First used as a drilling fluid in the 1960s, foam has endured a rather unrefined gestation period. Many of the obstacles facing foam drilling have been conquered through chemistry and mechanical innovations.
Foams serve many important functions in the petroleum production process, from their use as a drilling fluid to reservoir stimulation to secondary recovery. More so today than ever before, foam can be used to exploit more hydrocarbons from quickly diminishing reserves, and foam is an excellent tool for re-entering established wellbores. With the evolution of underbalanced drilling (UBD) technology, foam has become the medium of choice as a low-weight drilling fluid.
Anatomy of foam
If gas and a liquid are mixed together in a container and shaken, the gas phase will become a collection of bubbles dispersed in liquid; in other words, foam will form (Figure 1). Drilling with foam is the most versatile of all the reduced-pressure drilling systems. Foam is a mixture of fluid, surfactant, chemical additives and compressed gases (nitrogen, carbon dioxide, natural gas and air). It is a multiphase, metastable, compressible, non-Newtonian fluid, and has a consistency similar to that of shaving cream. The compressible bubble structure of foam provides up to 10 times the carrying capacity of many common liquid-based circulating systems. Due to foam's high carrying capacity, drilling operations with annular velocities as low as 70 ft/hr have proven effective. Experience has shown foam can handle more than 600 bbl/hr of downhole fluid influx. The effective fluid density of foam ranges from 0.05 to 0.5 specific gravity (sg). The density ranges are adjusted within the makeup of the foam by adjusting the liquid volume fraction (LVF) via injection of a liquid foaming solution and within the gas injection by adjusting the compressed-air pressure valve on the air-injection line. This valve restricts the flow down the drillstring and, therefore, slows the velocities in the annulus. Foam quality is the gas volume percentage used to define mist and foam. Mist is defined in the range 99.99% to 96%+ foam quality. Foam is from 96% to 55%. At qualities below 55%, the mixture is called aerated fluid.
Benefits of foam as a drilling fluid
A properly prepared foam solution has many beneficial qualities that make it an excellent drilling fluid. It is an effective low-weight drilling fluid and provides exceptional cuttings transport. Foam has been proven effective with low annular velocity, and it removes the possibility of lost circulation.
Low-weight drilling fluid. With foam, fluid densities as low as 0.05 sg (0.4 lb/gal equivalent circulating density) can effectively be achieved. This low density has two profound advantages: increased rate of penetration (ROP) and extremely low bottomhole pressures (BHP). With the recent emphasis on UBD, the effective density of the drilling fluid must be below the reservoir's pore pressure. In depleted, or low-pressure zones, the choice of fluids that function effectively at these low densities is rather limited. Conversely, relatively high fluid densities, up to 0.78 sg (6.5 lb/gal equivalent circulating density), can also be achieved with foam. Foam has a high friction factor, and when calculating BHP in dynamic foam flow, one sums the hydrostatic weight and friction pressure. Many wells have intentionally been drilled overbalanced with foam to eliminate unwanted influxes. Proper fluid chemistry is essential in these situations. If the foam breaks down (the viscosity of the foam is altered by the fluid influx), the friction factor decreases along with the BHP. A general understanding of fluid chemistry and the selection of the proper foaming agent is essential in assuring a successful project.
Cuttings transport. In the presence of a surfactant, a dispersion of small, tightly grouped gas bubbles within the water creates a motive fluid with the ability to transport up to 10 times the amount of cuttings of a single-phase fluid. Simply translated, an operator can increase the ROP and still maintain efficient hole cleaning. Along with its ability to carry cuttings, foam has the ability to displace large volumes of downhole fluid influxes. Recent foam drilling jobs have continued to operate effectively in the presence of 25 bbl/min of water influx.
So far, the discussion of foam has focused on its advantages in straight boreholes. Certain operators, however, might question the feasibility of foam in horizontal wells. Nonetheless, the unique fluid chemistry attributes of foam combined with the behavior of UBD offer two significant advantages in the horizontal market: unrivaled cuttings transport and a uniform circulation medium compatible with surveying techniques. UBD typically increases cuttings transport by increasing the viscosity and velocity of the drilling fluid in horizontal wells. Improved cuttings transport requires less weight on bit, allowing for a greater ROP and a higher footage/bit ratio. Lab tests have shown that in the cuttings-carrying continuum, the more homogeneous the drilling fluid, the better the carrying continuum. Conventional fluids, such as drilling mud, are homogeneous, but their carrying capability is limited. Of the UBD fluids, foam offers the best transport medium for carrying drilling cuttings and other debris up and out of the wellbore.
Annular velocity. Recent studies have focused on the detrimental effects of erosion caused by excessive annular velocities. Their basic conclusions suggest that when drilling most formations, there is a critical velocity that cannot be exceeded without adversely affecting borehole stability. Foam has proven effective with low annular velocities. Annular velocities as low as 100 ft/min have been proven sufficient to adequately clean the wellbore on many foam drilling operations. This gentle-flow medium is quite beneficial in velocity-sensitive shales and semiconsolidated formations. Further research is being conducted to better understand the relationship of foam and pore pressure diffusion, hoop stress and other technical borehole stability issues.
