The challenge of developing a water-based mud as a viable alternative for oil- or synthetic-based emulsion mud has presented itself to the drilling fluids industry for decades. Baker Hughes Drilling Fluids (BHDF) has achieved a breakthrough recently through a systems design approach to emulate the performance attributes inherent to emulsion mud in a new high performance water-based mud system (HPWBM). The development of the PERFORMAX HPWBM is representative of the innovative drilling fluids research and development effort at the newly created BHDF division. The new division was announced last September during the SPE conference in Houston. BHDF is responsible for the oilfield drilling fluids, completion fluids and fluids environmental services businesses. These businesses were formerly part of the Inteq division. The newly formed company supports its drilling fluids services with two major engineering centers, a network of area laboratories, and a worldwide distribution system, as well as a team of trained professionals at regional offices and well sites.

Emulsion mud attributes

The primary drilling performance attributes of emulsion systems are:
1) shale stability;
2) gumbo and clay stability;
3) cuttings stability and solids removal efficiency;
4) high rates of penetration (ROP);
5) minimized bit balling and accretion;
6) torque and drag reduction; and
7) minimized differential sticking.

Using a systems-design approach, BHDF selected additives such that PERFORMAX delivers drilling performance approaching that of emulsion muds.

Shale stability

The most important factor in maintaining shale stability is preventing pressure increases in the shale matrix. Emulsion muds are the ultimate membrane-generating systems and provide superior drilling performance primarily because of their ability to reduce pressure transmission in shale. The new fluid is designed to closely emulate the shale stabilizing mechanisms of an emulsion system by generating a selective membrane and creating an osmotic pressure difference between the mud and shale. Pore pressure transmission (PPT) tests were performed comparing it to conventional water-based mud (WBM) and synthetic-based mud (SBM). Performance similarities between these fluids in reducing pressure transmission in shale were observed.

Clay stability and cuttings encapsulation

Another area of "inhibition" is the stabilization of reactive clays and gumbo encountered when drilling upper hole sections. The inability to suppress hydration and dispersion of reactive clays and gumbo leads to problems such as bit balling, accretion, poor solids removal efficiency, high dilution rates and problems managing rheological and filtration control properties. An effective and proven mechanism to suppress hydration and dispersion is cationic exchange at reactive sites on clays. The new water-based fluid uses a mixture of polyamine derivatives for suppressing the hydration and dispersion tendency of reactive clays and gumbo.

Partially hydrolyzed polyacrylamide (PHPA) is used for cuttings encapsulation in the system. The combination of polyamine chemistry to suppress hydration and dispersion, and PHPA to encapsulate cuttings, has resulted in solids removal efficiency over 80% on onshore and offshore wells drilled with the new mud formulation. This efficiency, in turn, has led to lower maintenance and dilution rates, and lower well costs.

Drilling problems addressed

A wide range of drilling problems is addressed by PERFORMAX high performance water-based mud systems.

ROP, Bit Balling and Accretion. Emulsion systems are the "mud-of-choice" in many drilling operations because of the economic benefits derived from achieving high ROPs in combination with polycrystalline diamond compact (PDC) drill bits. PDC bits are typically used with emulsion systems because of extended bit life, increased footage and a reduction in the number of trips for changing bits. The new fluid contains an ROP enhancer, which is designed to preferentially "oil-wet" metal and shale surfaces using environmentally approved base fluids and surfactants.

Torque and Drag Reduction. Friction factors for invert emulsion muds typically range from 0.16 to 0.57 and 0.21 to 0.67 for water-based muds (average of cased hole and open hole). By comparison, friction factors for the new fluid range from 0.13 to 0.17, averaging 0.15. These friction factors values include both cased and openhole frictional forces.

Environmental Acceptance. The fluid fully satisfies environmental requirements for use in the Gulf of Mexico and the UK-sector of the North Sea.

Return Permeability. PERFORMAX has been extensively tested in return permeability tests and proven to be non-damaging to production formations in laboratory tests and verified in field production rates.

Field tests of the new system

A process of testing the fluid, gathering data and gaining experience on increasingly difficult wells was implemented. Emphasis was placed on comparison to offset well data as performance metrics. Testing on onshore and offshore (shelf and deepwater) wells provided a broad knowledge and experience base.

