High Resolution Overpressure Prediction from Seismic Velocity and Field Derived Formation Parameter in the Onshore Niger Delta.

ABSTRACT Occurrence of overpressured formation is common in Tertiary basins (e.g. Niger Delta), which can result in geomechanical problems, such as “kicks”, loss of expensive drilling mud, blowout, stuck pipe, reservoir quality damage, wellbore instability etc. if it is not accurately predicted prior to drilling. This study did a critically re-evaluation of the two commonly utilized methods and practices involve in pore pressure work in the industry. The main aim was to accurately predict pore pressure ahead of drilling using seismic velocity. This involve analyzing ten offset wells to probing the overpressure origin, deriving a site specific Bower’s parameter, establishing a relationship between velocity and pressure using well data, and accurate velocity analysis for pore pressure prediction. The velocity – density crossplot for all the wells analyzed showed a compaction disequilibrium as the origin of the overpressure. The lithology log derived from GR showed a typical sand dominated Benin Formation to a depth of about 9,200ft (TVD) and beyond that depth a shale dominated Agbada Formation. The overburden gradient ranges from 0.78 psi/ft. to 0.96 psi/ft. contrary to 1.0 psi/ft. fixed gradient sometimes used. Bower’s parameter (A = 7.5, B = 0.68) were derived specifically for the field, instead of the regional parameters (A = 8, B = 0.57) in use. The velocity pressure relationship established using the offset well data gave a good match with the measured pressure data where it’s available, which affirms the reliability of using acoustic velocity for pore pressure prediction. The onset of overpressure (at 10,200ft) corresponds with the 15.0 maximum flooding surface (which hinders fluid expulsion), further confirms compaction disequilibrium as the origin of the overpressure. Bower’s method at the shallow section (for Kappa 39 ST 1) gave a pressure ranging from 0.54 psi/ft. to 0.66 psi/ft. which is far greater than the mud pressure of 0.52 psi/ft. to 0.53 psi/ft. that was used in drilling the well (i.e. overestimate pore pressure). Whereas Eaton’s method gave a range of 0.44 psi/ft. to 0.46 psi/ft. for the same depth interval, which evidence that the offset well was drilled overbalanced, and not underbalanced as suggested by Bower’s prediction at that shallow depth. Seismic velocity analysis for pore pressure prediction and calibration with well data results in a good match of predicted pressure from seismic data with measured pressure for the offset well, which strengthens the assurance that seismic velocity can be relied upon to give an accurate pore pressure estimate where log data are not available. A safe drilling mud window interval between predicted pressure and fracture pressure was established also. It is therefore important to suggest based on the result of this work that Eaton’s and Bowers’ methods should be integrated for any deep exploration well, such that, the result of Eaton’s method can be relied upon at shallow section, where Bowers method tend to overestimate pore pressure. The origin of overpressures at the deeper section of the Onshore Niger Delta is an unresolved question, which should be given a priority as more deep exploration wells are been drilled.

