Cesium Formate: Results and Analysis of Drilling with a New High Density Unweighted Brine
Authors: Ramsey Mark S. , Shipp Jon A. , Lang Bill J. , Black Alan , Curry David 
Published: December 2007

Abstract
Full scale laboratory experiments were recently conducted to investigate drilling rates achievable with a new, high density, clear brine fluid, Cesium Formate (CsCOOH). These tests were conducted using industry accepted protocols, including bit selection, rock samples drilled, hydraulics, bit loading, and laboratory drilling apparatus. Control experiments using a hematite-weighted water based field mud were also conducted.
This paper presents the data with the authors' analysis. In addition, the authors discuss related prior work. The fluid of interest consists of a 18.7 ppg Cesium Formate with additives for viscosity and fluid loss control. These tests are believed to have used the highest density drilling fluid ever for published drilling tests conducted in a controlled full scale laboratory environment simulating downhole conditions.

Summary Results
Results of the tests show dramatic increases in drilling rates as compared with the control water based hematite field mud of similar density as the cesium formate mud. The cesium formate fluid was easy to handle and was relatively immune to contaminants. The cesium formate fluid produced more discrete shale cuttings. The formulations used exhibited high so-called "spurt loss", yet API fluid loss was low. Even so, the filter cake was of negligible thickness. No bit balling was observed for the cesium fluid drilling tests.

Introduction
Clear brine fluids have been used for many years in completions operations. More recently, clear brines have found use in drilling operations, particularly in drilling pay zones so as to minimize formation damage. Non-damaging or minimal-damage fluids used to drill pay zones are referred to as "drill-in" fluids.
A relatively new class of fluids known as formates have generated considerable interest due to their ability to achieve high densities and yet retain low viscosities. Cesium Formate is the highest density variety of these formate fluids.
Much laboratory work has been done to evaluate the physical properties of the formates, as well as to determine their health, safety, and environmental profile. In general, the findings have been encouraging, especially when compared to commercially available high density brines today.
Some wells have been drilled with the lower density formates, with excellent results. Drilling was relatively trouble-free and formation damage was believed to have been minimized.
With laboratory work on cesium formate being encouraging, and field trials of the lower density formates being successful, the need to evaluate possible drilling uses of cesium formate became apparent. Hence, this testing program was established to answer questions relating to drilling rate when drilling shales and sandstones with cesium formate fluids. The authors intent is that the data be used for economic evaluation of candidate wells. Additionally, some of the data may improve our understanding of drilling mechanics.
Cesium Formate Fluid Composition & Properties Cesium Formate is a clear, high density, low viscosity brine. The viscosity at 75 degrees F. and 19.2 ppg density is less than 3 centipoise.
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