Solids-Free Oil Base Fluids: An Option for Running Complex Completions?

Solids-Free Oil Base Fluids: An Option for Running Complex Completions?

N. Alfonzo, F. Almuqati

Saudi Aramco, Dhahran, Kingdom of Saudi Arabia

M. Dourado

SLB, Dhahran, Kingdom of Saudi Arabia

Summary – Extended-reach wells (ERD) pose a significant challenge for deploying complex completion assemblies due to operational constraints and friction reduction requirements. Completion assemblies limit the amount of weight and applicable torque that can be applied, creating the need to enhance downhole lubricity conditions. This work focuses on developing a weighted solids- free oil base (SFOB) alternative where lubricated brines cannot match well design requirements. The project aims to deliver an integral solution capable of covering friction reduction requirements, long-term stability, and rheological parameters and capable of being placed and removed without any hydraulic or chemical cleanup limitations. Thus, the lab work was divided into emulsifier screening, short- and long-term formulation optimization, and fluid parameter assessment. The evaluated fluids were selected to cover a wide range of densities as per the well design candidate requirement. Hydraulic simulations and specialized cleanup testing complemented fluid evaluation to identify the best technical option.

The suitable fluid formulations considered oil/water ratios from 4555 to 7030, using heavy brines as the internal phase to match density requirements of 9.9 up to 11.5 ppg. Room temperature and reservoir static aging tests at sequential periods up to 14 days have confirmed fluid stability. A reduction of 50% in the coefficient of friction was obtained on SFOB when compared to glycol- lubricated brines. The rheological profile of SFOB was reduced to mimic the lubricated brine rheology profile, aiming to obtain a similar pressure profile during displacements and facilitate any required cleaning operations. The designed SFOB achieved low-end readings resembling brine profiles while delivering fairly lower plastic and higher rheological readings.

This paper illustrates the development of a solids-free weighted OBM capable of working under an ERD-wide variety of conditions, whereas brine-based lubricated fluids show limited performance.

INTRODUCTION

ERD-focused projects have historically challenged the operational and technical boundaries of O&G field developments since the 1980s. Well designs and execution pose a complex combination of demands where the horizontal displacements tend to be multiple times longer than the vertical depth. The O&G industry defines ERD wells as those with a ratio above two times the horizontal displacement vs. the vertical reach.

Also, ERD wells tend to expose considerable lengths of reservoir with varying producing features, bringing the need to install complex completion assemblies capable of delivering flexibility in the wellbore production life. Nevertheless, complex completion assembly deployments bring more limiting factors to the execution operation. Their configuration tends to limit the circulation parameters, mechanical maneuvering, and difficult fluid displacements.

Singh et al. (2018) discussed the different optimization approaches for securing successful complex completion assemblies in ERD wells. This ensures proper wellbore condition, cleanliness, and stability prior to running the planned assembly. Thus, two different areas should be optimized as a whole, namely, mechanical and hydraulic.

• Mechanical encompassed all downhole and surface tools, helping to decrease torsional and axial restrictions. This could go from string redesign aiming for higher net downhole weight, a flexible assembly configuration, string reaming accessories, tools to allow partial or total string floating up to swivels allowing selective string rotation.

• Hydraulics embraces completion fluid selection, placement, and removal. The completion fluid selection would depend on its capabilities to deliver all required lubricity, being a reliable hydraulic barrier, stability, and flexibility until running the completion assembly and allowing delivery of the well with the selected packer or suspending fluid. Thus, the completion fluid could go from a conditioned reservoir drilling fluid, treated brines, or specialty fluids.

In terms of fluid options, Singh et al. (2018) and Beaman et al. (2016) demonstrated oil-based fluids offer the highest available lubricity and stability for ERD applications. Laboratory and field evidence have demonstrated the superior lubricity performance of OBM over conventional brines or WBMs. In these applications, OBM formulations are adjusted to reduce the solid load and minimize potential reservoir impairment.

Invert emulsions with a higher density internal phase have been used in Arabian Gulf (Mena and Gaber, 2017) and Nordic region (Hussain et al., 2021) areas with positive results. The operational sequence allowed the presence of solids for the completion run, and the fluid design accommodated a solid fraction as part of the considered fluid. Nevertheless, in complex completion assemblies requiring total absence of solids and lack of possibilities to float it while running it due to assembly configuration or well control requirements, OBM does not represent a suitable option.

