As ongoing gas production continues to deplete our shallow and conventional natural gas resources, sustained natural gas supply will increasingly depend on the domestic industry’s ability to safely and economically drill, complete, and stimulate wells in deeper, higher temperature, higher pressure, and harder geologic environments.
In addition, there are near-term industry needs to more efficiently drill and produce gas from conventional reservoirs to maximize productivity given limited rig, manpower, and service capacities.
To address these needs, the gas center oversees a wide range of research and development efforts designed to allow cheaper, deeper, safer, and less damaging drilling in a wide range of environments.
Drilling gas wells involves much more than simply "making a hole." It integrates complex technologies to cut rock, transport cuttings to the surface, control the influx of high-pressure gases and fluids into the wellbore, preserve the producing potential of the target formation, and minimize environmental damage.
Drilling is seldom straightforward; drillers regularly confront and solve highly complex technical and safety problems as the bit bores through rock layers of varying and sometimes unexpected hardness, temperature, and pressure.
Improving drilling performance is a high national priority as high costs and risks discourage development of the deep and low quality gas reservoirs that make up a growing portion of our national gas resource base.
Because 85 percent of the gas wells in the U.S. are drilled by independent operators, most of whom do not have the resources to develop or test new technologies, the gas center will take an active role in enabling the development of advanced drilling technologies.
Time is money on a drilling rig, and therefore a key focus of drilling research and development (R&D) is to increase the overall efficiency of drilling operations as a key method of controlling costs.
This can include longer lasting and more efficient bits, more accurate steering, more efficient downhole monitoring, casing-while-drilling completion systems, and other technologies. Deep drilling introduces significant additional challenges.
To support the nation’s growing need to access natural gas in very deep settings (more than 15,000 feet of drilling depth), R&D sponsored by the gas center investigates "smart" drilling strategies that can reduce rig down time and overcome the problems of high temperatures, high pressures, and hard formations.
However, simply "making hole" more efficiently will not be enough; drilling, completion, and stimulation systems must be designed so that damage to the target formation is minimized.
Therefore, the gas center supports the increased use of underbalanced drilling systems that preserve reservoir productivity.
Preparing a hole for production ("completion") means expensive and time consuming efforts to line the hole with casing and then perforate the casing adjacent to the target formation.
The gas center sponsors a number of projects, such as casing-while-drilling, that may revolutionize this aspect of rig operations. Finally, once the well is completed, the well is stimulated to increase productivity.
The use of common water/sand stimulation in shale-rich reservoirs, however, may unduly damage the formation. Therefore, the gas center is focusing on the commercialization of stimulation systems that use liquid carbon dioxide in place of water.
Drilling Cost Reductions:
Reducing drilling costs is a key goal of the drilling, completion, and stimulation program. Lower costs increase reserves and production by allowing marginal reservoirs to become economic targets.
Lower-cost drilling also commonly means faster drilling, and faster drilling frees up personnel and equipment so that the next well can be drilled.
The ability to drill hard, abrasive formations quickly and efficiently is being addressed by research on new drilling systems, bit wear reduction, and underbalanced drilling fluids that radically increase rates of penetration yet reduce formation damage.
NETL is supporting the development of several drilling systems that promise to increase drilling rates. One system eliminates the time to couple and uncouple drill pipe by using coiled-tubing drilling technology, resulting in a reduction of rig and personnel time.
Long-lasting and multi-purpose bits reduce drilling costs by reducing the time needed to retrieve and replace worn bits. The Department of Energy was instrumental in the introduction of polycrystalline diamond (PCD) bits in the mid 1980s.
Now, new hardening and diamond bonding technologies are being tested in the development of a high-strength thermally stable polycrystalline diamond (TSD) cutter design for advanced drag bits.
Drilling demonstrations using these bits with DOE's high-power slimhole mud motor have shown that they are more economical (faster and stronger) than conventional drag bits over a wide range of formation hardness.
Prospective horizons below producing reservoirs are a major contributor to reserve growth. However, these targets are not always large enough to justify the costs of a new well drilled from the surface.
Innovative high-power slimhole (less than 4-inch diameter) drilling systems currently under development may allow existing wells to be deepened to these zones.
Deep Drilling:
In the recent past, reservoirs below 15,000 feet drilling depths were generally not considered to be realistic targets: in fact, gas at these depths was typically not counted as part of the nation’s resource base.
Despite recent advances in drilling technology and efficiency that have made deep drilling a real possibility, only the very largest accumulations are economic to drill due to high costs and risks.
However, if deep drilling is to fulfill its role in expanding and sustaining production in the mid- and long-term, further advances in deep drilling technologies must be achieved.
Deep wells are extremely costly to drill because the time required to drill each additional foot dramatically increases with depth. One reason is reduced penetration rates from progressively harder formations.
