Description
ABSTRACT
In most of the papers and publications released, it is always noted that oil rim reservoirs are associated with several amount of challenges and limitations hence they are deemed to be unprofitable economically. Oil rim due to their sandwich structure in the presence of a large gas cap, a thin oil rim and an aquifer, oil and gas production can be reduced drastically hereby increasing several costs leading to lesser revenue than the initial expectation. This exploitation processes with the oil bearing layer and sandwich structure, reservoir performance and recover factor faces great challenges. Early water production from a thin oil rim reservoir can lead to corrosion of the equipment, deposition of salts, formation of gas hydrates, high cost of lifting the produced water (Anon, 2016). Early water production is inevitable but very much manageable and this is a key factor for a successful field development. (Sarkodie et al, 2014) provided a method of delaying and/or minimizing gas and water coning. Other challenges associated with oil rim reservoirs includes; spreading out of resources, complication of production mechanisms, invasion of different zones, smearing of oil and low oil recovery factor (RF < 18%) and field development is extremely costly. For the sake of this research project, a model of a thin oil rim reservoir was adopted for reservoir simulations in order to determine the optimum injection pattern on a thin oil rim reservoir at several injection rates. This simulation process was initialized using a reservoir simulator (Eclipse 100). This research starts by running several simulations on the model at different injection rates and patterns which gave rise to the cases discussed in this study. Through several runs, it is recommended that oil rim reservoirs with a large gas cap and underlying aquifer should be produced using the direct line injection pattern.
CHAPTER ONE
INTRODUCTION
- BACKGROUND OF STUDY
Oil rim reservoir which is referred to as a segment of oil/gas or gas/oil/condensate deposit contained by oil, the size of reserves is significantly smaller than the gas reserves of the two-phase deposit. Oil rim can vary in size which determines if it is commercial or not. The thin oil rim reserves are usually represented by column of oil which appears thin in the middle of a gas layer and an underlying aquifer. In an exploitation process of a thin oil rim reservoirs dates way back to 1965 (Weber, Klootwijk, Konieczek, & van der Vlught, 1978)therefore leading to successful developments in four of the different continents of the world (( (Kabir, 1998); (Bayley-Haynes, 2003); (Cosmo C. a., 2004); (Yaliz, 2002); (Forrest, 2005)). Over the past 30 years, thin oil column within oil rim reservoirs which lie beneath gas cap and lie above an aquifer have since developed economical stands since the introduction of horizontal wells and increased price of oil. (Irrgang, 1994)describes the coning of either gas or water while it is being exploited in commercial quantity, have developed and attracted attentions of several publications examining exploitation as well as different simulation model approach in line with other methods. As stated by (Kromah, M. & Dawe, R., 2008), it is noticed that above 70 percent of oil reservoir have presence of an aquifer present underneath the column of oil or a large gas cap. This configuration was later explained by (Silva & Dawe, 2010)to be either a dome shape with a overlying gas cap or a sloping shape with water at the edges.
| Oil rim |
| Gas cap |
| aquifer |
| aquifer |
Figure 1(a) The reservoir shaped in presence of a bottom water drive
| aquifer |
| Gas cap |
| Oil rim |
Figure 1(b) a truncated reservoir with the presence of an edge water drive mechanism
Also, due in line with the configurations of the large gas cap located on the thin oil column, the configuration for a thin oil rim reservoir was further explained to be either as a pancake structure or a doughed shape where the mobilization of an oil phase to the producing well depends on the mobility alteration exerted between the two phases (oil and gas) (Tiefenthal, 1994); (Cosmo & Fatoke, 2004); (Zifei, F., Heng, S., Angang, Z., & Xuelin, W., 2018); (Song, Wu, Hao, & Hou, 2017); (Augustine, Junin, Jeffrey, & Onyenkonwu, 2017); (Tiong-Hui & Cheng, 2018); (Owolabi & Ogungbamigbe,, 2017)) (see Figure 1.2).
| OIL PRODUCER |
| GOC & OWC |
| GAS CAP |
| OIL RIM |
| AQUIFER |
Figure 1.2 The dough and pancake structure for the two ideal oil rim reservoirs.
