For years attempts have been made in the drilling industry to increase the drilling efficiency and decrease the associated costs. The drilling efficiency can be evaluated by comparing applied energy, i.e., mechanical specific energy, with rock strength. The mechanical specific energy is defined as the energy required to destroy a unit volume of the rock. Over the years, this concept has been refined, and researchers proposed various models. Mechanical specific energy directly affects drilling efficiency, as excessive energy can lead to drill string vibrations and bit wear. In this study, a database was established by collecting drilling and log data from the Asmari formation in one of the oil fields of Iran. Various forms of specific energy were examined to develop the appropriate model based on operational conditions and the formation being drilled. Additionally, the confined compressive strength of the rock in the studied well was calculated. The results showed that the developed specific energy model provides a realistic energy value, as it includes all relevant parameters with an output close to the rock strength. Based on the comparison of mechanical specific energy with confined compressive strength, the optimal drilling parameters were determined: weight on bit ranges from 22.24 to 44.48 kN, flow rate ranges from 0.027 to 0.029 m3/s, torque ranges from 2522 to 3091 N·m, and rotational speed ranges from 160 to 180 rpm. Also, an inefficient drilling zone was identified in the studied well, where excessive applied energy compared to rock strength led to the drill bit damage and a significant reduction in penetration rate. The results highlighted the importance of drilling efficiency estimation in the drilling process, where an economic and technically feasible decision can be made by comparing the surface input energy with the rock strength.
Hole cleaning is considered as one of the most important drilling fluid functions. An efficient hole cleaning ensures a reliable well drilling practice with minimum troublesome problems. In this study, two main steps of hole cleaning, i.e., cuttings removal from under the bit and cuttings transport to the surface are discussed based on the drilling data of a shale formation. The traditional models for optimization of each step are presented. As the models require variety of input data, which are usually subjected to some extent of errors and uncertainties, the output of the model is also an uncertain parameter. Using Monte Carlo simulation, a simple probabilistic study was conducted to quantify the certainty level of the obtained results. Based on the result of this study, it is shown that for the proposed well, a good hole cleaning is expected. However, a more reliable decision for further hole cleaning optimization should be made considering the results of uncertainty analysis.
Article is devoted to creation of new compositions of process solutions for lining of directional wells at high temperatures. Developed compositions provide high durability characteristics of cement stone. It is shown that with an increase in packing density of cementing slurry components durability characteristics increase and porosity and permeability of cement stone decrease, whereas an increase in temperature and pressure leads to a significant increase in compressive and bending durability, which is associated with presence of quartz in them. It has been established that introduction of special structure-forming additives to the composition of developed cementing solutions allows formation of sedimentation-resistant cement systems that can provide an increase in durability characteristics of cement stone and, in general, quality lining of directional wells. Study of rheological properties of developed cementing compositions showed that the systems have high yield strength at increased temperatures and pressures. Developed compositions of water-based spacer fluids increase the cleansing degree for both casing and rocks surfaces from mud and clay cake residues, which improves the cementing quality of oil and gas wells. Mechanism for increasing the washing ability of spacer fluids and durability characteristics of cement stone, depending on composition and properties of their constituent components, is disclosed.
