21-10-2016, 12:21 PM
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Abstract
Any method that intends to minimize the uncertainty and enhances the preparedness to deal critical situation has always drawn attention in subsurface exploration. Showing no exception, in oil and gas drilling industry, pore pressure prediction has always been of immense importance as it provides insights to deal with probable high pressure zone, which are capable of creating disasters. The present study is an attempt to minimize such uncertainty in one of the exploration block of Indian Oil Corporation Limited in Cambay Basin where data availability is limited. With the limited data available in the nearby wells within the block and outside it, a study has been carried out which indicates the study area is likely to be in normal hydrostatic pressure regime. Accordingly, all drilling plans have been lined up. However, the model will be updated after drilling of the 1st well.
Concept
Formations penetrated during the drilling of an oil and gas well exhibit pressure which may vary in magnitude depending on geological setting, depth of occurrence, location and proximity to other structures.
Subnormal pore pressures are encountered less frequently than abnormal pore pressure and are often developed long after the formation is deposited. Subnormal
- σv is the vertical stress / overburden stress at depth TVD
- σb is the bulk density (including the water section above sea floor.
- g is the gravitational constant.
The input for the overburden stress estimation is the "best density", which is often a composite log from actual density measurement and a synthetic density where no density log is available (RBCP_COMP).
Pore Pressure
Pore pressure is the pressure on the fluids in the pore spaces of the rock. However, the above definition stands good for a reservoir rock like sandstone. For shale that has extremely small pores, intense chemical effects, dominance of bound water, make this easy definition unsatisfactory. Better answer for shale pore pressure is the fluid pressure in permeable zone in long-term equilibrium with the shale. This answer reflects the hazard that elevated shale pressures bring to drilling operations.
Depending on the magnitude of the pressure, it can be described as either being normal or abnormal or subnormal.
A. Normal Pore Pressure - Normal pore pressure is the pressure exerted by the column of the fluid extending from the surface to the subsurface being considered.
pressure may have natural cause related to the stratigraphic,
tectonic and geochemical history of an area or may have been caused artificially by the production of reservoir fluids.
Depositional effects
Diagenetic effects
Tectonic effects
Structural causes
Thermodynamic effects
D. Importance of Pore Pressure - Uncertainties cannot be avoided but preparedness to tackle the situation becomes very important. Pore pressure prediction is very important to determine the mud weight with which drilling is to be carried out. Improper analysis may lead to the following:
a. If the pore pressure is underestimated i.e. if mud weight is less than pore pressure, problems like caving, flows from Formation to wellbore, kicks, blowout, all threatening hole stability and safety issues.
b. If pore pressure is overestimated and actually if it touches fracture pressure of a Formation again hole stability is threatened and Formation is damaged. Furthermore, high mud weight damage the Formation (reservoir) hence basic purpose of well is not met.
c. Improper analyses of pore pressure, fracture pressure and overburden pressure leads to drilling complications like stuck pipe/lost in hole, low ROP (rate of penetration)