In reservoirs like this, the main reason for development is to produce and maximize the recovery coefficient for oil within limited outflows. Usually setting and trying to meet the conditions for the thin oil rim reservoirs which underneath a gas cap and a lie above an aquifer tends to pose several issues as a result of several limitations for an oil rim reservoir. Hence, in order to successfully develop a field and maximize production of oil reserves, before embarking on the construction of the gas cap above our thin oil rim column it is required that the maximum producible volume of oil is produced. The pre-occurrence of the three different phases in a reservoir which includes water, gas and oil give rise to several complexities in the mechanisms employed to produce from an oil rim reservoir. An oil rim reservoir is regarded as a complex structure due to reasons related to water injectivity, gas injectivity, oil degassing, retrograde condensations ( (Olabode, Orodu, Isehunwa, Mamadu, & Rotimi, 2018), (Davarpanah, Mirshekari, Behbahani, & Hemmati, 2018); (Siddhamshetty & Kwon, 2018); (Mohd Ismail, et al., 2018).; (Mjaavatten, Aasheim, Saelid, & Groenning, 2006), (Ogiriki, Imonike, Ogolo, & Onyekonwu, 2018); (Davarpanah, A feasible visual investigation for associative foam and polymer injectivity performances in the oil recovery enhancement, 2018); (Ogolo, Naomi, Molokwu, & Onyekonkwu, 2018)). In order to efficiently produce oil from the oil rim reserves, there are several driving factors which play a key role such as the presence of a large gas cap, the overlain solution gas expansion, and the resulting viscous withdrawal (see figure 1.3). This factors have a substantial part in the stabilizing the construction of a thin oil rim reservoir due to reinjecting of producer gas the pressure drop in the reservoir is kept under control as well as different section of this reservoir which is influenced by the effect of the strength the underlying strong aquifer. Also, The position and space which the column of oil is located from such perforations of the well have all played significant roles in oil stabilization and p
revented mobility of the oil ( (Fernie, Zhou, McCarthy, & Douglas, 2018); (Cosmo & Fatoke, 2004); (Zifei, F., Heng, S., Angang, Z., & Xuelin, W., 2018), ; (Song, Wu, Hao, & Hou, 2017); (Augustine, Junin, Jeffrey, & Onyenkonwu, 2017); (Tiong-Hui & Cheng, 2018); (Foudzer, Kilic, & Das, 2017); (Davarpanah & Mirshekari, A simulation study to control the oil production rate of oil-rim reservoir under different injectivity scenario, 2018))
Figure 1.3 Driving mechanism associated with an oil rim reservoir (Masoudi, Karkooti, & Othman, 2013)
Studying the thin oil rim reservoir performance with respects to the ultimate oil recovery factor could be linked to several factors such as reservoir parameters and operating and production strategies (Olabode, Orodu, Isehunwa, Mamadu, & Rotimi, 2018). As highlighted by Ibunkun (2011), factors affecting the productivity of oil rim can be grouped into 4 factors namely reservoir, geological, production and dynamic factor. These factors are also noted by (Vo, Waryan, Dharmawan, Susilo, & Wicaksana, 2000) while undergoing a study of factors affecting the thin oil column for an oil rim reservoir. (Uwaga & Lawal, 2006) identified a rapid decline in the production and recovery rate of oil the reserves of an oil rim composed of a large overlaying gas cap. (Wayne, 2003) constructed a matrix to proper understand the development concept of reservoir characterized by a thin oil column, a large gas cap and a strong aquifer, taking the relative size of the gas cap alongside the thickness into consideration also. (Masoudi, Karkooti, & Othman, 2013)discussed the importance and effects of developing oil rim under 4 different strategies namely; the sequential strategy, the swing strategy, the concurrent strategy and the gas cap blow down strategy. (Osoro et al, 2005) also further estimated that thin column of oil in an oil rim reserves are independent of the recovery factor and other factors including; fluid properties, initial reserves of gas and the geometry of the reservoir. (Olabode, Orodu, Isehunwa, Mamadu, & Rotimi, 2018)studied the various effect which would occur due to the injection of foam and Water Alternating Gas (WAG) on the success of the exploration of oil from the reserves of a thin oil rim reservoir. Several studies and researches have been made into the development, performance and productivity of an oil rim reservoir although most papers do not present the injectivity patterns used on an oil rim reservoir which is the basis of this research project.
The investment of resources, decreased oil or gas production and an increase in operating expenditure also mean lower revenues for all reserves containing oil. The management of this reservoir and improved oil recovery techniques face significant challenges with the use of these thin oil rim structures and their existence between the gas cap and water aquifer. (Silva & Dawe, 2010). This study is aimed at preventing the rise of this challenges by providing an injection pattern which would improve the oil recovery, operating expense and eventually increase the oil and gas production.
