Abstract

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.

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