Abstract

The efficiency of resistivity measurements for deriving water saturation in low salinity environments, spite of these measurement's depth of investigation, is a well identified issue in many areas in Colombia. In addition, when trying to evaluate movable hydrocarbons with multiarrays conductivity devices, the low contrast between formation water and filtrate salinities represents a limitation.

While these formation evaluation issues are known in vertical wells, a common assumption on the uniformity in the petrophysical properties along horizontal well sections tends to underestimate its potential occurrence. In this context, several wells with a proper design above well identified oil-water contacts and/or in the better structural positions within the reservoirs, are unexpectedly producing high water volumes in many areas.

In fact, preventing the high water production in horizontal wells and minimizing the risk of bypassing hydrocarbon accumulations in mature fields, are currently among the main concerns in several areas in Colombia. Given the new interest in some stratigraphic units that in the past were considered secondary targets, without much commercial importance, avoiding hydrocarbons underestimation is mandatory. The risk is based on the reservoir complexity and the current lack or reservoir characterization. Therefore, understanding the rock-pore fluids effect on petrophysical measurements and its impact on reservoir evaluation outputs, are among the main drivers in this work.

In order to attempt reducing the challenges described above, the incorporation of resistivity and salinity-independent measurements helped on the reservoir heterogeneities detection with a relation to high water saturations and to find additional hydrocarbon bearing intervals in several stratigraphic intervals. Since oil and water have different dielectric properties, borehole images are sensitive to rock texture and organic carbon is related to hydrocarbons, dielectric propagation with image logs were acquired in horizontal-open hole, while inelastic/capture spectroscopy was incorporated for cased hole conditions, as an attempt to reduce those uncertainties. The dielectric propagation provides a continuous saturation profile of water, movable oil and tortuosity, meanwhile porosity spectrum and sorting provided information on rock quality. Its integration assisted in the movable-oil, high water-saturated and better reservoir quality zones detection, confirming the presence of important reservoir heterogeneities. The inelastic/capture spectroscopy also unveils reservoir heterogeneities, whereas a time lapse approach provides some clues on saturation changes and oil mobility.

The present study demonstrates a level of reservoir heterogeneity not previously suspected, where the incorporation of advanced wireline technologies supported a better understanding on the relationship between fluids saturation, mobility and rock quality. Also, it helps to detect the presence of hydrocarbons in complex lithology and low resistivity zones. In terms or well completion design, these results open considerations for changing slotted liner-type completions to selective completions, for isolating high water and non-movable oil saturated intervals. It also provides information on reservoir matrix potential in tight/complex reservoirs.

Even in mature fields, further understanding on the high water cut causes is being achieved, meanwhile encountering new hydrocarbon bearing zones not detected with conventional technologies. The reservoir evaluation approach proposed in the present paper, constitute a valuable tool that helps operators to improve the initial reservoir characterization and well completion strategies.

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