Downhole pressure fluctuations due to the termination of flow in the annulus are of great concern. All conceivable actions must be taken to minimize the length of time circulation is stopped. When circulation is stopped over a period of time, the foam breaks back to water, and the gas phase expands toward the surface. In order to re-establish circulation of the continuous medium, the foam must displace the water in the bottom of the well, and the BHP must increase until the fluid begins to move up the wellbore. As the fluid is lifted out of the well, the BHP decreases, subjecting the formation to a fairly dramatic pressure drop. In pressure-sensitive formations, this may have an unfavorable effect on borehole stability.
Lost circulation. Foam flow in a porous medium is quite different from flow in the drillstring or casing. In a porous medium, immobile boundaries contain nearly every part of the flow. Figure 2 demonstrates in two dimensions how a porous medium prevents foam flow. Lost circulation is an economic and operational consideration when drilling with a single-phase fluid, and drilling with foam essentially eliminates these concerns. With the removal of lost-circulation possibilities, the likelihood of differential sticking of the drillstring is virtually removed.
Foam allows evaluation of the formation fluids while those fluids are commingled with the drilling fluid. As a result, the operator can benefit from more accurate identification and monitoring of where in the well the sample was taken. Aside from the potentially higher ROP, foam can drastically eliminate lost circulation in thief zones. With no need for lost circulation materials, it is easier to condition the fluids in UBD wells. Solids-free drilling fluids translate to faster ROP and thus overall cost savings. Inexorably linked with lost circulation and thief zones is differential sticking. Wells using foam experience no differential sticking of the drillpipe, translating to no lost drilling time or rig time caused by having to fish stuck pipe.
Flexibility of foam drilling. Foam is the most versatile of all the low-density drilling fluids. The water, LVF and percentage of gas can be easily controlled at the surface to achieve sufficient BHP to effectively drill the well. Many advantageous chemicals may be added to the fluid phase to address such problems as shale control, corrosion, borehole stability, downhole fires and other undesirable or hazardous conditions. With the high liquid volume of foam, downhole fire considerations are minimized, if not totally eliminated. Additionally, if nitrogen is used to generate the foam, rather than compressed air, the odds of a downhole fire are virtually eliminated.
The foremost advantage of foam is that an operator can produce the well while drilling. Because compressible fluid systems are dynamic, continual circulation of the drilling fluid is essential for optimum results. Foam also can reduce the damage to the producing formation by efficiently transporting fines and increasing borehole stability. Similarly, better cleanup of foam translates to maximum permeability and minimal skin damage and pore plugging. The ability to manage foam viscosity and shear for optimum performance at the bit also provides less damaging chemistry to the formation. Nonaqueous foams are available for protecting water-sensitive formations. When selecting a type of foam, specific formation geology and lithology must be considered.
Recyclable foam systems
Refinements in recyclable foam systems have led to successful foam drilling operations onshore and offshore.
A newly developed system allows fluids used in UBD foam systems to be continually foamed, defoamed and refoamed. The cycle can be repeated indefinitely. A positive benefit of this system is that the defoamer does not destroy the foaming agent. First, the defoamed fluid is cycled across a shale shaker to remove the large solids. Next, it is cycled through the cleaning equipment, a desilter and a centrifuge for complete removal of the solids. Then, an activator is added to refoam the fluid. Recyclable foam systems have four advantages: ease of containment, low water consumption, low chemical consumption and adaptability to multiple applications.
Containment. One problem with using foam in the past has been the necessity to excavate large earthen pits to capture the large volumes of foam coming out of the blooey line. On large-diameter wells, the equivalent of up to 600 gal/min of foam is needed to adequately clean the wellbore. In only 1 hour, this equates to 36,000 gallons of foam. Although some of the foam will break down over this period of time, the desirable characteristic of a worthy foaming agent is its ability to maintain foam volume over time. Recyclable foam systems are designed for the confines of an above-ground standard mud system, thus eliminating the effort and expense of excavating large pits. Installing earthen pits is not an option in many situations, including offshore drilling and environmentally sensitive areas.
Water consumption. On many drilling operations, the supply of fresh water is of foremost concern. The economics of water handling vary greatly depending on location, but the one common factor to all drilling projects is that water availability is expensive and can significantly increase project cost. Recyclable foam systems continually recycle the water, which greatly reduces overall water consumption. The systems operate in the absence of standard silicone-based defoamers. Disposal of water containing silicone-based defoamers is becoming increasingly difficult and expensive. Known environmental problems associated with silicone-based fluid have limited the options for proper disposal. In the recyclable systems, continually adding defoamer and activator forms an inert, non-toxic material that is highly water-soluble; therefore, no significant environmental concerns are involved. At the conclusion of the drilling project, the water can be disposed of in the normal fashion or saved to drill the next well. With proper fluid cleaning procedures and solids removal, the system fluid may be used indefinitely, positively affecting the bottom-line economics of future projects.