Over the years, thin oil reserves have been used gradually by the petroleum industry. Nonetheless, some operators also referred to the existence under a gas reservoir of a thin oil rim as a loss, which is required to produce by law. Developers would rather skip the oil rim and just produce gas, since it is not cost-effective to process oil.
In developing oil rims and manufacturing within the transitional capability zone, oil undertakings encounter many technical and commercial limitations that reduce their technical and commercial profitability. Among the various technical challenges are water breakdown and gas spread, complicated production and drive mechanisms, difficulty in understanding capillary transition and invasion zones, low recovery factor (usually below 18 percent), well type, well design, pf well boiling, and completion, lack of relevant capillary data ( i.e. thin oil) etc. (Masoudi, Karkooti, & Othman, 2013).
In order to ensure that an oil rim development is realized, especially those with a large gas cap, the development strategy can be developed in a small window, for either sequential or concurrent oil and gas production. The different emphasis of the owner is among other business questions by examining the productivity of the field life and the operator by looking at the revenue generated in the valid license period. Finally, this development is less expensive.
The use of this technology in reservoir operation, combined with active reservoir management and research simulation, well-performance monitoring and suitable injection strategies would therefore make the production of the oil rim a cost-effective and attractive project.
As mentioned earlier, there are several limitation and challenges associated with oil rim reservoir field development and production. This challenges have also been noted by researchers over time and reasonable solutions have been proposed to curb some of this issues. Usually in oil rim reservoirs which lie above an aquifer and lie below a large gas cap, there is low oil recovery hereby leading to several proposed techniques for improving recovery. This techniques includes; Injecting of gas at the Oil and water contact (OWC) (Ogolo, et al., 2017), injection of water at the Gas and oil contact zone (GOC) ( (Chan, Kifi, & Darman, 2011); and (Panda, Ambrose, Beuhler, & McGuire, 2009); ). (Ogolo, Naomi, Molokwu, & Onyekonkwu, 2018)proposed a technique which improves the recovery of oil from an oil rim reservoir using a horizontal well and studying the effect of combining several methods for improving oil recovery such as water injection, gas injection at the point of contact of Oil and water(OWC), water injection at the point of contact of oil and gas(GOC) in the reservoir but the proposed technique indicated that combining several methods doesn’t increase oil recovery efficiency significantly to defend the cost of applying two technique. In avoiding oil smearing into a different zone which would consequentially result in a decrease in the total volume of recoverable oil, maintenance of stable fluid contact in a thin oil rim reservoir (Ogolo, Naomi, Molokwu, & Onyekonkwu, 2018) and (Onyeukwu, Peacock, Matemilola, & Igiehon, 2012) proposed a technique which increased the total recovery factors of oil by simultaneous injection of gas and water up to 15% STOIIP (Onyeukwu, Peacock, Matemilola, & Igiehon, 2012)created a model for simulation which helps access trends in recovery of oil and several other parameters which would help understand oil rim performance even better. This model in topic focuses in using the incremental recovery of oil from the injection of gas and water into an oil rim using voidage replacement. (Onyeukwu, Peacock, Matemilola, & Igiehon, 2012) provided a very effective means of used to access oil in the reserves of oil rims in order to improve their recovery either by using a gas, water or WAG injection.
Over the years there have being several publications and papers on oil rim reservoirs either in a vertical or a horizontal well. There are papers which propose vertical wells are more profitable while other say otherwise. (Vander, 1991) proposed that horizontal wells would produce at a higher rate substantially than the normal conventional well. The process of drilling a horizontal well with several sand stack distributed in an oil rim reservoir is a very good alternative to the conventional well development and eventually would bring about an increase in the ultimate recovery (UR) of the oil field (Vander, 1991). Horizontal wells have been able to explore reserves which conventional wells wouldn’t be able to explore, the elevation present in a horizontal well is an important factor to consider in order to delay the breakthrough time of water and gas which have been present in the reserves of a thin oil rim for as long as possible (Vander, 1991). (Vander, 1991) also suggested that using a downhole device to shut flowing build up is recommended in order to decrease the effect of well bore storage. ( (Joshi, 1988); Giger et al, 1984) developed a concept of Oil productivity improvement factor (PIF) which was used to estimate the various benefits of a horizontal well. In horizontal well, oil usually flow from the region of high pressure to the region of low pressure which is a boundary control problem since the extraction of oil takes place at the boundary. Knowing this, (Sagatun, 2010) proposed a control scheme for the boundary by linearizing a Gas Oil Ratio (GOR) model to increase the total amount of oil which would be produced from a well before the inevitable gas breakthrough.
