Abstract

Post depositional modifications (diagenesis) of sandstones play an important role in controlling hydrocarbon discovery and production. Diagenesis can effect the ways production. Diagenesis can effect the ways in which geologists and engineers interpret, evaluate and develop sandstone reservoirs. In particular, diagenesis has a significant effect on several topics of importance to the oil industry including:

  1. Petrophysics

  2. Fluid System Compositions,

  3. Drill Stem Testing,

  4. Well Production, and

  5. Enhanced Recovery.

1) PETROPHYSICS

Diagenesis can control log shapes, leading to erroneous interpretations of depositional environment. It is important that the impact of diagenesis on log shape be understood so that accurate assessment of depositional environment can be made. Depositional environment Governs reservoir trend, shape, and at times dimensions and quality. It is, therefore, of importance to formation evaluation and field development. The importance of this control is heightened because of our necessary dependence on logs.

Post-depositional addition of various cements in the pore system, significantly effect gamma-ray log shape, porosity calculations from neutron and density logs, and also S, calculations from Archie's equation resulting in pessimistic interpretations of water saturations.

2) FLUID SYSTEM COMPOSITIONS

Drilling, completion and enhanced recovery fluids react with natural (authigenic) cements in rock pores and these reactions can effect the quality and production of any reservoir. This is particularly true in sandstones of West Texas, where a wide variety of natural cements are present. In some locations, sandstones are sensitive to fresh or sodium-based drilling and completion fluids because of the presence of the swelling clays illite-smectite or smectite. In other areas these clays are absent and no swelling problems exist.

Certain areas are characterized by sensitivity to either HCl or HF acids (whether used in mud clean-up or stimulation) due to the presence of specific sulfides or oxides in the pore systems. The sensitivities of sandstones to waters and acids are not uniform across West Texas, but vary from area to area and formation to formation. Thus drilling and completion fluids should be designed on a local not regional scale.

3) DST RESULTS

Diagenesis can have a significant effect on DST results. The presence of authigenic cements often causes poor DST'S. There are instances when DST results should be ignored and well completion encouraged. Poor DST's in the West Texas sandstones result from any or all of the following:

  • Secondary porosity,

  • Silica overgrowths, and

  • porosity,

  • Silica overgrowths, and

  • Illitic minerals.

Thus, poor DST results should be used to condemn a well only after the diagenesis of the sands is understood.

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