Chemical consumption. Since the recyclable foam system is a closed-loop system, the large majority of the foaming agent is recovered and available for reuse. Past experience shows a 50% to 90% surfactant retention rate depending on well conditions. Due to surface absorption on the drilled solids and the wellbore surface, some surfactant is invariably lost. The amount of overall absorption is determined by parameters such as:
solid rock type (sandstone or carbonate), mineral composition (clays, other minor or trace minerals), wetability, surface charge and specific surface area;
surfactant type, composition and solubility; and
temperature.
The absolute amount of surfactant absorbed per unit mass depends on the surface area of the drill rock.
Adaptability. The most outstanding feature of recyclable foam systems is simplistic operation. By being able to kill the foam so rapidly, the system will work within the confines of nearly all conventional drilling mud or aerated-fluid systems. One essential piece of equipment is a well-designed mud and gas separator. The separator is designed to separate the foam's fluid and liquid phases. Once the system fluid's pH is lowered, prior to entering the separator, the foam is destroyed, and all that remains are fluid, cuttings and gas.
The most important feature of the mud and gas separator is its ability to maintain a liquid level in the bottom of the unit. The function of this liquid leg is to increase the efficiency of the liquid-gas separation process. As the returned fluid-gas enters the unit, the velocity of the multiphase fluid decreases rapidly, the gas phase exits out the top of the unit, and the liquid and cuttings exit at the bottom. By maintaining a liquid leg in the unit, the gas more efficiently follows the path of least resistance, which is out the top. This liquid leg allows for near-complete desolation of the foam by disallowing any remaining foam to flow out the bottom of the unit onto the shaker screens. This separation technique is extremely important when drilling while producing oil reserves. Often the reservoir pressure is slightly above the bubble point, and as the foam brings the oil to the surface, the pressure decreases to below the bubble point. With an efficient mud and gas separator, the gas contained in the oil breaks out within the unit, thus eliminating the safety and handling concerns of excess natural gas carryover into the mud handling system.
The Galileo project
A major concern for operators in Lake Maracaibo is finding the most effective method of re-entering the field's numerous depleted reservoirs to maintain commercial production.
Earlier re-entry attempts made with conventional drilling fluid, drilling fluid with lost-circulation material, and nitrified drilling fluid were all unsuccessful. Each of these drilling fluids introduced expensive, counterproductive well conditions such as lost circulation, irreversible reservoir damage and insufficient solids carrying capacity. The latest re-entry attempt involved drilling a horizontal well from a coiled-tubing drill barge with foam as a circulation medium.
The well geometry consisted of 7in. casing set to about 5,500ft total vertical depth (TVD) and a 5in. liner set at 90° inclination to about 6,300ft TVD. The work included exiting the liner using coiled tubing equipped with a drilling bottomhole assembly (BHA). Next, a 41/8in. horizontal section was drilled with the most recent UBD technology available. The well then was completed with slotted liners or production screens.
Reservoir lithology was comprised of clean, consolidated sandstone and silty sandstone with intermittent shale stringers. Because the reservoir had been depleted, the equivalent circulating density ranged from 2.7 to 2.9 ppg foam. The four-well program used a 2.5-ppg average foam as a circulation medium.
Earlier drilling attempts had employed nitrified drilling mud, but because of the significant drop in annular velocity through 23/8in. coiled tubing in the 5in. liner and the 7in. casing, the solids carrying capacity was insufficient. The previously drilled reservoirs also contained equivalent circulating densities below the limits of nitrified fluids. A water-based foam was chosen for the circulation medium. Foam was created using filtered lake water, potassium chloride, permanent shale inhibitors, copolymer, buffering agent, recyclable surfactant and cryogenic nitrogen. The foam selected was a recyclable system, activated and reactivated by pH. Bottomhole foam varied from 80% to 92% quality with liquid pump rates from 15 to 30 gpm and nitrogen pump rates from 600 to 1,200 scf/min.
The 23/8in. coiled tubing contained an electric line, and a drilling BHA was used to drill the horizontal sections. The BHA was comprised of a 41/8in. polycrystalline diamond compound bit, a 31/8in. mud motor, a 31/8in. electric-over-hydraulic-operated orientor, nonmagnetic subs containing the survey tools, standard subs containing interior and exterior pressure sensors, a hydraulic disconnect, a check valve and a tubing connector.
The return system consisted of an electrically operated choke manifold, a four-stage separator, a vent stack, a water-storage vessel, an oil-storage vessel, a five-pit mud system containing shale shakers and centrifuges, and two storage tanks. Fluid from the separator flowed to the water-storage vessel and was transferred to the shale shaker while the produced oil flowed to the oil-storage vessel and was pumped to an existing production line.
This foam drilling project was an operational and technological success. Four horizontal wells were drilled without any problems related to the foam system, and the production results were two to three times greater than the anticipated value. A skin test was completed on the first well drilled, and the results showed 0% damage to the near-wellbore formation.
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