Core Flood Tests
Core flood tests can be conducted in various ways depending on the convenience and requirements of specific applications and/or the interpretation methods available. Core floods are usually accomplished under the conditions of constant pressure difference across the core plugs or constant flow through the core plugs. This provides convenience for interpretation of the experimental data by means of a mathematical model. However, maintaining constant pressure difference and constant flow conditions in actual test conditions may be difficult and may not be truly accomplished, in spite of the use of high-quality equipment.
Therefore, interpretation methods that can allow for variable flow conditions are preferred. Core flood tests can be conducted using single or multiple core holders. Multiple core holders may be run in series or parallel depending on the specific reasons. Parallel core holders may be required for various reasons. Similar core plugs in parallel may be flown simultaneously and certain formation damage effects, such as filter cake thickness, porosity, or precipitate quantity, can be measured separately and over different time periods for each core plug. Tests to establish the effect of the permeability contrast, such as for conformance control studies, require floods using parallel core plugs with different permeabilities (Prada et al., 2000). Core plugs may be run in series to simulate the effect of formation damage over long distances.
Core flood tests can be conducted in two ways:
1 interrupted core floods and
2 continuous corefloods (Haggerty and Seyler, 1997). In interrupted core flood tests, the fluid injection is interrupted at certain time intervals and permeability is measured. In continuous core flood tests, the effective permeabilities are measured during the injection process.
Laboratory Procedures for Evaluation of Formation Damage Problems
The laboratory procedures required for evaluation of common formation damage problems are described in this section according to Keelan
and Koepf (1977). They classified the frequently encountered formation damage problems into four groups:
1. The blocking of pore channels by solids introduced by drilling or by completion, workover, or injection fluids,
2. Clay-water reaction that yields clay hydration and swelling, or clay particle dispersion and pore plugging by movement with produced or injected water,
3. Liquid block that normally is caused by extraneous water introduced into the formation at the wellbore during drilling, coring, completion, or workover, and
4. The caving and subsequent flow of unconsolidated sands into the wellbore.
The Liquid Block Problem
As explained in this article by Keelan and Koepf (1977), "liquid block reduces effective permeability to the hydrocarbon." Before damage, the original mobile water saturation range in 0.20 < Sw < 0.80. After extraneous water incompatible with the formation invades the porous media, the irreducible water saturation raises to about 34% from its original value of 0.20. Hence, the line AA' shifts to the line DD'. Consequently, the relative permeability of the oil at the irreducible water saturation decreases from the original 0.9 value to about 0.3, which amounts to a three-fold reduction of permeability to the hydrocarbon. On the other hand, damage by clay hydration and fine particles plugging caused by incompatible extraneous fluid invasion increases the residual oil from its original 0.2 (Sw = 0.8) to about 0.26 (Sw = 0.74). Hence, the line BB' shifts to the line CC". The permeability to water also decreases. As a remedial action, Keelan and Koepf (1977) recommend treatments inferred by the capillary pressure equation:
where pc denotes the capillary pressure necessary for water retention, a is the surface tension between water and hydrocarbon, 0 is the contact
angle between the water and hydrocarbon, and r is the pore radius. Eq. 15-1 indicates that water retention can be reduced by workover schemes reducing the surface tension and/or increasing the contact angle to favor a less water-wet condition.
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