In this paper, a new drilling optimization procedure is presented that is designed to improve the drilling efficiency with positive displacement motors (PDMs) and PDC bits. This developed optimization method is based on predicting rate of penetration (ROP) from PDM outputs for any PDC bit design. More specifically, optimization is done for a hole section and optimum values of weight on bit (WOB) and surface RPM are obtained for the section. For given flowrates, estimated values of optimum WOB and surface RPM are used to calculate the corresponding motor differential pressures and the foot by foot ROP values. Also, the method is used to show how improper operational parameters selection can affect total drilling time. A case study was done to consider different PDMs with different lobe configurations and a set of fixed operational parameters.
From the new optimization technique, a pre-estimated motor differential pressure versus depth log can be generated for the optimized operational parameters. This information can further be used in hydraulics optimization and proper selection of bit nozzles. The optimization method can not only be applied to determine optimum operational parameters in drilling for one motor, it is also can be used to select the optimum PDM to drill a section most economically.
The presented method is verified by generating a confined rock strength log based on drilling data for a previously drilled well in Alberta. This foot by foot strength log is compared to a confined rock strength log generated as a follow up analysis by a commercially available drilling simulator package. Also, PDM differential pressure log is generated and compared to field recorded on bottom differential pressure values.
In this paper, a method to optimize the drilling operation with PDMs is presented with a field sample application. It is concluded that consideration of PDM performance/selection drilling planning phase will help to perform a safe and cost effective operation by preventing motor stalls and maintaining highest average ROP for the section. It also proves that by optimizing WOB and surface RPM values for a constant mud flow rate and pre defined bit wear at total depth, a maximum average ROP for the section can be reached for any PDM. The PDM with the lowest $/ft can then be selected based on the analysis.
Drilling simulation has gained a leading position in the well planning and the follow up process. The capability of applying different drilling scenarios in simulators is a powerful tool for drilling engineer to prepare an optimized drilling plan regarding basic criteria such as cost, time and safety. The rising application of Positive Displacement Motors (PDMs) in recent directional and horizontal drilling operations creates a demand for a tool which enables drilling engineer to simulate these operations in a pre-planning mode. Despite of similarities existing between conventional drilling simulation and PDM drilling simulation, there are some differences which can alterthe approach and goals. These dissimilarities are resultant of inseparable characteristics of PDM which exposes limitation on the drilling system hydraulics.
