MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF MINING AND GEOLOGY Pham Duc Thang A STUDY ON SUITABLE SOLUTIONS TO ENHANCED OIL RECOVERY IN THE LOWER MIOCENE FORMATION, WHITE TIGER FIELD Specialization: Petroleum Engineering Code No.: 62520604 SUMMARY OF TECHNICAL DOCTORAL THESIS Ha Noi - 2014 The study was completed at: Drilling and Production Department, Petroleum Faculty, Hanoi University of Mining and Geology. Advisors: 1. Assoc. Prof. Dr. Cao Ngoc Lam 2. Dr. Nguyen Van Minh Reviewer 1: Dr. Nguyen Huu Trung Reviewer 2: Dr. Nguyen Anh Duc Reviewer 3: Dr. Tran Van Tan The thesis will be defended in Assessment Committee of University level which meeting at Hanoi University of Mining and Geology in time hour date…. month year 2014 References to thesis at library: National library or Hanoi University of Mining and Geology library 1 1. Statement of the problem Lower Miocene is one of the major producers of White Tiger oil field which so far has been producing from June 1986. The formation now is in the final and declined phase with rising of high water-cut and water flooding occurred throughout most the object. The production is mainly by secondary method that is water injection to maintain reservoir pressure combining with some mechanical methods which was no longer effective as the first production phase. This formation has been produced of 6.36 million tons of oil with the oil recovery factor is now at 15.5%. The potential for increased oil recovery from lower Miocene after water injection is still very large. The amount of residual oil remaining in the formation is about 28.3 million tons of oil (accounting for 69% of initial oil in place). In order to ensure that achieved production plan for fields in the next years, the selection of appropriate methods to recover oil from lower Miocene of White Tiger field is urgently needed in this period. Production method by water alternating gas injection (WAG) was studied and selected that is an appropriate and most potential method for enhanced oil recovery in the lower Miocene of White Tiger field. This method not only increases the oil recovery factor (increase sweep, pushing efficiency of residual oil) but also take advantage of the low pressure gas quantity (level 2 after gas separator of 100 m3) which currently have burned partly in White Tiger field, contributing to environmental protection. According to the forecast, oil production of White Tiger field will be rapidly declined after 2012. In order to maintain the oil production due to forward plan in the coming years is an extremely difficult task but it is very urgent to meet the requirements and practices of production in Vietnam. 2. Objectives of the study To enhance oil recovery for lower Miocene formation, White Tiger oil field by applying water alternating gas injection (WAG). 3. Tasks of the study In order to achieve its goals, the tasks of the study to be solved are: - Overview of enhanced oil recovery (EOR) and water alternating gas injection method. - Status of production and selection of water alternating gas injection method for lower Miocene, White Tiger field. - Research by experiments and evaluate the efficiency of water alternating gas injection method for lower Miocene, White Tiger field. 2 - Reservoir model and rerervoir simulation of WAG injection for lower Miocene, White Tiger field. 4. Methodology of the study - Folder method: processing and analysis of geological, geophysical, drilling and production data to build an overview of research and application tertiary method of WAG injection for lower Miocene, White Tiger field. - Experimental method: buiding the exprimental model of pushing oil by WAG injection on the combination core of lower Miocene, White Tiger field to determine the technology indexes and evaluate the efficiency of enhanced oil recovery of this method. - Method of numerical simulation: model building, production simulation by specialized software, history matching and production forcast, preliminary evaluation of production efficiency of WAG injection to enhance oil recovery for lower Miocene objects, White Tiger field. 5. Scientific and practical significance 5.1. Scientific significance - A research, application project for a proposed solution to a rational production to enhance oil recovery for lower Miocene objects, White Tiger field from theory to experimental studies in the laboratory up to simulate production. - Find out optimization method to enhance oil recovery based on the actual production and reservoir conditions. The results of the thesis will contribute to clarify and enrich the EOR methods in general and in the lower Miocene, White Tiger field in particular. 5.2. Practical significance - The results of the study is urgently needed and meet the practical requirements of oil and gas production which is now EOR for the lower Miocene, White Tiger field as well as the others in Vietnam in declined phase. - WAG method has been applied very popular in the world, but in Vietnam is still in the research of application phase. Therefore, the results of the feasibility study will contribute to accelerating the investment in further studies of the oil companies and the premise for applying into practice on a large scale. 