Circulatability treatment in cementing

The condition of the drilling fluid is the most important variable in achieving good displacement during cementing. Regaining and maintaining good fluid mobility is the key. An easily displaced drilling fluid will have low gel strengths and low fluid loss. Pockets of gelled fluid, which commonly exist following drilling, make displacement difficult and should be broken up.

Drilling fluid evaluation for a cement job

Circulating or conditioning the drilling mud for at least two hole volumes, prior to cementing, is preferred. Varying pump rates during the conditioning process enhances hole cleaning. Pipe movement (reciprocation or rotation) helps to break up mud gels for greater displacement efficiency. Performing the steps discussed next conditions the drilling fluid for a cement job.

• Determine the volume of the circulatable hole, and evaluate the percentage of the hole that is being circulated. Good fluid returns do not reliably indicate the mobility of fluid in the annular space.
• When the casing is on bottom and before displacement begins, circulate the drilling fluid to help break the gel structure of the fluid, decreasing its viscosity and increasing its mobility. Condition the drilling fluid until equilibrium is achieved.
• Never allow the drilling fluid to remain static for extended periods, especially at elevated temperatures. When the drilling fluid is well conditioned (the drilling-fluid properties going in equal the drilling-fluid properties at the outlet), continue circulating it until the displacement program begins.
• Modify the flow properties of the drilling fluid to optimize its mobility and drill-cuttings removal.
• Measure gel strengths at 10 seconds, 10 minutes, 30 minutes, and 4 hours to examine the gel-strength profile of the drilling fluid. This testing would typically be performed during the job-planning stage. During conditioning just before the job, readings taken at 10 seconds, 10 minutes, and 30 minutes are typically sufficient. An optimum drilling fluid has flat, nonprogressive gel strengths (e.g., gel-strength values of 1, 3, and 7). Note: Gel strength is measured using a rotational viscometer. The unit of measure is lbf/100 ft². The test procedure is outlined in API RP13B-1, Recommended Practice Standard Procedure for Field Testing Water-based Drilling Fluids.
• Measure gel-strength development of drilling fluid to be left in the well at downhole conditions of temperature and pressure. At elevated temperatures and pressure, some drilling fluids gel to a consistency that prohibits removal. These increased gel strengths are not detectable at surface conditions. This testing should take place during the planning stages.

In deviated wellbores, a drilling fluid that has a higher viscosity at low shear rates may be required to help prevent drilling-fluid or wellbore solids from settling on the low side of the wellbore. The presence of large drill cuttings may also necessitate higher-viscosity fluids. This testing should take place during the planning stages.

See also

Cementing operations

PEH:Cementing