Abstract

Other things being equal the minimum footage drilling cost will ensue when penetration rates approach a maximum value. penetration rates approach a maximum value. One of the major factors controlling optimum penetration rates is the mud-column pressure. In penetration rates is the mud-column pressure. In planning a drilling program for drilling recent planning a drilling program for drilling recent sediments, it is critical to know whether a maximum penetration rate will occur when the differential pressure [difference between formation pressure and dynamic mud-column pressure] is overbalanced, balanced or pressure] is overbalanced, balanced or underbalanced.

A review of literature shows that from laboratory drilling tests the maximum penetration rate likely occurs at a balanced-pressure penetration rate likely occurs at a balanced-pressure condition. Rates tend to decrease when either an underbalanced or overbalanced condition is established. Results of single chisel tests tend to support these results.

Recently published field results are apparently incompatible with the above work in that they report drilling rates tending to increase as formation pore pressures exceed the dynamic mud-column pressure. The dynamic mud-column pressure has been defined in these reports as:

PP = PDYN = P STATIC + ABP PP = PDYN = P STATIC + ABP where pp = formation pore pressure, psiPSTATIC= static mud-column pressure, psiABP = annulus pressure drop due to flow, psi.

A review of known data suggests a balanced pressure drilling condition actually occur pressure drilling condition actually occur when

PP = PDYN = PSTATIC + ABP + PI + PR,

where PI = Impact pressure in a vertical direction under the bit due to bitrotation, psi. PR = impact pressure in a vertical direction under the bit due to bitrotation, psi.

Using this relationship, recently reported field tests become compatible with previously reported work and it is possible to conclude.

  1. A balanced pressure drilling condition can be defined as:

    PP = PDYN = PSTATIC + ABP + PI + PR

  2. The maximum penetration rate under many drilling conditions occur when formation pore pressure and dynamic mud-column pressures pore pressure and dynamic mud-column pressures are equal.

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