Runar Nygaard

Optimization of Multiple Bit Runs Based on ROP Models and Cost Equation: A New Methodology Applied for One of the Persian Gulf Carbonate Fields

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.

Methods of Using Logs to Quantify Drillability

Correlations between sonic logs and the formation drillability for different lithology types have been developed from data taken from 10 wells in North America. The gamma ray log was used in conjunction with drilling data to calculate the drillability. The drillability from penetration rate models is back calculated from bit design and reported field wear in conjunction with meter by meter operating parameters, formation type and pore pressure. Then this drillability was correlated with sonic logs for different lithologies as defined by the gamma ray log. The different formation types clearly show different correlations for the normalized correlations between drillability and sonic logs. The non-homogeneous lithologies are also correlated and normalized to rock strength from the sonic logs where the percent formation type mixtures are determined from the gamma ray. Data from multiple wells is presented showing the accuracy of the presented approach where more then 100,000 data points were statistically analyzed and evaluated in the development of the equations presented herein. The drillability from inverted penetration rate models has been verified to give good representation of rock strength based on comparison with triaxial laboratory data and makes the use of this model more versatile. The correlations provide improved estimations of rock strength which can be used in drilling performance simulation and wellbore stability studies.

Application of Rock Strength in Drilling Evaluation

Different sources can be used to develop rock strength information along the wellbore. Such strength information is important when assessing the stability of the wellbore, selecting mud weights and designing casing programs. However, there are other areas, especially in drilling, where rock strength information is applicable, but still underutilized. A methodology is developed to estimate drilling time and bit wear before drilling if rock strength is known. To estimate drilling time and bit wear, effects of other parameters like drilling parameters, well bore size and drilling bit design has to be normalized. This methodology has been used to estimate drilling time and estimate bit wear and further evaluate drilling performance while drilling. After drilling the additional information has been used to conduct a post analysis and transfer knowledge from well to well. The advantage of this methodology is it eliminates the effect of geological variability when comparing performance between wells and fields.

How to select PDC bit for optimal drilling performance

Selecting bit when you have multiple bit vendors giving their proposal can be a challenge for the operator. To justify the bit selection can be hard for the drilling engineer in a multiple vendor situation. The purpose of this paper is to show a systematic approach on how to select PDC bit based on quantitative measure by using a simple scorecard. When the drilling organization has agreed on the overall drilling objective for the well a scorecard is used as decision criteria for selecting bits. To quantify the input for each bit a drilling simulator was used. The simulator can, based on a rock strength prognosis for a well, predict the rate of penetration and bit wear for each bit based on the bit design. For other criteria which are more difficult to obtain e.g. ability to create dog leg a qualitative ranking was used. In the two field examples shown from the North Sea the method has worked well to give a reliable and transparent bit selection method. Using a scorecard also reduced the ambiguity among the bit company representatives on how the selection process was done.

Drilling Simulation Improves Field Communication and Reduces Drilling Cost in Western Canada

During the past two years a drilling simulator has been applied in a pre-simulated and follow-up mode on dozens of well in Western Canada. Using a commercially available rate of penetration (ROP) drilling simulator, the drilling learning curve of mature fields in Western Canada is significantly improved. The ROP drilling simulator optimizes the bit runs, drilling parameters and pull depths prior to spud by applying offset drilling data and records. The drilling cost is maintained consistent and or reduced from well to well and pacesetter performance is constantly copied or improved. After simulating and obtaining the optimum operating conditions for the individual drilling bits, the optimum parameters were sent to the company man and drillers in advance. This has shown to be an efficient way of communicating and obtaining optimal results. During drilling the home office is in communication with the field as daily updates of field parameters are compared to the preplanned scenario. These parameters include the operating parameters, calculated bit wear and updated rock strength compared to the pre-simulated values. This paper presents field examples where the presimulation and daily drilling data are compared. The improved communication points from the updates are given in examples as well as how this improves the overall performance. Introduction To be able to simulate drilling performance prior to actually drilling the wells, the drilling simulation