Migjie et al. (2010) described significant advances in brine lubricity improvement through soluble additives and exemplified the importance of brine selection to improve the potential overall system lubricity. However, attainable values might not represent a low enough lubricity coefficient for the application envelope of ERD with 25,000 ft/4,500 ft (Measured Depth-MD/True Vertical Depth-TVD) and above ratios. The required lubricity coefficient for target formulations lies below 0.12, where there is severe limitation to obtaining stable solutions for clear brines with density requirements in the range of 76 to 86 pcf.

Figure 1 – Effect of Friction Factor in the Surface Hook Load (example well from targeted in this study)

Figure 1 is an example of the dramatic impact the friction factor has when running strings in complex wells. At 19,000 ft, an optimized lubrication package was applied to the circulating system, and there was a notable improvement in torque and slack-off, allowing an efficient trip and further completion runs. Nevertheless, this lubrication package requires additional care to limit the risk of separation, along with further circulation steps.

As discussed in this literature review, a suitable lubricious fluid for running completions in ERD represents a critical piece or enabler to reach programmed depths. This takes more importance at shallow TVD, where there are mechanical limitations to pushing through a string weight or rig. On the other hand, execution envelope requirements for upper intelligent completion deployments in extended ERD impose stringent conditions on fluid design and field delivery. Suitable solutions should comply with: solids-free,

high lubricity, long-term stability, hydrostatic adjustment flexibility, and ease of fluid placement and removal once the completion has reached programmed depth. As discussed in the reviewed literature, brine-based fluids have limited capabilities, so there is a need to explore further fluid-lubricious options. Finally, this paper illustrates te development of a weighted SFOB capable of working under a wide variety of conditions and suitable for ERD smart completion deployments requiring clean fluid environments. This will represent a unique approach to untap more options for deploying smart completion in steadily more demanding ERD wells.

Fluid Design and evaluation

Like any drilling and completion fluid, the design criteria are driven by operational requirements to ensure all steps of operation are successfully executed. As previously mentioned, the driver for system development was running complex downhole completion equipment over an ERD well. This requested a very specific set of requirements listed and explained as follows:

• Mixable and pumpable under standard rig condition operations: the rigs are equipped with a basic set of equipment without flexibility in upgrades. So, adapting to rig conditions was a must.

• Solids-Free: The challenge of ERD completion requires a fluid with the lowest possible coefficient of friction, and the presence of solids substantially increases such value. Additionally, the lower completion screen had a tight limitation for solids utilization, so the density had to be achieved by balancing the concentration and type of brine with the volume of the external continuous phase.

• Low rheological profile: the operations may involve displacement in tight annular clearance, and such conditions demand a fluid with a low rheological profile to allow execution with operational pressure limits.

• Flexibility of density adjustments between 76 (10.1) and 86 (11.5) pcf (ppg): operational requirements dictated the density of the fluid to be used. For such density adjustments, changes in the type of salt, concentration of salt, and/or the oil/water ratio can be used. Such variations can have extremely detrimental effects on the rheological profile of the fluids and promote a quick destabilization of the system. This work focuses on the stated density range, but lower densities are much easier to achieve.

• Long-term stability: the main application of the fluid would be in running extremely complex downhole completion equipment, and such operations can last several weeks from the moment of spotting the fluid until the operations are finished. Stability is defined as no phase separation (emulsion stability) and no gelation (no significant rheology increment) while static for 14 days at downhole temperature.

Besides operational demands, the need to utilize the available products was also a critical factor for SFOB design. A Flat Rheology Oil Based System (FROBS; Alomair et al., 2021) is heavily used in the area of SFOB application, and combined with the also available ultra-high temperature emulsifier (UHT; Stamatakis et al., 2012), it enabled access to a robust set of chemicals for the formulation of the SFOB, delivering all required properties.

The testing protocol considered the operational design demands going from fluid design, integrity throughout period of time and rheological parameters.

The initial round of tests with 76 pcf was executed with five different formulations as follows:

Table 1- Initial Round of Formulations for 76 pcf

The fluids were mixed as per the concentrations stated in Table 1, and below are the images obtained a few hours after mixing.

Figure 2 – Phase Separation observed in Initial Fluid Mixing

Figure 1 clearly demonstrates that all fluids had a distinct phase separation after a few hours in static conditions. However, formulation 5 indicates that a combination of emulsifiers would enhance the stability of the fluid mixture and enable a viable solution.

The second round of tests, presented in Table 2, shows the increment in concentration of the FROBS emulsifier in the formulation with further increments of the FROBS Organophilic Clay.

