In-situ saturation monitoring (ISSM), using X-rays or gamma rays, has become a common method to determine fluid saturations in commercial coreflood experiments. The most common method in commercial laboratories entails 1D saturation measurements as a function of core-plug length and of experimental time. Laboratories often employ ISSM as the only method of determining fluid saturations, assuming an almost infallible accuracy of 1 to 2 saturation units (s.u.). However, as for all measurement methods, there are possible sources of uncertainty in ISSM data. Previous papers have discussed some of these uncertainties, such as X-ray drift, and inappropriate calibration scans or changes to core or fluid properties during testing. Despite this evidence, some laboratories continue to use ISSM measurements alone, assuming negligible uncertainty.
In the authors’ experience, uncertainties not only exist in measurement errors, but also may be introduced by inappropriate processing and interpretation methods. This paper first considers the stipulated 1 to 2 s.u. accuracy and the necessary signal-to-noise ratio, i.e., counts required, to achieve this; as well as providing a suggested approach, where plausible, to correct saturation data compromised by incorrect calibration scans. It also considers the uncertainties in use of ISSM production volumes in determining unsteady-state relative permeability; specifically, pre- and post-breakthrough data and the assumptions surrounding selection of breakthrough from flood-front scans. In addition, ISSM profiles are often used in coreflood simulation of relative permeability to aid correlation of the capillary end effect; incorrect data processing may compromise this correlation. The paper considers several sources of error in ISSM data and provides a recommended approach to acquisition, processing and interpretation of ISSM data for calculation of fluid saturations.
Increasing oil production by injection of designer water - also known as low salinity water - into a reservoir has recently attracted substantial attention from the oil producing community. The phenomenon has been studied by many researchers and low salinity water flooding is currently being applied in the field. On a macroscopic level, the effect can be parameterized as effective wettability modification to a more water-wet state but on a microscopic level, the effect is still not very well understood.
Most researchers agree that in sandstone rock, the mechanism is related to clay minerals but most of the experimental evidence is provided on the macroscopic scale (core flooding experiments) or even the field scale. Observations are not fully consistent and the predictability of the effect is limited. In a preceding publication [Petrophysics 2010, 51(5), 314-322] direct experimental evidence was provided for the detachment of oil droplets from a clay substrate upon exposure to low salinity brine.
The brine salinity for designer water flooding falls within a narrow window of opportunity: when too high, no additional oil production is observed; when too low, clay swelling and/or deflocculation may result in formation damage in the field. This raises the question whether there is a regime where oil is released with no or only minor formation damage and what the optimum salinity level for this would be. In this follow-up study, experiments are conducted on montmorillonite clay (which is a swelling clay belonging to the group of smectite clays) where the amount of released oil and the degree of formation damage are studied as a function of the salinity level. Starting at very high salinity (26,000 mg/L totally dissolved solids, TDS) no release of oil was observed and the clays remained stable. At very low salinity (2,000 mg/L TDS), up to 30% of the oil was released accompanied by substantial formation damage. There is, however, an intermediate salinity regime between 6,000 and 15,000 mg/L TDS where the formation damage is only very minor or not visible at all and still 10-30% of the initially attached oil is released. This is the regime of interest for field applications, although salinity levels have to be evaluated for the type of clay present in the formation rock.