Best described as a cross between foam and micelles, aphron-based drilling fluids can control fluid loss in mature reservoirs, enabling operators to reach total depth in productive zones beneath them.

A uniquely engineered drilling fluid system that combines certain surfactants and polymers to create a system of "microbubbles" has been credited with playing a major role in helping operators develop highly depleted reservoirs once deemed unprofitable.

The system was first used extensively in Venezuela's Lake Maracaibo, where it is credited with salvaging the economic prospects of reservoirs so depleted that well construction efforts often failed. Since then, the aphron-based fluid invasion control system has been used successfully in hundreds of wells worldwide.
Employed primarily to drill reservoirs prone to massive lost circulation, the microbubbles, also referred to as aphrons, are encapsulated in a distinctively viscosified system. Since these aphrons are non-coalescing, they create a microbubble network that blocks or slows the entry of fluids into the formation. The exclusive viscosity builds to create a resistance to movement into and through the zone, thus generating a true non-invasive and at-balance fluid. Field and test data confirm its enhanced hole-cleaning and suspension properties.

The technical problems associated with drilling the depleted reservoirs intrinsic to many of the mature fields throughout the world often make further development uneconomical. The water-wet sands that typify many of these zones generate seepage losses and differential sticking, both of which are extremely expensive to correct. Uncontrollable drilling fluid losses frequently are unavoidable in the large fractures characteristic of these formations. Furthermore, the typical laminated sand and shale sequences create conditions that can make drilling unduly expensive and dangerous when using conventional rig equipment. Consequently, these and a host of associated problems have led some operators to forgo continued development of these promising, yet problematic, reservoirs.

The majority of the lost circulation and differential sticking problems encountered when drilling these wells is attributed to the overbalance pressure generated when using conventional drilling fluids. The equipment required when using aerated muds or drilling underbalanced is often prohibitively expensive, and meeting safety requirements can be an exhaustive effort. In addition, these techniques may fail to provide the hydrostatic pressure necessary to safely stabilize normally-pressured formations above the reservoir. The aphron fluid technology is allowing operators a second chance at developing fields once deemed economically unfeasible.

What is an aphron?

Basically, an aphron is nothing more than a bubble containing a gas core and multiple micelle-like shells comprising various components. The purpose of these shells is to provide enhanced stability to the bubble by ensuring greater strength of containment for the gas core. This feature enables different performance benefits than single-shell bubbles in aerated fluids or foams.

Aphrons contain a gas nucleus of encapsulated air and compress when circulated down the hole. The internal pressure of these microbubbles increases at a rate proportional to the extreme pressure being applied. The combination of increasing pressure and temperature serve to energize the individual aphrons.

Once the bit exposes a depleted formation, the aphrons immediately aggregate within the pores of low-pressure zones. There, a portion of the energy stored within each aphron is released, causing it to expand. The expansion continues until the internal and external pressures on the wall of the aphron are in balance.

As the now-energized microbubbles enter the formation openings, they carry energy equal to that in the annulus. Once they crowd into an opening, external Laplace forces increase substantially, causing both aggregation and an increase in the internal low- shear-rate viscosity (LSRV) of the fluid system. The microenvironment created by this phenomenon assists in reducing fluid invasion.

Not only does the aphron technology draw upon the traditional benefits of high LSRV fluids, it further optimizes their performance capabilities. This enhancement is accomplished by lowering the high-end shear rate properties, and this translates into a significant reduction of frictional pressure loss at elevated circulation rates. This improvement broadens the opportunity window for the industry by expanding the utility of pressure-limited equipment. Standard tools such as coiled tubing and rig pumps can now deliver higher volumes of high-carrying-capacity fluid at similar or lower circulating pressures.

Aphrons vs. air or foam drilling

One of the more attractive features of the system is that it does not require any of the high-cost compressors, high-speed hoses and other extra equipment that must be used in air or foam drilling. The system employs conventional fluid-mixing equipment to form the tough and flexible microbubbles.
Once formed, these aphrons differ from the bubbles produced in air or foam drilling in two significant ways.

1. They do not coalesce into larger bubbles. While the aphrons are attracted to lower-pressure regions in the formation, they remain discreet from each other, forming a strong network of individual microbubbles. Further, each one is energized by bottomhole pressure and acts as a flexible "shock absorber" to protect formations from fluid invasion.

2. The microbubbles are tough and flexible, comprising a core of air surrounded by layers of a proprietary polymer and a tensoactive additive. Once formed, those aphrons that remain in the drilling fluid flow through the shale shaker and are recirculated downhole.