Table 2- Formulations Tested for 76 pcf SFOB

1

The formulation stated as F6 in Table 2 shows a very similar rheology profile to F7 while using a reduced set of chemicals, and for this reason, it was chosen to ease the logistics of SFOB fluid mixing. Figure 3 shows that both fluids remained stable without any signs of phase separation after 14 days of static at downhole conditions. As the fluid delivered all required properties, this stage was accepted as complete and moved to a design of 86 pcf.

Figure 3- 76 pcf SFOB after 14 days aging at downhole temperature

As presented in Table 3, the starting formulation for the 86 pcf fluid was the same concentration used for the best-performing 76 pcf fluid. However, given the lower non-aqueous to brine ratio, the observed rheologies were much higher. As this was anticipated to happen, several other formulations were run in parallel for an initial screening of the fluid, varying the amount of FROBS Organophilic Clay and UHT Fluid Emulsifier.

While the results after initial stress aging seemed promising with the SFOB 6, the 14-day stress aging results indicated an increment in rheology inadequate for the lower completion running operations. Nevertheless, the SFOB 6 with 86 pcf remained stable without phase separation for the whole static aging period (Figure 3).

Table 3 – Initial Round of 86 pcf Formulations

Figure 4 – 14 days static aging SFOB 6 – 86 pcf Non-Optimized fluid formulation

The following formulations focused on reducing the rheological profile in the static period while keeping the fluid stable during the required 14-day period at downhole temperature.

Table 4 – Second Round of Tests – 86 pcf SFOB

Table 4 shows the results of the second iteration, and while the fluid SFOB 7 demonstrates a low rheology profile, it also shows some phase separation, as can be seen in Figure 4.Error! Reference source not found.

Figure 5 Phase Stability of SFOB 9 and Phase Separation of SFOB 7

The phase separation in SFOB 7 can be explained by the reduction in the concentration of the FROBS emulsifier combined with the reduction in the FROBS organophilic clay concentration. The rheology profile of SFOB 8 was understood to be beyond acceptable after some days of testing and was abandoned. The SFOB-9 demonstrated a stable profile without any phase separation, and the rheology profile was essentially stable during the whole period of the test.

Lubricity is the most critical property to be delivered by the SFOB being designed, and as such, comparing it with industry-known high-performance products is a must. According to Foxenberg et al. (2008), the class of PLC (phospholipid compounds) can deliver a low coefficient of friction in brines while ensuring full dispersion up to high temperatures.

To assure adequate comparison, the reference effect of SFOB 9 was compared with the performance of pure brine and lubricated brine using a High-Performance Brine Lubricant (HPBL) as per Figure 5.

Figure 6- Coefficient of Friction of SFOB and Lubricated brines at 150 in.lbs

Figure 5 shows that the coefficient of friction of the SFOB 9 is superior to the highest standards of lubrication provided by the HPBL, with a reduction of 27.5% in the coefficient of friction when compared with the lubricated brine of 86 pcf.

Displacement Simulations

The SFOB will be used at a specific moment of the operation, and a proper design of displacement is required. To assure all pressures would remain within operational boundaries, proprietary software (VDS, or Virtual Displacement Software) was used to evaluate the impact of flow rate combined with the rheological profile in the various annular clearance scenarios.

The assessed scenario has two displacements for the proposed SFOB: the first is after the wellbore clean-out string reaches the bottom, the well is displaced from the brine that is filling the hole to the SFOB, and the second is after the middle completion is set in the hole and the SFOB has already provided the required lubricity to enable trouble-free trips in the hole.

The first displacement does not pose any pressure or flow rate limitations, as the well will be cased with a conditioning string and bit inside the hole. The second displacement is the one posing potential pressure and flow rate limitations. For the theoretical scenario, a limitation of 400 psi, and 3 bpm, was used in the simulation software.

‌Figure 7 – Displacement in hole – No Pressure or Flow Rate limit

As can be seen in Figure 7 – Displacement in hole – No Pressure or Flow Rate limit, the scenario is simulated with a pressure limitation of 4000 psi, and the VDS assures full utilization of the available pressure environment, indicating a potential flow rate of 12 bbl/min.

‌Figure 8 – VDS scenario using a limitation of 400 psi or 3 bbl/min due downhole tools operational limits.

Figure 8 – VDS scenario using a limitation of 400 psi or 3 bbl/min due downhole tools operational limits. shows the simulation considering the potential downhole tool limitations of pressure (400 psi) and/or flow rate (3 bbl/min), and it is observed that the VDS has adequate flow rate to accommodate the limitations throughout the displacement to assure operations are kept within limits. Both simulations demonstrate displacement feasibility within operational envelope.

