Use of a Decision Tree To Select the Mud System for the Oso Field, Nigeria
Authors: Dear S.F. , Beasley R.D. , Barr K.P. 
Published: December 2007

Original SPE manuscript received for review June 10, 1995. Revised manuscript received July 24, 1995. Paper peer approved July 26, 1995. Paper (SPE 29370) first presented at the 1995 SPE/IADC Drilling Conference held in Amsterdam, Feb. 28-March 2. Journal of Petroleum Technology, October 1995.

Far too often, the basis for selection of a mud system is the "latest, greatest" technology or personal preference rather than sound cost-effective analysis. The use of risk-vs.-cost decision analysis improves mud selection and makes it a proper business decision. Several mud systems usually are available to drill a well and, with good decision analysis, the cost-effectiveness of each alternative becomes apparent.

The Nigerian Natl. Petroleum Corp. and Mobil Producing Nigeria (MPN) recently began development of the Oso field, a condensate development funded by the World Bank. The budget authorized $187 million for development drilling of 19 wells and completion of 21 wells. Early drilling was very troublesome owing to wellbore instability, lost circulation, and stuck pipe. In addition, the mud cost was excessive because of high dilution rates and expensive mud products (especially barite, which was 50% of the final mud cost).

This paper describes how the drilling team used structured decision analysis to evaluate and select the best mud system for the project. First, Monte Carlo simulations forecast the range of possible results with each alternative. The simulations provided most-likely values for the variables in the decision tree, including reasonable ranges for sensitivity analyses. This paper presents and discusses the simulations, the decision tree, and the sensitivity analyses. The analysis compares several factors in six mud systems, including cost-per-barrel; dilution; trip time; rate of penetration (ROP); stuck pipe, lost-circulation, wellbore-stability, torque-and-drag, filter-cake, and carbonate problems; and environmental concerns. The analysis used variables based on actual data whenever possible. The results indicated that, in spite of necessary rig modifications and increased risk of lost circulation, a mineral-oil-based mud was the best choice.

Before introduction of the mineral-oil-based mud, a steady improvement had occurred in drilling performance, which improved dramatically after this mud was introduced. During the project, 21 wells were completed and 22 development wells were drilled (three more than originally planned). Total cost was $176 million, $28 million under the original budget and $11 million under the final budget. This paper presents field results that show the improvement in well, mud, and coring costs; average ROP; and trouble time. Through sound application of decision analysis, selection of the mud system becomes a sound business decision.

Introduction

The Oso field lies 15 miles off the southeastern coast of Nigeria in 50 ft of water. Although the field was discovered in 1967, development drilling did not begin until mid-1991. Initially, the wells were troublesome and very costly, and budget overruns to the World Bank funding seemed likely. It became apparent that any improvement in drilling performance would require significant changes.

Mud-related problems were apparent. There was high incidence of stuck pipe, high torque and drag, filter-cake buildup, and borehole instability. Wiper trips were nightmarish, with the drill string virtually being stuck on every connection when tripping from total depth (TD). In addition, the mud cost was excessive.

The drilling team felt that a different mud system was necessary for significant reduction of mud-related problems, but was undecided about which system. The team members (MPN Drilling Operations, MPN Drilling Engineering, MPN Fluids Group, the rig contractors, and the mud companies) made several recommendations primarily on the basis of prior experience, product availability, or personal preference.

The choice of the mud system had to be a good business decision, the one that added the most value to the project. The objective was not to reduce torque and drag, eliminate stuck pipe, avoid lost circulation, improve wellbore stability, or reduce the mud bill. The objectives were to minimize drilling cost and to maximize the value of the project. In fact, reducing the drilling cost was only an objective in so far as it also resulted in increased value for the Oso project. Structured decision analysis would help to ensure that nonbusiness-related issues and personal bias did not affect the choice and that the team remained focused on the company goal: maximum value for the project.

Mobil commonly uses forecasting, risk analysis, and decision trees on such problems as whether to drill, well-location selection, well configuration, and to fish or to sidetrack. However, the company had never used these decision techniques to select a mud system. Mobil and the drilling fluids companies had the considerable data required to predict the performance of the recommended mud systems, and MPN had the software required to perform the risk analysis and to create the decision tree. The drilling team decided to apply these decision-analysis techniques to the mud selection for Oso.

This paper describes how the team applied forecasting, risk analysis, and decision trees to selection of a cost-effective mud system. The analysis compares and evaluates alternative mud systems by use of historical data to establish probabilities of success and then constructs the decision tree with these probabilities. The paper also presents field results with the selected mud system.

Discussion

The first 13 wells for the Oso development used a potassium-lignite drilling fluid. This system proved costly, not only from the drilling-fluid costs but also from the trouble time associated with mud-related problems. Wellbore-stability models indicated that additional mud density would help prevent the shale problems. However, barite costs and dilution requirements were high, and lost circulation and differentially stuck pipe and logging tools, problems resulting from the use of higher mud weights, occurred frequently. Torque and drag were at the limits of the mechanical equipment, and filter-cake buildup across the production sands made running casing difficult. At best, running casing usually required washing to bottom. On occasion, the pipe would stick off bottom. From TD, to short trip to the shoe with only about 5,000 ft of open hole normally required 24 hours. Also, carbonate contamination caused excessive use of chemicals to control viscosity and fluid loss.

MPN initiated a study to select a different mud system. The team members made recommendations. Several mud systems were available that would "do the job." Without a structured method of decision making, the company faced the task of trying different mud systems on successive wells and comparing the results. This haphazard approach could prove to be very expensive. Mobil has a large database of mud systems used in Nigeria and worldwide, and the Drilling Div. decided that a structured analysis of the decision, with information from this large mud database, would be a valuable aid in selecting the best system for the Oso project - the system that would maximize expected value of the project. Judgment calls, made with structured thought processes and a large database, are significantly better than intuition and personal experience and bias.

The decision analysis included six alternative mud systems:low lime,potassium lignite,polyanionic cellulose (PAC)/lime,partially hydrolyzed polyacrylamide (PHPA),PHPA/glycol, andmineral-oil mud. The first step was to define the risks and costs associated with each mud system.

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