Stiff Foam Drilling

Stiff Foam Drilling Overview Stiff foam is a more viscous and stable form of foam that can be generated using surfactants alone. The reduction in horsepower requirements and low water consumption make the stiff foam a more economical option compared to other drilling fluids. In general, stiff foam drilling involves similar equipment and procedures to those in stable foam drilling. However, the flow properties of stiff foam are different from those of stable foam. In stiff foams, increased viscosity of the liquid phase provides a more stable structure. This structure allows higher quality foams to be generated without rupturing the cell walls. Rheology The concentrations of clays and viscosifying agents have a significant effect on the viscosity of the base liquids. According to the model of Hatschek ( 1910), the foam viscosity for qualities between 74 and 96 can be expressed as: Equation 1 [latex]\mu_{F} = \dfrac{\mu_{L}}{1-\Gamma ^{0.33}}[/latex] where: μF = foam viscosity, cp μL = liquid viscosity, cp Γ = foam quality, fraction As seen in this expression, the foam viscosity increases as the viscosity of the liquid phase is increased. Hole Cleaning Considerations High viscosity makes it possible to use lower annular velocities while maintaining acceptable hole cleaning efficiency. Several models are available to simulate cuttings transport in stiff foams. The chart shown in Figure 1 was generated by Garavini et al. (1971). This chart is used to obtain proper air and liquid injection rates for stiff foam drilling as a function of hole size. FIGURE 1 Surface foam qualities can be calculated using the air and liquid rates. Typically, stiff foam programs aim for surface foam qualities from 99 to 99.7 percent, and annular velocities in the range of 100 to 200 ft/min. According to the field and laboratory data from stiff foam drilling operations, the injection rates can show a significant variation for sufficient hole cleaning. Even though the air and liquid injection rates are reduced in stiff foam drilling, the increased cost of consumables might increase the overall cost. The circulating pressures in stiff foam drilling are not much different from those of regular foam. Because of its low density (caused by lower liquid volume fractions) stiff foam quality does not decrease rapidly. Therefore, it is not necessary to adjust the back-pressure in order to control the downhole foam quality. Recommended for You Nitrogen Drilling Stiff Foam Drilling Equipment Due to lower injection rates, there is less need for air injection capacity. A typical operation will require no more than 205 to 300 psi delivery pressure and a maximum of 460 scfm injection capacity. For liquid injection, most soap pumps on an air/mist drilling rig should have sufficient capacity for typical stiff foam drilling jobs. Different from conventional foam systems, a hopper system is usually required for an effective mixing of polymeric viscosifiers in water. Stiff foams have a longer half-life than stable foams. If no additional defoaming processes are used, size of the reserve pit should be increased. Tests have to be conducted to determine expected half-life and effectiveness of the proposed defoaming procedures. Viscosifiers Stiff foams were originally made by adding bentonite to water (10 to 12 lbs/barrel) to generate the viscosified base liquid. The most commonly used additives are as follows: HECHydroxyethylcellulose is a polysaccharide linear polymer based on a cellulose backbone. It is non-ionic.PACFast-mixing, high-activity, granular polymer. It increases viscosity, lowers fluid loss and prevents bit balling.Cypanis a synthetic material manufactured from petroleum feedstocks. It is not as complex structurally as the natural polymers. Usually has a straight-chain carbon backbone with different side chains.XCPolysaccharides manufactured from bacterial fermentation. Has extremely complex structures with high molecular weights.CMCCarboxymethylcellulose is a polysaccharide linear polymer based on a cellulose backbone. It is anionic with carboxylic acid groups.Viscosifiers additives PAC and Cypan are more resistant to minor saline and oil contamination than bentonite-based mixtures. Depending on the possibility of calcium contamination, it may be necessary to add soda ash to the liquid phase to stop the breakage of the base fluid. Increasing the polymer content increases the cost of consumables, but the higher viscosity that is attained allows lower injection rates, thus reducing the need for additional compression at the wellhead. On the other hand, an excessively stiff foam will be very difficult to break at the pit, causing foam control problems. Corrosion of downhole equipment is still a concern when drilling with stiff foam. Corrosion inhibitors compatible with the foaming agent and other potential contaminants should be added to the liquid phase. The high viscosity of the stiff foam can cause foam control problems. Because of its stable structure, it is more difficult to break stiff foams. Chemical defoamers have to be thoroughly mixed with the foam. The consumption of defoamer is also likely to be higher than for unstiffened foam. Recommended for You Dry Air Drilling Stiff Foam Drilling Procedures and Operating Considerations Most polymeric viscosifiers are added slowly to the liquid phase to ensure a dispersed mix. The polymer is added to the water, which circulates through the hopper. Large quantities of liquid may have to be mixed, and this operation can take several hours. Therefore, it is critical to use the tanks effectively during the mixing process. Stiff foam returns should have a reasonable consistency (similar to shaving cream). If slugs of liquid and air are seen, either the foam is incompatible with the formation fluid inflow or the foamer concentration is not sufficient. In both cases, the foamer concentration should be increased. If hole cleaning problems are experienced or water inflow occurs, the air component flow rate should be increased. Increasing the concentration of the foaming agent and viscosifiers in the injected fluid might also solve the problem. However, the liquid injection rate should never be reduced. Limitations of Stiff Foam Drilling Stiff foams are vulnerable to gas inflows, which can increase or decrease the quality to a point where the foam breaks down and cuttings transport efficiency is lost. This eventually might lead to a stuck drill string. If gas inflow is suspected, the drilling operation must be stopped and the liquid rate should be increased to establish foam returns. If the inflow is too fast, then the drilling operation can be switched to mist drilling. In terms of downhole fires, stiff foams are more stable than unstiffened foams. Since gas phase rates are low, it is easier to generate stiff foams using inert gases. This completely eliminates the risk of downhole fires and reduces the rate of downhole corrosion. The cost of the operation can be very high because of the cost of viscosifying agents. However, reduction in compressor power and low water volume requirements can sometimes offset the high cost of consumables. The presence of viscosifiers in waste water restricts its disposal. Because of the high viscosity, it is also difficult to re-inject the killed foam into disposal wells. Assessment 1. Stiff foams have a more stable structure due to the increased viscosity of the liquid phase. A .True ✔B .False 2. The flow properties of stiff foam are NOT different from those of stable foam. A .TrueB .False ✔ 3. What system parameter of stiff foam drilling is NOT much different than that of regular foam drilling? A .Air injection ratesB .Liquid injection ratesC .Overall foam viscosityD .Circulating pressures ✔ 4. What can you say about the stability of stiff foam compared to regular foam? A .Both have similar half-lifeB .Stiff foams have a shorter half-life than stable foamsC .Stiff foams have a longer half-life than stable foams ✔ 5. Which of the following additives is a high-activity, granular polymer, which lowers fluid loss? A .XCB .CMCC .HECD .PAC ✔ 6. Why is it difficult to re-inject the killed foam into disposal wells in stiff foam drilling? A .Because of high viscosity ✔B .Because of high gas contentC .Because of high fluid temperature Recommended for You Foam Drilling | What is Foam Drilling?