Case History: High Density Weighted Polymer Drilling Fluid Used in The Deepest Horizontal Well Dan 21# in Sichuan Basin of China
Authors: Li Jian , Zhang Jinhui , Luo Pinya , Xiao Yinxue , He Lun 
Published: December 2007

Abstract
This paper presents a hole cleaning, borehole stable, gas formation protecting, torque and drag program developed with high density weighted polymer drilling fluid in the deepest horizontal Dan 21# well in Sichuan basin of China. This program has be effective and been found critical to the success of Dan 21# well drilling progress.

Introduction
Dan 21# well, located in Sichuan basin southwest of China, is the deepest horizontal well (TVD: 10,269 ft, TMD: 11,926ft, Horizontal Section Length: 1,184 ft). From building section to horizontal section high mud density(16.52 lb/gal) has been used. The 13-3/8-inch (433.97mm) casing in set in 17-1/2inch hole at 108.42 m TVD to protect surface aquifers. The 12-1/4-inch hole is drilled to 2059 m TVD and run 9-5/8-inch casing. A 8-1/2-inch (216mm) hole is drilled to 2450 m TVD the kick off point, encourted water reactive sloughing shale formation Fixianguan. Drilling fluid density was increased to 16.52 /b/gal for wellbore stability. To the TMD 3024m 42 inclination, 7 inch (177.80 mm) liner is put down. A 8-1/2-inch hole is drilled to 3334m TMD finish the building section which then for 6-inch horizontal section. The horizontal section is to 311m long, and TVD 3130 m, TMD 3635 m at 94 inclination. 7-inch casing is tied-back to the surface and then run 5-inch production liner and 4-1/2-inch slotting liner to 3632m TMD. Figure 1 shown the well profile.
The borehole problem can be considerably more severe in Dan 21# horizontal well high weighted fluids than with low weighted fluids. Therefore how to provent cuttings bed form and barite sag, to keep water sensitive and abnormal pressure shale formation wellbore stable in building section, to protect gas formation from damaging, to solve torque and drag problems at high density and solid content condition are the key point of drilling fluid technique. This paper give a detail description of drilling fluid program, approach and application results as well.
By using amphoteric polymer (such polymer contain anionic (negative) and cationic(positive) groups at one molecular chain), which has special molecular structure with long molecular weight and possesses an amphoteric ionic character, excellent rheology and inhibition performance are observed in weighted drilling fluids.
With the research results of rehological properties of drilling fluid at low shear rate and initial structure stress, the hole cleaning evaluation approach and well site controlling standard were established to prevent cuttings bed and barite slide downward.
Shale formation borehole stability has been observed, drilling fluid demonstrates lower coefficient of friction and performs more effectively at lower loads. Bridging additives which particle size distribution can match the size of porethroat and fractures have been used, when drilling in high pressure gas pay zone. This system protects producting formation form damage by limiting fluid loss, fine solids migration and solids invasion.

High Density Weighted Polymer Drilling Fluid System
The high density weighted polymer drilling fluid is developed recently years in response to increasing. Penetration rate for deeper well with weighted fluid intervals. Amphoteric polymer had been used to replace dispersed additive in weighted fluid. It has anique properties with respect to inhibition and rheology for deeper well drilling. P. 145