Çatlaklı rezervuarlarda su enjeksiyonu ile petrol üretimi üzerine deneysel araştırmalar

Abstract

Bu çalışma gözenekli ortamda petrolün su tarafından kılcal kuvvet ve yoğunluk tartanın yardımı ile ötelenmesi üzerine yapılan deneysel bir araştırmayı içerir. İki farklı kuvvete dayanan bu öteleme imbibition olarak adlandırılır ve çatlaklı rezervuarlardan su ötelemesiyle yada su itişimiyle üretim yapılıyorsa önemli bir yere sahiptir. Sahaların üretim performansları laboratuarda yapılan imbibition deneylerinden yararlanılarak tahmin edilebilir. Bu çalışmada, literatürdeki deneyler temel alınarak, mâtrikslerin tamamen su içinde kaldıkları durum deneysel olarak incelenmiştir. Karotlar her deney öncesi, aynı standarta uyularak aynı ön işlemlerden geçirilip imbibition deneylerine hazırlanmışlardır. Yapılan üretimin belirlenmesinde gerekli duyarlılığı sağlama ve olayı etkileyen parametrelerin çok ve ölçülmesi zor olması, yorumlama konusunda güçlüklere neden olmuştur. Bir diğer önemli sorun ise deneylerin uzunluğudur. Ancak literatürde yapılan bazı deneyler incelendiğinde kullanılan karotlar yüksek geçirgenlik, büyük gözeneklilik yanında petrollerin de daha küçük akmazlık ve özgül yoğunluğa sahip olmaları üretim zamanlarını kısaltmıstır. Bu çalışmada ortalama 1.5 ay süren bir deney süresinin hazırlık çalışmalarıyla 2 aylık bir döneme uzadığı olmuştur. 20'nin üzerinde deney yapıldığı göz önüne alınırsa bu konudaki güçlük daha iyi anlaşılır. Deneylerde Batı Raman ve Şahaban sahalarından alınan rezervuar karotları ve Yeniköy ile Karakuş petrolleri kullanılmıştır. Kalker matriks bloklarını temsil eden bu örnekler dışında özellikle litolojinin ve geçirgenliğin etkisini incelemek üzere kumtaşı karotlarla da deneyler yapılmıştır. Genellikle % 100 petrole doymuş karotlar yanında kalıcı su doymuşluğunda petrol içeren karotlar da kullanılmıştır. Genel değerlendirmede özellikle gözeneklilik, geçirgenlik, ıslatımlıl.k, su tuzluluğu, petrol özgül yoğunluğu ve diğer bazı parametrelerin ımbıbıtıon'a etkileri incelenmiştir. Bu yaklaşımın dışında orijinal petrol ve karotun kullanıldığı B. Raman ve sahası için ölçeklendirme yardımıyla bir performans tahmini çalışması da yapılmıştır. Ancak burada yorumlamada ıslatımılığın neden olduğu belirsizliği belirtmek gereklidir. Rezervuardaki orijinal ıslatımlılığın bilinmesi güçtür ve muhtemelen deneylerde kullanılan karotların ıslatımlılığı da farklıdır Çalışma ve sonuçları, özellikle kısmi imbibition ve optimum hız konusunda eksiklikler içermesine rağmen, önemlidir. Bu önem bu konuda ülkemizde yapılan ilk ve tek çalışma olmasından kaynaklanmaktadır. Konu üzerine yapılacak eklemelere, bağlı konularda yapılacak deneylere zemin hazırlayacak bilgi ve donanım birikimi elde edilmiştir ve 3. ve 4. bölümde ayrıntılı olarak anlatılmıştır

