Case histories from Saudi Aramco fields are cited to demonstrate how enhanced rock strength calculations from open hole logs and core data, has aided Polycrystalline Diamond Compact (PDC) bit optimization. Cost per foot reduction was a challenging target to achieve with rotary, turbine or Positive Displacement Motor (PDM) in drilling deep, high temperature hard formations.
With the increasing number of rigs deployed to drill deep gas production and exploration wells, Saudi Aramco has utilized state of the art PDC bits available in the market to minimize drilling costs.
It is well understood that the deeper we drill, the higher the temperature and rock compressive strength. This results in higher mud weight for most cases, slower rate of penetration and thus frequent bit trips that add to the well cost.
The use of PDC bits has greatly expanded over the last decade. The reason for this is that PDC bits generally drill faster, stay longer on bottom than a roller cone bit, and perform well for a wide range of difficult drilling conditions. Moreover, they are less likely to leave junk in the hole since they have no moving parts. Typically, PDC bits drill twice as fast as roller cone bits and can often replace two or three roller cone bits in places where they are successful, Since 1989, PDC bit life has increased 115% to an average of 3629 ft by mid 1995. The 13.7% annual increase in the average operational life of a PDC bit clearly explains the growing popularity of these tools.
It is a common knowledge that PDC yields the best results in soft to medium formations. Unlike roller cone bits, PDC cutting action is by shearing the rock not crushing. Therefore, less weight on bit is required to shear the rock. This shearing action leads to a higher rate of penetrations in rocks like shales and limestone. PDC bits can be destroyed in hard formations such as granite or quartz or by nodular formations such as chert or pyrite.
The objective is to drill a well with a minimum feasible cost at the highest standards of safety and well integrity. This can be achieved by increasing the rate of penetration (ROP) and reduce the cost per foot (CPF) by using PDC bits on rotary, PDM motor and turbine.
This paper will present case histories from Saudi Aramco fields and illustrate how open hole logs, mud logs, offset bit records along with core data were utilized in analyzing the rock compressive strength and internal angle of friction to predict the drillability of the formation in order to optimize bit selection.