Evaluation of Viability of a Crude Oil Reservoir Using Petrophysical Parameters (a Case Study of Agbara Oil Well Reservoir in the Niger-delta Basin)
Chapter One
OBJECTIVES OF THE STUDY
- To estimate the quantity of recoverable hydrocarbon in a reservoir and evaluate the recovery factor, by using the related petrophysical parameters.
- To improve the efficacy of volumetric method in reservoir evaluation by using the area-depth concept model.
- To determine the productive oil zones in a particular reservoir , by the use of Archie’s equations.
- To develop initialization algorithm for determination of pressure gradient in a reservoir.
- To evaluate the basic economic measures in relation to oil production.
CHAPTER TWO
LITERATURE REVIEW
BACKGROUND OF THE STUDY
The volumetric method for estimating oil in place is based on log and core analysis data to determine the bulk volume, the porosity and the fluid saturations, and on fluid analysis to determine the oil volume factor. Under initial condition 1 ac-ft of bulk oil productive rock contains:
But for oil reservoirs under volumetric control there is no water influx to replace the produced oil, so it must be replaced by gas, of which its saturation increases as the oil saturation decreases. If Sg is the gas saturation and Bo is the oil formation volume factor at abandonment conditions, then 1 a c-ft of bulk rock contains;
Where 7758 barrels is the equivalent of 1 ac-ft (Allan and Sun, 2013).
The volume element of a reservoir that is considered porous is called the rock porosity and the fraction of the pore space that is occupied by connate water is called the connate water saturation Swc. Hence the pore space filled by hydrocarbon called the hydrocarbon pore volume (HCPV), is given by;
The summation of the hydrocarbon pore volume over the gas occupied region will give the volume of the gas initially in place while the summation over the oil region will give the oil initially in place all being in reservoir volume (Green and Whillite, 2014). In a reservoir evaluation using the volumetric method, the reservoir system is considered to be a container whose volume represents the quantity of oil in place. The porosities and fluid saturation are obtained from core analysis.
For an oil system, there is an amount of water present from origin called connate water. This is the water in the oil and gas bearing parts of a petroleum reservoir above the transition zone. This water is important because it reduces the amount of pore space available to oil and gas and it also affects their recovering. Connate water is generally not uniformly distributed throughout the reservoir but varies with the permeability and lithology. (Schlumberger Well Evaluation Conference, (SWEC), 2011).
Thus, the fluid saturation with the systems is given by:
A volumetric balance states that since the volume of a reservoir (as defined by its initial limits) is a constant, the algebraic sum of the volume changes of the oil, free gas, water, and rock volumes in the reservoir must be zero(Everdingen et al, 1953). For example if both the oil and gas reservoir volumes decrease, the sum of this two decreases must be balanced by changes, equal in magnitude to the water and rock volumes.
The ratio of the initial gas cap volume to the initial oil volume, symbol is given as:
The value of m is determined from log and core data and from well completion data, which frequently helps to locate the gas oil and water oil contacts. The ratio m is known in many instances much more accurately than the absolute values of the gas cap and the oil zone volume (Firroozabadi, 2016). In the evaluation of the reservoirs that are produced simultaneously by the three major mechanisms of depletion drive, segregation or gas cap drive and water drive, it is of practical interest to determine the relative magnitude of each of these mechanisms that contribute to the production of oil in the reservoir.
The principal problems in preparing the contour map are the proper interpretation of net sand thickness from the well logs and the outlining of the productive area of the field as defined by the fluid contacts, faults, or permeability barriers on the subsurface contour map.
In the evaluation of the approximate volume of the productive zone in a reservoir with the application of a planimeter, the reservoir is considered to be in the form of a container. The entire reservoir may be taken to be in the form of a pyramid, of which the oil volume is taken to be the volume enclosed by the pyramid. The volume of the frustum of a pyramid is given.
CHAPTER THREE
RESEARCH METHODOLOGY AND DATA ORGANIZATION
AREA-DEPTH CONCEPT MODEL
This model is developed in order to improve the efficacy of the volumetric method in reservoir evaluations. Area-Depth concept involves a mathematical analysis of the reservoir geometry. It uses subsurface and isopachous (horizon) map based on the data from the electric logs, cores, drill stem and production tests. A subsurface contour map shows lines connecting points of equal elevations on the top of a marker bed and therefore shows geologic structure. The engineer uses this map to determine the bulk productive volume of the reservoir. The volume is obtained by planimetering the areas between the isopach lines of the entire reservoir or of the individual units under consideration.
MODIFICATION OF FORMULA FOR ESTIMATION USING VOLUMETRIC METHOD
The fraction of the volume element V that is porous is called the rock porosity ø, and the fraction of the pore space that is occupied by connate water is called the connate water saturation, Swc.
CHAPTER FOUR
RESULTS AND ANALYSIS
The Agbara oil well reservoir investigated using the area-depth concept model was viable. This is because, considering the related petrophysical parameters involved in the evaluation, the reservoir was found to contain an oil deposit of 295.8 x 106 stb.
