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COMPARATIVE ANALYSIS OF RESERVE ESTIMATION USING VOLUMETRIC METHOD AND MBAL ON NIGER DELTA OIL FIELDS
ABSTRACT
In Petroleum reservoir study and management, an accurate description of the volume of fluid present is very important in quantifying the resources and selection of production techniques, rates and overall management of the reservoir. The information obtained is also the basis for resource development and plan making hence the need for cross examination to mitigate inherent problem resulting from overestimation or under estimation. It is expected that the oil and gas production are measured with confidence since industry’s revenues are based on oil and gas sales. In this work, software called MBAL was used to compute the oil originally in place by performing a non linear regression of average pressure against cumulative oil production on the data from two fields ( Z and Y) in the Niger Delta. The STOIIP estimates obtained from the analysis of field Y were 2075MMSTB, 5212.78MMSTB and 1772.5MMSTB for volumetric method, MBAL and the Monte Carlo while field Z gave about 43MMSTB, 26.5MMSTB and 45.45MMSTB respectively.Differences in the basic assumptions for each method, gross averaging procedures Inadequate available data, data inconsistency, type and impact of the predominant drive mechanisms were identified among other factors that may be causative to the substantial differences between the results of volumetric and material balance estimates of oil and gas initially in place.
TABLE OF CONTENT
CERTIFICATION ii
DEDICATION iii
ACKNOWLEDMENT iv
ABSTRACT v
TABLE OF CONTENT vi
CHAPTER ONE
INTRODUCTION 1
1.1 Background of study 1
1.2 Statement of the problem 5
1.3 Objective of the study 6
1.4 Significance of the study 6
1.5 Scope and limitations of the study 7
CHAPTER TWO
LITERATURE REVIEW
2.1 General Review 8
2.2 Material Balance Theory 12
2.2.1 Derivation of The Material Balance Equation 13
2.2.2 Material Balance Expressed as a Straight Line 14 CHAPTER THREE
METHODOLOGY
3.1 Brief Introduction 20
3.2 Approach 20
3.3 Volumetric Method Estimation of OOIP 21
3.4 Theory of MBAL 21
3.4.1 The MBAL Software Tool (The M-BALTM 10.5) 22
3.5 Method/ Procedure 24
3.5.1 Material Balance Workflow oil in place determination 24
3.6 Data Requirements and Input 25
3.6.1 Data Collection and Organization 25
3.6.2 Data Formatting and Consistency Checks 25
3.6.3 Reservoir System and Correlations 26
3.6.4 Data Analysis and Validation 26
3.7 History-Matching 26
3.7.1 Analytical Method 27
3.7.2 Graphical Method 27
3.7.3 Energy Plot 27
3.8 Assumptions Made 28
CHAPTER FOUR
DATA PRESENTATION AND RESULTS ANALYSIS
4.1 Data Used 29
4.1.1 PVT Data 29
4.1.2 Geological/Petro physical Data
29
4.1.3 Relative Permeability Data
30
4.2 Results and Discussion for Field Z 30
4.2.1 Brief history of Field Z 30
4.2.2 Results Obtained for Z 31
4.2.3 Results of History Matching 31
4.3 Results for Field Y 34
4.3.1 Brief History of Field Y 34
4.4 Summary of Volumes and Other Reservoir Parameters
37
4.5 General Result Analysis and Discussion 38 CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION
5.1 CONCLUSION 41
5.2 RECOMMENDATION 43
NOMENCLATURE 44
REFERENCES 45
APPENDIX 48
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background of Study
The origin of petroleum is attributed to the decomposition of organic matter associated with sediments. The type and amount of petroleum generated in a basin depends on the nature of the organic matter in the sediments, the abundance of the organic matter, the extent to which the organic matter was matured during burial and the nature and type of environment in which the organic matter accumulated.
The amount of organic matter in the source rock determines the volume that would be converted to petroleum and some fractions of this volume do not migrate into the reservoir for eventual trapping.
The petroleum engineer is often faced with the challenges of accurately determining this volume of hydrocarbon that is contained and the volume that is economically recoverable.
An accurate description of the volume of fluid present is very important in quantifying the resources and selection of production techniques, production rates and overall management of the reservoir throughout its life and also enhances adequate schedule control. The information obtained is also the basis for resource development and plan making.
All reserve estimates involve uncertainty depending on the amount of reliable geological and/or production data available and the interpretation of those data. Because the geology of the sub-surface cannot be examined directly, indirect technique must be used to estimate the size and recoverability of the resources. While new technologies have increased the accuracy of these techniques, significant uncertainties still remain. These uncertainties reduce from the exploratory stage to the ultimate recovery and abandonment as a result of acquisition of more data that enhance description of the reservoir. The level of uncertainty in making such estimates is affected by the following factors
(Olatunige G, 2007)
- Reservoir type
- Source of reservoir energy
- Quantity and quality of the geological, engineering, and geophysical data
- Assumptions adopted when making the estimates
- Available technology and
- Experience and knowledge of the evaluator.
The accurate determination of this volume forms the basis for investment into any project and would also inform the selection of production techniques, production rates and overall management of the reservoir through its life and also to enhance adequate schedule control.
Over the years reserves have been underestimated or overestimated leading to huge loss of revenue, resources, higher operational cost, abandonment of unrecovered petroleum resources and some other times affected economic plans with respect to production rates.
The Saudi Aramco put the reserve of Saudi Arabia at 209 billion barrels, In May, 2006 Saudi Aramco announced the details of an increment in the production capacity to 12.5mbbl/d by 2009 and 15mbbl/d by 2020. In December, 2006 it reported that the existing fields sustained an average decline of 6 to 8 percent above estimation requiring about 700,000bbl additional capacity each year to compensate for the natural decline. The rapid depletion was reportedly due to higher water cut which affected her budgeting and planning.
