Innovative Technology for Mature Field Reactivation
Authors: Salazar Giovanni , Fernandez Fabian , Franco Julian , Bustos Ulises , Chapellin Alvaro , Kovarskiy Egor 
Published: November 2020

In a difficult context of low oil prices, current investments are aimed to cash flow maximization through the oil production increase in old reservoirs with complex structures and a lot of technical requirements. In addition, data acquisition requirements for uncertainty reductions is carefully revised on a case by case basis. This is the current situation for Caguan-Putumayo Basin located in the southeast region of Colombia.

The targets are Caballos, Villeta and Pepino Formations, consisting in reservoirs composed of sandstones, carbonates, clay types mixtures, organic matter presence and matrix porosities in the ten percent range and below. Most of wells in this field, that were drilled some decades ago, lacks from electrical logging data in Villeta and Pepino formations -nowadays of increasing interest-. This is because in the past the main target was the Caballos reservoirs. In some wells, the only available information is natural gamma ray and a cement bond log. Considering that a representative formation evaluation lies in reliable mineralogy, porosity, saturation and permeability estimation, we carry out a cased hole petrophysics with ultimate available wireline technology. Regarding petrophysical methods, the archie-based approaches are difficult to apply there due to low formation salinity, variable tortuosity and potential wettability changes (in the carbonatic sections). Additionally, due to matrix mineralogy mixtures it is impossible to accurately determine porosity with just nuclear logs and fixed matrix values. Consequently, erroneous porosity means errors in saturation calculation. Besides that, there are a lot of operational constraints caused by the state of old wells.

At the beginning of the project, the solution to overcome the limitations described above and fully characterize these reservoirs in cased hole conditions, relied on advanced nuclear spectroscopy measurements accompanied by solid methodology for resistivity and salinity independent hydrocarbon saturation determination and by a multidisciplinary approach to select candidates and improve workover execution. Later in the project timeline, it was possible to incorporate advanced nuclear magnetic resonance logs, which brings valuable information on longitudinal relaxation times distribution (T1) and transverse relaxation times distribution (T2). These measurements, carried out with a new wireline device with sophisticated design and special pulse sequences acquisition strategy, provided with relevant information on rock quality and reservoir fluids analysis. We present preliminary findings on hydrocarbon behavior, enabling to validate the nuclear spectroscopy-derived saturation and the potential capability of those hydrocarbons to be commercially produced.

The main advantages of the new approach for tight formation evaluation are: First, the oil saturation determination is a resistivity/salinity/tortuosity - independent approach since it is computed from total organic carbon measurement; Second, the derived oil saturation is consistent with all available data, because conversion of dry weight total organic carbon into oil saturation is performed through a simultaneous inversion of all measurements into matrix-porosity-fluids volumes (ELAN); Third, this method allows to account for all clay dependency and other lithology effects. The hydrocarbon obtained with the spectroscopy-based petrophysical model was benchmarked against cutting descriptions and nuclear magnetic resonance fluids analysis. The integration with advanced nuclear magnetic resonance was an added value to the project, leaving to a new reservoir characterization step which is currently under study.

From a production point of view, the implemented solution is robust and assures data in almost any operational conditions, both for new and old cased hole wells. As a result, the applied solution allowed to significantly increase success rate of improved oil recovery (IOR) activities in Putumayo basin.