Also, for the recovery factor evaluation of the reservoir, it was found the amount of oil recovered, out of this deposit depends on the presence or absence of aquifer influx. This was seen from the calculation of the ultimate recovery (UR) of the oil from the reservoir. For example, the amount recovered for a case where there is a strong aquifer influx was 153.82 x 106 stb. The amount of recovery for a case where is no aquifer influx was 136.1 x 106 stb. These results showed that the reservoir investigated was economically viable.
This method of evaluating reservoir viability is believed to be more economical and direct in the calculation of Stock Tank Oil Initially In Place(STOIIP) and the Ultimate Recovery Factor (URF) analysis. Although it depends on the type of the reservoir equipment used and other uncertainties in measurements of the petrophysical parameters.
CHAPTER FIVE
DISCUSION
In the evaluation of a reservoir, the contractor has to consider the viability of the reservoir. This is done by estimating the quantity of stored oil in the reservoir. That is , the calculation of the Stored Tank Oil Initially In Place (STOIIP), by taking into account , the geometry and the related petrophysical parameters of the reservoir. It is not feasible to extract all the oil stored in a reservoir, no matter the sensitivity or accuracy of equipment used and the experience of the drilling contractors. At abandonment condition, oil is still left in the well. So, before drilling a particular well, the engineers have to justify the economic level of the reservoir. This is achieved by estimating the recovery factor, which is the amount of hydro-carbon that is deemed recoverable from a particular reservoir. This recovery factor (RF) analysis helps companies to know the maximum quantity of oil that can be drilled out from the well. It also helps to now the budget estimates to be made before production starts. Also in a reservoir with variable quantity of hydrocarbon at different zones, the stored oils in these zones can be calculated by the combination of related petrophysical parameters using Archie’s law.
In this study, area-depth concept model was used to estimate the STOIIP in a reservoir. The recovery factor of Agbara oil well was properly analyzed by this concept. It was shown that the reservoir is economically viable. This model was developed to analytically estimate the initial oil in place and the recovery factor in a particular reservoir. This method of reservoir evaluation could be seen to be more economical and direct although its efficacy depends solely on the sensitivity and accuracy of the equipment used for the analysis of the well logs and cores. It also depends on the experience of the drilling engineers and the reservoir conditions, if the related petrophysical parameters are considered and measured accurately. In this research, initialization algorithm was developed for determination of pressure gradient in a reservoir. This pressure check helps the engineers to have an idea of the reservoir conditions in order to prevent lost circulation and kick effects, during drilling.
On the economic analysis of a reservoir, if the production quantity N, is known, the expenses to be incurred are the capital expenditure which are the cost facilities such as off sure platforms and pipeline arrangement. Also, the operating expenses like the salaries of workers and maintenance costs are to be analyzed.
CHAPTER SIX
CONCLUSION
Almost all oil and gas produced today comes from accumulation in the pore spaces of reservoir rocks. The amount of oil or gas contained in a unit volume of the reservoir is the product of its porosity and the hydrocarbon saturation. In addition to the porosity and hydrocarbon saturation, the volume of the formation containing the hydrocarbons is needed in order to determine if the accumulation can be considered commercial.
In this study, the initial oil and gas volumes was calculated from depth contour maps (horizon maps), reservoir rocks and fluid properties of the Agbara oil well reservoir, which was used as a case study. The results obtained from the analysis of the area – depth graph and cumulative bulk volume graph, shows that the fluid saturations in the reservoir structure was clearly estimated. Also, the recovery factor estimates was done. These estimates are obtained by making certain assumptions regarding displacement efficiency, residual saturations, abandonment columns and drive mechanisms based on experience in an operating area. Usually, these are by correlations based on reservoir and crude properties. Sweep efficiencies depend on oil and reservoir properties obtained from correlations based on local experience and simulation studies. Residual saturations depend on rock type obtained from measured averages from different rock types. Abandonment column height depend on reservoir/ oil characteristics and well type.
To evaluate the viability of a reservoir, it is useful to know how easily fluid can flow through the pore system. This property of the formation, which depends on the manner in which the pore spaces are interconnected, is its permeability. This project has shown the determination of initial oil reserve in a reservoir and the recovery factor estimates, using fix values of related petrophysical parameters. It also explains how logs are used to obtain valuable information about permeability, lithology and connate water saturations, and to distinguish between oil and gas.
Resistivity measurements, along with porosity and water resistivity, are used to obtain values of water saturation. Saturation values from both shallow and deep resistivity measurements were compared to evaluate the viability of an oil reservoir formation.
The results obtained from the evaluation of Agbara oil well showed that the reservoir is economically viable.
RECOMMENDATIONS
From the findings of this research, I wish to recommend as follows:
(A) There is need to carry out intensive research on reservoir evaluations in order to explore and evaluate the economic and technological improvements and advantages of alternative energy sources like oil shales and the biomass-fuel plants over fossil fuels.
(B) I also recommend that more drilling related areas be covered in the future with a more comprehensive view on oil management.
(C) In reservoir evaluations, I recommend that there is need to improve on the quality of the scientific log reading tools as the reservoir data needed for evaluation depend on the accuracy and precision of this equipment.
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