Displacement of drilling mud or well service fluids and removal of debris, rust, scale, and other solids from the wellbore prior to the final completion of oil and gas wells is an integral—but often under-appreciated—step in the successful well-construction process.

Halliburton Baroid is preparing to change that perception by introducing new patented displacement systems and software that underscore the benefits that engineered displacement treatments can bring to the bottom line.

Since the earliest days of the oil and gas industry, the foremost technical considerations in designing wellbore displacement programs essentially have been limited to pumping specified volumes of spacer fluids to physically separate incompatible drilling fluids from completion fluids. Even today, many operators and completion engineers have unrealistic expectations of displacement chemicals, not recognizing the role engineering the displacement design plays in chemical performance. Most displacement software currently used generates displacement designs based on subjective input data.

Operators in general are comfortable with simplistic approaches in part because less-than-optimal displacements generally do not result in catastrophic losses. However, several field case histories have shown that displacement processes can be deceptively complex with extremely high risks of nonproductive time (NPT).

Displacement costs can escalate quickly in the form of excessive rig time, unnecessarily large volumes of displacement fluids, and high fluid-disposal fees. This is especially true in deep water and other logistically-challenged drilling frontiers where total costs at the well-site can amount to hundreds of thousands of dollars per day. Displacement and solids removal is a key operation in well completions as any residual solids can potentially damage the producing formation or impair the function of completion tools.

Quantifying displacement performance
Because of the potentially high cost of poorly designed displacement programs, Baroid for the past four years has been vigorously researching the critical factors that can influence displacement effectiveness. Building upon earlier fluids research by Halliburton's Cementing PSL, Baroid spent literally thousands of hours testing each chemical constituent used in displacement treatments to determine its effectiveness against all types of water-, oil- and synthetic-base drilling muds currently used in the industry.

The performance of spacer chemicals and displacement fluids were tested at various shear rates to determine the cleaning characteristics as a function of shear rate. The amount of contact time required for effective cleaning was determined for each shear rate. Tests also were conducted to determine the maximum amount of surface area each spacer chemical can clean before it is consumed.

The maximum chemical effectiveness of Baroid cleaning chemicals were determined by repeatedly exposing known volumes of spacer materials to a deposited wall cake of known area and calculating the percentage of wall cake cleaned in each cycle, until the performance of each chemical began declining. This provided a volume per square foot value that is used to calculate how much chemical is required to clean the total surface area in the well while still maintaining reactive effectiveness. These data are embedded in an integrated approach to displacements that enables completion engineers to model the displacement design as a function of both the chemical and physical forces at work in the displacement process. Displacement performance is dependent upon how the displacement is designed around the characteristics and limitations of the specific chemicals being utilized.

Baroid's rigorous testing activities demonstrated that all these parameters are interrelated and are integral to the performance of a displacement program. In addition, Baroid also determined the set of physical parameters most likely to optimize the application of each spacer or displacement chemical when applied against a specific mud system. In addition to a family of new, more effective chemical spacers and flushing fluids, a new, highly-engineered approach to designing and implementing displacement treatments has emerged from Baroid's intensive testing regimen. This knowledge is providing the framework for a more effective, more efficient displacement service to be introduced commercially this year.

Achieving displacement goals
The first and foremost function of the displacement spacer train is to remove all of the existing wellbore fluid and separate incompatible fluid systems. However, ensuring that existing wellbore fluids are removed is complicated by the tendency of many drilling fluids to gel in response to elevated downhole temperatures.

Baroid has found that to optimize drilling fluid removal, a spacer treatment should be engineered based upon the rheological properties of the initial spacers used and the pumping rate needed to create the shear stress required to mobilize the drilling fluid and move it to the surface. Optimizing the removal of whole mud helps improve displacement performance by leaving less residual for chemical cleaning spacers. A poor result in this phase can render the chemical design inadequate if overwhelming volumes of solids are left behind for chemical flushes to remove.

The second phase of completion fluid displacements is to remove all remaining wall cake and residual solids. This phase of displacement is engineered differently than the first, where chemical flushes are designed to provide sufficient volume for required contact time and surface area coverage as a function of the shear rate and pump rate.

