Resolving the Petrophysical and Production Challenges Associated with Complex Low-Porosity, Low-Salinity Fractured Carbonate Reservoirs – A Case Study

Abstract

Abstract The evaluation of fractured low-porosity, low-salinity reservoirs has always posed different exploration and production challenges. Resistivity based models for determining saturation have significant uncertainties due to poor hydrocarbon-water resistivity contrast further exacerbated by low porosity. Near-wellbore reservoir stimulation is very common in such reservoirs with low-matrix porosity to enhance inter-connectivity between existing fractures. However, understanding the efficiency of such operations is usually critical for efficient production optimization especially when unexpected water production is experienced. In the presented case study, three main challenges were to be resolved: complete petrophysical assessment, evaluation of stimulation efficiency and understanding of the source of produced water. To achieve this, a multidetector pulsed neutron tool (MDPNT) along with a temperature sensor was deployed after reviewing the well history, well logs, mud logs and field structural maps. Saturation uncertainty modelling was done to establish the number of logging passes and associated logging speed. The acquired pulsed neutron log was integrated with open hole logs, borehole image data, cement bond log and temperature data for a holistic characterization of the reservoir. Spectra gamma ray, mineralogy, sigma, carbon-oxygen and oxygen activation data were acquired from the pulsed neutron measurement. Mineralogical data showed the dolomitization of the primary lithology which was limestone across the reservoir zone. Oil saturation was computed from carbon-oxygen measurement as sigma saturation had significantly high uncertainty due to low water salinity. Mineralogical data was used in saturation model to correct for the impact of lithology on the saturation results computed from the carbon-oxygen measurement. Porosity data was computed from the pulsed neutron porosity indicators. This was used to confirm the stimulation efficiency by comparing it with the open hole porosity measurement. The impact of the stimulation on the flow dynamics of the produced water was identified. The source of the water production was detected from the oxygen activation data of the pulsed neutron measurement. This was corroborated by the acquired temperature measurement and cement bond log. Borehole image log was integrated with the other data to understand the fracture density and possible impact on the water production. Finally, perforation/completion zones where optimized based on the integrated evaluation of the acquired data. The value of data integration was shown in this case study in resolving the challenges associated with the complex carbonates of the upper cretaceous South Anatolia basin. The importance of pre-modelling and incorporation of mineralogical data in the saturation model to reduce saturation uncertainty was highlighted. Furthermore, multiple sensors were combined to ascertain the water production source in the well. Consequently, completion zones were properly optimized to decrease water production while significantly improving hydrocarbon production.