Trakya Havzası Doğal Gaz Değerlendirmesi

Abstract

Bu çalışmada, jeolojik olarak doğal gaz potansiyeli taşıdığı kanıtlanmış olan ve halen arama ve üretim faaliyetlerinin sürdürüldüğü Trakya Havzası için doğal gaz değerlendirmesi yapıldı. Doğal gaz oluşumunun jeokimyasal yorumu ve Trakya Havzası'nın jeolojisi gözden geçirildi. Bugünkü durum itibariyle ; gaz üretimi gerçekleştirilen rezervuarlann jeolojisi, üretilen doğal gazın satışa sunulabilir hale getirilmesi için uygulanan gaz proses tekniği ve pazarlama konulan tartışıldı. Sorunlar ve öneriler bölümünde, Trakya Havzası doğal gaz üretim işlemlerinde karşılaşılan ; gaz kuyularında sıvı birikmesinden ve formasyondaki hareketli partiküllerden kaynaklanan ve kuyuların üretebilirliklerini azaltan, üretim ve rezervuar yönetimine ilişkin sorunlar ve çözüm yolları tartışıldı. Enerji kaynaklarının daha ekonomik olarak geliştirilmeleri ve daha verimli olarak kullanılmaları, mühendisler için her zaman ilginç olan konulardır. Bölgede doğal gaz üretim ve dağıtım işlemlerinde yaygın olarak uygulanan gaz basıncının düşürülmesi işleminde enerji kazanımı sağlayan eş-entropi genleşmesi yöntemi ve marjinal gaz sahalarının ekonomik olarak üretilmesi konusunda sıkıştırılmış doğal gaz taşımacılığı yöntemi incelendi. Trakya Havzası yeraltı su kaynaklarında görülen metan gazı sorunu kapsamlı araştırma gerektiren bir konudur. Bu çalışmada, sorun ana hatları ile gözden geçirilerek olası nedenler ve alınması gereken önlemler önerildi. Türkiye'de enerji planlamasının, kuşkusuz önemli konularından olan yeraltı gaz depolanmasında Değirmenköy gaz sahasında pilot uygulama önerildi.

The Thrace Basin is the general area where gas deposits have been found in Turkey. The exploration and natural gas production activities of The Turkish Petroleum Corporation (TPAO) which is the national oil company increased in last ten years as a result of the development of the gas market after the start of gas export from the former Soviet Union in 1986. The purpose of this study is to evaluate and summarize the natural gas potential of the basin, and to discuss the related problems of gas production operations. In this study, the geochemical process of natural gas generation, the geological structure of the basin and the descriptions of the gas fields are reviewed. The related problems of gas production such as liquid loading and fine migration are discussed. An energy efficient gas pressure reduction process in the gas production and the distribution operations is suggested. The use of the compressed natural gas transportation for the economical development of the marginal gas fields is discussed. The problem of the methane gas occurrence in the meteoric waters of the basin is reviewed. A pilot project concept to provide the operating of the Değirmenköy gas field before depletion as an underground gas storage field is also suggested. Organic matter which is the remains of once-living plants and animals forms some part of almost all sedimentary rocks. This organic matter deposited in sediments is on a path toward its own ultimate destruction which involves the generation in the sediments by biochemical and thermochemical processes of natural gases. The first step in the decomposition of organic matter occurs immediately after deposition, and is biochemical (microbial). This the attack on the organic matter by many kinds of bacteria under aerobic and anaerobic conditions.The principal products are carbon dioxide, hydrogen sulfide and methane gases. There is a succession of biochemical zones in the organic rich sedimentary environment. That is an aerobic oxidizing zone just below the surface of deposition which produces carbon dioxide, an anaerobic sulfate reducing zone, commonly generating carbon dioxide and hydrogen sulfide, and an anaerobic methane producing zone. Eventually, with depth, bacterial action in sediments ceases, due to rising temperature and increased compactness of the rock. What this limiting depth is probably quite variable. In any case, biogenic methane can be found to depths of thousands of meters, due to subsequent deep burial of originally near surface generations in younger sediments. A second mode of methane gas generation is through thermocatalytic decomposition of organic matter under the influence of rising temperature. The processing of the raw natural gas produced from the wells is technologically the separation of some gases from the gas mixture for the purpose of either purification or recovery. All of the natural gas processing methods are either equilibrium limited or diffusion controlled. An oil company after gas discovery decides the processing needs according to the types of natural gas products. Generally, three gas product options are available, namely ; transportable gas, sale gas and natural gas liquids. Actually, the market demand and economic parameters determines the choice of the plant product type. The facility type depends on the production rates, fluid properties, sale and disposal requirements, location and operator preferences. XIII The Thrace Basin of Turkey is geologically a tertiary basin deposited over magmatic and metamorphic rocks. Over the past 40 years more than three hundreds of wells were drilled for the exploration and the development of oil and gas deposits. Osmancık, Mezardere, Ceylan, Soğucak and Hamitabat formations have reservoir rock properties bearing hydrocarbon potential where traps formed. The geochemical studies indicates that Hamitabat and Mezardere formations are considered to be source rocks. The first commercial discovery of natural gas is the Hamitabat field in 1970. TPAO discovered the Umurca field in 1982, the