BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
Gel Strengths
Gel strength measurements are made with the V-G meter and describe the time-dependent flow
behavior of a drilling fluid. Gel strength values must be recorded at 10-second (initial gel) and 10-
minute intervals. One additional gel strength value should be recorded at 30 minutes. Gel strengths
indicate the thixotropic properties of a drilling fluid and are the measurements of the attractive forces
under static conditions in relationship to time. Plastic viscosity and yield point, conversely, are
dynamic properties and should not be confused with static measurements. However, gel strengths and
yield point are somewhat related in that gel strengths will typically decrease as the yield point
decreases.
Gel strengths occur in drilling fluids due to the presence of electrically charged molecules and clay
particles which aggregate into a firm matrix when circulation is stopped. Two types of gel strength
occur in drilling fluids, progressive and fragile. A progressive gel strength increases substantially with
time. This type of gel strength requires increased pressure to break circulation after shutdown. A
fragile gel strength increases only slightly with time, but may be higher initially than a progressive
gel. The NEW-DRILL ® system is characterized by fragile gel strengths that are high initially but are
very fragile. f gel strength measurements are taken after a 30-minute time period, the progressive or
fragile nature of the gel strengths can be easily determined. Progressive and fragile gel strengths are
illustrated in Figure 1–14.
Gel strength in a drilling fluid is dependent upon chemical treatment, solids concentration, time, and
temperature. There is no well-established means of mathematically predicting gel strengths in any
fluid system. Generally, gel strengths will increase with time, temperature, and increase in solids. If a
fluid system is not sufficiently treated for temperature stability, the gel strength developed after a bit
trip becomes a major factor in the pressure required to break circulation, and in the magnitude of swab
and surge pressures. Additionally, initial gel strength in a weighted fluid system must be sufficient to
prevent settling of weight materials. Therefore, the drilling fluids technician must be concerned with
having sufficient initial gel strength, yet not having excessive long-term gel strength.
Gel strengths assume great importance with regard to suspension properties under static conditions
and when performing swab and surge analysis. When running a drill string or casing into the hole
it is necessary to overcome the gel strengths. Gel strengths also affect the ability of a fluid to
release entrained gases. At times it may be necessary to break circulation at intervals while
running into the hole rather than to initiate flow in the entire wellbore at the same time in order
to minimize the pressure spike to initiate circulation.
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 1-21