South Texas

PERFORMAX was used in South Texas to evaluate the system as an alternative to oil-based mud (OBM). Here, water-based muds have performed poorly in the 121/2-in. interval due to shale instability, with several days of operating time typically lost to reach total depth and log the well. Wellbore instability problems with WBM typically lead to an average of 2 to 3 days of lost rig time during logging operations. It reduced pore pressure transmission and allowed the operator to successfully drill, stabilize and log the interval at a mud weight comparable to that of OBM. The interval was drilled in record time and reached total depth with the lowest mud weight of any offsets, including OBM. The interval drilled with the new fluid also had the highest average footage per day reported on daily reports. Figure 1 shows a comparison of average washout, from caliper logs, for the new HPWBM, OBM and one of the better WBM offsets.

Gulf of Mexico deepwater

The new fluid was also used on a deepwater well in the Gulf of Mexico. The system performed well in drilling the 121/2-in. interval at a maximum angle of 45? by stabilizing reactive clays and gumbo at high ROP. A total of 2,543 ft (775 m) of 121/2-in. hole was drilled, with 957 ft (291 m) (38 %) drilled while sliding and 1,586 ft (483 m) (62 %) while rotary drilling. ROP varied from 20 ft/hr to 200 ft/hr (6.1 m/hr to 61 m/hr) with instantaneous ROP above 300 ft/hr (91.5 m/hr) using a rock bit (Figure 2). The average ROP when sliding was 85 ft/hr (26 m/hr), while the average ROP when rotary drilling was 94 ft/hr (28.6 m/hr).
Bioassay (LC50) results were well above the minimum required for discharge (30,000 ppm), ranging from 225,000 ppm (mud plant) to greater than 500,000 ppm for the field mud. Friction factors were calculated while drilling and building angle to 45? and compared favorably with values obtained for SBM offsets. Return permeability testing showed the fluid's low potential for formation damage, with results greater than 92 % return permeability. Flow line temperatures averaged 54?F (12?C) and it was processed over solids control equipment without problems.

High performance from a water-based formulation

A new HPWBM, designed to emulate the performance characteristics of OBM/SBM, has been developed.
The HPWBM has been successfully field tested on onshore and offshore (shelf and deepwater) wells and proven to be performance and cost competitive with OBM/SBM.

The HPWBM has eliminated the environmental risks and costs associated with waste management of OBM/SBM.

The system is environmentally friendly and has been approved for use in the Gulf of Mexico and UK-sector of the North Sea.

The HBWBM cleans up easily prior to completions and has proven to be non-damaging to producing formations.

Comprehensive fluids technology

BHDF offers an environmental quality assurance process known as Fluids Environmental Service (FES). This process helps customers meet performance and environmental objectives by taking a comprehensive, engineered approach in providing drilling and completion fluids, solids control and waste treatment, as well as cuttings handling and disposal. The drilling industry has been working for some time to improve environmental performance and meet increasingly strict environmental standards by replacing oil and synthetic based emulsion systems with water base fluid systems.

For more challenging applications, BHDF has been on the forefront of developing synthetic-base emulsion systems for drilling in tough downhole conditions, such as deepwater wells, while meeting stringent environmental requirements. One example of a "compliant fluid" is the SYN-TEQ CF system, which has been developed specifically to meet discharge requirements in the Gulf of Mexico.

Complimenting the SYN-TEQ CF system, the company also is introducing innovative invert emulsion systems for drilling in hostile environments, such as deep, high pressure/high temperature (HP/HT) and highly deviated wells. The MAGMA-TEQTM HP/HT fluid is an emulsion-based system offering unique suspension properties to avoid density differentiations (barite sag) frequently encountered in deviated wells.

The concept of reservoir drill-in fluids was introduced in 1994 with the PERFFLOW system. Since then, the company has developed several new drill-in technologies. The SCIFLOWTM DIF system maximizes hydrocarbon recovery by minimizing solids and fluid invasion into depleted or fractured reservoirs. The company also has developed proprietary software to determine the optimum particle size distribution necessary to bridge pore openings that may otherwise be damaged during reservoir drill-in.

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