I.Pressure Concepts Depth: Measured Depth (MD). True Vertical Depth (TVD). I.Pressure Concepts Force: ((Force = Mass X Acceleration)) DENISTY: ((= Mass (or Weight) Per Unit Volume)) Pressure • • • • • • ((Is an Expression of Force Per Unit Area)) Hydrostatic Pressure. Frictional Pressure. Annulus Pressure. Well Bore Pressure. Differential Pressure. Bottom Hole Pressure. Pressure Concepts Balance: ((Refer to the Relation Between the Formation Pressure and the Hydrostatic Head)) Balanced. Under Balanced. Overbalanced. II.ROCK PROPERTIES Porosity. Saturation. Permeability. Overburden. Formation pore pressure. Compaction. Matrix stress. 1. Porosity Matrix Pores Porosity= Pore Space Volume Bulk Volume. Porosity Generations Primary porosity: • HPP 40%. • At 10000’, shale porosity reaches 5%. Secondary porosity: • Those ones created after initial deposition. • Either by dissolving or fracturing. 2. Saturation • 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. 3. PERMEAPLITY • Is a measure of the ease with which a fluid will flow through a formation. • Q= k*a* (p1 – p2)/v*l. • K = QVL/A(P1-P2) in millidarcies. • The larger the porosity the higher the permeabilty. 4. Overburden • Overburden load: Is the combined weight of the formation matrix and the fluids overlying a formation. • Overburden pressure: Is the pressure exerted by the overburden load upon underlying formations. Compaction It is PRINCIPALY a process of mechanical rearrangement Factors Affecting Compaction Rate 1. 2. 3. 4. 5. 6. Rate of deposition. Tectonic forcess. Formation permeability. Lithology. Diagenesis. Osmosis. Overburden Overburden pressure Matrix Stress Pore Fluid Pressure 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. • 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… Normal Pressure Matrix Pores Expelled fluid Normal Pressure equal Formation pressure in magnitude to the hydrostatic pressure of a column of pore fluid that reaches from the surface to the vertical depth of the formation. Abnormal Pressure Matrix Pores Abnormal Pressure greater Formation pressure 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. • Restrict the vertical an lateral movement of pressure. • e.g. evaporates, faults .. 3.Equilibrium. S = Mv + P S= overburden. Mv= Vertical matrix stress. P= formation pore pressure. Equilibrium Overburden pressure Matrix Stress Pore Fluid 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. Normalized Drilling Rate 1. 2. 3. 4. 5. 6. 7. 8. Weight on bit. Rotary speed. Bit diameter. Bit type. Rock type. Mud properties. Differential pressure. Hydraulics. “d” Exponent R/60N= (12W/10^6D)^d -Log(R/60N)=d*log (12W/106D) -d= Log(R/60N)/ log (12W/10^6D) R=rop (ft/hr). N= rpm. W=wob (ib) D= bit diameter (in). Corrected “d” exponent Dc = d*(n/mw) N= normal pp Mw= mud weight Factors Affecting dc 1. 2. 3. 4. Bit wear Lithologic change Drastic changes in drilling parameters. Bit types. Flow Line Temperature Formation fluids tend to act as thermal barriers or insulators which impede the normal flow of heat from the earth’s core to the surface. Flow Line Temperature 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 1000 FP=Sf-(Sf-n)*(DCo/DCn)^m 2000 TVD (m) FP= fm pressure Sf= overburden at wanted depth N= normal pp. DCo= observed dxc DCn= normal dxc 1500 2500 Top Overpressure 3000 NCT 3500 0.1 1 Dc Exponent 10 Resistivity Sonic 1000 1500 1500 2000 2000 TVD (m) TVD (m) 1000 2500 re s pons e in cas ing Cycle Sk ipping Top of Ove rpre s s ure 2500 Top Ove rpre s s ure 3000 3000 NCT NCT 3500 3500 0.1 1 RILD (ohm.m) 10 100 Sonic (usec/m) 1000 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. 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 Fracture pressure Fr={V/(1-V)}*(S-P)+P Leak off Test Application of pressure to a formation to determine its fracture pressure, without fracturing the formation. Frp= MWt + LOP TVDt*.052 Thanks for your attention