Can an all-in-one mud work?

Due to peaks in directional and logging tools, the scope for game-changing drilling technologies is narrowing. Increasingly, the industry is focusing on other areas such as expandables, casing while drilling and mud systems.

Greater production, environmental exigencies and economies of scale represent major drivers. In the case of muds, greater production because permeability is retained; environmental exigencies because drilling fluids must be environmentally friendly. So the need for fluids that minimize reservoir or environmental damage is unquestioned. That follows, but economies of scale? Casing and bits can be bought in bulk, but the question of standardized muds seems more complex. Typically, formation characteristics in individual wells and wellbore conditions vary so much from top-hole to reservoir that the idea of a universal mud seems almost naive.

Most mud companies offer as many drilling fluids as there are problems; wellbore instability, lost circulation and reservoir invasion are diagnosed and treated separately. But this approach always has been challenged by standardized systems that promise rationalized inventories and logistics. Operators have long sought noninvasive systems that not only stop losses, but also retain reservoir permeability. One way of achieving this is through underbalanced drilling (UBD). However, this practice is only valid in certain applications due to economic and technical restrictions. Another way of minimizing reservoir invasion is by using effective noninvasive fluids. Although the term noninvasive may not suit the purists - there is always some liquid invasion at the reservoir surface - it sums up the concept: particles do not invade the reservoir.

In overbalanced conditions, whether static or dynamic, all drilling fluids exert pressure on the formation. What is special about the noninvasive fluid is that fluid particles behave differently than those of conventional fluids. Due to the nature and distribution of conventional fluid particles, the overbalance pressure forces the particles to invade the rock. Over time, the invasion deepens, often causing bridging, filter cake formation and poorer permeability. Particles in noninvasive fluids act differently. Once in contact with formation, they are instantly attracted to each other and form a seal. This seal is no more than a few millimeters thick, remains at the surface of the rock and prevents particle invasion and protects permeability. What is unique about the seal is its reversibility. Once reservoir flows are initiated, the sealing membrane easily comes off the rock surface, enabling uninhibited production. Because there is no internal filter cake, a high percentage of the original permeability of the rock can be expected.
Laboratory and field research shows that with this type of sealing mechanism, formation invasion is limited to an initial period only. The solids content of the fluid is low, typically less than 10 ppb, and this permits the formation of a thin and impermeable membrane at the reservoir face. Instead of bridging the formation and enabling further invasion, the noninvasive fluid seals it. Because the particles inside the fluid are attracted to each other, they congregate at the surface of the rock to form a seal. This means drilling fluid cannot enter rock pores, but at a given underbalanced reservoir, fluids can flow back, removing the sealing membrane at the surface rock. Different formations with varied pore size distributions can be handled with the same fluid composition due to congregation. The system is biodegradable and nontoxic, but also offers a series of benefits:

* heterogeneous permeable formations including microfractured shale can be sealed;
* leak-off pressures can be increased;
* the range of mud weights can be widened; and
* the overburden and reservoir can be drilled with the same fluid, including completion.
Leading the way in noninvasive technology is Impact Engineering Solutions.

"By effectively sealing against a permeable formation while drilling, the noninvasive fluid develops what can be called virtual casing," said Vice President of Technology Dr. Helio Santos. Field applications for several operators have highlighted a marked reduction in stuck pipe incidents, fluid losses, wellbore instability problems and minimized formation damage. An indirect benefit was benign effect on internal flow paths of bottomhole assembly components and a lower incidence of mud packing. Operators have estimated substantial savings due to not having to run a casing string and as a result of reduced mud transport and storage costs. Several state oil companies, multinationals and independents, such as Ecopetrol and Chesapeake, have used the noninvasive fluid.

The fluid comprises a viscosifier, a dynamic fluid loss reducer (not for reduction of API filtrate) and a lubricant. A fourth additive is recommended in severe loss zones, when drilling formations with fractures and vugs, for example. It should be kept as contingency and used immediately to restore circulation. In addition to these products, the final fluid should have a pH of around 10, salinity as desired to inhibit shales (any kind of salt and any concentration can be used, even formates), and weight material as needed (barite or hematite). As the sealing membrane is located at the surface of the rock and the particles do not penetrate deep into the rock, the seal offers extremely low resistance once overbalanced pressure is removed. As flowing starts out from the reservoir, the seal is washed away permanently.
The technology is relevant in many drilling situations, not just minimizing reservoir damage but also in dealing with problematic interbedded shale formations or reservoirs with different pressures exposed in the same openhole section.


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