The objective of the logging campaign was to explore the hydrocarbon deposits in low-permeability clastic reservoirs utilizing an advanced logging suite and a high-tech wireline formation tester (WFT). The exploration well, which had a diameter of 146 mm and was drilled to a vertical depth of 4750 m, had a temperature of 147 degrees Celsius downhole.
Despite the challenging geological and downhole conditions: low permeability reservoir, high reservoir temperature, small wellbore diameter, and very high salinity drilling mud, the advanced logging suite data, which included nuclear magnetic resonance (NMR) and cross-dipole broadband acoustic logging, was successfully acquired. The WFT, which consists of a unique radial probe technology for efficient sampling of extremely low-permeability formations and a downhole fluid analyzer to determine the nature of the inflow and make rapid operational decisions in real time, was then used to perform downhole fluid analysis and sampling stations.
Oil and gas-saturated reservoir intervals were identified, their porosity, water saturation, and permeability were evaluated, and an optimal open hole WFT program was produced as a result of the extended logging suite's interpretation. Special technological operations were performed during well logging in this well, allowing for the acquisition of high-quality NMR data under extreme conditions. Rapid processing and interpretation of well logging data, performed without any a priori reservoir data, allowing for the identification of the best permeability intervals in oil and gas-saturated reservoirs and the design of an optimal WFT program in an open hole. Downhole fluid analysis and sampling stations were performed with WFT with pressure build-up to determine fluid mobility to validate the presence of productive intervals. Thus, in several downhole trips in a relatively short period of time, potential hydrocarbon layers were assessed in an open hole section, and in the presence of sufficient fluid mobility, the saturation of the interval was determined by WFT. It is important to note that downhole gas samples were acquired in intervals having fluid mobility less than 0.06 mD/cP, where the conventional well test would most probably fail to induce a flow without stimulation.
An integrated approach using modern well logging methods made it possible to solve the set geological challenges in the extreme conditions in this exploration well, where traditional methods would introduce significant uncertainties.