Log Determination of Residual Oil Saturation in the Mauddud Zone, Bahrain Field
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Estimation of hydrocarbon reserves is governed by natural, technological and economic factors. Re-estimations and reviews continue almost till the end of exploitation in many instances. Estimation of oil reserves, when a field is in its initial stages of development is a relatively easy task. The estimation will not invite critical or close scrutiny, as the oil production at that stage is far lower than the reserves calculated. However as the field matures and the cumulative oil production approaches the estimated value of reserves, the accuracy in the estimation of reserves becomes crucial and more important. Also the estimation process itself is a challenging task, in the light of uncovering geological complexities and reservoir dynamics during the development and the exploitation methods employed to produce the oil. Bahrain field, the first discovery in the Arabian Gulf region, has been on production since 1933. It is a multi-stacked field with 16 oil and 4 gas reservoirs. Most of them are carbonate reservoirs. The nature of the fluids varies from tarry oil in the Aruma formation occurring at shallow depths to dry gas in the deep Khuff zones. The geology of the field is complex with a large number of faults on the structure. The major oil zone Mauddud - an oil wet system -has been on gas injection from 1938. The gas injection has lead to the transfer of oil and gas into layers above and below through the faults. Though many reservoirs of the field have reached mature stage, there are some zones containing oil resources still unexploited due to the heterogeneities, complex nature of the fluid distribution, unfavorable rock-fluid characteristics, compartmentalization, thin oil rim overlain by gas and underlain by water, etc. This paper brings out the challenges posed by the above situations while attempting to estimate and classify oil reserves. Introduction Bahrain field, discovered in 1932, has 16 oil reservoirs and 4 gas reservoirs (Figure: 1).1,2 The fluid content of the reservoirs vary from the tarry oil in Aruma to dry gas in the Khuff and pre-Khuff zones (Figure: 2). Most of the above reservoirs are carbonates and some of them are clastics. The carbonates of the Cretaceous have shale limestone sequences, while the Jurassic and Precambrian Khuff have dolomitic limestone embedded in anhydrites. Based on the depth of occurrence, the oil reservoirs have been separated into three groups. The shallow zones are the zones from Aruma to Ab, which are poor producers in general. The middle zones comprise of the Wara sand Ac, the major oil producer -Mauddud limestone and the Nahr Umr sands. These are prolific reservoirs and are known as Bahrain zones. The remaining deeper reservoirs, Kharaib, Arab zones and Fadhili are termed as Deep zones, which have moderate productivity. The geology of the field is extremely complex with a large number of faults, especially in the shallow and Bahrain zones. Also some reservoirs are fractured. In a single field, there are large variations in the rock properties and the fluid properties. This makes the work of estimating and classifying the oil reserves a challenging task.
Estimation of hydrocarbon reserves is governed by natural, technological and economic factors. Re-estimations and reviews continue almost till the end of exploitation in many instances. Estimation of oil reserves, when a field is in its initial stages of development is a relatively easy task. The estimation will not invite critical or close scrutiny, as the oil production at that stage is far lower than the reserves calculated. However as the field matures and the cumulative oil production approaches the estimated value of reserves, the accuracy in the estimation of reserves becomes crucial and more important. Also the estimation process itself is a challenging task, in the light of uncovering geological complexities and reservoir dynamics during the development and the exploitation methods employed to produce the oil. Bahrain field, the first discovery in the Arabian Gulf region, has been on production since 1933. It is a multi-stacked field with 16 oil and 4 gas reservoirs. Most of them are carbonate reservoirs. The nature of the fluids varies from tarry oil in the Aruma formation occurring at shallow depths to dry gas in the deep Khuff zones. The geology of the field is complex with a large number of faults on the structure. The major oil zone Mauddud - an oil wet system -has been on gas injection from 1938. The gas injection has lead to the transfer of oil and gas into layers above and below through the faults. Though many reservoirs of the field have reached mature stage, there are some zones containing oil resources still unexploited due to the heterogeneities, complex nature of the fluid distribution, unfavorable rock-fluid characteristics, compartmentalization, thin oil rim overlain by gas and underlain by water, etc. This paper brings out the challenges posed by the above situations while attempting to estimate and classify oil reserves. Introduction Bahrain field, discovered in 1932, has 16 oil reservoirs and 4 gas reservoirs (Figure: 1).1,2 The fluid content of the reservoirs vary from the tarry oil in Aruma to dry gas in the Khuff and pre-Khuff zones (Figure: 2). Most of the above reservoirs are carbonates and some of them are clastics. The carbonates of the Cretaceous have shale limestone sequences, while the Jurassic and Precambrian Khuff have dolomitic limestone embedded in anhydrites. Based on the depth of occurrence, the oil reservoirs have been separated into three groups. The shallow zones are the zones from Aruma to Ab, which are poor producers in general. The middle zones comprise of the Wara sand Ac, the major oil producer -Mauddud limestone and the Nahr Umr sands. These are prolific reservoirs and are known as Bahrain zones. The remaining deeper reservoirs, Kharaib, Arab zones and Fadhili are termed as Deep zones, which have moderate productivity. The geology of the field is extremely complex with a large number of faults, especially in the shallow and Bahrain zones. Also some reservoirs are fractured. In a single field, there are large variations in the rock properties and the fluid properties. This makes the work of estimating and classifying the oil reserves a challenging task.
