Integrated Multiwell Approach for Heterogeneity Mapping and Production Optimization: A Case Study from Ratawi Formation of South Fuwaris Field, Partitioned Zone between Kuwait and Saudi Arabia

Abstract

The Ratawi Formation is one of the well-known hydrocarbon bearing reservoir in many parts of the Middle East. South Fuwaris Field in Partitioned Zone between Kuwait and Saudi Arabia is one of the examples where Ratawi is the main pay. This reservoir has shown immense potential in-terms of production from different parts of this field. However, sharp water cut increase in some of the wells has challenged the existing understanding of heterogeneity distribution in the field. An integrated approach was applied to characterize the secondary porosity using the high resolution image logs, core data and open-hole logs. A workflow was developed to map the heterogeneity in the field and calibrate it with the available production data. The Ratawi Formation was deposited in a ramp, platform setting. It is broadly divided into an upper Ratawi Shale Member and a lower Ratawi Limestone & Ratawi Oolite Member. Major discoveries were made in the lower part of the Limestone Member and Ratawi Oolite at nearby Wafra Field in 1953. Since then, major efforts have been given to understand and map the complex porosity and permeability in the field in order to sustain the production. Though at places, presence of open fracture has been reported from the image logs, however multi-well study reveals no major impact of fractures on production. An innovative approach was implemented to extract the secondary or macro porosity formed due to leaching, using high resolution image data. Initially total porosity was estimated using high resolution borehole image log and subsequently a cut off was used to segregate the secondary porosity. The permeability was then derived from macro porosity using a complex transform. Core derived total porosity and permeability shows excellent match with the image log derived results. A geostatistical approach was applied through the static model building process to generate property maps using secondary porosity, effective porosity and derived permeability logs. A Clear degradation of reservoir property has been observed towards the NNW and SSE part of the field as it lies on the flank of the gentle anticlinal structure. Available production data validates the production rate is controlled by secondary porosity in Ratawi Oolite Formation. New wells drilled based on the property maps shows promising production results from Ratawi Oolite Formation. Single porosity geo modeling using the conventional open-hole logs has always been the first approach adopted by the industry. However, in mature carbonate reservoirs, well wise production always brings surprises. A novel approach was adopted to quantify the secondary porosity from the borehole image log which was further calibrated with the core data. Permeability was also calculated considering the secondary porosity as one of the key controller. Production data shows excellent match with the secondary porosity and permeability derived 2D maps; which has bolstered the confidence of releasing new well locations with better productivity.