Mohammad Monzurul Alam

Permeability prediction in chalks

The velocity of elastic waves is the primary datum available for acquiring information about subsurface characteristics such as lithology and porosity. Cheap and quick (spatial coverage, ease of measurement) information of permeability can be achieved, if sonic velocity is used for permeability prediction, so we have investigated the use of velocity data to predict permeability. The compressional velocity from wireline logs and core plugs of the chalk reservoir in the South Arne field, North Sea, has been used for this study. We compared various methods of permeability prediction from velocities. The relationships between permeability and porosity from core data were first examined using Kozenys equation. The data were analyzed for any correlations to the specific surface of the grain, Sg, and to the hydraulic property defined as the flow zone indicator (FZI). These two methods use two different approaches to enhance permeability prediction from Kozenys equation. The FZI is based on a concept of a tortuous flow path in a granular bed. The Sg concept considers the pore space that is exposed to fluid flow and models permeability resulting from effective flow parallel to pressure drop. The porosity-permeability relationships were replaced by relationships between velocity of elastic waves and permeability using laboratory data, and the relationships were then applied to well-log data. We found that the permeability prediction in chalk and possibly other sediments with large surface areas could be improved significantly using the effective specific surface as the fluid-flow concept. The FZI unit is appropriate for highly permeable sedimentary rocks such as sandstones and limestones that have small surface areas.

Nuclear magnetic resonance and sound velocity measurements of chalk saturated with magnesium rich brine

The use of low field Nuclear Magnetic Resonance (NMR) to determine petrophysical properties of reservoirs has proved to be a good technique. Together with sonic and electrical resistivity measurements, NMR can contribute to illustrate the changes on chalk elasticity due to different pore water composition. In this study we relate NMR data to changes in P-wave velocity and electrical resistivity. Core plugs from outcrop Stevns chalk, of 44% porosity, were divided into groups of three and saturated with deionized water, calcite equilibrated water, as well as sodium chloride and magnesium chloride solutions of the same ionic strength. Saturation with a solution that contained divalent ions caused a major shift on the distribution of the relaxation time. The changes were probably due to precipitats forming extra internal surface in the sample. Sonic velocities were relatively low in the MgCl2 solution saturated plugs.

Burial diagenesis of deep sea chalk as reflected in Biot's coefficient

Burial diagenesis of chalk has been widely studied, but little agreement has been reached on by which mechanism porosity declines, and on how to calculate the deforming stress in the most informative way. Data from Ocean Drilling Program show that calcareous ooze transforms to chalk and chalk to limestone as burial increases and porosity decreases. The porosity decrease is accompanied by an increasing velocity to elastic waves, and consequently a decreasing Biot’s coefficient, as estimated from velocity and density of core samples. When the effective burial stress is normalized to total horizontal cross sectional area, the porosity is found to decline as a function of stress. The porosity trend proceeds smoothly from ooze over chalk to limestone. By contrast, when vertical effective stress is normalized to grain contact area, each lithology shows a distinct porosity-decline - stress pattern. In the ooze, we find that the natural compaction causes an increasing stress on grain contact area, indicating that the ooze particles become strongly strained. In the chalk section, contact cement is probably the reason why particles become less strained as porosity declines. In the limestone, stress on particles apparently is low and not correlated with porosity, probably because the pore-filling cementation in this interval causes Biot’s coefficient to decline as burial increases. Limestone from the water zone of the North sea Chalk Group follows the same stress trend as deep sea limestone. These results indicate that by normalizing effective stress to grain contact area, we can get information about the mechanism behind burial related diagenetic porosity decline.

Nuclear Magnetic Resonance and Elastic Wave Velocity of Chalk Saturated with Brines Containing Divalent Ions

Nuclear magnetic resonance (NMR) has proven a good technique for measuring pore size distribution in reservoir rocks. The use of low field NMR together with sonic and electrical resistivity measurements, can contribute to illustrate the effect of adsorbing ions on chalk elasticity. NMR is useful for the study of the physical and chemical phenomena within saturated cores and sonic velocity is intimately connected to density and elastic constants of the rock. In this study we relate NMR data to changes in P-wave velocity due to ion adsorption. Core plugs from outcrop Stevns chalk, of ~45% porosity, were divided into groups of three and each group was saturated either with deionized water, calcite equilibrated water, or sodium chloride, magnesium chloride and calcium chloride solutions of the same ionic strength. Saturation with solutions that contain divalent ions caused major shifts in the distribution of the relaxation time. Core samples saturated with calcium chloride solution relaxed slower and those saturated with magnesium chloride solution relaxed faster than the rest of the samples. Along with the changes in relaxation the samples experienced smaller velocities of elastic waves when saturated with MgCl2 solution. Rock samples saturated with brines containing salts experienced lower electrical resistivity.

Wettability of quartz surface as observed by NMR transverse relaxation time (T2)

Injection of optimized water composition (smart water) is an advanced water flooding method for Enhanced Oil Recovery (EOR). Low saline waterflooding has proven successful in sandstone reservoirs. However, there is still controversy on the mechanism of smart water flooding. We studied the wettability property of the quartz surface by using Nuclear Magnetic Resonance (NMR) method. The principle of this method is that protons in water relax faster when it comes close to solid surface. We observed that quartz is highly water wet. A layer of water (bound water) forms on the quartz surface when they are mixed. The amount of bound water increases with the second power of specific surface area of particles. Addition of metal ions to the water in some cases increases the amount of bound water for same amount of surface area. We studied the affect of Ca2 , Mg2 and Na ions and observed that among these ions, the Mg2 ion produces the maximum amount of bound water while the Ca2 ion produces the least. The method demonstrated in this study could be used to decide the optimized water composition for waterflooding in sandstone reservoirs.