Coaxing Information from Petrophysics and Integrated Data Analysis: The Menggala South Field Case History, Sumatra, Indonesia

Abstract

Summary We developed a reservoir model for potential infill locations in this mature oil field. Prior to a 3D seismic survey expected recovery was over 60% and the field had a limited amount of reserves based on the old 2D seismic lines. The new seismic study showed the crest of the structure was missed during the initial field development. This study was commenced to delineate new well locations and determine if one or more horizontal wells were feasible. This study consisted of revising the petrophysics data from 11 wells, building an earth model and making a simple simulation model. The field had wells drilled over a fifteen year period from 1972 to 1987. This paper outlines the efforts to obtain a usable log suite including permeability, Swi and Sorw. Information from nearby fields and the initial field simulation using average properties were included in this analysis. This information was included into the earth model and subsequently the simulation model. Introduction When minimal data (wireline, core) are available in a given field, general concepts and approaches may be used to derive petrophysical answers that fit the overall model for the region in given stratigraphic layers. These answers (porosity, perm, irreducible water saturation and residual oil saturation) may be modified in the earth model building process to address shortfalls in preliminary simulation runs. This "looping" between the petrophysicist, the earth model builders and the simulation engineer provides a framework and process to achieve a coherent model result. In this case permeability and SCAL data are missing or exist in insignificant quantity, answers may be derived using available data and subsequently verified and supported with data from offset fields. Zones that are known to be at irreducible water saturation from supporting data (well tests, obvious sharp oil-water contact from resistivity and porosity) can be used to estimate irreducible water from Simandoux1 then use these answers in an empirically derived correlation between irreducible water, porosity and permeability, i.e. Coates2. This result may be verified with core data from offset fields. One caveat is the assumption that all rock types are represented within zones at irreducible water saturation. As with many oil fields in Sumatra, there exists a definitive relationship between increasing clay content and decreasing permeability. Zones at residual oil saturation below a moved oil-water contact seen in late-drilled wells in the field were used to estimate residual oil. These values are roughly verified by the water saturation verification via derived permeability. Missing wireline data may be synthesized from existing data. For example a rough relationship can often be found between bulk density and normalized gamma ray. It is very important to normalize the statistics of the gamma ray in a given interval that exists in the entire field for consistency. Pseudo curves can be used to insert reasonable values in intervals of poor data quality or if necessary used as a substitute curve. Background Menggala South field is located in central Sumatra (see Fig. 1). The structural closure is relatively small; only eleven wells had been drilled at the time of this study. Cumulative production was 53.4 MMBO as of September 2001 and original oil in place is estimated to be 94.8 MMBO. New 3-D seismic indicated a possible up dip location for a new well. Consideration was given to a horizontal well. To evaluate the horizontal well case, an earth model was created and simulation runs utilized to evaluate the potential of a horizontal well to reduce the possibility of water coning. A model was quickly built and simulations run, modifications made to input data and additional runs completed before making a recommendation.