Practical Drilling Technology Integrating surface systems with downhole data improves underbalanced drilling

Larry Comeau
Sperry-Sun Drilling Services
Nisku, Alta.

Improving the underbalanced drilling operation and obtaining quality data in real time can help eliminate damage to the formation and increase ultimate production.

Recent advances in drilling technology have made it possible to drill horizontal wells underbalanced more safely and effectively. This technology has greatly reduced the potential for skin damage to the bore hole. Experience from western Canadian underbalanced horizontal drilling clearly demonstrates that a well bore's initial productive potential is very accurately predicted from its productive behavior during drilling operations.

Underbalanced drilling has numerous benefits:

• Prevention of formation damage

• Increased penetration rates

• Increased bit life

• Elimination of differential sticking

• Additional reservoir information during drilling

• Ability to run well production tests at any point during the drilling of the horizontal well bore

• No exotic muds or chemicals required.

In most cases, the total benefits from underbalanced horizontal drilling outweigh conventionally drilled horizontal wells.

The western Canadian oil industry has used horizontal drilling technology extensively during the past 10 years, and during the past few years, closed-loop underbalanced horizontal drilling has increased tremendously. In 1995, 323 wells were drilled with this technique, up from 246 wells in 1994, 129 wells in 1993, 33 wells in 1991, and none in 1990.

Many aging reservoirs in western Canada have had steadily declining reservoir pressures. It has been recognized that in drilling horizontal wells in these pressure-depleted reservoirs, extensive formation damage was occurring the longer the reservoir was exposed to overbalanced drilling conditions.

Well productivity is related to the length of the horizontal section; the longer the horizontal interval, the higher the productivity. This correlation has increased the popularity of multilateral drilling techniques to increase reservoir exposure.

Production engineers had once speculated that most oil production in a horizontal well came from the heel of the well because this was the portion of the well subjected to the highest drawdown pressure. Recent findings however, have suggested otherwise.

Many production engineers now believe that severe formation damage occurs at the heel of the well, decreasing toward the toe of the well. A study by Ahmed and Badry demonstrated that only 16% of the completed section of a horizontal well may actually be producing, and the productive section was located at the toe of the well. The reasons for this have a great deal to do with the pore pressure of the formations being drilled and the formation exposure time. The lower the formation pore pressure, the higher the risk of formation damage using conventional drilling technology, and the heel of the well has a much higher formation exposure time than the toe of the well.

A large number of the horizontal wells drilled in Western Canada today are in oil fields that have been producing for more than 40 years. In many cases, these reservoirs have been managed without the use of pressure maintenance. The result has been significant pressure depletion with significant oil left in place.

Why drill underbalanced?

In conventional drilling, the drilling fluid density is planned such that the hydrostatic pressure is high enough to hold back the oil and gas to ensure that the well cannot flow during drilling. This method is referred to as overbalanced drilling. Problems occur when the drilling fluid creates enough hydrostatic pressure to force drilling fines into the formation, causing formation damage.

In some instances, drilling fluid flows into the formation at such a rate that very little or no drilling fluid returns to surface. This problem is referred to as lost circulation. The formation may become packed off with cuttings, other solids, or lost circulation material added to the drilling fluid to control fluid flow into the formation. Low pressure and fractured reservoirs are more susceptible to damage.

Consider the following example of a typical horizontal well in western Canada. The well has a vertical depth of approximately 1,200 m and a mud weight of 1,065 kg/cu m. The hydrostatic head at the heel of the well is 12.5 MPa (1,800 psi). When this pressure is compared to a typical pore pressure of 7 MPa (1,000 psi) for this depth of reservoir, the problem of formation damage becomes evident. In rare instances, pore pressures have been found as low as 3.5 MPa (500 psi). Damage to such reservoirs can be extensive.

The permanent skin damage in a well bore can only be partially corrected, especially in open hole or slotted liner completions. In carbonate reservoirs, treatments can effectively dissolve solids that plug the formation. In sands, however, fewer effective treatment options are available.

The cost for such a service ranges $100,000-$300,000 in Canada, depending on the length of the well and type of treatment required. It is also important to note that there are no guarantees this treatment will be effective enough to justify the cost. The real answer lies in preventing damage through improved drilling technology.

Canadian operators recognized that current technology was not allowing them to fully exploit their reservoir assets, and they began to search for technological solutions that would allow them to improve their rate of return. Underbalanced drilling was seen as a technology that could reduce or eliminate formation damage and improve well productivity. To be successful, the productivity improvement and subsequent production revenue increase through the use of underbalanced drilling would need to outweigh the cost and benefit of stimulation treatment. Underbalanced drilling would also need to be done in a safe manner to ensure that well control was maintained and personnel were not at risk.

