Applications of the Wojtanowicz et al. Model
In general, formation damage may be a result of a number of mechanisms acting together with different relative contributions. But the Wojtanowicz et al. (1987, 1988) analysis of experimental data is based on the assumption that one of the potential formation damage mechanisms is dominant under certain conditions. Therefore, by testing the various diagnostic equations given in this article derived by Wojtanowicz et al. for possible mechanisms involving the laboratory core damage, the particular governing damage mechanism can be identified. They have demonstrated that portions of typical laboratory data can be represented by different equations, indicating that different mechanisms are responsible for damage.
The portions of the experimental data for damage by foreign particles invasion with low particle concentration drilling muds (0.2%, 0.5%, and 1.0% by weight) can be represented by Eqs. Tl-1 and 2, successfully, revealing that the pore surface deposition and pore throat plugging mechanisms are dominant during the early and late times, respectively. Tl-3 provides an accurate straight-line representation of the core damage with injection of suspensions of high concentration drilling muds (2% and 3% by weight) of foreign particles, revealing that the dominant formation damage mechanism should be the pore filling and internal cake formation.
The data plotted in this article shows that the sizes and concentrations of the particles of the injected suspension significantly affect the durations and extent of the initial formation damage by pore surface deposition (Eq. Tl-1) and later formation damage by pore throat plugging (Eq. Tl-2) mechanisms.
In this article show that the damage of the core by particle-free ammonium nitrate/alcohol-based completion fluids are due to pore surface deposition and pore surface sweeping, because the data can be satisfactorily fitted by straight-lines according to Eqs. Tl-6 and 5, respectively. Figure 10-14 showing the plot of data for the combined effects of pore surface deposition and sweeping according to Eq. Tl-4, indicates the effect of the flow rate on damage. As can be seen, the rate of formation damage increases by the flow rate.
Wojtanowicz et al. (1987) explain this increase due to about a fivefold increase in the value of the release coefficient, ke, as a result of about a threefold increase in the flow rate from 3 to 10 ml/min. The best estimates of the intercept and slope values obtained by the least-squares regression analysis for the cases analyzed are presented by Wojtanowicz et al. (1987) Using these parameters in the relevant equations representing these cases, the relative retained permeability curves vs. t are plotted in Figure 10-15 for comparison. As can be seen, pore filling by cake formation causes the most severe damage.
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