DRILLING TECHNOLOGY Reservoir damage problems multiply in horizontal drilling

Drill-in, completion fluids remedy some borehole problems, but more work needed on drilling fluids

Leonard LeBlanc
Editor

More extensive damage can be done to reservoir productivity in the drilling of horizontal wells than in vertical holes because of the greater exposure of the reservoir to borehole fluids. The remedies for that damage, if available, can be costly.

Frustrated completion engineers make the point that the additional cost and effort in drilling horizontal sections to enhance productivity come to naught if the reservoir has been damaged by borehole fluids, the formation skin or filter cake is difficult to break, or the section has been poorly cleaned.

The need to develop outlying reserves from existing offshore structures has drawn a host of technologies that are geared to drill horizontal and extended reach boreholes efficiently. Producers are encouraged by the additional production obtained from these horizontal sections, but many are now learning about potential production in those same wells that has been permanently shut off by poor drilling or completion practices.

Balancing needs

In fairness to drillers, providing the right environment downhole while ensuring safe well control and avoiding serious torque and drag problems is never as easy as planned. Logging-while-drilling and measurement-while-drilling equipment behind the drill bit have greatly improved drillstring and lithology information, but interpreting that data and providing a formation-compatible drilling fluid or drill-in fluid at the right time is always a judgment call. Obtaining a core upon entry to the productive formation and conducting core-fluid reaction tests (with both drilling and completions fluids) provides the site-specific information needed, but the time and expense may not be cost-effective. There is rarely an ideal solution.

After years of drilling horizontal sections, a number of technical papers finally are addressing the provision of a necessary fluid chemistry, weight, and pressure environment while drilling long horizontal sections.

One conclusion is that much more planning, better quality control, or reaction to changing downhole conditions is needed in drilling horizontal sections. The matching of drilling and completion fluids to reservoir conditions is much more subjective than most producers feel comfortable with.

Reservoir damage

The physical parameters needed in productive reservoirs - permeability and porosity - make the formation vulnerable to drilling fluid invasion, particularly where fluid pressure greatly exceeds reservoir pressure. Generally, for over-balance conditions, the longer that drilling fluids occupy the horizontal section, the greater the depth of invasion.

Thus minimization of the damage consists of keeping the invasive fluids and solids within a tight radius of the borehole axis, either by tight time constraints or careful drilling practices.

Two reasons that under-balance drilling is being examined for offshore horizontal sections is the reservoir damage phenomena and the situation in which lost circulation materials sent down to revive circulation have plugged up formations permanently.

Fluid invasion by as much as two ft from the borehole in a porous reservoir can be remedied often by simple dissolution of the filter cake and flowing the well, however invasions beyond that often require expensive treatment measures, mostly fracturing and sidetracking.

Formation damage such as pore space plugging depends upon the type of fluid and entrained solids. Deposits can include asphaltenes, waxes, polymers, swelling clays, hydrates, and any number of solid materials that make up drilling fluids. Altering the pressure and temperature can create solids, such as gas hydrates, in the near-borehole formation.

Needed solutions

The ideal filter cake deposited by the drilling fluid will partially block leakoff of drilling fluids, minimize flow-through of solid particles and polymers, and break up or wash off the formation face easily upon application of completion fluids.

In some cases, the filter cake cannot be washed off or chemically broken down. If the filter cake has a low permeability, it will not accept solvents very easily. Then, completion engineers have to rely on collapse and dissolution of the filter cake during initial production. With the greater number of gravel and sand completions being installed, flowing the filter cake is no longer considered a good backup option.

More research is being conducted on completion and drill-in fluids to blend well control characteristics with clean-up capabilities. The successful transition from maintaining an adequate filter cake to dissolving that filter cake is an important one for drilling and completion engineers. An important factor in that transition is laying down a filter cake that has sufficiently large particles to permit solvents sufficient entry, yet small enough to restraint rapid leakoff during drilling operations.

A second front of investigation is the development of lost circulation materials that dissolve after time or upon reaching certain temperatures. That way, one element of well control can be overlooked in devising a plan to drill and complete a horizontal section.

Some producers have a policy of switching to drill-in or completion fluids as soon as drillers are certain the productive formation has been contacted. Others simply believe in the capabilities of completions fluids, including breakers, acids, oxidizers, and solvents, to break down the filter cake, regardless of the strength of adherence to the borehole walls.

References

Woodhouse, R., Opstad, E., Cunningham, A., "Vertical Migration of In vaded Fluids in Horizontal Wells," Courtesy of BP Exploration.

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