Organophilic-Clay-Free Invert-Emulsion Fluid Helps Drill Record-Length Well in the UAE
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This paper discusses the successful design, laboratory testing, and performance of an innovative, low-solids, organophilic-clay-free invert-emulsion fluid (OCF-IEF) used to drill the reservoir section of an extended-reach-drilling (ERD) well. This specially designed drill-in fluid helped maintain the key ERD factors within the specifications necessary and set new limits for drilling performance, thus maximizing the horizontal-section displacement and reservoir drainage and production output.
ERD is an operative practice for drilling high-angle wellbores with long horizontal displacement. Typically, a well design is classified as ERD when the horizontal displacement is at least two times the vertical-displacement true vertical depth (TVD). The longest well drilled in the UAE had a measured depth of 35,800 ft, with 18,800 ft of horizontal displacement and a 7,894-ft TVD (Fig. 1). This technique is considered one of the more technically challenging in the industry. However, because of increased reservoir contact, it is generally preferred to conventional well designs.
Proper fluid selection is fundamental during the ERD-well-planning phase. Software support is necessary to predict the well conditions and simulate the fluid behavior to maintain the equivalent circulating density (ECD) lower than the fracture gradient, maximize hole cleaning, provide wellbore stability throughout the well execution, and prevent losses. The different drilling parameters used to drill the horizontal section should also be considered during the planning phase to help enable correct simulations and optimize the execution accordingly.
During drill-in-fluid design, formation damage and production rates should be considered while maintaining the optimal mud properties for drilling and rate-of-penetration (ROP) enhancement. A comprehensive testing and validation process was compiled using a systematic approach to customize this specific well design. Clay-free invert-emulsion fluids (CF-IEFs) are designed to provide a much stronger gel structure than traditional IEFs. However, with the application of force, these gels break easily, thus reducing the wellbore stresses and pressure spikes. Progressive yet fragile gel strengths are important elements of a CF-IEF system that allow the cuttings to be suspended while maintaining low ECD and minimal circulation pressures compared with a clay-laden system. The organophilic clays and organophilic lignites are replaced with various organic polymers to achieve different fluid behaviors. Acid solubility to minimize formation damage was another important system-design criterion. This helped minimize formation damage and maintain proper bridging by use of ground marble with the correct particle-size distribution (PSD), determined by specialized software. Calcium bromide (CaBr2) was selected as the internal phase of the fluid, to minimize the amount of solids in the system and provide the density necessary for wellbore stability while maintaining the plastic viscosity (PV) at the lowest possible value.
The primary objectives of the drill-in-fluid design for this well included, but were not limited to, the following:
- Reduce the ECD by reducing the amount of solids in the fluid. This was achieved using a CaBr2 brine for the internal phase of the fluid.
- Achieve tight fluid loss using properly sized bridging material determined by specialized software.
- Maintain low sag factor because of longer liner-running times, with the fluid being static for a prolonged time.
A drill-in clay-free fluid system was used previously in the field. The system demonstrated superior performance and confirmed the achievement of all the key factors necessary to drill long lateral sections. However, an extremely long section (18,800 ft) presented additional challenges to be addressed. The system design showed high reliability during laboratory testing, but, during field applications, additional footage affected several factors, such as ECD, torque management, tool capabilities, fluid stability over an extended period, and logistics.
Fluid Properties. During the planning phase, significant focus was placed on the fluid properties. Long horizontal sections necessitated fine tuning of the fluid properties, such as mud weight (MW), rheology profile, PV, yield point (YP), low-shear YP (LSYP), yield shear stress, gel strength, percentage of low-gravity solids (LGS%), and high-pressure/high-temperature filtrate.
MW. Maintaining the hydrostatic pressure higher than the reservoir pore pressure and lower than the reservoir fracture gradient at all times was crucial. The primary challenge was maintaining stable MW throughout the entire section. The ultrafine cuttings generated affected the MW stability negatively during the phase execution. The MW fluctuations, in turn, directly affected the ECD values.
