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 The paper has not been peer reviewed.

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.

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.

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