An Inhibitive, Bentonite-Free Polymer System Cuts Well Costs In Custer County, Oklahoma
Authors: Williams D.G. , Jones S.C. 
Published: December 2007

Abstract
Michigan Wisconsin Pipe Line Company, in the latter part of 1976, initiated a comprehensive evaluation of their active Custer County, Oklahoma drilling program. Their objective was to improve overall well cost by increasing penetration rates, while maintaining wellbore stability.
This paper reviews some of the problems associated with drilling wells; in this area, and goes into the decision made to use an inhibited bentonite-free polymer system to drill through the production intervals. Comparisons are given for penetration rates and mud cost per foot for several wells drilled with the inhibited bentonite-free polymer system as opposed to the fresh polymer system as opposed to the fresh water, dispersed muds used previously. Additional information includes effects on wellbore stability, total well cost savings, and typical mud properties for the polymer mud.

Introduction
Michigan Wisconsin Pipe Line Company, in the latter part of 1976, initiated a comprehensive evaluation of their active Custer County, Oklahoma drilling program. Their objective was to reduce overall well cost by increasing penetration rates while maintaining wellbore stability. Both, increasing penetration rates and maintaining wellbore stability are directly related to the type of drilling fluid used.
A review of the various intervals drilled in already completed wells indicated that achievement of the objective could best be accomplished by changing the type of drilling fluid used through the production interval of the wells. Drilling the production interval encompasses penetrating production interval encompasses penetrating the Atoka, Morrow and Springer formations. Problems encountered through this interval Problems encountered through this interval are abnormal pressures of varying magnitudes, sloughing shales in the Atoka and Springer formations and lost circulation throughout the interval. Footage drilled in this interval is 2200' (670.5 m) to 2500' (762 m).
The wells were being drilled with dispersed fresh water muds weighted with barite. Muds weights varied from 12.0 to 14.0 lbs/gal (1437.6 to 1677.2 Kg/m3) with the majority of wells requiring densities of 12.5 to 13.0 lbs/gal (1497.5 to 1557.4 Kg/m3). Penetration rates averaged 3.0 ft/hr. (.9 m/hr.)
A generalized casing program for these Custer County wells calls for 7 5/8" (19.36 cm) intermediate casing to be set in the top of the Atoka formation and drilling a 6 1/2" (16.51 cm) hole through the production interval.

FLUID SYSTEM: WEIGHTED POLYMER
After consultation with a major drilling mud supplier, it was decided to drill the production interval on the next well, the Tharp #1, with an inhibited, clay free polymer system, tradenamed IMCO DRIL-S. polymer system, tradenamed IMCO DRIL-S.The consensus was that this system would be effective in increasing penetration rate due to the overall reduction of suspended colloidal solids resulting in lower plastic viscosities and better control of plastic viscosities and better control of plastic viscosity to yield point ratios. plastic viscosity to yield point ratios. The polymer system was prepared for the Tharp #1 using a sodium chloride brine with chloride content of 170,000 ppm. Initial mud weight for displacement was 10.2 lbs/gal.