Rheology Profile. PV, YP, LSYP, low-end readings, and yield shear stress are all key parameters during the execution of ERD wells. To maximize hole cleanup, these values should be properly planned and maintained throughout the entire section. The nonorgano-clay viscosifiers used in the formulation can provide a flatter rheology profile over a full range of shear rates and can provide a fragile thixotropy to the emulsion gel structure when fluid flow is initiated. All of these parameters were managed properly by adding a novel polymers package that allowed for rheology-profile modification as dictated by the well.
Gel Strengths. The pressure necessary to break the drill-in-fluid gels is one of the primary concerns during ERD-well execution. High amounts of energy can be necessary to break the gels if their structure is strong. Application of high pressure to break the gels can induce losses, which are difficult to manage and cure in such well types. Drill-in fluid with fragile gel-strength structures helps minimize the pressure necessary to allow the fluid system to regain movement after pump-off. The minimal gel structure demonstrated by the drill-in-fluid system during the execution helped minimize induced-loss issues, which can be common with conventional oil-based mud.
LGS%. The amount of solids in the mud system that cannot be removed from the solids-control equipment increases the ECD value substantially. The objective was a maximum of 5% LGS during section-drilling operations. However, because of the presence of CaBr2 as the internal phase, calculation of the LGS% was altered, and 4 to 10% LGS values were recorded while drilling.
PSD. Fluid design was based on the limestone-reservoir permeability, which ranged from 10 to 50 md. To deliver a fluid system with the proper bridging capabilities and minimal effect on the solids concentration that directly affects the ECD, software support was used to design the bridging package. Sized calcium carbonate (marble) was used to achieve a D50 ranging from 5 to 50 µm. However, previous experience in the field demonstrated that shifting the D50 in the higher range of the window (25 to 50 µm) helped maintain the LGS% and mitigate the presence of colloidal solids.
Hole Cleaning and ECD. In this long lateral section (18,800 ft), hole cleaning and ECD were key factors that were evaluated in advance. Rheology optimization and software validation were necessary to understand the section limitations without exceeding the fracture gradient. A Herschel-Bulkley rheological model was used to improve representation of the actual fluid used during the drilling operations.
Fluid Stability. The process of pulling out of the hole from 35,800 ft necessitated significant time, particularly when operations such as back reaming were planned, and an additional extended period was necessary to run the completion string. During this time, the drill-in-fluid system was maintained in static condition at 200 to 220°F. This indicated that the system should be thermally stable for an extended period to help prevent potential sag issues. It was important to determine the susceptibility of the fluid to barite sag in the field. To evaluate the fluid behavior, a static aging test was performed at 220°F.
The drill-in-fluid system showed minor value fluctuations, with an overall thermal stability for the entire rheology profile and emulsion stability. Additionally, the sag index was recorded to determine the probability of bridging/weighting-material separation in the drill-in fluid during a prolonged period. The results showed stability during the 7-day test. The results of the static aging test were confirmed during the first circulation after 30 days from the end of the drilling phase. No pressure spikes or settling was observed during circulation before setting the liner hanger.
Friction Factor (FF)
Running 35,800 ft of tubulars into the well, with more than 18,000 ft of tubulars on the low side of the well, generates friction. The drill-in fluid should provide proper lubricity to minimize this. During operations, it was evident that the drill-in-fluid system provided a low FF and mitigated stress issues in the tubulars.
During section execution, an FF of 0.1 to 0.2 was recorded. Because of the low FF recorded, the completion string could be run with high confidence, without additional measures to reduce the FF. This was confirmed by running 19,007 ft of slotted liner without exceeding torque-and-drag values.
ROP. The ROP is typically a manageable variable. A higher ROP is generally preferred to minimize drilling time and potential tool failure and to reach the production zone faster. The drill-in fluid is directly affected by ROP variation. A higher ROP usually generates more cuttings and solids and consequently affects MW, PV, and ECD management. A properly designed drill-in fluid should increase the ROP and properly handle the solids without affecting the fluid properties. The following ROP records were achieved while drilling the longest well in the UAE:
- Maintained 2,103-ft/D average ROP throughout the section
- Achieved 2,584-ft/D maximum ROP
The properly designed drill-in-fluid system was able to manage the amount of solids generated without affecting the drilling parameters.
Organophilic-Clay-Free Invert-Emulsion Fluid Helps Drill Record-Length Well in the UAE
01 May 2018
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