This study investigates oil recovery by water injection in naturally fractured reservoirs. Production mechanisms occuring in fractured reservoirs are more complex than that in conventional reservoirs. The fractured reservoirs exhibit double porosity system behaviour. The presence of the high permeability fracture system in fractured reservoirs is identified by a sharp and horizontal two phase contact. When the production begins in the reservoir different main production zones such as gas invaded zone, gassing zone, under saturated zone and water invaded zone can occur. Imbibition process takes place in water invaded zone. If a reservoir rock is fully saturated by a wetting phase the displacement of this phase by a nonwetting phase will be related to the magnitude of capillary pressure and to the corresponding decrease of the wetting phase saturation. This displacement is defined as the drainage displacement and the relationship between capillary pressure and saturation is known as the drainage capillary curve. However, imbibition capillary pressure develops when reservoir rock is saturated with a nonwetting phase which must be displaced by a wetting phase. For a conventional pore distribution, imbibition capillary pressure is approximately half of the magnitude of the drainage pressure The imbibition mechanism is characterized by two important forces viz. capillary and gravity forces. If oil is the nonwetting fluid that saturates the matrix and if water is a the wetting fluid that saturates the fracture then capillary and gravitational forces contribute to the imbibition process. If the water-oil contact is at the lower face of the matrix block, the forces displacing oil are known as the capillary forces. If the water oil contact is above the bottom of the matrix block the difference in the specific weights between the water in fractures and the oil in matrix generates a gravitational force acting as a driving force for the displacement of oil from the matrix block. In this case Vll capillary forces act as a second driving force in addition to the gravitational forces. Parameters such as rock homogenity, reservoir lithology, interfacial tension between rock and fluid, fluid viscosity, mobility ratio and also the water advancement velocity affect the total production versus time relationship during oil displacement by imbibition process. In a triple system consisting water, oil and rock, if water tends to displace the oil, then the rock is defined as water wet. In such a triple system, a force balance can be written as At = 0,0 ~ - Com cos(0) If 9 >90 the rock is defined as oil wet If 0 < 90 the rock is defined as water wet and if 0=90 then the rock is defined as neutral. If the reservoir rock is strongly water wet (9=0) then imbibition mechanism is feasible. Therefore, the wettability must be examined carefully. Wettability of the core sample taken by the reservoir may be affected from any contamination during the contact with other fluids. Also, certain additives used in drilling and completion fluids may alter the wettability of the core. Surfactants in invert oil emulsion drilling muds and the contact with air are known to alter the wettability of the core. Therefore cleaning methods must be carefully examined before putting the samples on testing. Once the core is cleaned, the wettability of the core can be restored by saturating with synthetic formation brine and uncontaminated reservoir crude followed by aging of the cores at the reservoir temperature. In this study, the efficiency of solvents for cleaning uncontaminated plugs was investigated. For this purpose, three different cleaning methods were examined. In the first method, contaminated plugs were cleaned by using benzene-aseton with 5 to 10 pore volumes. The second method involved use of chromic acid to make the core strongly water wet. The third and our standart method of cleaning procedure utilizes distillation/extraction using Soxhlet apparatus. The two solvents successively used are chloroform and a 50/50 mixture of toluen/methanol. Upon completion of the cleaning procedure the cleaned plugs were dried in oven at 400 #C. Vİ3İ The wettability of the plugs was evaluated by a modified USBM method which includes two steps. The first step of the USBM method is brine drive and the second one is oil drive. Here Yeniköy oil was used in oil drive and a synthetically prepared B.Raman brine. After carrying out two steps, the graph of capillary pressure versus water saturation is obtained. The areas under a set of two curves which are called oil drive and water drive curves are used to calculate the wettability. The logarithm of this ratio is defined as the USBM index (W)as W=log(A1/A2) where A-j and A2 are the areas under the oil drive and the brine drive curves, respectively. As mentionad before, there were two important forces in imbibition process and one of them is the capillary pressure. When the two immiscible fluids are in contact in porous media, pressure difference occurs in the contact line and this pressure difference is called as the capillary pressure. The relationship between interfacial tension and capillary pressure is given as Pa-Pb =a(ir4£) where R1 and R2 are main curvature and 6" is the interfacial tension Capilary forces will be large if pore size is small while gravity forces will increase with increasing the height of the matrix block. As a result out of the two important components the gravity forces will govern the imbibition in the case of small matrix blocks and reduced pore size. The experiments were designed to investigate the influence of fluid properties such as viscosity, specific gravity, surface tension, and also the rock properties such as porosity permeability and wettability. The performance of imbibition mechanism was evaluated on three types of reservoir plugs; these are Bati Raman, Guney Sahaban and Bentheim. The litologies of these plugs are carbonate, carbonate, and sandstone, respectively. Also, they have different sample size, length, permeability and porosity. Using such a large range of rock properties permitted us to effectively compare the above parameters on imbibition. IX In the displacement experiments three types of water were used to investigate the role of water. These are distilled water, synthetic brine that represents formation water and 1 % of CD128 surfactant solution. The effect of temperature on imbibition was tested by experiments carried out at 18 °C, the room temperature and at 65 °C corresponding to the reservoir temperature of B. Raman. Therefore all fluid properties were measured at 1 8 and 65 °C. The results of the experiments carried out to define the fluids are tabulated in the text. Two types of methods were designed to measure the oil produced by imbibition. One of them is weight measuring method and the other is volume measuring method. The components of weight measuring method are electronic balance that has sensitivity of 1/1000 gm., wire basket, and oil bath. All weights are measured in water as grams versus time and using the following relationship (the produced oil was transformed to the volume scale in terms of cm3): V= Am/A ç where Am denotes the weight increased upon displacement of oil and A