(Davarpanah & Mirshekari, A simulation study to control the oil production rate of oil-rim reservoir under different injectivity scenario, 2018) studied the means which can be employed to properly influence the rate at which the thin oil rim reserves are being produced under the 6 different patterns used for injection by using a model created on eclipse. This paper studied the different scenarios for the optimum scenario but also failed to include the injectivity pattern in determining the optimum scenario which this research project intends to do by also increasing the amount of scenarios from 6 to 10 different injection patterns to determine the most profitable injectivity pattern before the start of production in a thin oil rim reservoir.
- STATEMENT OF PROBLEM
Drilling of a thin oil layers is always regarded as a high expenditure development and is a Problem of established dispute in the oil companies that expect to invest. The oil rim reservoir is a type of unconventional reservoir, occupied by an enormous gas cap and a strong aquifer that offers a complex environment for the production of the oil reserve.
For a thin oil column which is one of the main characteristics of a thin oil rim reservoir, field development and oil production present extra challenges. In the light of stabilization of the underlying water and overlaying gas, optimizing oil recovery in limited oil rims was also a problem (Silva & Dawe, 2010) The key goal of field production is to ensure that the full amount of reserves is extracted and that the desired oil recovery level is always accomplished with minimal costs. This is also impossible to have this requirement with a thin oil rim reservoir with the normal enormous gas cap and a strong aquifer. Therefore, it is important to deliver the maximum volume of reserve oil before the coning-effect happens for a productive project in these reservoirs. According to the designation of the three processes in the reservoir, oil, water and gas, the process of processing and injection in a crude rim reservation is highly complex. The cause of the problem is the crude oil degassing, water and gas injection as well as retrograde condensation ( (Olabode, Orodu, Isehunwa, Mamadu, & Rotimi, 2018), (Davarpanah, Mirshekari, Behbahani, & Hemmati, 2018); (Siddhamshetty & Kwon, 2018); (Mohd Ismail, et al., 2018).; (Mjaavatten, Aasheim, Saelid, & Groenning, 2006), (Ogiriki, Imonike, Ogolo, & Onyekonwu, 2018); (Davarpanah, A feasible visual investigation for associative foam and polymer injectivity performances in the oil recovery enhancement, 2018); (Ogolo, Naomi, Molokwu, & Onyekonkwu, 2018)).
1.2.1. PRACTICAL PROBLEMS
(1) Water and Gas Coning and Break-through issue
(Uwaga & Lawal, 2006) noted that problems such as means of completion, estimation of reserves and production policy are present with a gas cap and an underlying aquifer in the reservoir for thin oil rims. Water and gas coning typically increases the final output of oil below the commercial and economic levels . In oil rims with developable oil columns, the policy of producing concurrently and blowing down the gas cap after oil recovery has been implemented.
(2) Complex Production Mechanism
The main reason behind the development of oil rim reservoir have been to produce the oil column without adversely affecting the production of the gas and water present. The classic upset is to maintain production of the gas without affecting the oil production (which is the ultimate recovery of the oil) in the long run. It is common to use a reservoir with a wide gas cap to only ensure production from the oil rim and to hold the gas cap in place at the same time to sustain the pressure from the reservoir. (Sascha & and Marc, 2008) noted the challenges affecting the development and production of Wide and relatively growing gas cap in oil rims with a small underlying aquifer present as:
(i) In an attempt to maximize oil recovery, the oil production must come before that of gas as the best option for the reservoir
(ii) Late oil production or gas development can result into gas deferment making the project very unattractive economically as the main gas sales and gas condensates are lost and,
(iii) High pressure is usually mounted on prompt delivery of gas supply for both domestic and foreign use due to the late gas production.
At the same time, oil and gas production requires the continuous extraction of oil and gas from a specific reservoir from the start of the development process to the abandonment phase through different production tubing, either via separate pipes, dual string completion or using a single conduit.
The current scale and behavior of the gas cap and aquifers are some of the major challenges facing the sustainable development of oil and gas in reservoirs at the oil rim, as are the geometry of the oil rim reservoir, present fluid properties, variabilities, comportability of reservoir process and gas departure. It is also important to note that the movement of the oil is essentially determined by two dominant drives:
(i) The underlying aquifer
(ii) Drive for expanding the gas cap (i.e. gas cap size)
In a nutshell, the aquifer extends and forces the oil rim up.
This implies that before development starts, thorough understanding of the existence of the oil rim reservoir and its parameters should be developed and properly evaluated.
(c) Oil Smearing
Depending on the policy governing the production, oil present in the rim can begin to smear in or shift into the gas cap. It is due to the large gas and aquifer oil rims. When oil is lost in the gas tank, low oil recovery is inevitable.
