The TekTote delivery system enables fast, reliable application of pre-mixed viscous fluid loss chemicals, minimizing requirements for rig deck space and excess personnel. (Image courtesy of BJ Services Co.)

One common source of non-productive time (NPT) in chemical fluid-loss solutions is mixing fit-for-purpose crosslinked solutions and waiting for polymer hydration. Proper quality control of materials and mixing procedures is important to ensure successful fluid loss control, but offshore operations may value time at the expense of quality.

New technologies enable timely application of engineered fluid loss chemicals with quality control assured at land-based facilities, eliminating the need for extra qualified personnel and nonproductive mixing time offshore.

An expensive problem

When the wellbore hydrostatic pressure exceeds the reservoir pressure, fluid losses occur. In high-permeability, high-porosity formations, fluid losses can be significant and can result in major unexpected costs. Fluid losses can compromise well control, damage the formation and increase fluid costs, disposal costs, NPT, and time to sales (because of increased fluid recovery after the completion).

The ideal solution to fluid loss is a mechanical barrier, such as a valve, a plug, or a multiple-component completion tool system. However, if the mechanical system fails or cannot be used (such as when tripping pipe or tubing in or out of the well), chemical fluid-loss pills can serve instead.

Chemical systems generally are one of two types: linear gels and crosslinked fluids. Either can be used with or without solids.

Linear gels use viscosity to reduce the fluid’s ability to flow through the formation. When overbalance pressure and reservoir permeability are relatively low, these systems can control fluid losses without significant penetration into the formation. More typically, however, the systems must enter the porosity of the formation to develop resistance to fluid flow. This invasion can create formation damage that persists even after the gel is broken.

In addition, linear gels typically reduce losses rather than stopping them because they do not create a compressed gel barrier at the formation face. To improve performance, especially when fluid losses are very high, solids such as calcium carbonate can be added. The solids comprise a range of particle sizes, designed to bridge the pore throats of the formation and aid in forming a nearly impermeable filter cake. The filter cake is later removed by soaking in an appropriate breaker. In porous, permeable or fractured formations, however, the fluid can invade deep into the formation, where the breaker may not reach it, leaving permanent formation damage.

Unlike the linear gels, crosslinked polymer gels do not invade the formation. Instead they create a viscous barrier or plug at the sand face (or inside a sand screen, depending on the pill design and completion process). This minimizes formation damage, even in high-permeability, high-porosity sands. The material can be removed with a breaker, typically a dilute acid. Generally, crosslinked polymer gels have higher regain permeability than linear gels in compatible formations. When necessary, solids also can be added to crosslinked fluids.

The best system is the one that most effectively controls the fluid losses without damaging the formation.

This means the system must be compatible with both completion brine and formation mineralogy and reservoir fluids. Brines containing zinc, in particular, require engineered fluid-loss solutions to avoid interference with the crosslinker. Temperature can also affect system stability, with temperatures above 225°F (107°C) typically requiring specialty systems.

Crosslink concerns

Crosslinked polymer gels are often the best fluid loss control choice because of their minimal formation invasion and excellent regained permeability after breaking, but they can be difficult and time-consuming to achieve on a rig. Good quality control is essential. Polymer quality, blending process, and crosslink time are all critical to achieving consistently good results.

In particular, mixing these systems offshore can be problematic: Rig hands are encouraged to minimize NPT, and waiting for a crosslink to hydrate properly may seem very unproductive, especially when thousands of dollars of rig time are being spent every hour. However, pumping a crosslinked fluid-loss pill too early eliminates many of the fluid’s advantages over linear gels.

When fluid is being lost to the formation, however, it is difficult to wait for a crosslink. So when experience suggests that fluid-loss control might be needed during completion operations, a pill is generally prepared in advance. If the chemicals are to be mixed offshore, this means the operator pays for the chemicals regardless of whether they are pumped. In many cases, disposal costs are also involved. It also should be noted that when blending crosslinked polymer pills on location, once the crosslinking agent is added, the pill must be pumped immediately to avoid incomplete crosslinking.

A novel technology changes the economics. The TekTote delivery system comprises two sealed tanks in a compact but rugged skid (Figure 2). Available with capacities up to 6 bbl, the tanks contain pre-crosslinked TekPlug crosslinked fluid-loss control systems that require no additional mixing or on-the-fly additives. After blending with brine, the system can be pumped into the workstring and spotted as designed. The skid includes all necessary lift straps, hoses, adapters, and connectors. It is simply rigged up to a pneumatic pressure source and a discharge line.

Because the fluid-loss control system is pre-crosslinked, there is no need to wait for a complete hydration or crosslink. In addition, the quality control takes place onshore before reaching the rig, mitigating delays due to incorrect mixing or inadequate hydration. In addition, because the delivery system is sealed, the operator pays only for the material that is pumped, rather than all the material that is mixed.

Deep, hot losses

In an example of the technology’s use in the field, an operator was completing a well in the deepwater Mississippi Canyon area of the Gulf of Mexico (GoM). Upon perforating the well around 20,650 ft (33,226 km), the well began to experience 150 bbl/hr losses of the 10.4-ppg CaCl2 completion fluid.

A bottomhole temperature of around 225°F (107°C) and permeability near 1,000 md limited the chemical fluid-loss choices, but the operator had TekPlug XL material on the rig. Two six-barrel pills were circulated into place, and they controlled the fluid loss to <3 bbl/hour with 2,200 psi overbalance differential pressure. Subsequently, the well was frac packed without incident.

Storm damage cure

Another mature well in the GoM was severely damaged in Hurricane Katrina. Storm surge had knocked a bulkhead into the christmas tree, bending it almost 90º from vertical and preventing pumping downhole. The damage, however, extended further downhole, where the stress had dislodged the packers that sealed the annulus.

The hydrostatic pressure of the 8.6-ppg seawater in the well bore was too high for the depleted formation to hold back, and the well suffered severe, unquantifiable fluid losses. To maintain well control, the operator hung a fire hose in the annulus and continuously pumped seawater. Still, some gas was managing to escape from the formation through the breached annulus.

Lacking an alternative solution, the operator asked the service company to try plugging the breach with TekPlug XL HD material at 13.5 ppg. A TekTote unit was delivered to the rig and attached to a hose, which was run into the annulus next to the fire hose. Six barrels of TekPlug XL HD were pumped into the top of the well bore and allowed to sink into place. Twelve hours later, the well was not showing gas at the surface and the fluid losses stopped, indicating the breach had been sealed.

The US Minerals Management Service eventually allowed the operator to rely on the TekPlug material for temporary abandonment of another well in the same field during repairs. In this well, the operator was still able to pump through the tubing to the zone. The TekPlug material was pumped into place and left there until a rig was available for remediation operations.