density values are consistent for sandstone at around 2.5 g/cc while the gamma shows a drop in potassium which could be inferred as a cleaner (less clayey) sandstone, of greated effective porosity. The spikes in pore pressure values could be due to higher specifi water gravity (because of higher concentration of dissolved salts and calcium) or it could be because there of an increase in hydrostatic head along the horizon and higher relative water permeability.
An alternative explanation is that it the pore pressure spikes are due to trapped gas. If the pore spaces are partly gas saturated then this fits in equally well with the curve indications.This anomaly exists at the 270 m mark and will be the target of a resistivity analysis to provide a stronger case for the presence of a distinct water horizon or a gas saturated zone to be discussed in the second post in this series.
In conclusion this method of finding pore pressure zones using drilling parameters is straightforward and easy to apply for its original purpose of locating high pressure areas and aiding the design of petroleum well construction. Even a quick-look geological understanding is aided by clues derived from the d’exponent method although a thorough geological interpretation is too complex to be fully satisfied by this methodology.
- Jorden, J.R. and Shirley, O.J.,1966 : “Application of Drilling Performance Data to Overpressure Detection”, Journal of Petroleum Technology, p1387-1394, Vol.18, No.11, Nov 1966.
- Rehm, B. and McClendon, R. 1973. Measurement of formation pressure from drilling data Paper No. SPE 3601
- Tan C.P., Willoughby D.R., Zhou S., Hillis R.R., 1993:An analytical method for determining horizontal stress bounds from wellbore data International journal of rock mechanics and mining sciences & geomechanics abstracts [0148-9062],1993 vol:30 iss:7 pg:1103 -1109
- Zamora, M. 1972: Slide Rule Correlation Aids ‘d’ Exponent Use. Oil and Gas Journal, December 18, 1972.