A second key reason is the frequent need to pull longer and longer drill strings from the hole to change bits and gather data about a well's progress.
In addition, drilling becomes more risky as the bit penetrates high-temperature and high-pressure strata. NETL's program in deep drilling addresses these problems through new bit technology and smart drilling systems.
The advanced mudhammer drilling system is being developed for use in directional drilling in deep formations. This bit integrates several bit technologies into one bit that can drill formations of different types, thereby reducing the need to change bits to maximize penetration rates.
The bit also incorporates downhole monitoring technology that allows real-time measurement of bottom-hole location. NETL is also sponsoring the development of composite drill pipe technologies that allow the drill pipe to serve as a conduit for the transmission of data from the bit to the surface.
In addition, NETL is working to develop improved measurement-while-drilling and logging-while-drilling tools that can operate at temperatures up to 195oC and 175oC, respectively.
Underbalanced Drilling Systems: As drilling proceeds, operators fill the advancing borehole with fluids. These fluids carry cuttings away from the bit.
Additives such as barite are typically mixed into the drilling fluid to make it denser and capable of controlling downhole pressures. In many environments, operators will "overbalance" to provide added safety.
However, encountering porous and permeable zones while overbalanced can lead to a stuck drill pipe and loss of drilling fluids into the formation.
Both problems can add significantly to costs, and lost drilling fluids can also permanently damage the formation, reducing the productivity of the well.
Some environments, such as established gas producing areas, pose limited downhole pressure risk. Technologies that allow drilling to proceed at or below reservoir pressure ("underbalanced") can result in faster penetration, and limited or no reservoir damage.
Underbalanced drilling is especially desirable for heavily drilled areas where the reservoirs are partially depleted. In these cases, reservoir pressure may be less than the minimum hydrostatic pressure obtainable with standard drilling fluids.
To correct this problem, the Department of Energy is assisting in the development of an advanced underbalanced drilling fluid that uses encapsulated glass beads as an additive.
The glass bead mixture retains the ability to remove drill cuttings while lowering the drilling fluid density, thereby reducing damaging influx into the formation.
To answer an urgent need by industry for assistance in foam drilling, (where the drilling fluid is a mixture of air, water, and a surfactant), a personal computer-based foam drilling model is under development.
This model assists drilling engineers to efficiently design and execute foam-based underbalanced drilling operations. Proper modeling can determine the correct mixture of foam ingredients for maximum cutting removal and drilling performance.
Completion Systems:
The largest tangible cost to drilling and producing a natural gas well is associated with the cost of casing and production strings (tubulars).
NETL continues to work with industry to develop innovate alternatives to typical casing programs. For example, NETL is developing a technology to eliminate the need for surface and intermediate casing by using casing-while-drilling (CWD) methodology.
CWD enlists a polymer that is activated at the drill bit, resulting in the creation of an impermeable seal along the wellbore face. CWD has the potential to eliminate the time and expense of running steel surface and intermediate tubulars.
"Intelligent" completion systems are being developed to determine the types of fluids being produced prior to and after completion.
A down-hole fluid analyzer is being developed to provide real time information on the types and proportions of oil & gas, without interrupting production.
Using permanent remote sensing and fiber optics, the analyzer can monitor the well's performance.
Production abnormalities can be detected earlier in the life cycle of the well and can be corrected before becoming major problems.
Work is also being conducted on a high-energy/low-damage casing perforation system to improve wellbore to producing formation contact without reducing conductivity.
Stimulation Systems:
The vast majority of gas wells in the nation’s emerging gas plays do not naturally produce gas at sufficient rates to make the well economical.
It is typically necessary to stimulate the well to increase conductivity between the wellbore and high-permeability zones and/or natural fracture systems.
The most prevalent stimulation method, "hydrofracking", generally involves inducing artificial fractures in the reservoir through injection of high-pressure water.
Sand is also injected and provides a means of propping the fractures open once the water is retrieved.
In many settings however, hydrofracking is ineffective or even damaging, because the water causes shale grains prevalent in marginal reservoirs to swell, thereby reducing reservoir permeability and/or porosity.
Research conducted for NETL is evaluating and characterizing various new and innovative stimulation fluids that avoid this problem.
For example, NETL is actively demonstrating the potential of well stimulation that uses liquid CO2 in place of water. Like water, the liquid CO2 can effectively carry sand into the induced fractures; however, unlike water, the liquid carbon dioxide reverts back to a gas at reservoir pressures, leaving only sand in the induced fracture system.
Although the CO2 sand stimulation is costly, well clean-up costs are reduced because there is no need to recover the injected water.
Most importantly, well productivity is enhanced because there is no permeability-damaging reaction between the water and clay minerals occurring naturally in the formation.
Source: www.fe.doe.gov
© 2001 Mena Report (www.menareport.com)