6. New contribution of the study 3 - The thesis has found Minimum Miscible Pressure (MMP) between oil and associated gas of Miocene formation, White Tiger field and the solutions to reduce MMP by mixing associated gas with available low pressure gas at field to ensure the feasibility of applying the results in practice. - Thesis has evaluated the efficiency of the method of Water Alternate Gas (WAG) injection on physical modeling and reservoir simulation for Miocene formation, White Tiger field. 7. The defending of new conclusions - MMP of oil with associated gas of Miocene formation, Bach Ho field is 350 bar. MMP will achieve exactly the current reservoir pressure is 255 bar by mixing 40% of low pressure gas (gas separation of level 2 at field) with 60% of associated gas. And while associated gas is enriched by LPG with different mixing ratio, respectively: 5, 10, 20, 30 and 40% mol, MMP can be decreased by respectively 315 bar, 291 bar, 238 bar, 185 bar and 140 bar. - WAG injection before water injection could recovery from 70.5 to 80.2% oil in the core sample, while only water injection has recovery efficiency of about 55.5 to 60.5%. WAG injection at the time after the water injection will make more oil recovery from 15.9 to 17.8%. The results of simulation shown that the WAG injection schemes on northern Miocene models have oil recovery factor increased from 2-10% and water cut down from 90-45% compared to water injection method. 8. Database The thesis was studed on the basis of documents, research reports, review of geological, geophysical, designed production for the Miocene formation, White Tiger field in particular and the White Tiger field in general and a lot of material reports, studies, laboratory analysis of core samples, fluid is taken from the Miocene formation, White Tiger field of the authors; summarized reports of exploration and production activities in Vietnam's continental shelf of Vietnam oil and Gas Group; articles and scientific studies of the domestic and foreign authors which ware published in professional journals. 9. Structure of the study The dissertation consists of an introduction, four research chapters, conclusions and recommendations, list of published works of the author and reference list, appendices. The entire contents of the thesis are presented in 109 pages, including 23 tables, 85 graphs, drawings, and 91 references. 4 Chapter 1 OVERVIEW ON ENHANCED OIL RECOVERY AND WATER ALTERNATING GAS INJECTION METHOD 1.1. Production phases Production process in the oil field can be divided into three phases: primary, secondary and tertiary phases. For each phases, the technologies applied in the field will be varried. 1.1.1. Primary production phase As initial reservoir pressure is greater than hydrostatic pressure or when the initial reservoir pressure greater than the total pressure loss of the flow from bottom hole to the surface, and then field oil will automatically flow by its natural energy. 1.1.2. Secondary production phase Process of water or gas injection to maintain reservoir pressure after primary production phase called secondary production. These oil recovery methods are also known as incremental recovery of conventional oil (Conventional EOR). 1.1.3. Tertiary production phase (EOR- Enhanced Oil Recovery) EOR methods with purposes to increase energy supplement for reservoir, also create favorable conditions for oil recovery process by the interaction of injected fluids (usually not in reservoir) with oil and rerservoir rock. This interaction can reduce the surface tension between phases, dilates significantly oil, reduce oil viscosity, reduce the possibility of the oil wettability, increase sweep efficiency, reduce viscous fingering by maintaining oil mobility and gravity segregation between interacted fluids. Oil recovery efficiency is calculated by the following formula: E = E A * E V * E D (1.1) In which: E A : Area Sweep Efficiency E V : Vertical Sweep Efficiency E D : Displacement Efficiency 1.2. Water Alternating Gas injection method (WAG) 1.2.1. Minimum Miscible Pressure (MMP) When gas injection to push oil, it occurs the process of mixing between gas and field oil. The ability to mix and the efficiency of pushing oil depend on the injection pressure. There is a critical point in the relationship between the oil recovery factor and injection pressure that at this pressure 5 with high oil recovery and minimum injection pressure. At this injection pressure, the oil is almost recovered and therefore injection pressure increased higher than that, recovered oil is negligible. 1.2.2. The mechanism of mixing oil and gas The mechanism of mixing oil and gas are classified into two main mechanisms that are first contact miscibilty (FCM) and multible contact miscibilty (MCM). In which multible contact miscibilty is devided into two types that are condensing drive and vaporing drive. Factors affecting the minimum miscible pressure are reservoir pressure, reservoir temperature, reservoir oil components, reservoir oil density, injection gas composition (enriched gas, lean gas and natural gas) and injection gases (CO2, HC and N2). 