Predicting the potential rate of penetration (ROP) for all bit types, fixed and roller cone, has been accomplished by applying specific energy theory and mechanical efficiency as a function of rock strength. After the apparent rock strength is accurately determined from either open-hole log analysis or core measurements, the ROP based on work and power input into the bit efficiency can be calculated. The influence of bit type and torque is represented by the parameters of coefficient of sliding friction and mechanical efficiency. Specific energy theory is not new and has been used for bit performance assessment for years. The technical paper will describe a unique and revolutionary approach to establish relationships for the sliding coefficient of friction, mechanical efficiency, weight on bit, and rpm as a function of rock strength, and then to use these relationships to predict a reasonable and achievable ROP with the associated bit torque for all bit types. Because the development of the model requires extremely accurate and controlled drilling data, we performed full scale simulator tests, using several rock types with confined compressive strength (CCS) between 5,000 psi to 70,000 psi. The CCS approach better represents the apparent rock strength to the bit. The use of CCS has enabled us to identify key relationships and to develop globally applicable algorithms for bit performance prediction that require minimal or no calibration. The ROP model has been field tested and validated on wells drilled and the predicted ROP value closely correlates with the actual ROP value for all bit types. The paper will illustrate how drilling performance can be enhanced by selecting the optimum drilling bits for any well or hole section during the planning phase or operation phase for optimization, thereby eliminating the learning curve and reducing both drilling time and cost.

The Serang Field, offshore East Kalimantan, Indonesia was discovered in 1973. Production started in 1993. Although it is a mature field with 120 wells, drilling still faces challenges in the form of severe lost circulation and stuck pipe events. Side tracks often need to be drilled and drilling budget overruns can be significant. The 2006 drilling campaign of 5 wells saw the loss of several BHAs and needed 4 sidetracks, due to stuck pipe events. In order to improve the drilling operation and to uncover root causes of the losses and pack-offs, a detailed geomechanical evaluation was conducted. The study began with a comprehensive data audit and drilling event review. All the captured information and log data was then used to create a Mechanical Earth Model (MEM) for wellbore stability planning of future wells. Key findings of the study were that lost circulation occurred in high permeability zones in shallow carbonates (8-1/2" intermediate hole section) and were not caused by drilling induced fractures. A contingency plan was developed to drill to section TD with sea water, should losses occur. Breakouts, aggravated by low mud weights, lost circulation events, high deviation and slim hole contributed to poor hole cleaning, which caused the packoff incidents in the 6 1/8" sections. Safe and stable mud weight windows were established to mitigate hole collapse and stuck pipe. Since losses can occur even at low mud weights but do not usually have severe consequences, the team decided it was more important to focus on avoiding breakouts and improve hole cleaning with higher mud weights, improved drilling fluids with stress cage system to drill through depleted reservoirs, controlled drilling, ECD management to prevent formation breakdown and the use of Rotary Steerables to improve hole cleaning. The recommendations of the study were incorporated during a 3 well drilling campaign in 2009. All 3 wells were drilled and completed within budget and without stuck pipe incidents.

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