The Blanca field in Ecuador has entered a new era with the introduction of superior logging-while-drilling (LWD) capabilities including capture spectroscopy and Sigma measurements. These new measurements provide a step-change from the traditional openhole logging methods previously used in this field and have proved to be crucial in determining hydrocarbon saturation and carbonate content in the target sandstone reservoir by reducing the high level of uncertainty in formation evaluation historically associated with the Blanca field.

The Blanca 04 and 05 wells are positioned in the young Sub-Andean sedimentary basins,

Figure 1. Petrophysics log showing resistivity and sigma saturation computation. (Images courtesy of Schlumberger)
known for the complexity of the depositional sequence and an active history of tectonic stress. The integrated suite of data provided by the EcoScope multifunction LWD service, which includes gamma-ray, density and neutron porosity, resistivity, capture spectroscopy, and Sigma measurements, enabled accurate identification of the zones with the highest hydrocarbon saturation and best producibility potential.

The capture spectroscopy measurement not only corroborated the wellsite lithology interpretation obtained from cuttings, it also captured new data about formation properties that could not be explained from interpretation based on the conventional gamma-ray, density and neutron measurements. The EcoView integrated petrophysical solution utilizing all of the multifunction LWD measurements significantly improved reservoir characterization.

Block 27 is located in the northeastern part of the Ecuadorian Oriente sedimentary basin between the borders of Colombia and Ecuador. In this area, formations preserve a Phanerozoic depositional sequence marked by abrupt variations of the shoreline on the shallow marine platform of the eastern sedimentary basin. These formations show lateral and vertical changes of facies that interrupt the predominant low-energy marine sedimentation.

Wellbore stability issues are often encountered and bring a significant level of risk during logging operations with conventional cable-conveyed technology. These issues, documented in areas of Peru, Colombia and Ecuador, generally result in lengthy openhole logging operations, repetition of wiper trips for hole conditioning, sticking pipe events and frequency of situations where logs are not acquired, leaving important reservoir zones without proper characterization.

In the main target of the Blanca 04 well (the Napo-1 sandstone reservoir), an unexpected low-resistivity profile with a maximum value of only 7.5 Ohm meters (Ohm.m) was observed in the zone during drilling. Such a low resistivity range did not provide a good correlation with logs from a nearby offset well (Blanca 02), where the resistivity in the target zone was approximately 60 Ohm.m. This raised questions regarding the hydrocarbon saturation in the zone and resulted in a pessimistic scenario for the well.

The availability of the Sigma measurement proved to be critical. The Napo-1 zone found in this field normally contains formation water salinity around 51,000 ppm sodium chloride equivalent, which provides good conditions for the use of Sigma in hydrocarbon identification. The Sigma measurement obtained in real time from the multifunction LWD service indicated values of 12-13 capture units corresponding to either hydrocarbon or a fresher formation water with much lower salinity than expected.

When both resistivity and Sigma measurements were used to calculate water and hydrocarbon saturations for the Napo-1 zone, the agreement in results derived from these two independent methods provided encouraging production expectations for this well.
In the Blanca field the Napo-2 sandstone reservoir underlies the Napo-2 limestone sequence. The boundary between these two depositional sequences is often transitional, with a gradual decrease in carbonate and increase in sandstone content. In many cases, where only gamma-ray, resistivity and density-neutron measurements are available, it is extremely difficult to accurately quantify the carbonate content within the Napo-2 sandstone reservoir.

The capture spectroscopy measurement was the key to determining the carbonate content in the Napo-2 sandstone. The elemental concentrations were also fundamental to the
Figure 2. Three-dimensional image and caliper interpretation.
petrophysical interpretation as they were used to calculate matrix properties necessary for calculating porosity and hydrocarbon saturation more accurately. The matrix properties derived from elements include matrix density, matrix neutron and Sigma matrix response.
The EcoScope service provides enhanced integration of formation evaluation data with drilling optimization. The service combines a set of measurements specifically designed to aid and improve drilling performance, including accelerometers for shock and vibration, and annular pressure, temperature and caliper information while drilling. In addition, the pulsed-neutron generator (PNG) permits high-quality neutron porosity data to be acquired without the use of an americium-beryllium chemical source and provides the neutron flux necessary for the capture spectroscopy and Sigma measurements.

With its ability to acquire logging data as close as possible to the bit, application of the service in two deviated wells in the Blanca field meant that the wellsite team (reservoir and drilling groups) were constantly updated on the position of the well within the stratigraphic column. The Blanca 04 and 05 wells were drilled with 97¼8-in. PDC bits and water-based drilling fluids. Based on real-time data obtained and verified while drilling, the drilling operation on the Blanca 05 well was stopped 177 ft (54 m) earlier than originally planned, reducing risks of unnecessary drilling in unstable formations while saving significant rig time. Because the formation evaluation measurements were obtained prior to significant invasion or wash-out, high-quality log data and a more accurate petrophysical evaluation were obtained.

The ability to obtain a true caliper in the well, which had not been possible previously, resulted in an improved understanding of the wellbore geometry. Two ultrasonic sensors located above the stabilizer and placed 180° apart provide the information to derive an average real-time caliper measurement. The ultrasonic caliper data was also binned into 16 sectors, allowing for estimation of eight azimuthal calipers used to visualize the borehole shape. Density measurements were also used to estimate standoff from the sensor to the borehole wall.

Caliper and image data acquired in the Blanca 04 well through the conglomerate transition zone located above the primary interval of interest (the Napo-1 sandstone reservoir) displayed important effects of wellbore instability. Zones of borehole enlargement and breakout could be clearly identified from the azimuthal density, gamma-ray and ultrasonic measurements. The oriented images provided a 3-D representation of the well bore in response to the formation stress conditions and drilling fluid balance.

Stimulation operations on both wells also benefited from the porosity, permeability and mineralogical data provided by the multifunction LWD service. The vast amount of data provided from logging the Blanca 04 and 05 wells was also used to improve the reservoir model and stimulation operations on a nearby well.

The LWD service permitted City Oriente to successfully acquire high-quality petrophysical data for reservoir evaluation without spending additional time and risk of exposure to unstable formations. Sigma and spectroscopy measurements were the key to accurately determining porosity, fluid saturation and permeability, even in formations exhibiting mixed or complex lithologies. The permeability values estimated from these measurements were also used to help define zones with the best productive potential and were later found to be in excellent agreement with production test results.

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