Reductions In the Productivity of Oil And Low Permeability Gas Reservoirs Due to Aqueous Phase Trapping
Authors: Bennion D.B. , Bietz R.F. , Thomas F.B. , Cimolai M.P. 
Published: March 2010

Abstract
Many hydrocarbon bearing reservoirs exhibit the potential for significant productivity reductions due to adverse relative permeability effects associated with the retention of invaded aqueous fluids. These fluids could include-water-based drilling mud filtrates, completion fluids, fracture fluids, workover fluids, kill fluids or stimulation fluids (including-spent acid).
This paper identifies potential mechanisms behind phase trapping and identifies particular reservoir types which tend to be susceptible to this type of formation damage, most notably low initial water saturation gas reservoirs and strongly oil-wet oil reservoirs. Laboratory techniques to investigate the severity of aqueous trapping and various remedial techniques are described, and two field case studies illustrating the potential for permeability impairment due to invasive aqueous trapping are presented. One case study describes a series of wells completed in the Paddy formation and the second in the Cadomin formation in the Deep Basin area of central Alberta (both gas producing zones). Laboratory case studies documenting the phenomenon of aqueous, phase trapping in strongly oil-wet porous media are also presented.

Introduction
Oil and gas bearing formations are potentially susceptible to many different types of formation damage(1–6). In this paper we are exclusively concerned with damage associated with aqueous phase trapping (or water trapping or blocking as it is often referred to). To understand the concept of aqueous phase trapping, it is essential to differentiate between the concept of initial (often referred to as connate) aqueous phase saturation (Swi) and irreducible aqueous phase saturation (Swirr).Initial aqueous phase saturation is the initial average fractional portion of the pore space which is occupied by water. The value of the initial aqueous phase saturation is controlled by numerous factors, including reservoir geology, depositional history, temperature, wettability, height above free water contact and pore size distribution. The key point to differentiate in this area is that the initial aqueous phase saturation is not necessarily, and often is not equal to the irreducible aqueous phase saturation and can be either higher or lower than the irreducible saturation. It is in the second case, where the Swi is less than Swirr where productivity reductions due to phase trapping can occur.Irreducible aqueous saturation represents that saturation which is forced to exist in the reservoir by capillary mechanics. Once again, the value of the Swirr is determined by parameters such as the reservoir morphology, pore size distribution, pore throat size distribution, wettability, surface roughness, etc. We often obtain estimates of Swirr through the use of air-brine or air-mercury capillary pressure tests, Although these values often provide good approximations to Swirr they may be poor indications of actual Swi.
FIGURE 1: Mechanism or aqueous phase trapping. (Available in full paper)

Mechanism of Aqueous Phase Trapping
Figure 1 provides an illustrative example of a set of relative permeability curves. The diagram is applicable to either an oil or a gas reservoir. Examination of Figure 1 indicates that, if the zone of interest is at some aqueous saturation greater than the irreducible value of 45%, aqueous trapping will not be a severe problem because the reservoir is already initially highly saturated with water and may even be producing free mobile brine.