In January 2002, Shell was disclosed to have erroneously overestimated her reserves by 20% which resulted in a sharp fall in the shares of the group’s two holding companies; Royal Dutch Petroleum Company of Netherland and Shell transport and trading company of London
Many volume estimation approach has been used and developed for estimation of reserve including; analogy, volumetric, decline analysis, material balance calculations etc.
This research work uses two major approaches; the volumetric and the material balance methods to determine the volume of oil in place in order to compare the STOIIC values and analyse causes for the possible variations resulting from the estimates. An extension is also made to determine the volume using the flowing material balance approach which is an extension of the material balance method. It is important to adopt two or more methods of reserve estimation so that adequate comparism can be made to arrive at an accurate estimate of the reservoir.
The volumetric method provides a convenient approach to determining the oil in place per acre foot of the bulk reservoir rock. The estimation with this approach are often based on known geological factors from other areas thought to be sufficiently similar to the area under study applied to a reservoir description based on site specific interpretative data. The volumetric method entails determining the physical size of the reservoir, the volume with the rock matrix and the fluid contact within the void space. This provides an estimate of the hydrocarbon in place from which ultimate recovery can be established by using the appropriate recovery factor. Each of the factor used in the calculation have inherent uncertainties that when combined cause significant uncertainties in the reservoir estimate.
The material balance is based on the principle of the conservation of mass:
Initial HC originally in place = fluid produced + remaining fluid in place.
Material Balance is a volumetric balance which states that since the volume of a reservoir (as defined by its initial limits) is a constant, the cumulative observed production, expressed as an underground withdrawal, must equal the expansion of the fluids in the reservoir resulting from a finite pressure drop. In principle, it consist of producing a certain amount of fluids, measuring the average reservoir pressure before and after the production, and with the knowledge of PVT properties of the system, calculate a mass balance.
The material balance uses actual reservoir performance data. In order to generate a traditional material balance plot, the well is shut-in at several points during its producing life to obtain the average reservoir pressure. However, this is sometimes impractical and usually, the duration of shut-I is often not enough to obtain an accurate measurement. The flowing material balance uses the concept of boundary dominated flow or pseudo-steady state flow, as well as flowing pressures and rates to calculate original hydrocarbon-in place. The magnitude of various factors in the material balance equation indicates the relative contribution of different drive mechanisms at work in the reservoir.
The flowing material balance shows that the reservoir pressure could be obtained from flowing pressures for wells producing at a constant rate. Knowing the flow rate and the flowing sandface pressure at any given point in time converts the measured flowing pressures that exist in the reservoir at that time. Once this is done, the classical material balance equation becomes applicable and a conventional material balance plot can be generated using this calculated average reservoir pressure and the corresponding cumulative production rate to calculate the oil in place.
The procedure described by the flowing material balance does not require the well to be shut in instead it utilizes information obtained from standard good operating practices (production and flowing pressure) to quantify the oil in place. The flowing material balance is the p/z plot of the flowing pressure (as opposed to the shut in average reservoir pressure) versus cumulative production. A straight line can be drawn through the data and then a parallel line drawn through the initial oil in place.
1.2 STATEMENT OF PROBLEM
Due to the uncertainties and complexities that characterize accurate description of the volume of fluid present which forms a very important part of reserve exploration and development, adequate management of the reservoir, guide for the selection of production techniques and rates, schedule control and also the basis for development policy and decision making and the consequent loss of revenue, resources, time and disruption of planned production rates and economic plans if wrong estimates are made, there is need for careful and adequate study to obtain values that would enable comparative analysis and reveal the parameters or factors that hinder accurate reserve quantification, at the same time recommend a sequence that would address the problem.
The world’s largest third oil company (Shell), in terms of production after more in depth study trimmed its oil reserve from 19.5 billion estimated at December 2002 to 15.6 billion cutting its reservoir life from 13.4 years to 10.6 years
The studies and reviews indicated that the proved reserve disclosed did not in all cases properly reflect the maturity of development project but overestimation. According to J.P Morgan, the bulk of the downward reserve adjustment stems from overestimation on the fields in Nigeria and Australia (over 50% adjustments). Over one billion of oil equivalent proven reserve was overestimated in Nigeria.
No doubt, several research work have been carried out with high degree of success, but they may not have predicted rightly the causes militating against the wrong estimate commonly encountered and a reliable approach that would adequately resolve it as it relate to the Niger Delta.
1.3 OBJECTIVES OF THE STUDY
This project is aimed at using the data obtained from the oil fields in the Niger Delta to:
- Obtain the stock tank oil in place for reservoir using the volumetric, and material balance
- Compare the results obtained; identify possible reasons for varience
- Identify the constraints and challenges in these methods of estimation that results in the discrepancies observed in the estimated values, their relevance, also to recommend a possible sequence of estimation approach for a particular type of reservoir.
1.4 SIGNIFICANCE OF THE STUDY
It is expected that the analysis from this estimation will be useful in minimizing the various errors militating against precise reserve estimation resulting to underestimation or overestimation of reserve. It would also be able to recommend a more robust, cost effective and time saving approach and possible sequence that would enhance a more accurate determination of oil in place.
1.5 SCOPE OF THE STUDY
As a result of stringent measures in obtaining data from multi-nationals and the limited time for this research, only oil fields in the Niger Delta was considered and the result and recommendations were based on the few data collected.
COMPARATIVE ANALYSIS OF RESERVE ESTIMATION USING VOLUMETRIC METHOD AND MBAL ON NIGER DELTA OIL FIELDS