1.2.3. The mechanism of pushing oil by water and gas (reservoir model) For lower Miocene formation, pushing and trapping oil properties are often studied on double channel model (model-Double and More Slobod, 1956). On the structural double model is simulated by two large and small channels. This model is the water wettability model. The process of pushing oil occurs on the model as follows: For WAG injection method, mechanism of sweep and pushing oil occur over the entire small and large channel by the water and gas alternating injection which aim to increase the efficiency of oil recovery compared to model of water injection or gas only. This demonstrates the efficiency of WAG injection on the double channel model. In other words, WAG injection is reality effective on all kind of sediments. Particularly, miscible gas injection process may be more convenient when the surface tension between gas and oil is 0. In summary, the mechanism of pushing oil in capillary force model dominantly as the sedimentary rocks which applied WAG injection will have significant effect. For most types of rock that have small permeability or high capillary force, the efficiency of WAG injection is larger show. Oil Oil Oil water water Water water Water gas Gas Gas Oil Oil water 6 1.2.4. Mobility ratio When a fluid pushing the other, the mobility ratio (M) is defined as the ratio of the mobility of displacing fluid ahead to that the mobility of displaced fluid behind. If this ratio is greater than 1, this means pushing not meet requirement as a caused result of viscous fingering. And for those of mobility ratio less than or equal to 1, progress will occur as pushed piston. In which: M : Mobility ratio of WAG injection.  chat- day : Mobility of water and gas.  chat-bi-day : Mobility of water and oil. K w , K g , K o : Permeability of water, gas and oil, mD. μ w , μ g , μ o : Viscousity of water, gas, oil, cP. 1.2.5. The factors effect on the efficiency of WAG injection 1.2.5.1. The effect of injection speed Blackwell et al (1960) observed that at the surface of pushed face of oil/gas, viscous fingering was happened quite seriously, meanwhile at the surface of pushed face of pushed face of water/gas was happened stable. However, under high injection speeds then more oil was trapped by the gas passing over oil without sweep. This behavior occurs because gas moved too fast to water. With slower injection speed, water moved faster than gas that caused reducing of gas sweep efficiency. With optimum injection speed, gas and water will move at the same speed and the sweep effiency of oil is the most effectively. 1.2.5.2. The effect of injection slug size The size of the injection slug can be defined by injection volume for each injection slug. In general, the volume of each slugs are calculated as a percentage of pore volume (PV) or hydrocarbon pore volume (HCPV). The total gas volume used for injection is the total volume of the injection slugs. The majority of researches and applied results on the field, injection slug size are applied to approximately 5% PV. According to that reseach, total gas slug with about 40% PV is optimal for gas quantity when applied in WAG injection.     Sowavg w w o o Swavg w w g g wo wg daybiChat dayChat KK K K M                               (1.2) 7 1.2.5.3. The effect of WAG ratio (water and gas ratio) WAG ratio is determined by ratio between water slug volume and one gas slug volume. In some researches before, WAG ratio is defined the percentage of water to gas slug. In scope of this study, WAG ratio will display under ratio of water volume with one gas volume. 1.2.5.4. The effect of wetability on the efficiency of oil recovery According to the study on structured reservoir model with the different wetabilities of Sohrabi et al at Heriot-Watt university that published on 2001 [75] was shown that the efficiency of WAG injection is higher than that in water and gas injection on all model. On the types of watability model as water wetability, oil wetability and combined wetability then the efficiency of oil wetability model is highest and the efficiency of water wetability model is lowest. 1.2.5.5. The effect of reservoir characteristics on the efficiency of oil recovery According to geologist, almost reservoirs were flooded in water for a long time due to sedimental process. Therefore, oil reservoir is often hetrogeneous and has a complex structure such as not the same permeability and porosity with the different directions. There are divided into two main effects on oil recovery that are the effect of Kv/K h ratio and the effect of sedimental layers. 1.3. WAG projects in the world Gas injection projects were implemented in so many places in the world. Almost projects applied WAG injection by hydrocarbon gas (HC). Magnus and Ula fields are operated by BP at northern sea that are new fields applied WAG injection and achieved success [57, 89]. In Vietnam, WAG injection method by hydrocarbon was studied and has been implementing for pilot test at Rang Dong oil field and achieved the initial satisfactory results [7, 62]. Based on oil properties, field reserves, the distance of gas pipeline and investment costs for Magnus, Ula and Rang Dong fields it would be considered to apply completely WAG injection for Miocene formation, White Tiger field. 