Cesium Formate - The Beneficial Effects of Low Viscosity and High Initial Fluid Loss on Drilling Rate - A Comparative Experiment
Authors: Ramsey Mark S. , Shipp Jon A. , Lang Bill J. , Black Alan , Curry David 
Published: December 2007

Abstract
Full scale drilling tests using industry accepted protocols have been conducted to investigate drilling rates achievable with a new, high density, environmentally acceptable, clear brine fluid. Importantly, control experiments using a hematite-weighted water based field mud were also conducted.
This paper presents the data and the authors' analysis, with an emphasis on fluid loss measurements conducted during the tests. The fluid of interest consists of approximately 18.7 ppg Cesium Formate with polymeric additives for viscosity and fluid loss control.
It is believed that the fluids used in these tests are the highest density ever used in published full scale laboratory drilling tests.

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 formulations used exhibited apparently high so-called "spurt losses", yet API fluid loss was low. The authors attribute the exceptional drilling rates to both the low base fluid viscosity and high initial fluid loss of the cesium formate based drilling fluid.

Background
Cesium Formate, a new compound being actively tested in laboratories today for possible oilfield application, is chemically classified as an alkali-metal salt of formic acid. It offers properties that in many respects are superior to alternative compounds such as Zinc Bromide and Calcium Bromide.
Due to its high solubility in water, a "clear brine" of Cesium Formate in water can reach 19.8 ppg (2.4 Sg) density. In achieving this high density, cesium formate retains exceptionally low viscosity. Referring to Figure 1, a sodium formate liquid at 1.2 Specific gravity (Sg, water=1 g/ml) has a viscosity of only slightly over 2 cP. Potassium formate at about 1.4 Sg has about a 3 cP viscosity, and is only about 10 cP or so at 1.5 Sg. Cesium Formate's viscosity at a density of 19.2 ppg (2.3 Sg) is less than 3 pseudo-centipoise.
The advantages of a solids-free system are significant. The plastic viscosity of a mud is greatly affected by the quantity of suspended solids. Figure 2 shows suspended solids concentrations for barite and hematite water based muds as density is increased. Being free of particulates, clear brine systems in general will exhibit lower plastic viscosities than conventional mud systems. This will result in lower equivalent circulating densities (ECDs) and lower parasitic pressure losses down-hole resulting in greater drilling efficiency. These two effects are particularly important in slim hole drilling where, because of the reduced pipe and annulus diameters, minimizing down-hole pressure losses (and hence ECDs) is essential to ensure good rates of penetration while preventing fracturing of the formation.
Of increasing importance to today's production operations, the formates are considered to be more environmentally friendly and less toxic than available alternatives, and have been shown to be essentially non-corrosive. The formates are largely compatible with conventional oil field polymers, and are thought to be relatively non-damaging to productive formations as compared to conventional fluids, making them excellent candidates for selected "drill-in" fluid markets.
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