Geological Perception of Pore and Fracture Prediction: Case Histories from the Gulf of Mexico
Selim S. Shaker
Geopressure Analysis Services, Houston, TX
Most pore and fracture pressure predictions in the Gulf of Mexico are based on indirect measures of petrophysical properties such as velocity, resistivity and density. The geological framework of a prospect dictates the methods and algorithmic relationships used to predict pore pressure. All the prediction techniques focus on low permeability shale, clay and marl. Conversely, permeable sand and sandstone does not follow the same effective stress laws.
The main inputs for pore and fracture pressure prediction are 1) normal compaction trend, 2) top of geopressure, 3) overburden as primary principal stress, 4) effective stress exponent, and 5) coefficient of fracture matrix stress. Valuations need to be incorporated into the prediction model for expected hydrocarbon columns, pressure decay, and failure of structural elements.
Age and clastic lithology are primary contributors to the main building blocks of pore and fracture pressures predictions. Transgressive envelopes of a pressure profile usually are determined by the repetition of the embedded sand layers in the subsurface.
Structural frameworks, especially diapiric salt and shale, influence the principal stress and pore pressure gradients. Moreover, the resulting top of geopressure may be shallow. Faults permit communication between the subsurface geopressure cells and therefore occasionally lead to regression and breach of reservoirs.
Several prediction models have been used to represent selected wells in the shelf and deep waters of the Gulf of Mexico. They exhibit the importance and necessity of incorporating the geological building blocks for attaining comprehensive forecasting results.
AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004