Oil & Gas Institutes in Kerala,Kochi,Pathanamthitta,Kollam – Techshore Inspection Services – PRESURE INVOLVED IN DRILLING

PRESSURE EXERTED BY THE FLUID THAT GET ‘S TRAPPED INSIDE THE FORMATION ROCK IS KNOWN AS FORMATION PRESSURE.DEPENDING UPON THE FORMATION PRESSURE GRADIENT, IT IS CLASSIFIED INTO TWO PRESSURES:-

  • NORMAL FORMATION PRESSURE
  • ABNORMAL FORMATION PRESSURE

NORMAL FORMATION PRESSURE

BY THE DEPOSITION OF ROCK LAYER OVER ONE OTHER,WATER FROM FORMATION IS SQUEEZED OUT OF THE PORES DUE TO OVERBURDEN,THIS PHENOMINA IS CALLED AS COMPACTION.AS THE FORMATION WATER ESCAPE THROUGH THE FORMATION ROCK,IT FORMATION PRESSURE ALSO CHANGES DOWN TO NORMAL VALUE.MOSTLY ACCEPTED NORMAL VALUE IS RANGING  FROM 0.433 psi/ft TO 0.465 psi/ft.THUS THE PRESSURE WITH IN THIS RANGE IS KNOWN AS NORMAL FORMATION PRESSURE.

         ABNORMAL FORMATION PRESSURE

THE FORMATION PRESSURE WHICH RANGES ABOVE THE NORMAL VALUE ,ie 0.465 psi/ft  IS KNOWN AS ABNORMAL FORMATION PRESSURE.IT IS CAUSED BECAUSE OF THE REASON THAT DUE TO OVERBURDEN OF FORMATION WATER IS FORMATION ROCK TRIES TO ESCAPE ,THIS GET TRAPPED BY IMPERMEABLE FORMATION THAT DOES NOT ALLOW ITS NORMAL ESCAPE,HENCE ABNORMAL FORMATION PRESSURE IS FORMED.THE CAUSES OF ABNORMAL PRESSURE IS DUE TO.

  1. UNDER COMPACTION
  2. ARTESIAN EFFECT
  • FAULTING
  1. SALT DOME
  2. GAS CAP EFFECT
  3. CHARGED SANDS

 HYDROSTATIC PRESSURE

THE PRESSURE EXERTED ON THE FORMATION TO OVER COME THE FORMATION PRESSURE BY STATIC COLUMN OF MUD IS KNOWN AS HYDROSTATIC PRESSURE.HYDROSTATIC PRESSURE IS MAINTAINED BY  THE DENSITY OF MUD.HYDROSTATIC PRESSURE OF THE MUD DEPEND UPON THE TRUE VERTICAL DEPTH OF THE WELL AND MUD DENSITY.HYDROSTATIC PRESSURE IS EXPRESSED IN psi.

HYDROSTATIC PRESSURE = TVD (In ft) * MUD DENSITY (ppg) * 0.052

WHERE, 0.052 IS THE CONVERSION CONSTANT

PRESSURE GRADIENT

WHEN THE PRESSURE OF A FLUID IS CALCULATED FOR A GIVEN UNIT LENGTH IT IS KNOWN AS PRESSURE GRADIENT.PRESSURE GRADIENT IS THE PRODUCT OF MUD DENSITY (in ppg),CONVERSION FACTOR AND UNIT LENGTH.

PRESSURE GRADIENT = MUD DEMNSITY (in ppg) * UNIT LENGTH (in ft) * 0.052

                   SWAB AND SURGE PRESSURE

WHEN THE DRILL STRING IS LOWERER FOR DRILLING PURPOSE OR THE AT THE TIME OF TRIPPING IN, THE STRING EXERT A PRESSURE ON THE MUD COLUMN, THIS IS KNOWN AS SURGE PRESSURE.SURGE PRESSURE IS ALWAYS TAKEN AS +VE.

WHEN THE DRILL STRING IS RAISED DURING DRILLING PURPOSE OR THE AT THE TIME OF TRIPPING OUT, THE STRING LOSES A PRESSURE ON THE MUD COLUMN, THIS IS KNOWN AS SWAB PRESSURE. SWAB PRESSURE IS ALWAYS TAKEN AS –VE, BECAUSE THERE IS LOSE OF PRESSURE.

                       BOTTOM HOLE PRESSURE (BHP)

BOTTOM HOLE PRESSURE IS DEFINED AS THE SUM TOTAL OF ALL THE PRESSURE THAT ACTS ON THE BOTTOM OF HOLE.THAT IS THE PRESSURE EXERTED AT THE BOTTOM OF THE HOLE.

 

BHP = HYDROSTATIC PRESSURE + SURFACE PRESSURE +/- DYNAMIC PRESSURE

DYNAMIC PRESSURE IS THE PRESSURE EXERTED BY THE PIPE MOVEMENT, ie SWAB OR SURGE PRESSURE OR EITHER DUE TO FLUID MOVEMENT, ie BY ANNULAR PRESSURE LOSSES.

BHP DURING DIFFERENT WELL OPERATIONS:-

  • DURING DRILLING

BHP = HYDROSTATIC PRESSURE + ANNULAR PRESSURE LOSSES

 

  • WHEN NOT CIRCULATING

BHP =   HYDROSTATIC PRESSURE DUE TO MUD COLUMN

  • WHILE TRIPPING IN

BHP = HYDROSTATIC PRESSURE + SURGE PRESSURE

 

  • WHILE TRIPPING OUT

BHP = HYDROSTATIC PRESSURE – SWAB PRESSURE

MAXIMUM ALLOWABLE ANNULAR SURFACE PRESSURE (MAASP)

MAXIMUM ALLOWABLE ANNULAR SURFACE PRESSURE IS DEFINED AS THE MAXIMUM PRESSURE AT SURFACE THAT A CASING CAN HOLD WITH COMPLETE SAFETY.

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