A Novel Approach for Improved Accuracy in Water Volume and Reservoir Quality Assessment During Field Development in Organic Shales

Abstract

Abstract Complexity in petrophysical interpretation in organic shales is related to the presence of inorganic and organic pores that house hydrocarbons, kerogen, and water. Advanced workflows were proposed for assessing hydrocarbon types and reservoir maturity. The water volume and saturation assessment are routinely done with archie equations, using resistivity, nuclear and/or sonic measurements. With proper matrix corrections on porosity computation and representative inputs of formation water salinity (Rw), wettability (N) and tortuosity (M), water saturation can be calculated. In organic shales, however, the archie equations parameters can be highly variable across a formation and, a well-known thermal maturity effect on resistivity logs (higher maturity tends to cause a drop in resistivity), leads to incremental uncertainty on formation water assessment. During field development stage in Vaca Muerta formation, Argentina, a complete electrical logging acquisition is carried out in pilot wells. With this information, landing zones for placing the horizontal-producer wells are defined. Since the criteria for landing zone definition is strongly conditioned by water saturation, we focus on uncertainty reduction in water volume estimation as a first step and then calculation of a new practical reservoir quality indicator that matches local production behavior. Given the independence to archie equations and similar depth of investigation, dielectric dispersion and nuclear magnetic resonance logging became of increasing interest for water and hydrocarbons volumes and saturation computation. In Vaca Muerta shale oil targets, we developed an integrated method based on dielectric dispersion, magnetic resonance, formation capture cross section and spectroscopy-derived formation chlorine for early assessment of representative water volume and saturation for best landing zone selection with producible hydrocarbons and minimum free water. We built a matrix model from spectroscopy dry weights, nuclear and NMR logs, solving for minerals, kerogen, and organic/inorganic matrix-corrected porosity. With a clustering technique applied on data from a new high-resolution NMR T1-T2 processing and from Dielectric inversion, with inputs from total formation capture cross section and spectroscopy derived chlorine dry weight, we obtain two independent water volumes. Through iterative process, a representative water volume is achieved whenever differences are below two porosity units. Then, incorporating inorganic/organic corrected porosity, formation total water saturation is derived. Additionally, by splitting volumes of pore water and hydrocarbon in both organic and inorganic pores, we compute a continuous reservoir productivity index that considers both producible hydrocarbons and combination of bound oil and inorganic pore water.