SFOB Re-Usability

The nature of the SFOB is such that it can be stored and re-used in multiple operations with minimum interventions such as infrequent shearing to assure homogeneity of the fluid every 30 to 60 days and proper screening to remove any undesirable solids from the SFOB prior to storage. If any contamination occurs, the fluid is easily treated to regain its original properties.

If storage is not the preferred choice, the SFOB can be mixed with regular FROBS drilling fluids as a re-use approach. The below direct laboratory assessment is a straight mixture of a 1:1 ratio of SFOB (86 pcf) and a FROBS (100 pcf) reservoir drilling fluid (barite-free).

Figure 9 shows the properties of the mixture immediately after a mix of both fluids, showing complete miscibility and no indication of incompatibility. The additional adjustments would depend on the temperature of use, as SFOB shares several of the chemicals used in the FROBS.

Figure 9 – Mixture of SFOB 9 and 100 pcf FROBS

OBM Clean-up Assessment

Optimized hydrocarbon recovery depends on the efficient displacement and removal of the SFOB fluid and related residues from the wellbore before a solid-free brine is placed. The main challene with cleaning up invert emulsion oil-based mud lies in its tendency to form stable emulsions that are resistant to traditional cleaning methods. These emulsions can trap contaminants and prevent effective separation of the mud from water or metal, making cleanup efforts time-consuming in order to filter the brine and costly, resulting in an obstructed production flow.

Quintero et al. (2008) discussed the properties needed in a cleaning fluid. High cleaning efficacy, resistance to emulsion, and compatibility with the completion fluid are all requirements for the displacement fluids. These fluids are designed to water-wet the completion assembly and remove debris, cuttings, and other contaminants from the wellbore. Laboratory tests need to simulate downhole conditions with the designed SFOB to represent the cleaning efficiency of the displacement fluids. The lab tests are conducted to evaluate factors such as viscosity, cleaning efficiency, and the chemical compatibility of cleaning fluids. These tests help to make informed decisions about which cleaning fluids will be most effective in this particular drilling environment.

Another factor to take into consideration is the displacement of the cleaning fluid into the wellbore. One of the main challenges in fluid displacement is ensuring that the cleaning fluid reaches all parts of the wellbore. This can be particularly challenging in ERD wells where gravity may not assist in moving the fluid throughout the entire length of the wellbore. Another challenge in fluid displacement is maintaining the desired rheological properties of the cleaning fluid throughout the process. Changes in temperature,

pressure, and chemical interactions can all affect these properties, potentially leading to reduced effectiveness of the cleaning process.

The clean-up assessment is executed through laboratory analysis, with the final fluid to be used in the hole and the available surfactants to be implemented in the operation. Ideally, such exercises are done with the available material, and the results are uploaded to the VDS software to aid in the simulation and obtain an enhanced assessment of the operation.

‌Figure 10 – Cleaning Assessment to be used in VDS tool

As can be seen in Figure 10, the chemical cleaning assessment is executed through a visual and weight inspection of the rheometer bob after it is exposed to the fluid (in this case, the SFOB) and the cleaning chemical. Under the evaluated conditions, the SFOB could be removed within expected operational requirements.

CONCLUSIONS

Solids-free oil-based fluids enable extending the design and execution envelope for completing extreme ERD wells where lubricated brines do not meet lubricity, and stability requirements. Also, it is a valid option to be a low-solids-free drilling fluid with the capability to be adjusted for a solid-free completion fluid while completing ERD wells. The following points summarized the worked done:

• The emulsion stability was confirmed with the exposure of the SFOB for periods of up to 14 days to a downhole temperature environment where no phase separation was observed and the fluid rheology remained stable to execute any operation required.

• The proper design of the displacement is required to ensure all pressures remain within operational boundaries. Proprietary software was used to evaluate the impact of flow rate combined with the rheological profile in the various annular clearance scenarios.

• Laboratory tests demonstrated feasibility to achieve water-wet environment after displacing SFOB for delivering the wells suspended with a selected brine.

• The designed SFOB uses chemicals that are widely used in the region and are fully compatible with fluids regularly used, being capable of being converted to regular FROBS or to an ultra-high temperature non-aqueous fluid. Given the nature of the SFOB, it is also easily recovered from operations and can be stored to be used in following completion operations.

Target operations consider running upper completion inside lower completion with total isolation from producing formation. Thus, to have a general approach, this work could be complemented with return permeability studies to assess formation interaction with potential selected producing formations. Also, further study can be done to convert the SF OBM to a drill in fluid, allowing to minimize the fluid usage footprint with a single solution for drilling and completion phases.

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