Improving the rate of penetration (ROP) is one of the key methods to reduce drilling costs. Several ROP models have been developed and modified based on the concept where unconfined compressive strength (UCS) is inversionally proportional with the rate of penetration. These models can predict the rate of penetration of different bit types in an oil or gas field with a reasonable degree of accuracy. The ROP model studied herein relates the rate of penetration to operating conditions and bit parameters in addition to the rock strength. Also, the effects of bit hydraulics and bit wear on rate of penetration are included in the model. In this paper, the drilling performance was optimized, using the ROP models, for upcoming wells in one of the Persian Gulf carbonate fields. Based on previous drilled wells a rock strength log along the wellbore is created and modified to mach the the new well survey. The rock strength is back calculated from the ROP model which includes bit design and reported field wear in conjunction with meter by meter operating parameters, formation lithologies and pore pressure. By conducting a number of simulations a learning curve was constructed to obtain the optimum bit hydraulics, best combination of operational parameters and the most effective bit design. Based on the proposed ROP model, a simple and useful simulator was developed. This methodology can be used in pre-planning and post analysis to reduce drilling cost where previously drilled wells exist. Introduction The objective of any well drilling is to drill in the shortest time in compliance with safe operations and environmental regulations. Selecting and using the best combinations of equipments and techniques to accomplish this goal is "Drilling Optimization". The main function of drilling engineer is to recommend drilling procedures concerning routine rig operations such as drilling fluid treatment, pump operation, bit selection, and any problems encountered in the drilling operation. In many cases, the use of the drilling cost equation can be useful in making these recommendations. The calculation of cost per foot is done by the conventional cost equation as follows: Equation (1) where Cf is drilled cost per unit depth ($/ft), Cb is the cost of bit ($), Cm is downhole motor cost ($/hr) and Cr is the fixed operating cost of the rig per unit time ($/hr) independent of the alternatives being evaluated. In addition, ?D represents the depth drilled (ft), tr is the bit rotating time (hr), tt is the total trip time (hr) and tc is the pipe connection time (hr). According to the drilling cost equation, the key to minimize the drilling cost would be the increase of rate of penetration (ROP). Based on the recorded data from the previously drilled wells in a given field, one can predict the rate of penetration in that field using a predictive ROP model. Having this predictive ROP will give a more reliable estimate of drilling future wells. Therefore, the more reliable and precise ROP model, the more practical results will be.

Sarvak formation is the second major carbonate reservoir in Iran. There are several geological, petrophysical and geophysical investigations which have been carried out on this important reservoir. In this work, Sarvak is studied to find productive zones. At first, four different methods were used to identify producing intervals from well log data and well test results. Then, final zoning is generated by integrating outputs of these four methods. One of them is the conventional cutoff based method; the other three methods are based on flow equation, Bayesian and fuzzy theories. Thereafter, by considering the classification correctness rate of each classifier in each well and technique of majority voting, a unique zoning for Sarvak formation is presented. Based on the final zoning, the whole Sarvak interval is divided into seven zones. Three of them are classified as oil producing zones, two of them cannot be classified as conventionally producing zones, and the remaining two are water producing. Zone number 2 not only has the highest production rate, but also is the most homogeneous zone among the productive zones. The novelty of this research is using well test results in defining productive classes, which improves the certainty of classification in comparison with previous works that were based on core analysis and log data.

The thermal history of the sedimentary successions at the wells in the northern Persian Gulf have been studied using apatite fission track analysis and vitrinite reflectance data. These data were used to identify and quantify episodes of heating and cooling which have affected the sections penetrated by these wells. This information was synthesised to provide a thermal history framework for the wells, within which the history of hydrocarbon generation, as well as regional structural development, can be understood. Preliminary hydrocarbon generation histories are presented for the Soroosh and Nowrooz oilfields and nearby areas.Modelling of hydrocarbon generation histories based on the AFTA-and VR-derived thermal histories, assuming a dominant Type III kerogen for possible Albian Kazhdumi Formation source rocks and a dominant Type II kerogen for possible Neocomian Fahliyan (Lower Ratawi) Formation source rock, suggest that local sourcing of oil from the Kazhdumi Formation is unlikely. The most likely source rock for oil in the Burgan Formation reservoir at is interpreted to be the Fahliyan Formation based on the available data. On the other hand, speculative modelling of the Hendijan-1 well down-dip from the Nowrooz field does allow some oil to be generated from the Kazhdumi sequence at that location, and this might be available for migration to the Nowrooz field.

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The sandstones of the Early Triassic non-marine Rewan Group in the southern Bowen Basin represent a potential reservoir for hydrocarbons. They are mainly lithic in nature, with a large proportion of volcanic rock fragments and contain quartz grains of variable size and an authigenic clay matrix. The results of petrographic analysis show a dual provenance basin-fill pattern (cratonic-volcanic). Reservoir qualities vary considerably, but are best developed in the Quartzose and Conglomerate Intervals where partial quartz cement retarded compaction and secondary porosity developed by dissolution of labile material and flushing of clay minerals. The presence of different clay minerals in pore networks may have a significant impact on petrophysical properties of the reservoir and hence affect reservoir productivity.