North Marmara offshore field in 1988, The Karacaoğlan field in 1989. Over the last three years the Değirmenköy, Karaçalı, Silivri and Yulaflı gas fields have been discovered and developed. Currently, TPAO lias over 8 billion sm3 of producible gas reserves and 1.5 million sm^ daily gas production capacity in the Thrace Basin.. The biggest gas deposit have been found in the Hamitabat field. However, the most favorable rock properties of porosity, permeability and have been encountered in the North Marmara field. The Umurca field gas was recognized as biogenic because of a relatively high ratio of 12C to 13C. The discovery of the commercial shallow gas deposit of the Danişmen formation at the 450 meter depth in the Değirmenköy field caused some new views in the exploration activities in the basin. In this study, along with a brief reservoir geology, gas and rock properties, production characteristics and history of all gas reservoirs of the basin reviewed. The North Marmara Gas Field is the first offshore field in Turkey. The production project has been started in April 1995, and the first gas production will begin at the end of this year. An unmanned, minimum facility wellhead platform will be installed into the sea and processing facilities will be on the shore. The natural gas compositions found in the Thrace Basin are sweet and dry. Therefore, the only processing requirement of the produced gases for the sale gas are the phase separation and dehydration. The phase separation is done by the gravity type separators at the wellheads. The glycol absorbtion method is used to dehydrate the gas at the central stations. The leanless of the reservoir gases do not justify any natural gas liquids recovery processes. A very well developed industrial gas market is available in the basin for the utilization of the field gas productions. One of the most important operating problems in the Hamitabat field is the liquid loading of the wells as the flowing velocity of the gas slows due to low bottomhole pressure from natural depletion. The accumulation of the liquid will impose an additional back pressure on the formation that can significantly affect the production capacity of the well. In low pressure wells the liquids may completely kill the well ; and in the higher pressure wells there can occur a variable degree of slugging or churning of the liquids. Numerous methods have been proposed for the liquid unloading, or dewatering of gas wells. Some of the methods that have been successfully used in field operations are beam pumping units, plunger lift, gas lift, small tubing string and surfactant injection.The current practice in the Hamitabat filed is to produce through periodic pressure buildups and/or to open the wells to the flare. It is very obvious that the this practice for the unloading of the wells is not an economic method due to gas losses to the atmosphere and production time losses due to shut-in periods. The low productivity wells in the Hamitabat field experiencing liquid loading problem are good candidates for a plunger lift installation. The plunger acts as a mechanical interface between the gas and liquid in a well, with the gas forcing the plunger up the tubing, thus displacing the fluid above XIV it. Liquids are brought to the surface by the movement of the plunger, eliminating fall-back increases the efficiency of the well's gas to lift liquids and maintains a lower bottomhole pressure. The results are usually an increase in production. The plunger lift system produces most wells to depletion. It is simple and inexpensive to install. Plungers are available in various configurations. Conventional use of plunger lift is in wells with an annulus that is open. The annulus is used to store gas during shut-in period. When the well is opened, the stored gas serves as an operating gas. A rule of thumb that has been used for years to estimate the volume of gas necessary for a succesful operation is, 400 scf per barrel per 1000 feet of lift. However, no consideration was given to the back pressure above the head of liquid. So this rule should be considered as the minimum gas requirement. The fluctuation in pressure and rate, and the decrease in the productivity were observed during the extended flow period of the deliverability testing operation of the gas discovery well of the Değirmenköy-1 completed in the Osmancık sandstone. The cause and effect analysis was performed and concluded that the only reasonable explanation of this phenomenon is the fine migration and pore throat plugging. Fine particles are very small particles of loose solid materials present in the pore spaces. These particles can be incorporated in the formation as it is deposited during geologic time such as illite, chlorite and kaolinite type clay minerals or they can be introduced into the formation drilling and completion operations. Regardless of their mode of entry, they cause formation damage. These particles are free to migrate through the pores along with any fluids that flow in the reservoir. They can concentrate at pore throats causing plugging when they are carried a lot in high flow rates. The same problem but with different results has also been observed in the Karaçalı and Yulafli wells. At low production rates fine particles can align themselves to work their way one by one through the pore throaths. At high rates, the randomly distributed fines in rapid motion interfere with each other and could form bridges. The disappear of the fluctuation production characteristic in the Değirmenköy-1 well after the rate decrease is an example of these bridging and nonbridging rate dependent conditions. The observations