TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Mauddud carbonate reservoir is the main oil producer in Bahrain oilfield that has been undergoing crestal gas injection with natural aquifer support at the flanks. It has been producing since 1932 and currently in a very mature stage. Due to its rocks wettability characteristics (preferentially oilwet nature), the residual oil left behind gas and water fronts range from 20-70%. One of the potential options to recover portion of this residual oil is through Chemical Flooding possibly Alkaline-Surfactant-Polymer (ASP) flood. A project was hence initiated to investigate its technical and economical feasibility. It was decided to carry out field experiments to gauge the reservoir response through well-bore treatments before proceeding with a full-scale study and field pilot implementation. Initial field-testing on single-well huff-andpuff basis with experimental recipes provides sufficient positive indications for the project to proceed.The results of these initial tests are presented in this paper.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractManagement of mature oil fields requires accurate formation evaluation information. The ability to acquire essential data through casing adds a new dimension to reservoir characterization and affects the reservoir development of a mature oilfield like the Bahrain field. New logging tools and technology fill the gap in evaluation, especially for monitoring water saturation changes over time and for detecting bypassed oil pockets across the zone.In this paper we present the results and validity of a logging through casing run in a depleted well completed in a sandstone reservoir in the Bahrain oilfield. The reservoir has a complex lithology of high-permeability sandstone with thin interbeds of siltstone and shale, erratically deposited but prolific in oil production. The objectives of the data acquisition were to monitor oil depletion through casing and to study the effect of lithology on the water encroachment pattern across the zone.In this paper we describe the interpretation procedures and data validation of the information obtained from an advanced suite of logs run through casing. The paper also focuses on how this new information has resulted in a review of potential further development of this pay in one part of the field.

Geochemical data of oil integrated with geological and engineering data have contributed to an understanding of the mechanism and relative timing of hydrocarbon emplacement in shallow oil zones (Rubble, Ostracod & Magma) of Bahrain's Awali oil field. These zones contain heavy as well as light oil. It is observed that the different oil types reside in different reservoir compartments. To efficiently develop and produce from these reservoirs, it is required to know the current distribution of the oil types and whether they result from different charging histories (gradations developed during initial or late stage migration) and or post-emplacement alteration (i.e. water washing, biodegradation, gas stripping, gravity segregation or de-asphalting). The primary objective of the study was to determine the vertical and lateral continuity of the reservoir intervals to geographically identify areas for development drilling and EOR processes. A geochemical study involving an advanced gas chromatography technique was used which included:comparison of abundance of "inter-paraffin peaks" between the different oil samples,determination of the magnitude of the compositional differences between the oil "Star Diagrams" and "Cluster Diagrams", and"Slope Factor Analysis" where the relationship between molar concentration and molecular weight (carbon number) are compared between the oils. The Geochemical evaluation provided compositional data that helped to:Characterize the difference between the oils.Verify reservoir compartment-talization.Reveal the level of biodegradation, charging fluid type andrelative charge histories for the different fluid types.Identify the geographical distribution of the different fluid types and their relation to potential migration pathways (e.g. faults). In addition to providing information concerning the lateral continuity within specific reservoir intervals and vertical continuity between intervals, the geochemical data also indicate three charging events with oils of differing thermalmaturity:Primary: Charges of extremely immature oil wereintroduced along a nearly linear path which traverses the field from SE to NW. These lines could be the major fault (N-S) and fractures providing initial migration paths.Secondary: Oils of increasing maturity (mixtures of light& heavy oil) were introduced progressively, as the basinsubsided and the oil source matured. It is possible that the light liquids condensed from gas at the sites where theypresently occur, the oils acting as a stripping liquid.Tertiary: (Emplacement of highly mature oil). Thisprocess has occurred predominantly in the Mauddud oil bearing formation, produced primarily close to the perimeter of the field. Back Ground on Geochemical Techniques to assess Reservoir compartmentalization The geochemical approach adopted in the study is based on well established proposition that oils from discrete reservoirs tend to differ from one another in composition.[1–5] These compositional differences exist for one or more of the following three reasons:Processes like biodegradation, water washing, and evaporative fractionation that affect oil composition after oil enters a reservoir.Oil composition expelled from a source rock changes as the source interval is progressively buried and the thermal maturity of the source and the generated oil increases.More than one source rock (or source facies) may contribute oil to an accumulation.

