1. Porosity MatrixPores Porosity= Pore Space Volume Bulk Volume.
Porosity Generations Primary porosity: HPP 40%.HPP 40%. At 10000’, shale porosity reaches 5%.At 10000’, shale porosity reaches 5%. Secondary porosity: Those ones created after initial deposition.Those ones created after initial deposition. Either by dissolving or fracturing.Either by dissolving or fracturing.
2. Saturation Is that fraction of pore volume that is filled with a given fluid.Is that fraction of pore volume that is filled with a given fluid. A 20% sw = 20% of the pores are filled w/ water & 80% w/ other fluid which may be oil or gas.A 20% sw = 20% of the pores are filled w/ water & 80% w/ other fluid which may be oil or gas.
3. PERMEAPLITY Is a measure of the ease with which a fluid will flow through a formation.Is a measure of the ease with which a fluid will flow through a formation. Q= k*a* (p1 – p2)/v*l.Q= k*a* (p1 – p2)/v*l. K = QVL/A(P1-P2) in millidarcies.K = QVL/A(P1-P2) in millidarcies. The larger the porosity the higher the permeabilty.The larger the porosity the higher the permeabilty.
4. Overburden Overburden load:Overburden load: Is the combined weight of the formation matrix and the fluids overlying a formation. Overburden pressure:Overburden pressure: Is the pressure exerted by the overburden load upon underlying formations.
Compaction It is PRINCIPALY a process of mechanical rearrangement
Matrix Stress Is the resistance of the formation matrix to compaction expressed in psi or psi/ft.
Overburden Gradient This is an expression of the pressure unit per unit depth. e.g. psi/ft.psi/ft. ppg..ppg..
5.Formation Pressure ((Is the Fluid Pressure Found Within the Pore Spaces of the Formation)) Can Be Expressed As an Average Vertical Pressure or Equivalent Mud Weight. E.G. psi, ppg, G/cc…Can Be Expressed As an Average Vertical Pressure or Equivalent Mud Weight. E.G. psi, ppg, G/cc…
Formation pressure greater than the magnitude of the hydrostatic pressure of a column of pore fluid that reaches from the surface to the vertical depth of the formation.
1.Concept of Continuous Deposition This concept assumes that the compaction rate of any formation is the same as for all formations of a given type. This mean that shales for example deposited at different times and buried to different depths are considered to have compacted at the same rate as long as the formation pore pressure remained normal.
2. Pressure Seal ((A zone of low permeability acts to trap the pore fluids within a formation.)). May be either physical or chemical.May be either physical or chemical. Restrict the vertical an lateral movement of pressure.Restrict the vertical an lateral movement of pressure. e.g. evaporates, faults..e.g. evaporates, faults..
3.Equilibrium. S = Mv + P S= overburden. Mv= Vertical matrix stress. P= formation pore pressure.
4.Transition Zone This is an argillaceous sequence of gradual pore pressure increase above a permeable high pressure formation.
Causes of Abnormal Pressure Rapid subsidence and sediment accumulation. Thermal expansion. Tectonic movement. Reservoir altitude. Clay digenesis. Repress ring of shallow reservoirs.
Signs of Abnormal pressure Normalized drilling rate (Drilling models). Change in rotary torque. Change in drag. Shale density. Gas analysis. Flow line temperature. Size and shape of cuttings. Open hole logs.
Size and shape of cuttings A cutting drilled near balanced conditions will be larger and more angular in shape than one which is drilled under conditions of greater overbalance.
Pore Pressure Calculations 1.Depth of seal FP=Sf-[(Se-n)/(TVDe/TVDf)] FP= fm pressure Sf= overburden at wanted depth Se= overburden at depth of seal. N= normal pp. TVDe= depth of seal TVDf= fm depth
Pore Pressure Calculations 2. Eaton equation FP=Sf-(Sf-n)*(DCo/DCn)^m FP= fm pressure Sf= overburden at wanted depth N= normal pp. DCo= observed dxc DCn= normal dxc
III- FRACTURE PRESSURE Is the pressure needed to make a fracture in a formation. The orientation of the produced fracture depend on the orientation of the principal stress of the fracture point.The orientation of the produced fracture depend on the orientation of the principal stress of the fracture point.
Principal Stresses At any point in the formation there exists a stress regime consisting of three perpendicular stresses Sig1 maximum, sig2 intermediate, sig3 minimum.
Fracture development The fracture is developed perpendicular to the minimum stress
Poisson Ratio The ratio of transverse strain to axial strain V=change in breads/original breads change in length/original length