in the Değirmenköy testing operation demonstrated that the common field practice of producing at a maximum rate would appear to increase the likelihood of the formation damage during production. Bridges can be distrupted by pressure disturbances which is considered to be the case in the Karaçalı well. In this well, the amplitude and frequncy of the fluctiations were much higher the Değirmenköy and the during the production period productivity of the well increased. Besides mechanical bridging particle wettability and interfacial forces influence particle mobility when multi fluid phases are present. Particles will move only if the phase that wets them is moving. These mechanism help explain why the fluctiations decreased following the onset of the water production. The oil and gas industry does not just produce energy. The industry also consumes large quantities of energy in transporting hydrocarbons to the consumer. As an example, the natural gas must enter up to a high pressure for effective pipeline transportation and then reduced in pressure at delivery points to make it usable to the consumer. A significant energy is lost during the process of isenthalpic pressure reductions at regulators. The turboexpander is a simple machine in which gas at high pressure is expanded to achieve a lower pressure. The released energy drives a turbine wheel and generates power or mechanical work. Wasted energy at reduction points on gas pipelines can be converted to usable energy by a turboexpander. Generally, 15% or more of the total energy available in the pressure reduction process can be recovered and used. The utilization of the turboexpanders for gas pressure reduction has widespread application in Canadian gas market. XV This subject were discussed in the Section 5.3 and a sample calculation done for the glass factory which is one the largest consumers in TPAO gas network reveals the net energy gain as an economic advantage. There are some marginal gas reservoirs belonging to the TPAO in the Thrace Basin. These are too small to support a pipeline investment, and not operated. The compressed natural gas transportation means the transfer of the processed and compressed natural gas in tube trailer. The effectiveness of the CNG transportation depends on the maximizing the volume of the tube bundles along with the minimizing the weights of them. There is an optimum pressure and optimum tube diameter to achive that result. CNG transportation technology offers new profit opportunities for gas producers, natural gas disributors and industrial energy users. Some applications are production of gas from wells which have no pipeline system, continuation of gas service during pipeline repair work, barging of gas from offshore. According to the results of the calculations in the Section 5.4, five tubes can be mounted on CNG trailer creating a total capacity of approximately 4000 sm^ of natural gas. In the case of four trip per day for a distance of 50 km, 16 000 sm-* of daily gas transportation can be realized. A preliminary cost analysis demonstrates CNG offers an alternative that will allow such sources to be produced profitably. The high gas content in meteoric groundwaters especially in the central and southern parts of the basin where oil and gas deposits have been found is the reason of the several injurious explosionsThe presence of gas in groundwaters in a basin where gas deposits have been found is a normal situation from the stand point of hydraulic theory of hydrocarbon migration. However, the systematic relations between the position of gas deposits and regional groundwater flow patterns should be investigated for the Thrace Basin. In this study, some procedures for safe drilling and production of groundwaters containing methane are recommended. A typical well completion scheme for a well with gas to produce water and gas separately is also suggested. A simple gas anchor type downhole separator could be used for this purpose. Underground gas storage is the process that matches the constant supply from long distance pipelines to the variable demand of markets, which are subject to weathers. In underground storage, natural gas is injected into subsurface reservoirs when market demand falls below the supply available from the pipeline. It is withdrawn from storage to supplement the pipeline gas when demand exceeds that supply. The depleted gas, gas-condensate or oil fields, aquifers and salt caverns are possible canditates for underground storage prospects. The depleted gas fields are the best prospect if available simply because it is closest to the natural environment for pipeline gas. The absolute minimum requirements for any storage project are the sufficient pore volume for necessary storage capacity, sufficient permeability to provide the necessary minimum deliverability and caprock of adequate integrity to ensure against leak and migration. There is not any underground storage field in Turkey besides the fact that the temperature dependent peak gas consumption rates increased as a result of the development of the domestic gas market in recent years. The gas reservoirs in the Thrace basin are attractive to construct underground gas storages for technical and economical advantages. XVI A pilot project concept to provide the operating of the Değirmenköy gas field which is reefal limestone with suitable rock properties, before depletion as an underground gas storage field is possible and suggested. The main consideration is the proper scheduling of the injection and re-production periods and rates. This project would also provide the operating experience of the underground gas storage reservoirs.