TABLE OF CONTENTS

TITLE PAGE I

CERTIFICATION II

APPROVAL PAGE III

DEDICATION IV

ACKNOWLEDGEMENT V

ABSTRACT VI

TABLE OF CONTENTS VII-XII

LIST OF FIGURES XIII-XV

LIST OF TABLE XVI

CHAPTER ONE

1.0 Introduction 1

1.1 Problem statement 6

1.2 Justification 6

1.3 Project aim and Objectives 6

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1.4 Scope of Work 7

1.5.1 Impact of Overpressure on Drilling 7

1.5.2 Impact of Overpressure on Petroleum Systems 10

1.6 Regional Geology of Niger Delta 10

1.7 Study Area/Data Set 19

1.8 Worldwide Example of Overpressure 22

1.9 Overpressures in Niger Delta 22

CHAPTER TWO

2.0 Review of previous works 24

2.1.1 Eaton 25

2.1.2 Zhang 27

2.1.3 Bowers 27

2.1.4 Miller 30

2.1.5 Lopez et al 30

2.1.6 Hoesni 30

2.2.0 Introduction to the Pore Pressure System 32

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2.2.1.0 Pressure 32

2.2.1.1 Pore or formation pressure 32

2.2.1.2 Normal or hydrostatic pressure 33

2.2.2 Porosity & permeability 34

2.2.3 Compaction 34

2.2.4 Normal compaction trend (NCT) from porosity logs 35

2.2.5 Normal compaction trend (NCT) from sonic log 35

2.3.0 Overburden pressure (OVB) 37

2.3.1 Kick 38

2.3.2 Effective stress 38

2.3.3 Fracture pressure 39

2.4 Mechanisms of Overpressure Generation 41

2.4.1 Stress related mechanism 42

2.4.1.1 Disequilibrium compaction 42

2.4.1.2 Tectonics stress 44

2.4.2.0 Secondary pressure mechanism 44

2.4.2.1 Fluid expansion unloading mechanism 44

2.4.2.2 Aqua-thermal expansion 45

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2.4.2.3 Clay diagenesis 46

2.4.2.4 Hydrocarbon maturation/generation 47

2.4.2.5 Oil to gas cracking 48

2.4.3.0 Fluid movement and buoyancy mechanisms 49

2.4.3.1 Osmosis 49

2.4.3.2 Hydrocarbon buoyancy (density contrasts) 50

2.5.0 Methods of pore pressure prediction 56

2.5.1 Seismic method 56

2.5.2 Drilling rate 57

2.5.3 Shale density 57

2.5.4 Temperature measurements 59

2.5.5 Wire line logs (petrophysics) 59

2.5.6 While drilling method 62

2.5.7 Direct pressure measurement 62

2.5.8 Basin modeling 62

2.6.0 Data description 62

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2.6.1. Wire line logs 62

2.6.2 Caliper log 63

2.6.3 Natural gamma ray tool 63

2.6.4 Sonic log 64

2.6.5 Resistivity and conductivity log 65

2.6.6 Density log 65

2.6.7 Seismic data 67

2.6.8 Formation pressure measurement 67

2.6.9 Reservoir characterization instrument (RCI) 68

2.7.0 Repeat formation tester (RFT) 68

2.7.1 Modular formation dynamics tester (MDT) 71

2.7.2 Leak off test (LOT) 71

CHAPTER THREE

3.0 Methodology 72

3.1 Data Loading and Quality Checking 72

3.2.1 Overburden stress 72

3.2.2 Shale volume 73

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3.3 Workflow 74

3.4.1 Log filtering 75

3.4.2 Transferring shale intervals to porosity indicating datasets 75

3.4.3 Normal compaction trend 75

3.4.4 Vp – Rho cross plot 76

3.4.5 Deriving a relationship between velocity and pore pressure using Eaton’s and Bower’s

method 77

3.5.0 Seismic velocity analysis for pore pressure prediction 77

3.5.1 Seismic velocity calibration with well data 77

3.5.2 Pore pressure/ fracture pressure prediction 78

CHAPTER FOUR

4.0 Results and Discussion 79

4.1.0 Overburden pressure 79

4.1.2 Shale volume 81

4.1.3 Log filtering 81

4.1.4 Vp shale filtered trend 81

4.1.5 Normal compaction trend 87

4.1.6 Vp – Rho cross plot 90

4.1.7 Offset well prediction 96

4.2.0 Velocity analysis for pore pressure prediction 98

4.2.1 Seismic to well calibration 98

4.2.2 Seismic pore pressure prediction for the offset well (kappa 39 st 1) 101

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4.2.3 Seismic pore pressure prediction for the exploration deep well

(kappa deep IX) 101

CHAPTER FIVE

5.1 Summary 107

5.2 Conclusion 108

5.3 Recommendation 109

REFERENCES 110