Emerging Trends in Pressure Prediction
Authors: Alberty Mark W. , McLean Michael R. 
Published: October 2008

Recognition of some recent observations regarding pressure prediction may significantly impact traditional methods used to predict pore and fracture pressures. These observations include the influence of clay diagenesis upon compaction trends and pressure models, the influence of unloading upon pressure prediction methods, and the influence of whole mud systems upon apparent fracture gradient behavior in sands.

Many basins have smectite/illite-dominated shale. It has been observed that smectite and illite have significantly different compaction trends. Transition from smectite to illite is primarily driven by temperature and therefore the composite compaction trend will depend on the temperature gradient in the region. Properly interpreting and defining the compaction trend through knowledge of this process can significantly impact well planning pressure prediction techniques.

A study of recent wells indicates that the horizontal stress at a given depth below mud line is typically greater in regions subject to erosion. The closer to mud line the more notable the effect. The leak-off tests used to determine the horizontal stress magnitude imply that virtually all the horizontal stress built up during deposition remains after erosion. Compaction trends used to calculate pore pressure need to account for the unloading effects of erosion on the sediments under investigation. However, with little or no unloading of horizontal stresses, the reduction in mean stress in response to erosion may be far less than our algorithms previously assumed.

The design of an appropriate casing program is dependent on fracture resistance of intervals to be drilled as well as the pore pressure. The fracture resistance of sand is seen to be dependent on the presence or absence of solids in the drilling fluid.

Incorporation of the above observations and principles in pressure and fracture gradient prediction will result in significantly different well design needs.

Introduction

The prediction of pore and fracture pressure along a proposed well path are key inputs to well design. Accurate predictions of these parameters can have significant impact on the cost of wells, both from design and operational perspectives. Recent observations have an important impact upon the prediction of these properties.

Compaction trends are most accurately characterized as plots of effective stress versus porosity. However, they are typically presented in terms of interval velocity or transit time versus depth, in which case acoustic velocity is used as a proxy for porosity and effective stress is correlated to depth on the basis that the overburden is normally pressured. Thus, at each point the compaction trend shows the interval velocity or transit time for the appropriate lithology at that depth if it were normally pressured. Different practitioners may substitute for acoustic velocity other porosity sensitive log measurements such as resistivity, density, nuclear magnetic resonance determined porosity, and drilling exponent.

The key to success in predicting pressure is to accurately define the compaction trend that is appropriate for the formation being analyzed. A number of factors can affect these curves including the clay type and the relationship between vertical stress and mean stress.