Underbalanced techniques

Air drilling systems have been used to drill surface holes in hard rock drilling locations in North America since the 1800s. In the late 1980s and early 1990s, a significant number of horizontal wells were drilled in the Austin chalk using a technique known as flow drilling. This technique allowed the well to flow while drilling, but the control system was not a completely closed loop system. The Austin chalk could be successfully drilled underbalanced using brine and killed using a weighted brine. Because many of the reservoirs in western Canada have much lower reservoir pressures, this approach to underbalanced drilling was not applicable.

In 1991, a study was undertaken by Veteran Resources Inc. to develop parameters and equipment specifications for safely creating an artificial underbalance with nitrogen gas to drill a horizontal well. The first closed-loop, artificially underbalanced horizontal well in Canada was drilled by Beau Canada Exploration and Veteran Resources in May 1992. A parasite endless-tubing sidestring and a closed-loop surface system were used to induce and control the underbalanced condition. This system consisted of the following:

Significant delays in data transmission from the mud pulse tool will occur if a compressible medium, such as air, is allowed to enter the drill pipe. If air enters the drill pipe, the compressible medium must be circulated past the pulse generator on the MWD tool in order for detection to be reestablished. This could result in the MWD prematurely activating once pumping begins, and the transmission of initial survey data before detection is established on surface. If this occurs, the pumps must be shut down and the survey data resent.

After introduction of the parasite string technique, two other processes began to emerge, the use of microannulus injection and two-phase gas flow down the drill pipe.

Microannulus injection

The microannulus technique emerged from the reluctance of some operators to use a parasite string (Fig. 4 [113702 bytes]). Its design allowed for the injection of gas at the beginning of the horizontal section, reducing the amount of nitrogen required to induce the underbalanced condition.

Most microannulus casing techniques require larger intermediate casing sizes, however. An example well configuration would be 177.8-mm (7 in.) intermediate casing set to horizontal with a 139.7-mm (51/2 in.) slim-line microannulus casing string installed inside of it. A special spool is placed at the casing flange to accommodate the additional casing string. Gas is injected into the annulus between the 177.8-mm casing and the 139.7-mm casing.

Some operators install two joints of 139.7-mm casing on the bottom of the 177.8-mm casing to allow packers to be set to ensure well control when the 139.7-mm casing is being withdrawn from the well. Once again, this technique was developed primarily around the requirement of mud pulse telemetry for the use of an incompressible fluid in the drill pipe.

Drill pipe injection

While using the parasite gas injection and microannulus gas injection techniques, operators often inject nitrogen into the drill pipe to aid hole cleaning and reduce the required volume of nitrogen to induce an underbalanced condition (Fig. 5 [117019 bytes]). When this drilling mode is used, no detection is obtainable from the conventional mud pulse telemetry MWD tools because of the presence of a compressible medium. The maximum allowable gas concentration before detection is lost occurs at approximately 90,000 ppm.

Drilling technology

Conventional downhole motors have operated successfully in underbalanced drilling operations. An air or two-phase flow motor was developed to maximize the torque available to the bit and reduce the potential of severe damage to the downhole motor. The result has been a more reliable motor capable of optimizing the torque available to the bit. When downhole motors are run on two-phase flow, the combination of gas and fluid needs to be determined to ensure that the motor is not overrun.

One benefit realized from underbalanced drilling has been an increase in penetration rate two to six times greater than the normal rate in a given field. For example, on the first well drilled by Beau Canada at Twinning where a closed-loop underbalanced drilling system was used, the penetration rate increased by 320%. The casing shoe was drilled out, and 25 m of hole were drilled conventionally with produced native crude at a rate of 10 m/hr. Nitrogen injection was started, and the penetration rate increased to 42 m/hr.

Another secondary benefit of underbalanced drilling is increased bit life. In some instances bit runs exceeding 80-100 hr have occurred with 120.6-m (43/4 in.) roller cone bits. This longer bit life results from the very clean drilling fluids which provide improved lubrication for the bearing surfaces. In addition, during underbalanced drilling operations, less wear occurs on the cutting surface of the bit because the cuttings are not reground as they usually are during overbalanced drilling.

A replacement for mud pulse telemetry systems was needed to optimize underbalanced drilling operations and to minimize the costs associated with the implementation of this technique. Acoustic telemetry signal transmission technology was evaluated, but this device proved noncommercial with current technology.

