New Flat-Rheology Synthetic-Based Mud for Improved Deepwater Drilling
- E. van Oort (Shell EP Americas) | J. Lee (M-I SWACO) | J. Friedheim (M-I SWACO) | B. Toups (M-I SWACO)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 26-29 September, Houston, Texas
- Publication Date
- Document Type
- Conference Paper
- 2004. Society of Petroleum Engineers
- 2.2.3 Fluid Loss Control, 1.6 Drilling Operations, 3 Production and Well Operations, 1.12.6 Drilling Data Management and Standards, 1.7.6 Wellbore Pressure Management, 2.5.2 Fracturing Materials (Fluids, Proppant), 1.11 Drilling Fluids and Materials, 5.3.2 Multiphase Flow, 2.1.7 Deepwater Completions Design, 1.14 Casing and Cementing, 1.6.10 Running and Setting Casing, 4.2.4 Risers, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties)
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In deepwater drilling operations, synthetic based mud (SBM) is the preferred drilling fluid because it delivers high drilling rates and excellent wellbore stability. However, lost circulation problems caused by adverse elevation of viscosity and increase in equivalent circulation density (ECD) as a function of temperature and pressure often result in significant operational cost increase. Controlling the pressure and temperature dependence of SBM rheology thus is an important step towards achieving successful and cost-effective deepwater drilling operations.
This paper highlights the development and application of an improved SBM that optimally balances the requirements for excellent rheological properties, ECD control, and improved barite suspension. Unlike conventional SBMs, the new SBM exhibits a constant ("flat") rheological profile over a wide temperature and pressure range. For instance, certain key rheological parameters, such as 6-rpm reading, yield point, and gel strengths, remain virtually unchanged when temperature and pressure are varied. This flat-rheology characteristic allows for a higher viscosity to be maintained without negatively affecting drilling rate or ECD. Moreover, cuttings carrying capacity and barite suspension properties are greatly improved.
The flat-rheology profile is achieved through the usage of a re-designed package of emulsifiers, rheology modifiers and viscosifiers. The emulsifier package minimizes the impact of drill solids on the rheological properties of the new SBM. Optimal rheology modification is achieved through minimal use of organophilic clays. Moreover, a new rheology modifier also reduces the key viscosity parameters at low temperatures while raising them at high temperatures. The viscosifier is used to provide the desired enhancement in overall viscosity and suspension capacity.
Extensive field data from Gulf of Mexico (GoM) wells (including drilling performance data, ECD and downhole pressure data, etc.) shows the benefits of the new flat-rheology SBM (FR-SBM) system, including lowering of ECD (resulting e.g. in lower SBM mud losses), improved cuttings evacuation from the well and prevention of barite sag problems. The operational success of FR-SBM has led to rapid acceptance in the field, allowing it to displace conventional SBM in Shell's GoM deepwater drilling operations.
The utility and reliability of synthetic based muds (SBMs) as drilling fluids has been well documented in the past decade1-3. Often it is the fluid of choice in deepwater environments due to its superiority in achieving high penetration rates and maintaining desired wellbore stability. One drawback to the use of SBMs is their potential for lost circulation, especially when the mud has been static for an extended period of time.
In the deepwater environments, water temperatures easily reach 40°F (5°C) and below. This low-temperature environment effectively cools down the drilling fluid, significantly increasing fluid viscosity, which in turn impacts equivalent circulating densities (ECDs) and surge pressures. Narrow drilling margins (i.e. the window between fracture gradient and pore pressure) encountered in deepwater drilling operations often make such rheological increases intolerable, resulting in severe losses of SBM and thus significant increases in fluid cost and rig time.
One method of managing these critical ECDs is to manage the fluid rheology. In general, a thinner fluid yields lower ECD's. However, fluid rheology considerations not only have to address ECD control but also must take into account cuttings removal and barite suspension for specific wells. Thus, the main challenge to fluid design in most deepwater drilling operations consists of effectively balancing fluid rheology for hole cleaning, ECD and barite suspension simultaneously. This concept is illustrated in Fig. 1. Since fluid rheology is the common denominator of all these issues, a re-design of SBM to minimize the temperature dependence of rheological properties was believed to be a promising approach to alleviating the difficulty routinely encountered in achieving the desired balance among all these issues.
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