Formation damage concerns in reservoirs with ultralow water saturations have been over dramatized; perhaps, because the mechanisms of permeability reduction as a result of extraneous fluid invasion are not well understood. It has been proposed that some gas reservoirs contain less than reducible water, Swrl and as such, have additional pore volume which contributes to hydrocarbon permeability. Consequently, introduction of fluids during drilling, completion and workovers could result in more permeability loss in these reservoirs, than in reservoirs which contain water saturations controlled by capillary pressure.
This paper demonstrates, with laboratory data and with Poiseuille's theory, that for water-wet reservoirs (which most gas reservoirs are, since gas is strongly non wetting) areas of porosity where bound water exists, contribute little to the overall permeability. Several permeability measurements performed on clastic, limestone and dolomite core samples with and without connate water saturation demonstrate that minimal permeability reduction occurs due to irreducible water saturation. Even in extremely low permeability cores less than 0.5 md, the effect of irreducible water saturation on gas permeability was only a 20 percent reduction. These results suggest that permeability damage is only slightly worse in desiccated gas reservoirs than in conventional gas reservoirs. Furthermore, permeability impairment as a result of fluid invasion is more a function of effects on macroporosity than microporosity.
With the increased activity in natural gas exploitation in Canada, much attention has been drawn to the potential for formation damage as a result of introduction of drilling and completion fluids, particularly in low permeability gas reservoirs. A perception exists that damage may be more severe in reservoirs which are initially at ultra-low water saturations, i.e. lower than what might be predicted from capillary pressure measurements. This thinking differs from the general consensus that microporosity, which is the major portion of pore volume controlling water saturation in a water-wet reservoir, contributes little to permeability. This paper reviews various literature on the relationship of pore size and water saturation with respect to contribution to permeability. Laboratory permeability measurements on cores from four formations are presented and compared to other published data, to demonstrate the effect of water saturation on effective permeability, and how it relates to capillary pressure and pore throat size. Finally, a technique will be presented, based on Poiseuille's theory, on how to calculate the contribution to permeability as a function of pore throat size using air/mercury capillary pressure data.
Thomeer1 and Swanson2 developed empirical techniques for predicting absolute permeability from capillary pressure data. They recognized the insignificant contribution to permeability of smaller pores compared to the contribution of larger pores.
Swanson's method of characterizing capillary pressure curves uses the inflection point (point A in Figure A) on the lower portion of the capillary pressure curve to predict permeability. He argues that this inflection point represents the maximum product of pore throat size times effective flow area. The value of this maximum, which is inversely proportional to the distance of the inflection point from the lower left-hand corner of the axes, is proportional to permeability.