1.3.1. Magnus field with WAG injection project Magnus field has started producing from 1993 and applied water injection to maintain oil production of 150 thousands bbl/day until 1995. Residual oil saturation after water injection is 25%, OIIP is 1.5 billions bbls. Field has been applied 4 wells for WAG injection from 2002, minimum 8 miscible pressure (MMP) is 345 bar. Gas pipeline is 400 km of length from gas fields in the west that is operated by BP. In 2010, oil recovery added by WAG injection is 11.5 million barrels and contributed up to 40% oil recovery for entire field. 1.3.2. Ula field with WAG injection project Similar to Magnus project, the system of EOR injection of Ula field have advantage conditions to apply with the key factors of reservoir engineering and commerce. Ula field started producing from 1986 and injected water from 1988. The research shown that WAG injection could be recovered from 8-10% OIIP compared to water injection. Residual oil saturation is high (35-50%), large OIIP (1 billions bbl) which are advantage conditions to inject WAG. The gas source for WAG injection is at the field and adding from Tambar and Blane field and one more Oselvar gas field in 2012. The commercial negotiation has made sure to carry out WAG injection. In 2010, the addition of produced oil by WAG injection was more than 23 million barrels (accounted for 2.4% OIIP) and contributes to 60- 70% oil recovery of entire field. 1.3.3. Pilot test project by hydrocarbon injection at Rang Dong field, Viet Nam Rang Dong field began producing from August 1998 in two main subjects that are Miocene sandstone formation and fractured basement rocks. Up to 2010, the field was produced about 82 million barrels of oil and 80 million cubic feet gas from lower Miocene formation with ultimate oil recovery factor is 26.7%. The average of oil production currently is approximately 16,000 barrels/day and 15 million cubic feet gas/day, the water cut is 55% on average [7]. Water injection was carried out in 2006 and so far field is now in the declined phase. According to the initial assessment, the efficiency of WAG injection method can increase oil recovery in this formation of about 10 million barrels in the period of 2011-2020, equivalent to 35% of the total oil recovery. WAG injection method has been studied by experiments with MMP of associated gas is about 331 bar. In general, WAG injection method performed the following mechanisms: - Increased pushing efficiency of oil recovery by maintaining reservoir pressure. Mechanism of gas mixed with oil will reduce oil viscosity that leads to increase pushing efficiency or increase oil recovery factor. [...]... injection gas is measured by gas meter equipment (gas meter-FQT400) Dead oil were put in 1 cylinder (T2) has an accuracy of 0.1 cc During the experiment, an electronic scale was reading the weigh of cylinder Thus, oil and gas ejected in the experiment was strictly controlled A constant flow pump Gilson (P1) with a maximum pressure of 400 bar can be achieved automatically and controlled by computer Flow may... device measures the amount of oil, water and gas obtained 3.4.2 Experimental procedure The core samples after drying, extracting and measured prorosity and permeability were saturated by formation water Anh then, these core samples were installed in the core holder respectively All system is put in oven to heat and reach reservoir conditions by increasing slowly overbuden pressure and out put pressure... injection method to enhance oil recovery is an appropriate solution and most feasible in the current period 1 2 3 4 5 6 7 8 THE LIST OF RESEARCH WORKS OF THE AUTHOR Nguyen Huu Trung, Pham Duc Thang, Ho Anh Phong (2003), Thermally insulated submarine pipeline, laid for the Bach Ho field using polyurethane, Proceeding of Science and Technology Conference Vietnam Petroleum Institute 25 years of construction... Conference Petroleum Technology and Human Resources - 2012, Hanoi University of Mining and Geology, Ha Noi, pp 57-58 Pham Duc Thang, Nguyen Van Minh, Tran Đinh Kien, Cao Ngoc Lam, Nguyen The Vinh, Nguyen Manh Hung, Hoang Linh Lan (2013), A study to enhance oil recovery by water alternating gas injection (WAG) for lower Miocene, White Tiger field, Scientific-Technical Journal of Mining and Geology, Hanoi, . Lam 2. Dr. Nguyen Van Minh Reviewer 1: Dr. Nguyen Huu Trung Reviewer 2: Dr. Nguyen Anh Duc Reviewer 3: Dr. Tran Van Tan The thesis will be defended in Assessment Committee. accuracy of 0.1 cc. During the experiment, an electronic scale was reading the weigh of cylinder. Thus, oil and gas ejected in the experiment was strictly controlled. A constant flow pump Gilson. drying, extracting and measured prorosity and permeability were saturated by formation water. Anh then, these core samples were installed in the core holder respectively. All system is put