Displacement

Introducing a clear brine fluid to a well after drilling operations can be a critical step in a successful well completion. This section is designed to provide an overview of the topic. For more information, consult the "Displacement" section in Chapter 8.

Objectives

• Protect the formation by developing a completely solid free environment in which to carry out well completion or workover operations
• Ensure that drilling fluid constituents do not come into contact with CBFs
• Separate the two systems to maintain the integrity of the drilling fluid and the CBF
• Reduce standby rig costs caused by unnecessary filtration time

Factors Affecting

• Mud type
• Pressure constraints
• Environmental discharge limitations
• Time constraints due to rig operations or cost

Discussion

Clear brine fluids are incompatible with water based, diesel oil based, and synthetic oil based muds. When they are mixed, this incompatibility generally produces a viscous, unpumpable mass due to flocculation of the mud by high salt content of the brines. Should this reaction take place downhole during the displacement, the flow resistance and pump pressure will increase dramatically, and pumping operations may have to be suspended due to excessive pressures. To avoid this reaction, drilling muds and CBFs must be separated when the mud is being displaced from the well.

The two general methods by which drilling fluids are removed from the well prior to the introduction of a CBF are indirect displacement and direct displacement.

Predisplacement Activities. There are seven major activities that must be performed prior to conducting displacement operations. These predisplacement activities must be undertaken in the case of both indirect and direct displacements.

The seven major predisplacement activities are as follows:

1. Selecting Displacement System. Pay careful attention to the design of the displacement system chosen. TETRA has developed two chemical systems, TDSP and TETRAClean, both of which can be configured for use in either direct or indirect displacements.
2. Cleaning Surface Equipment. Clean all surface equipment so that it is completely free of solids and residual water. Active pits should be cleaned, completely dried, and covered.
3. Verifying Rheology. Check the drilling fluid rheology and thin the mud to promote complete removal of solids.
4. Ensuring Flow Path is Clear. Ensure that the flow path is clear by circulating or drilling out settled solids.
5. Calculating Pressure Differentials. Calculate pressure differentials along the flow path to reduce overpressuring casing or tubing. Reverse circulating during displacement will also result in a large pressure drop at the base of the working string due to the highly turbulent flow conditions at that point. Higher pumping pressures may be required.

   With single salt brines, large differentials are less likely in lower pressure wells using lightweight mud and brines. But spacer densities should be carefully designed to minimize these pressure effects.

   With two salt and three salt brines, pressure differentials between heavier brines and the lightweight surfactant rinse stage of a displacement system can result in large pressure differentials between tubing and casing.

6. Running Wellbore Cleanup Tools. Run brush and scraper tools to the casing bottom.
7. Maintaining Flow. Do not stop pumping at any time during displacement until the returns indicate a continuous flow of the CBF.

Water Based Mud

In order to develop a completely solid free environment in the well prior to completion operations, all traces of solid laden drilling fluids must be removed. Two options, direct and indirect displacement, are available to the operator when a water based mud has been used.

Indirect Displacement. This technique can be used with water based muds or sometimes with synthetic oil based muds because of the need to discharge rinse water containing residual mud constituents.

An indirect displacement technique consists of:

1. displacing the mud from the hole by making a single pass with seawater or lease water,
2. circulating seawater with a surfactant added to remove the final mud residue, and
3. installing the clear brine fluid with a spacer separating it from the seawater.

Indirect displacements are carried out when drilling fluid constituents can be safely jettisoned to the ocean and adequate rig time is available to allow for recirculation until returns are clean. Sufficient circulation time must be expended to avoid contact between residual drilling solids and the CBF. Contamination of the CBF by solids will cause delays, as the solids will have to be filtered from the CBF. If solids are not entirely removed, the completion could be jeopardized if solids are allowed to invade the perforations and/or producing zone. Solids may also settle around the packer, making it difficult to remove.

Additionally, the presence of solids can induce a form of concentration cell corrosion known as crevice corrosion, which can lead to EAC issues. A further consideration is the possible generation of H2S or sulfur from additives associated with the mud solids. These sulfur containing contaminants can lead to sulfide stress cracking (SSC).

Direct Displacement. This technique requires slightly more attention to detail; however, because it can be carried out in a much shorter period of time, it reduces rig time lost while circulating.

TETRA's TDSP direct displacement system is a three stage process:

1. TDSP I---Mud Removal Stage. This first stage consists of a weighted spacer designed to push the mud from the hole. This spacer is very viscous and should have a higher yield point than the mud being displaced, which will ensure separation of incompatible fluids and maximize the hole cleaning ability. The density of the TDSP I phase will be determined by the density of the drilling mud.

   The volume of TDSP I should provide for at least 1,000 feet of coverage in the largest annular section of casing.

2. TDSP II---Surfactant Wash Stage. Stage two consists of a turbulent flow spacer with a concentrated surfactant which disperses any residual mud from casing and tubing surfaces.

   The annular velocity should be greater than 180 ft/min, and the volume of TDSP II should provide at least 2,000 feet of coverage in the largest annular section of casing.

3. TDSP III---Viscosified Sweep Stage. The third stage consists of a spacer used between the surfactant wash spacer and the completion fluid. This stage promotes the removal of residual materials dispersed by the surfactant wash. The rheology of this stage is designed to maximize lifting capacity.

Diesel Oil Based Muds and Synthetic Oil Based Muds

Diesel oil based and synthetic oil based drilling fluid systems often require the use of direct displacement. An additional oil based pad should be placed between the mud and TDSP I when a CBF is to follow an oil based mud system; however, other than this, the procedures are the same.

TETRAClean Displacement System

Developed for use under the stringent environmental regulations of the North Sea, the versatile TETRAClean displacement system is used for well cleanup after water based, diesel oil based, or synthetic oil based drilling muds. For ease of use, the TETRAClean system is mixed as a single viscous pill, usually in the range of 200 to 250 bbl. The highly effective, concentrated pill reduces the need for additional pit volume. Depending on brine chemistry, TETRAVis HEC polymer or BioPol polymer may be used to build viscosity. The TETRAClean 105 surfactant package and TETRAClean 106 activator are added to the viscosified brine. The pill is run after a compatible spacer and pumped at a rate high enough to achieve turbulent conditions.

The TETRAClean system can be used without restriction in the UK North Sea, as the system has an environmental Chemical Hazard Assessment and Risk Management (CHARM) rating of Gold. An in depth discussion of displacement and the TETRAClean system can be found in the "Displacement" section in Chapter 8.