JPT Cased-Hole Solution Assesses Tight Reservoirs – Journal of Petroleum Technology

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The complete paper presents a solution that assesses tight matrices and natural fractures at a level not previously achieved. At the tight-matrix level, advanced nuclear spectroscopy is carried out with a new pulsed-neutron device that achieves simultaneous time- and energy-domain measurements. A new resistivity- and salinity-independent methodology is presented for obtaining gas saturation by a measurement known as fast neutron cross section (FNXS), oil saturation from the total-organic-carbon log, mineral volumes solved from formation elemental concentrations from the energy domain, and porosity from the hydrogen index obtained from the spectroscopy time domain.

Reservoir Challenges

The reservoirs under analysis belong to the Colombian Pauto Complex field (Fig. 1). Wells drilled in the area have confirmed a staked structure architecture, in which three main thrust sheets can be differentiated: Miche, Guamalera, and Pauto Main. The present study was focused on the Mirador formation of the Pauto Complex, which can be described as a fluvial and shallow marine environment deposit system. The Mirador formation is divided into two zones by Middle Eocene unconformity and is almost totally composed of quartz grains and mainly cemented by silica and significant kaolinite content. An additional characteristic of this reservoir is the presence of natural fractures that contribute strongly to well productivity. The main production is from the Mirador formation and is characterized by highly complex heterogeneity and anisotropy because of a multidiagenetic process, the accumulated hydrocarbon of which is a gas rich in condensate with variations in composition.

Fig. 1—Location of the Pauto Complex in Colombia.

The reservoirs are located at average depths of approximately 17,000 ft in the eastern foothills of the Colombian Andes, a region characterized by high-tectonic-stress regimes and reservoirs intercalated with reactive shales. Those factors lead to challenging drilling conditions where the combination of oil-based-mud systems and small wellbore diameters (approximately 6 in.) at the target zones are the only way to minimize wellbore-stability problems. Data acquisition is also challenged because logging-while-drilling technologies are sometimes limited in small wellbores and openhole wireline logging is sometimes jeopardized by hostile hole conditions. Under these scenarios, the authors explored formation evaluation in cased-hole conditions with wireline-conveyed technologies for both matrix and natural-fracture systems.

The described spectroscopy device is wireline-conveyed and consists of a nonradioactive/high-energy pulsed-neutron generator and a highly sensitive set of multidetectors equipped with advanced electronics for carrying out complex neutron-burst schemes for accurate gamma-ray spectral analysis. This achieves a broad mineral-spectrum analysis, highly significant for two key formation evaluation parameters, porosity and saturation. The porosity typically is derived from neutron measurements incorporated in the same tool, where the matrix components over the interest zones provided by this spectroscopy device ensure an accurate porosity computation. The technology provides gas saturation from a hydrogen-index-independent gas-detection device and the liquids/hydrocarbon fraction from the carbon measurement; other rock elements ­containing carbon also are considered for a precise saturation computation.

The borehole electrical images combined with sonic measurements are the primary method for static evaluation of naturally fractured reservoirs in openhole conditions. The vertical resolution of borehole images on the order of a few millimeters enables visualization and determination of spatial orientations of natural-fracture events; meanwhile, sonic logs provide information on the natural fracture aperture at depths of investigation greater than those of the borehole images. When dealing with cased holes, however, the borehole image cannot be acquired, and conventional sonic logs cannot measure behind the casing in a proper manner. In this project, the authors present the incorporation of sonic-based seismic images and their integration with acoustic dispersion analysis, allowing detection and assessment of the natural-fracture system behind casing steel and deep into the reservoirs.

Slim Spectroscopy Technology for Matrix Analysis

The multifunction spectroscopy tool has an outer diameter of 1.72 in., allowing it to fit through most completion restrictions. Detector-resolution degradation at high temperatures is minimal, eliminating the need for a flask and a larger tool diameter. The configuration consists of a pulsed-neutron generator (PNG) and four detectors. The first detector, the compact neutron monitor, is largely sensitive to fast neutrons and is placed adjacent to the high-output PNG, where it measures source output in an accurate manner. The second and third scintillation gamma-ray detectors, named as near and far, respectively, feature lanthanum bromide (LaBr3) scintillators. The fourth and farthest-spaced detector (deep), uses an yttrium aluminum perovskite scintillator. The combination of LaBr3 detectors with a high-output pulsed-neutron source and advanced pulse-counting electronics is the enabler for high spectroscopy performance. The measurement can be described in two domains: energy and time.

