Recent field trials of the Raptor cased-hole reservoir evaluation system in the Adriatic Sea gas fields offshore Italy demonstrated the power of a new multidetector-array pulsed neutron formation gas measurement tool. This instrument is designed specifically for greater sensitivity to formation fluids and to provide calibratable curves for use in formation gas analysis. These attributes are particularly valuable in either mature or developmental surveillance programs aimed at maximizing production.

In this application the Raptor tool provided behind-casing views of the reservoir hydrocarbons and quantified the volume of gas in the porosity while being run in complex single- and dual-string completions.

Technology overview
Unlike typical dual-detector instruments, the new generation of multidetector pulsed neutron (MDPN) technology uses a detector array of much larger dimensions. This allows the sampling of a larger volume of the neutron-gamma transport field and yields a greater volume of investigation and greater sensitivity to the formation fluids. The tool response is characterized not only for the attributes of the reservoir mineralogy and fluids but also for the exact wellbore geometry and borehole fluid conditions.

Every well logged by the tool goes through this computerized characterization in the Weatherford modeling factory that contains more than 400 computer processors. Based at the Fort Worth, Texas, R&D facility, the modeling is automated to handle the roughly 10,000 data files associated with the modeling process for a single logging characterization in about 8 hours. Most of the time the characterization is completed before the well is logged, but if unsuspected conditions are encountered in the wellbore, new characterization can be complete in less than a day.

The logging procedures in the field follow standard practices except for the instrument calibration, which includes calibrating both the absolute value of the measured curves and the oil and gas sensitivity of the Raptor readings.

The process of getting the final oil and gas saturation answers uses new algorithms for the high-sensitivity response characterization and for shale petrophysics. The analysis workflows are designed to strengthen the mathematical and petrophysical handling of the shale effects on the saturation answers.

Field trial details
The main challenge encountered along the Adriatic coast was finding producible hydrocarbons (gas) in old wells in mature reservoirs. The field for this trial was characterized by thin-bedded Pliocene sand-shale sequences showing good areal continuity. While looking for bypassed production targets, the producer faced issues such as distinguishing between gas sands at original saturation (production targets), flushed saturation (wetted sands being produced by another well) or pressure-depleted sands (sands in hydraulic isolation that are being produced by another well and that reduce pressure as a result). This logging initiative reports the first application of the Raptor array pulsed neutron tool and the associated interpretation technology applied in the single- and dual-string completions offshore Italy.

The logging campaign involved data acquisition from two candidate wells, A1 and A2 (Figure 1):
• Well A1 completion is a dual-string completion with 7-in. production casing and dual 2.375-in. production tubing strings. In the logged interval, the annulus was brine-filled, and the tubing was water-filled. The tool was logged in the long string; and
• Well A2 completion is a single-string completion with 7-in. production casing and 3.5-in. production tubing. In the logged interval, the annulus was brine-filled, and the tubing was water-filled.

The Raptor detector array is configured with four spectroscopic lanthanum bromide gamma ray detectors. It also has a fast neutron detector distributed axially along the tool body, coupled with high-count-rate electronics. This array generated two new principal measurements for use in formation evaluation: a fast neutron normalized burst ratio and a capture ratio, both constructed as a ratio of the count rates of the nearest and farthest detectors.

These measurements were made using a generator burst timing mode called N-Vision that is different from both the sigma and CO burst modes. Conventional sigma and CO measurements—in which the four detectors of the array each produced a sigma and CO curve—also were made. These four signals are mixed according to the requirements of the logging objectives. The N-Vision mode also yields a simultaneous sigma measurement.

A typical section of the Raptor answer product is shown in Figure 2. The track of interest is the saturation track, which shows the quantified remaining gas saturation from the tool (red curve) and the original openhole gas saturation (in blue). The remaining gas saturation is the primary indicator of the remaining producible gas content of the sands.

Two sands are highlighted in the figure. In the first sand, the saturation curves clearly show that the remaining gas saturation is about 30% of the original. This sand has been produced by another well and flushed with formation water. In the second sand, however, the saturations are essentially equal, indicating producible gas remaining in this sand. Identification of production targets such as these is the object of the logging activities.

The Raptor tool provided a coherent and clear evaluation of production targets while logging in very complex borehole environments involving both single- and dual-string completions. The high sensitivity of the detector array plays a large part in this result, and the innovations in the petrophysical aspects of the analysis workflows pay big dividends. The high-accuracy response characterization adds assurance that the tool response is correct for the operator’s reservoir and wellbore.

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