A solution was found through the development of an electromagnetic measurement while drilling system which allows signal transmission through a compressible medium (Fig. 6 [112292 bytes]). Electromagnetic MWD shows great potential for use in underbalanced drilling worldwide, but in some locations it has experienced a significant number of depth limitations and formation sensitivities. Some of these problems should be solved with the introduction of a new-generation device that is currently in semicommercial operation in western Canada. This system includes the capability to add repeaters to increase the depth range and reduce the sensitivity to lithology.

Surface monitoring system

Coupled with the introduction of the elecromagnetic MWD system, a drilling optimization system has been developed. The surface monitoring system monitors all required surface parameters and provides the capability to commingle downhole data such as bottom hole pressure and directional data (Figs. 7 [136029 bytes] and 8 [113498 bytes]).

The continuous monitoring of all parameters at one central location also provides the capability for alarm activation should any parameter vary over preset operational limits. This system allows the operator to maintain complete and safe control of the entire drilling operation.

A valuable byproduct of this comprehensive underbalanced drilling-monitoring system is the information which can be used in post well analysis and well data archives. The surface monitoring system is also an important tool for interpretation of information related to geosteering.

The information provided by the monitoring system may be used to interpret the point when the well is underbalanced and can be correlated through time and depth to provide a variety of interpretations of what occurred during the drilling process (Fig. 9 [99450 bytes]). A unique, continuous qualitative reservoir permeability log may be derived from the depth, surface tank level, and nitrogen injection rate data. This type of log very accurately locates and quantifies the most productive portions of the reservoir. Tank volume, fluid density, and fluid sample data may be used to locate zones with higher producible water contents, which assist in making completion decisions.

Changing hydrocarbon gas flow rates, determined from continuous total gas monitoring and comparisons between the nitrogen flow rates into and the gas flow rates out of the well bore, allow changes in the reservoir fluid volatility to be interpreted for proximity to potential gas/oil contacts.

Perhaps the single most important contribution provided by this integrated surface monitoring system is the ease with which nonproductive reservoir intervals may be quickly detected. Many instances have occurred where the drill bit has become trapped within a nonproductive fault plane, and early recognition of this fact has resulted in minimal lost productivity as the drill bit is steered into a better reservoir.

Similarly, many zones may be drilled rapidly with good oil shows from the drill cuttings but have extremely poor permeability. Many conventionally drilled horizontal wells are poor producers simply because of this inability to assess permeability during drilling.

Experience from western Canadian underbalanced horizontal drilling clearly demonstrates that a well bore's initial productive potential is very accurately predicted from its productive behavior during drilling operations. Decisions crucial to the economic success of any ongoing underbalanced horizontal well program can therefore be made as each well is being drilled rather than having to wait the weeks or sometimes months after drilling is completed for such data to be available from a conventionally drilled horizontal program.

This integrated approach of adding specially developed downhole sensor and transmission capabilities in conjunction with a surface drilling optimization system has resulted in the capability to effectively manage and understand the underbalanced drilling environment within a closed-loop system.

Acknowledgment

The author wishes to thank Veteran Resources Inc., in particular D. Jewitt and B. Schmidt, and R. Hay and D. Gust of Sperry-Sun Drilling Services of Canada for their insights and help in developing this article.

Bibliography

1. Haas, R.C., and Stokely, C.O., "Drilling and Completing a Horizontal Well in Fractured Carbonate," World Oil, October 1989.

2. Stayton, R.J., and Peach, S.R., "Horizontal Drilling Enhances Production of Austin Chalk Well," paper No. 19984, presented at the International Association of Drilling Contractors/Society of Petroleum Engineers Annual Drilling Conference, Houston, Feb. 27-Mar. 2, 1990.

3. Mehdizadeh, A.M., Hashemi, R., and Caothien, S., "Laboratory Investigations of the Effects of Formation Damage in a High-Permeability Reservoir," paper No. 23826, presented at the Society of Petroleum Engineers International Symposium on Formation Damage Control, Lafayette, La., Feb. 26-27, 1992.

4. Renard, G., and Dupuy, J.M., "Formation Damage Effects on Horizontal-Well Flow Efficiency," Journal of Petroleum Technology, July 1991.

5. Economides, M.J., and Ehlig-Economides, C.A., "Discussion of Formation Damage Effects on Horizontal-Well Flow Efficiency," Journal of Petroleum Technology, December 1991.

6. Ahmed, U., and Badrey, R., "Production Logging as an Integral Part of Horizontal Well Transient Pressure Test," paper No. 20980, presented at the SPE European Offshore Petroleum Conference, The Hague, Oct. 22-24, 1990.

The Author