• Theoretical development of the rock strength correlation needed in the ROP modeling where both the overbalanced and underbalanced drilling condition is considered.

  • Prediction of bottom hole pressure for UBD operations with aerated or foam drilling

  • Correlation from the confined compressive strength (CCS) under either underbalanced or overbalanced drilling conditions to uniaxial drilling strength.

  • Verification using actual drilling data to predict ROP and compare it to field data from UBD operations using the ROP models.

Accurate knowledge of rock strength is essential for drilling optimization and rate of penetration (ROP) prediction. Conventional drilling simulators provide a tool to generate rock strength for the drilling engineer to further model and study the effect of different drilling parameters which can optimize the overall drilling process performance. Experience shows that the best correlation is generated from actual drilling data which consider the effect of drilling and bit design parameters on rock strength. The work presented herein focus on generating rock strength based on the geological and drilling data from offset wells to generate the rock strength for the underbalanced drilling (UBD) condition. The scope of the paper is divided into the following phases:The application of this work links a UBD bottom hole prediction program and drilling rock strength in a ROP drill bit model, which results in a UBD drilling simulator that is a great preplanning tool for drilling engineers.


Rock Strength Confinement Well planning by analysis of bit performance based on log-based rock strength has become standard practice in the drilling industry. There are accepted methods in the literature to calculate rock confined compressive strength (CCS) based on rock unconfined compressive strength (UCS) and pore pressure.

(Equation in full paper)


  • Annular frictional pressure losses

  • Mixture density

Estimation of bottomhole pressure (BHP) is one of the most important tasks during UBD. Accurate prediction of BHP is the key to successful underbalanced drilling operations. Two of the most important parameters in BHP calculation:These are very difficult to evaluate due to multiphase flow behaviors, where gas, liquid and solids flow simultaneously with different flow rates. The following criteria are used as basic guidelines for the development of foam and aerated drilling hydraulic optimization and BHP calculation program.

  • Back pressure needs to be kept as low as possible for a convenient BHP control.

  • In UBD operations, solid concentration is preferred to be kept at less then 4%. 3. Bottom hole Kinematical Energy per unit volume should be more than

  • (lb-ft/ft3) for better cutting removal.

  • The gas and liquid volumetric flow rate needs to be kept as low as possible to reduce the cost.

  • The aerated mud velocity should not be too high to avoid the wellbore erosion.

From the first law of thermodynamics and neglecting the velocity change in a vertical conduit: The BHP including hydrostatic and frictional pressures is then solved numerically using the equation (4).

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