The third function of the spacer train is to provide additional carrying capacity to sweep solids out of the hole and to pre-treat the completion fluid with flocculants to assist in solids settling and help ensure maximum filtration rates. Any excess solids remaining following displacement can hamper displacement performance by requiring greater circulation and/or filtration time. While failed displacements are obvious, often the displacement can lead to unseen cost increases to operators where it cannot be objectively validated that several hours might have been saved.

Baroid, through its research, has established the relative shear rates, contact times, and annular velocities required to achieve all these objectives for each chemical, when applied in direct, indirect and modified displacement programs against all mud types. Baroid displacement software provides the tools to help determine the most efficient spacer-train compositions and volumes to achieve high cleaning efficiency over the entire surface area of wellbore tubulars. Displacement performance is enhanced further through software validation of the spacer parameters and pump rates recommended to create the hydraulic energy required to most efficiently accomplish the task.

New displacement software
Baroid's new software is designed to help ensure a properly engineered displacement service and can enhance the effectiveness of any displacement treatment. Proper fluid flow is critical to proper displacement, and the software can calculate the required pump rate for initial mud displacement for maximum removal before chemical flushes. Then, based upon research data, the software specifies the recommended spacer train as a function of the mud type, calculates required pump rates for chemical cleaners, and designs spacer volumes as an integrated analysis of contact time, shear rate, surface area coverage, and physical separation.

Further analysis defines the maximum pump pressure anticipated and, as a function of pump rate, the maximum pumping horsepower required to help ensure adequate pumping power is available to handle the anticipated pressures at the recommended pump rate.

In order to optimize chemical concentrations and the formulations of spacers and flushes, the Baroid displacement software can define potential deficiencies by analyzing each spacer to determine the flow regime, annular velocity, and vertical separation distance it will occupy in each annular section, any one of which can lead to a failed displacement. Any spacer parameter not meeting minimum specifications is highlighted to immediately catch the attention of the reviewer.

In addition, the software can model the displacement with variable pump rates helping operators save money and reduce displacement risk by eliminating the need for multi-function circulating tools.

Aggressive displacement recommendations
The enhanced understanding of the reactions of chemicals when used to displace any available drilling mud system, the new in-depth understanding of the pressure and flow dynamics during displacement treatments, and the more precise displacement designs possible through Baroid's new, highly engineered displacement service, can remove most of the guesswork from the task of transitioning from drilling to completing wells.

Together, these new tools are enabling Baroid completion engineers to recommend extremely aggressive displacement programs, in which smaller volumes of more effective chemical spacers are prescribed, helping reduce circulating and filtering times while minimizing disposal costs.

In a recent application on the Outer Continental Shelf of the Gulf of Mexico, Baroid displacement performance provided savings to an operator by reducing spacer costs by 45 percent, trimming 20 hours of rig time, and decreasing disposal volumes by 75 percent compared to five previous displacements in the same field by a competing oilfield supply company. In another instance, Baroid performed a displacement without incident at a deepwater well where failed displacement services by another contractor had resulted in 67 hours NPT, clear evidence of the debilitating effects failed displacements can inflict upon high-cost drilling projects.

In the past four years, the amount of NPT attributed to Baroid displacement treatments on 82 deepwater wells totals less than 0.05 percent of the total NPT.

In seven engineered, direct displacement programs involving synthetic-base fluid in seven deepwater wells, Baroid achieved full displacement in fewer than two circulations on average and recorded average displacement, circulation and filtering times of slightly less than four and a half hours per well. In addition, the volumes of spacer fluids used averaged about 20 percent of total well fluid capacity, significantly reducing both chemical and disposal costs.

Baroid's new, engineered displacement service is able to consistently achieve such unprecedented results because it is the only displacement technology that takes into account all the variables that can affect displacement performance. These factors include the reactions of displacement chemicals with whatever fluids are encountered in the well bore, as well as the pressure and flow parameters resulting from recommended spacer constituents and pumping rates.

The benefits that Baroid's new displacement technology can bring to the oil and gas well construction process will become more apparent when the newly-branded displacement service is launched in the next few months.