A number of methods are used in the industry to detect and evaluate overpressured formations. The ability to detect and recognize overpressured formation is critical in conducting efficient and safe drilling operations.
Observed changes in the properties of rocks, especially shales, can be used to evaluate the overpressured zones. Variations of rock properties can be detected by geophysical methods, wire-line logging techniques, surface measurements on the drilling mud and shale cuttings, and monitoring of several drilling parameters.
The best approach for the detection and evaluation of overpressured formations is the study of a combination of several measured parameters since relying on one type of data parameters since relying on one type of data can result in misinterpretations.
In the worldwide search for oil and gas, abnormally high pressure zones (geopressures) have been encountered in numerous countries on several continents. In our present discussion such geopressures, or abnormally high subsurface pressures, are defined as any pressure which exceeds the hydrostatic pressure of a column of water containing approximately 80,000 ppm total solids. ppm total solids. For the oil industry, occurrence of geopressures are important in many respects.
1. Worldwide experience indicates a significant correlation between the presence and magnitude of geopressures and the shales and ratio of sedimentary sections penetrated by the drill bit.
2. In some areas, the distribution of oil and gas appears to be related to the regional and local pressure environments.
3. Knowledge of the expected pore pressure and fracture gradient is the basis pressure and fracture gradient is the basis to make the best of modern drilling techniques, i.e., efficiently drilling wells with correct mud weights and proper casing programs. This also prevents a breakdown of exposed formations and contains the high-pressure fluids in deeper formations, thereby reducing blowout hazards.
4. Much of the extra cost in the search for and development of hydrocarbon reserves is for drilling fluid and casing programs. An additional, quite expensive item is the properly selected completion method which properly selected completion method which must be effective, safe and allow for killing of the well. Here, too, reliable pore pressure and fracture gradient data are a prerequisite.
5. Last but not least, geopressures are also an important factor in several aspects of reservoir engineering. This includes consideration of compressibility and failure of the reservoir rock and the possibility of water influx from the overlying and underlying shale formations.
For years the drilling industry dreamed of a method to predict and evaluate geopressures. Today, a number of methods, both surface and subsurface techniques, have become available for geopressures detection and measurement. These overpressured intervals and as much prior knowledge of them before drilling as is possible - are critical to conducting efficient and safe drilling operations.