--> Abstract: Porosity Prediction Using a Simple Mechanical/Chemical Compaction Calculator for Clastic Reservoir Rocks, by Andrew Pepper, Niall J. McCormack, Erich Heydweiller, Jennifer Wolters, and Caroline Burke; #90082 (2008)
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Porosity Prediction Using a Simple Mechanical/Chemical Compaction Calculator for Clastic Reservoir Rocks

Andrew Pepper1, Niall J. McCormack1, Erich Heydweiller2, Jennifer Wolters1, and Previous HitCarolineTop Burke1
1Exploration, Hess Corporation, Houston, TX
2Colorado School of Mines, Golden, CO

Forward prediction of porosity is an important component in the pre-drill and post-drill evaluation of reservoir storage capacity.
In this paper we summarize the construction, calibration and application of a simple porosity calculator within Hess Corporation since 2004. Mechanical and chemical compaction occur independently and concurrently.

Rock description requires a QFL-based compositional breakdown; the components being quartz, feldspar, hard lithics (most igneous and metamorphic rock fragments), and soft lithics (sedimentary rock fragments, micas). Grain size is an important parameter in both mechanical and chemical compaction, with degree of grain coating an additional important parameter in chemical compaction. The volume of matrix grains (<0.02 mm) is subtracted from the inter-granular volume (IGV) to derive final porosity.

Mechanical compaction is handled using a strain vs. effective stress relationship. Our model ignores the minor differences in mechanical behavior between quartz, feldspar and hard lithic grains; but does account for the effect of soft lithics, which compact more readily. For a given composition and effective stress, coarser-grained rocks compact faster.

An optinmized kinetic model predicts chemical compaction via dissolution and re-precipitation of the quartz, via an optimized kinetic model, as a function of time temperature history. These kinetics predict that time is much more important in quartz precipitation than in kerogen maturation. Rocks with finer grains (larger surface area/volume ratio) cement faster than courser grained rocks; cementation is retarded as degree of grain coating increases.

Over the last 2 years we have applied and tested the model in a wide variety of basins and plays. Where the burial/thermal history and rock composition are well constrained pre-drill, model predictions have been quite accurate.

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