The composition of the system

There is nothing complicated about building the system or maintaining its rheological properties downhole. The high-LSRV base fluid consists of a high-yield, stress-shear-thinning polymer coupled with proprietary components that create and stabilize the aphrons within the system. An exclusively formulated aphronizer is employed to reach the desired concentration of microbubbles, which typically is 8 to 14% by volume. As the concentration builds, it is not uncommon to see the Brookfield LSRV increase to between 120,000 and 160,000 cP.

Once the system is circulating, the rheological properties are maintained easily to provide optimum hole cleaning, cuttings suspension, and a high degree of control over the invasion of whole drilling fluid.
Case histories

Perhaps no region better illustrates the problems associated with drilling depleted reservoirs than Lake Maracaibo. Water-wet sands that frequently triggered costly seepage losses and differential sticking typify many of these zones. Some contain micro-fractured sandstone formations where uncontrollable losses of whole drilling fluid were the norm, rather than the exception. Others are characterized by laminated sand and shale sequences, which create the conditions for slow, dangerous and unduly expensive drilling. Attempts were made with underbalanced drilling, but in addition to the extra time and equipment required, wellbore instability led to failed well construction and thus seriously degraded project economics.
The early wells drilled in the Lake Maracaibo area employed underbalanced drilling techniques combined with special casing designs to insolate the Miocene and Eocene formations. Yet, the hole instability problems associated with this drilling technique rendered this project unprofitable. Consequently, the operator looked for any alternative that would return profitability to this mature field by reducing the drilling days and enhancing the production rate.

The aphron-based system was recommended as an alternative drilling fluid to drill both the normal and subnormal pore pressure sections, while simultaneously maintaining wellbore stability and controlling mud losses. The main challenges placed upon this system were the whole mud losses and hole instability problems associated with drilling normal and depleted pressure intervals and the cost of an extra casing run.

The first aphron-based system field trial was performed in the reservoir section of the VLA 1321 well. This well was characterized by a formation pressure gradient of 0.15 psi/ft to 0.30 psi/ft.

The aphron-based system was displaced to drill the reservoir section after the 133/8-in. casing was set at 5,477 ft (1,670 m) and the interval was drilled to 6,855 ft (2,091 m). The total section length was 1,378 ft (420 m), and throughout this interval 390 ft (119 m) had been cored with 91% recovery. At the section total depth (TD) three logging runs were made, and the 95/8-in. casing was run without any problems. No mud losses were experienced during during drilling, logging, running casing or cementing. The formation gradients ranged from 0.15 psi/ft to 0.33 psi/ft, while the mud gradient varied from 0.39 psi/ft to 0.41 psi/ft.
After the success of the first field trial, various operators used the system in several fields in the Lake Maracaibo. The system has been adjusted and fine-tuned continuously as lessons were learned from each specific field and application. The system applications ranged from:

• drilling low-pressure / low-fracture-gradient sections;
• drilling fractured and highly permeable sections;
• reservoir drill-in applications; to
• drilling normal and low-pressure sections (high density applications).

The low-pressure applications were mainly in the depleted reservoirs in the Miocene formations. Owing to the depleted nature of the sandstone formations, the aphron-based system was formulated to bridge the porous media and prevent lost circulation problems. All the wells drilled for this type of application showed a pore pressure ranging from 2.5 lb/gal to 5.0 lb/gal and equivalent circulating density (ECD) between 9.5 lb/gal and 10.0 lb/gal. No mud losses were experienced.

After its success in Venezuela, the system was later utilized to control downhole mud loss in a depleted reservoir in the Dutch sector of the North Sea. The key issues of this project were:

• excessive overbalance drilling conditions (>5,000 psi) leading to the risk of lost circulation; and
• open perforations in the upper producer, requiring temporary sealing during drilling.

The aphron system allowed the well to be drilled successfully to TD without any drilling fluid losses.
While the system has been used primarily offshore, it made a successful onshore debut in the Elk Hills development in California, where severe lost circulation in earlier wells had frequently prevented the operator from reaching total depth. By displacing the current fluid with the aphron system, the operator dramatically reduced losses per well with a resulting 10% reduction in drilling time. More importantly, the operator was able to successfully reach the targeted TD, which had not been possible in many of the earlier wells because of the inability to regain and maintain circulation.

The Aphron ICS design software is used to tailor the system to each individual well. By inputting pore pressure, well depth and the type of thief formation expected, engineers can tell whether a bridging agent is needed. The software provides a risk analysis to show the operator the aphron system's probability for success.

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