Formation pore pressure determinations from log properties in carbonate environments has always been a difficult task. They do not compact uniformly with depth as do shale's, nor is it necessarily true for the fluids to be in support of the overburden when abnormally pressured, as in the classical sand shale abnormally pressured environment. Consequently, the traditional, somewhat straight forward, techniques of pore pressure determination from log properties do not, as such, apply.
We have developed an approach to determine formation pore pressures in carbonate environments utilizing sonic velocity trends. This technique has been utilized on approximately 20 wells to date, as of this writing with very good results. Wells analyzed have ranged in pore pressures, at total depth from 10 PPG to 18 PPG and have been as deep as 23000 feet. It has also been possible to identify pressure regressions.
The approach will be illustrated in detail, and the results of its use on a number of wells will be presented and explained. Mud weights required to drill these wells will also be illustrated as well as any pressure control data such as a kick.
In just about any environment, there tends to be a mixture comprising a rock's makeup. Generally speaking, in all sands there is some shale, in all shale's there is some sand, in all carbonates there is some sand and shale and so forth. Thus, when we speak of a rock as being a sandstone for instance, we are referring to its primary compositional nature.
When we analyze a gamma ray log in classical sand shale environment, we look for a gamma ray response to the right for shale indication, and to the left for an indication of sandstone.
When we analyze the gamma ray in a carbonate environment, the peak responses to the right we will consider to be the result of some shaliness within the rock matrix. It will be in these intervals where we will concentrate our efforts.
The first step is in determining lithology tops. This is done by displaying the gamma ray and sonic logs in a one inch equals one thousand foot scale. In compressing data like this, a smoothing function need be applied to avoid a blur of data. Lithology tops are then determined by picking the points where either the gamma ray or sonic shows a change in the general trend. This process is illustrated in figure 1, with lithological tops indicated with the dark horizontal lines. The well utilized in this figure is in the Destin Dome area of the Gulf of Mexico.
The gamma ray and sonic are then displayed in a one inch equals one hundred foot scale. Again smoothing may be required. The lithology tops previously determined are translated to this display. Next to this data we display an unsmoothed version of the gamma ray, as well as an SP, resistivity and conductivity curves as in figures 2 through 6.
