Different Techniques to Calculate Horizontal Permeability and Initial Water Saturation: Case Study

Abstract

Abstract A case study is presented to compare results between a deterministic and probabilistic hybrid flow unit approach (FU) based on core-log analysis and classical models based on log and petrophysical interpretations in Chihuido de la Sierra Negra Field. Neuquén basin, Argentina. The base of this paper comes from an exhaustive and detailed reservoir characterization model performed by a multidisciplinary team following the FU approach1. The objective of this work is to obtain more realistic values of OOIP and to compare different calculation techniques to achieve reliable absolute horizontal permeability and initial water saturation values. The permeability obtained from different approaches was tested with several core data points available in the field. The main conclusion is that the permeability derived from FU was more accurate than the generalized relationships. The initial water saturation calculated was validated with field data and preserved cores. Besides, values obtained by FU-capillary pressure also were more precise than those models using straight well log data. These improvements in reservoir description have provided the opportunity to increase the amount of OOIP and justify reliable recovery factors. The accuracy in the determination of initial water saturation and absolute permeability shown on this work allow us to apply this technique in any numerical simulation model. Introduction Permeability is one of the most important parameters in basic reservoir engineering. It is used for completion purpose, stimulation strategies, waterflooding projects and for complete characterization of reservoirs to be applied in representative simulation models. Many studies were done on permeability. They have been conducted on clean and shaly formation in order to get better correlation and estimation. Some of them take into account the heterogeneity involved in different pore geometries and the amount and type of clay. Initial water saturation is one of the key factors to achieve more accurate OOIP. Common calculations derive water saturation from logging tool responses. Meanwhile, other approaches estimate it from capillary pressure curves. The upcoming numerical simulation project of Chihuido de la Sierra Negra Field give us the challenge to review the reservoir characterization done before and check different techniques to calculate horizontal permeability and initial water saturation. For that purpose two new cores were added and tested with the prediction model. A sensitive study has been performed taking into account contrast in heterogeneity, difference in depositional environments and also the quantity of data available. Field Description Chihuido de la Sierra Negra is located in Neuquén Basin at 200km to the Northwest of Neuquén City. It is the largest oil field in Argentina. It have been under waterflooding since 8 year. Current production is around 12,000 m3/d of light oil (33–35 API) and 70,000 m3/d of water distributed in 700 producer wells. The water injection is about 90,000 m3/d with 550 injectors. It has three main sand clean reservoirs (less than 8% of clay) within Huitrin formation (lower Cretaceous). These sand reservoirs were deposited in a combination of different environments such as aeolian dune; fluvial channels and sandbars and they exhibit complex variations of pore space-related properties. These variations reflect not only the original depositional process but also the diagenetic and tectonic changes. The producing intervals are in an average of 1,200 m(MD) and listed from top to bottom are: the Troncoso Inferior (TRI), the Agrio Superior (AGS) and Avile (AVI). The TRI is the main reservoir with more than 60% of OOIP. TRI and AGS are subdivided in 10 geological layers. The gross thickness of this several layers plus AVI reservoir is around 200 m.