SPE Members Abstract The implementation of a welltesting program of Build Up and Draw Down tests conducted on wells producing with high GOR from a thin tight oil zone has revealed a unique and consistent communication pattern between three communicating reservoirs. Analysis of the tests showed conflicting response; in Draw-Downs, for instance, Constant Pressure Boundaries (CPB) were detected, whereas in Build Ups, barriers were encountered for the same well. The tests revealed the presence of a threshold pressure which controls communication between the reservoirs and hence the high gas production rates from the tight formation. The welltesting program shed light on the drive mechanism of the tight formation and explained the conflict between the production and pressure data experienced throughout the history of the reservoir. Based on the findings which were further supported by a simulation model, interference tests, and statistical distribution of production methods in the area of interest, a secondary recovery method was designed to provide a solution. This paper highlights the effective role of welltesting in understanding the drive mechanism of a complicated reservoir and utilizing the information in improving its recovery. Introduction Being under gas injection for over 50 years, the Mauddud reservoir developed a thick gas cap in the central area of the field. With the presence of many faults in the area, the Mauddud gas cap caused early gas breakthrough in the overlaying reservoirs, Aab and Ac, causing wells to produce dry gas and the reservoirs to have very poor recoveries. Despite the confirmed communication between the reservoirs, the static pressure of the overlaying reservoirs, Aab and Ac, never reached that of Mauddud gas cap even after months of shut in. Therefore, in order to understand the communication patterns and develop a plan to optimize oil production from both reservoirs, a welltesting program was implemented on Aab wells in the area. The tests, mainly Build Ups and Draw Downs, revealed a unique and consistent communication pattern through threshold pressures across fault planes. The tests also revealed the role of the intermediate zone, Ac, in providing a conduit to gas flow from Mauddud to Aab. The process was confirmed by a simulation model, interference test and a study on the statistical distribution of naturally flowing vs. artificially lifted wells in the area of interest. The simulation study indicated that if Aab wells are left shut in for several years, the pressures between Aab and Mauddud would eventually equalize indicating the threshold pressures to be acting only initially. Based on the above, it was possible to quantify the short-term threshold pressure between the three reservoirs. This further allowed the development of an engineered recovery scheme whereby, waterflooding Ac is expected to increase the pressure of Aab and Ac to be always below the minimum threshold pressure, thus; eliminating or minimizing Mauddud's gas cap encroachment into both reservoirs. Background Geology. As shown in Fig. 1, the three layered reservoirs under study; from bottom up: Mauddud, Ac and Aab, belong to the Wasia group of the Middle Cretaceous age. The Mauddud limestone thickness averages 110' and has a permeability in the range of 100–300 md. It has a gas cap thickness in the range of 20'–80' in the Area of Interest (AOI) and is overlain by the Wara formation which varies from 60 to 95' in thickness. Its erratic sandstone facies, Ac, distribution has a good permeability of 200–300 md and varies from 0 to 60' in thickness. The limits of this sand body forms the AOI of this study. P. 315^

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper describes a creative and pragmatic approach that improved the production from one of the most difficult reservoirs in the Bahrain field. The Middle Cretaceous Ab zone is a thin, tight, highly faulted, and irregularly-fractured limestone reservoir. The difficulty with this 15 ft, 1 md reservoir has prevented an efficient recovery. The average production of wells is 15 bopd. This has prompted a detailed integrated study plan to increase the wells' productivity. Welltesting was key to understand the reservoir dynamics. A comprehensive welltesting campaign revealed a flow mechanism controlled by a fracture network. Fracture modeling and simulation failed to give clues on how to improve the productivity. However, the new approach that links transient welltesting and production data with fracture network indications derived from seismic interpretations has resulted in improving the productivity considerably. This was accomplished through re-entering old wells and designing special trajectories to intersect productive open fractures. The productivity was significantly increased to 60 bopd with sustained performance. The paper describes in detail our approach and methodology to understand the reservoir and its drive mechanism and to increase productivity and recovery starting from analyzing core data up to designing special configuration wells. The paper further highlights the pitfalls of the conventional workflow approach in modeling such difficult reservoirs.