Rock Typing: Key Approach for Maximizing Use of Old Well Log Data in Mature Fields, Santa Rosa Field, Case Study

Abstract

Abstract The Santa Rosa field is located in the greater Anaco area in the Eastern Venezuela Basin. A total of 280 wells have been drilled, 65 with modern porosity logs. This situation makes the reservoir very difficult to be evaluated using only well log classical techniques. In the studied reservoir have been completed 74 wells, the production started in 1950, and a gas injection project was implemented in 1955. With the purpose of validating reserves and optimizing exploitation plans a petrophysical model based on rock type characterization was developed using existing cores, logs and production data. This model was extrapolated to the rest of the wells. Rock type characterization was based on mercury injection capillary pressure data, which allowed the determination of pore throat profiles for each rock type and the dominant interconnected pore system, which corresponds to a mercury saturation of 35% in a capillary pressure curve. An empirical relationship was used to related conventional porosity and permeability with pore throat, and was used to classify rock types. The upscaling procedure from core to logs was accomplished through a relation between gamma ray and neutron logs with core porosity and permeability in a key well. These relations were found to be dependent on rock type, and were used to extrapolate core characterization to those wells with only gamma ray or neutron logs. Porosity obtained from wells with neutron/density logs was compared to porosity obtained from the gamma ray - neutron -core porosity relationship, with very good results. Maps of rock type distribution were used to identify reservoir zones with better quality rock. These maps along with facies maps from the sedimentological study helped delineate the reservoir limits, in order to validate and identify prospective areas for future drilling. The characterization of a reservoir into rock type integrating geological, petrophysical and production data is fundamental for the development of exploitation plans. Introduction Effective petrophysical models require the integration of core, log and production data, in order to understand the variations in properties such as porosity, permeability, capillary pressure, geometry and fluid content of the rock. The Santa Rosa field is located in the Greater Anaco Area in the Eastern Venezuela Basin (Fig. 1). In the study reservoir 7 wells have modern porosity logs and 3 wells have cores. With the purpose of validating reserves and optimizing exploitation plans a petrophysical model was developed. Rock type characterization was based on storage and flow capacity and was estimated from routine core porosity and permeability data combined with capillary pressure results.