(d) Low Recovery Factor (below 18%)
(e) Transition and Invasion Zones
(f) Spread Out Resources
(3) PRACTICAL PROBLEMS FROM WATER AND GAS INJECTION
During implementation of water injection, excess water injection could lead to several practical problems which includes;
- Formation of gas hydrate
- Corrosion
- Salt decomposition
- Clay swelling
- Scaling problems could occur in a situation where sea water comes in contact with fresh water
For gas injections several issues also persist such as;
- Presence of CO2 which is present in gas used in stripping of oxygen can be a corrosive agent for materials
- Unlike water injection, high pressured gas injection procedures are always expensive
- Premature breakthrough of gas due to gravity difference
1.2.2. ECONOMIC AND BUUSNESS CHALLENGES
(a) Oil Rim Development Perspectives.
Due to the scale of the overlaying gas cap and underlying aquifer, and since it has excessive liquids (i.e. water and gas), investments are usually small because of the strength and thickness of the oil-rim reservoirs.
(b) Confusion between the initial gas development and oil production.
Masoudi (2012) development plan identified that a gas production when the oil rim is less than 30 feet, would lead to a jeopardy in the production of oil and operating options Such as the producing rates and types of well completions. Hence, in a case of early gas commitment, gas production is encouraged while oil production is initiated at a later time and stage but not concurrently. Oil recoveries from such schemes are usually low due to reduction of the reservoir pressure which should have been depleted by producing the gas.
(c) Expensive Field Development and marginal economy
A production strategy is economically less attractive for reservoirs with a gas cap expansion drive especially for thin oil-rim reservoirs. It is feasible and should still be considered but due to complexity and the low recovery rates of oil rim reservoirs, they tend to be unattractive and furthermore expensive in the long run.
1.3 AIMS AND OBJECTIVES OF THE RESEARCH
1.3.1 AIMS
The purpose of this research project is to simulate water and gas injection patterns and several injection rates in a thin oil rim reservoir to determine the injection patter and rate that are more profitable for the purpose.
1.3.2 OBJECTIVES
The project objectives are as follows;
- Properly modify an existing oil rim reservoir model from the Niger delta region.
- Initiate the model by developing it under a concurrent production of oil and gas.
- To implement different Water and Was injection rates at different injection pattern to estimate the oil recovery
- To decide which of this Gas and Water injection pattern would be most profitable for an oil rim reservoir.
The above objectives should try to resolve the difficulties and limitations in a thin oil rims reservoir. The final result of the simulation is to determine the most profitable pattern of injection into the reservoir model, which is done by means of simulating variations in the six distinct injection patterns.
- SIGNIFICANCE OF THE STUDY
Due to the nature of oil rim reservoirs, oil recoveries might be low even under best optimization practices of production parameters. It is normal to augment or maintain reservoir pressure through injection schemes. Studies have shown various oil recoveries through secondary and enhanced oil recovery schemes. The best way to optimize an injection scheme is to consider the injection pattern and location of injector wells (which is paramount in case of dipping reservoir). Thus a proper pattern of injection should serve as a basis for implementing injection schemes in oil rim reservoirs for optimum recovery. The purpose of this study is to simulate the six different injection scenarios at various injection rates for an oil field in the Niger Delta region to select the optimum scenario which has the most profitable oil production.
- SCOPE OF STUDY
The scope of these study is to investigate oil recoveries in a thin oil rim reservoir from the Niger delta region of Nigeria through different injection patterns at various injection rates of gas and water using a black oil simulator model. Thus, this is a predictive work and the intricacies of reservoir traits, behavior and properties are not fully considered as the oil rim model used in the study is already incorporated with these functions.
Due to the presence of an infinite boundary condition for a reservoir, a simulation would be executed using a software called ECLIPSE to sufficiently determine the most profitable injection pattern which can be used to adequately optimize oil recovery in a pre-occurring oil rim located in Niger Delta region when simulated at various injection rates. The simulation will be carried out by modifying a detailed reservoir simulation model of a preexisting thin Oil Rim Reservoir using the software, defining and varying the six different injection patterns for optimization in oil rim reservoir. The effects of each pattern would be observed and would serve as a means to adjudicate which would be the most profitable pattern for optimization.
- STUDY AREA
This research study would cover a modified model of a pre occurring oil rim located in Niger Delta region. Several simulations would be conducted in order to determine the most effective and profitable injection pattern and injection rate amongst the six different injection scenarios.
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