Geopressure and Porosity – Examples from the Chukchi Sea and the National Petroleum Reserve, Alaska

ABSTRACT:

The relationship between porosity and effective stress has long been recognized and exploited for the purpose of geopressure (i.e., formation fluid pore pressure) analysis. However, this relationship is frequently overlooked because of the practice of using porosity proxy functions such as resistivity, velocity, interval transit time, density, and others. Most comparisons of pore pressure analyses are performed though the use of proxy curves, rather than the underlying porosity values. Also, many methods exist for the calculation of porosity from various data types. To reconcile pore pressure estimates from disparate data sources, it is suggested that the underlying porosity proxy function should first converted to porosity, and then analyzed for geopressure. It is proposed that more meaningful results can be obtained if the following processing workflow is followed: 1. Obtain well petrophysical data and seismic interval velocity data 2. Establish normal hydrostatic pressure based on formation fluid density 3. Identify lithologies likely to be encountered 4. Perform appropriate petrophysical analysis of porosity sensitive curves (primary v secondary porosity, porosity texture, cementation, environmental corrections, etc.) 5. Calculate lithostatic (overburden) pressure using fluid and rock densities 6. Determine a porosity normal compaction trend analysis using compaction and effective stress relationships 7. Estimate formation fluid pressure determination The conversion of sampled data to porosity, using rigorous petrophysical techniques, prior to pore pressure determination has the additional benefit of extending the utility of other methods, such as the Bowers’ Method. Accurate determination of porosity dovetails into many rock property relationships such as permeability and compressive strength, further extending the utility of this approach.

BIOGRAPHY:

Since 1988, Mark Herkommer has focused his professional career on the analysis of seismic and geological data for pore pressure, fracture gradient and wellbore stability (PP/FG/WBS) analysis and interpretation. He has been involved in most geopressure and geomechanical related phases of well planning and drilling. He currently operates Qube Tech as its CEO.

Mark is actively involved providing Real-Time Geopressure-Geomechanics (RT-GPGM) Services, PP Event Investigations, and Pre-Drill Wellbore Feasibility analysis. At the wellsite, his proven methodologies for surveillance integrate Advanced Surface Data Logging measurements with LWD to provide real-time PP/FG/WBS surveillance for unconventional onshore and deep-water offshore wells.