Measurement of the erodability of drilling fluid deposits

Claim 1

A method of measuring the shear stress required at the walls of a well bore to erode drilling fluid deposits formed thereon as a result of the well bore containing a drilling fluid and penetrating one or more permeable formations, which method comprises the steps of: (a) introducing a drilling fluid into a test apparatus (20) having a permeable section (24,26) simulating a permeable section of a well bore: (b) maintaining said drilling fluid in a static state in said permeable section (24,26) at a pressure and for a time period such that drilling fluid deposits are formed therein; (c) circulating said drilling fluid through said permeable section (24,26) at progressively increasing flow rates and maintaining each of said flow rates for a time whereby a pressure drop of said drilling fluid through said permeable section stabilizes while measuring said flow rate, said pressure drop, the viscosity, the temperature and the density of said drilling fluid; (d) determining the stabilized pressure drop measured in step (c) below which no significant erosion of said deposits takes place by calculating and comparing the well bore size equivalents to said stabilized pressure drops; and (e) determining the minimum shear stress required to erode said drilling fluid deposits corresponding to the pressure drop below which no significant erosion takes place determined in step (d).

Claim 7

A method according to any of claims 1 to 6, wherein the drilling fluid is circulated through the permeable section (24,26) in accordance with step (c) at three or more progressive flow rates.

Claim 8

A method according to any of claims 1 to 7, wherein the well bore size equivalents to the stabilized pressure drops are determined in accordance with step (d) based on the relationship: D e = 2fLV²ρ g c Δp wherein: D e is the equivalent diameter through which the drilling fluid is flowing; f is the friction factor of the drilling fluid based on the drilling fluid viscosity and temperature; L is the length of the flowing area; V is the velocity of the drilling fluid; ρ is the drilling fluid density; g c is the gravitational constant; and Δp is the stabilized pressure drop across the length of the flowing area (L); where the above variables are in consistent units.

Claim 9

A method according to any of claims 1 to 8, wherein said minimum shear stress required to erode said drilling fluid deposits which occurs at the pressure drop below which no significant erosion takes place is determined in accordance with step (e) based on the relationship: τ w = D e Δp bne 4L wherein: τ w is the minimum shear stress at the wall required to erode said drilling fluid deposits; D e is the equivalent diameter through which the drilling fluid is flowing; ΔP bne is the pressure drop across the length of the flowing area (L) below which no significant erosion takes place; and L is the length of the flowing area; where the above variables are in consistent units.

Claim 10

A method according to any of claims 1 to 9, wherein the erodability of the drilling fluid deposits formed by said drilling fluid is determined based on the relationship: E df = 1.991 X 10²⁴Aa (4a²)(12h²)τ w wherein: E df is the erodability of the drilling fluid deposits; τ w is the minimum shear stress at the wall required to erode the drilling fluid deposits; A is 3 x 10⁻²⁰ joules; a is the average radius of particles in the drilling fluid deposits; and h is the separation distance between particle surfaces; where the above variables are in consistent units.