Matrix Analysis. The spectroscopy data acquisition is processed at the computing center. Once data quality control, spectral stripping, and oxide closure model processing is achieved, the petrophysical steps proceed as follows:

  • Mineral volumes are obtained from the dry weight element fractions
  • Porosity is derived from nuclear measurement inputs with simultaneous corrections for mineralogy and matrix variations.
  • Oil-vs.-water saturation is obtained alternatively from carbon measurements or from carbon/oxygen ratios, whereas gas volume and saturation are obtained from FNXS measurement.

Also, a continuous permeability is computed from mineral volumes and porosity. The formation evaluation work flow is a 100% resistivity-/salinity-sigma-independent analysis.

Sonic Reflective Waves for Natural Fracture Analysis

A multiarray sonic device enables an application the authors call seismic imaging that extends the area of investigation for natural fracture detection and assessment to 30–120 ft, depending on various factors, providing information on the fracture systems from an undisturbed zone in the reservoirs while maintaining the vertical resolution of electrical logging.

The typical acoustic log measurement is the formation slowness, which is based on the first arrivals of refracted train waves. However, in sonic imaging, the train waves acquired from a special acoustic-acquisition mode that the authors call a borehole acoustic reflection survey (BARS) is processed to extract the reflected waves. The reflected components respond to acoustic impedance contrasts (acoustic discontinuities) in the formations, either very close to the wellbore wall or from some distance away from the wellbore and from geological features that do not necessarily cross the wellbore trajectories. The sonic imaging methodology resembles a 2D surface seismic, where the wireline sonic device operates in the kHz range with its transmitters and receivers located in the same tool, then inside the wellbore. The wireline acoustic tool consists of a device with three monopole sources, two dipole sources, and a 13-receiver array section, logged with an inclinometer device to orient the acquired waveforms. Demonstrating the methodology’s ability to weather environmental challenges, data acquisition in the present project was conducted in cased-hole conditions, where sonic pulses had to penetrate casing steel and cement.

The sonic imaging techniques developed for surface seismic data can be applied to BARS data to extract and enhance the energy reflected from the formation and position the reflectors relative to the borehole. The higher frequencies and transmitter/receiver spacings used with wireline devices provide formation images with higher resolution than those obtained with conventional seismic methods.

Even though the BARS methodology was designed to generate acoustic images from reflectors, the method can be used to detect natural fractures inside the formations. The process of acquiring and processing BARS data is discussed in ­detail in the complete paper.

Conclusions

Reservoirs with very low porosities (where almost all petrophysical methods do not apply) and natural fractures now can be evaluated with the integration of advanced multifunction spectroscopy measurements and borehole acoustic-reflection-derived images in cased-hole environments.

At the matrix level, the solution consists of a simultaneous analysis, where formation elemental concentrations are combined to solve for lithology, mineralogy, clay volume, clay types, matrix-corrected porosity, and saturation in tight rocks. The gas saturation is solved from a hydrogen-index-independent FNXS measurement that demonstrates sensitivity to gas even at low porosities, while the carbon redistribution among the matrix components containing this element enabled solution for liquid hydrocarbon saturation. The approach proposed in this paper is independent of water salinity, salinity variations, and reservoir tortuosity in the case studies presented in the complete paper. The case studies demonstrate the capability of obtaining accurate information and representative formation-evaluation outputs in cased-hole conditions, opening opportunities in mature fields or new wells where openhole logs cannot be acquired.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 197423, “Cased-Hole Solution With Fast Neutrons and Sonic Reflective Waves in Tight Reservoirs,” by Ulises Bustos, Schlumberger, Carelis Moya, Equion Energia, and David Rose, SPE, Schlumberger, et al., prepared for the 2019 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 11–14 November. The paper has not been peer reviewed.