Bahrain oil field being the first oil discovery in the gulf region in 1932 is now in a mature stage of development. Crestal gas injection in the Mauddud reservoir has continued to be the strongest drive mechanism since 1938. Over the last five years, gas injection and fluid production rates have grown three folds with expanded drilling, workovers, and high volume lift activities however there are significant opportunities to increase oil production and optimize gas injection. Since 2009, a growing number of wells in Mauddud including newly drilled wells were being shut-in due to high gas to oil ratio (GOR) within the limited surface handling capacity. 30% of active producers accounting for 25% of oil production were shut-in by year 2014. Proper evaluation of this opportunity followed by the field implementation has helped to significantly improve the oil production rate, reserves base, and understanding of the Mauddud reservoir fluid dynamics. To manage the voidage replacement ratio (VRR) associated with the rapid increase in oil producers, gas injection in the Mauddud reservoir has grown from 180 mmscfd to 500 mmscfd between years 2009-2015. A phased optimization program has been implemented to identify the conformance issues, impact of faults, and for optimization of surveillance systems in order to maximize the oil recovery. The VRR optimization program has also shown opportunities to preserve the source gas from Khuff reservoir which is a valuable resource for the Kingdom of Bahrain’s future. This paper will describe the integration of gas shut-off workovers, selective gas compression, and VRR optimization as excellent gas management tools for maximizing recovery potential in the Mauddud reservoir.
This paper presents a novel approach to estimate the recovery of oil by the stripping of the lighter components by the injection gas, using the fluid characterization models derived with semi-continuous thermodynamics method in earlier studies. Bahrain Field is an asymmetrical anticline trending in the North - South direction. The sedimentary column extends from Cambrian Saq sandstone to the Miocene reefal deposits exposed on the surface. The field contains twenty-two reservoirs with the hydrocarbon content varying from tarry oil in Aruma to dry gas in Khuff reservoirs. The geology of the field is extremely complex with a large number of faults occurring in the Wasia group formations, which contain the major oil reservoir of Bahrain field, the Mauddud reservoir. The field was discovered in 1932 and gas injection in the under saturated Mauddud reservoir was initiated in 1938. Over the years of injection a secondary gas cap has developed. The dominant recovery mechanism is gravity drainage with crestal injection of gas from the Khuff zones. The reservoir being heterogeneous and structure complex with a large number of faults, most of the injected gas gets produced. In addition to the above recovery mechanism, stripping of the lighter components in the oil by the "dry" injection gas, is significant. In fact the associated gas produced is enriched with the stripped oil and forms the major feed stock for the LPG recovery plant. To understand the recovery mechanisms and to provide fluid characterization for the simulation work in progress, characterization of the reservoir and injection fluids were carried out in earlier studies. These fluid models were validated with the available experimental data satisfactorily. This paper describes how these fluid representations were used along with the production and the compositional data of the associated gas produced, to simulate the gas injection process. Also with the current strategy of development of the field, future oil recoveries expected from the stripping process have been estimated. Introduction Bahrain Field discovered in 1932 (Figure: 1), is an asymmetrical anticline trending in the North - South direction. The sedimentary column extends from Cambrian Saq sandstone to the Miocene reefal deposits exposed on the surface. The field contains twenty-two reservoirs (Figure: 2) upto the Pre-Khuff level, with the hydrocarbon content varying from tarry oil in Aruma to dry gas in Khuff reservoirs. The geology of the field is extremely complex (Figure: 3) with a large number of faults occurring in the Wasia group formations, which contain the major oil reservoir of Bahrain field, the Mauddud reservoir. The reservoir is continuously on production since discovery of the field. The dominant recovery mechanism is gravity drainage with crestal gas injection. The reservoir energy is supplemented by an aquifer, which is active from the north and south directions. Gas injection started in 19381 with Arab gas and with the development of Khuff reservoir2 in early 70s, the injection gas was switched from Arab to leaner Khuff gas. The associated gas production until 1964 was only the solution gas. From 1965 the free gas from the secondary gas cap developed with continued gas injection since 1938, started breaking through in the oil producers. This gas is considerably enriched with lighter hydrocarbons stripped from the oil contacted by the gas as compared to the injection gas. Oil recovery has been estimated using conventional reservoir engineering assuming the oil to be black oil. In these recovery calculations, the oil stripped by the lean injection gas is not accounted.
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