Criteria for calculation of drilling fluid consumption during offshore horizontal wells drilling
Mirzayev Zaur Gabil, student master's degree;
Efendiyeva Leyla Zohrab, researcher
Scientific adviser: Shmoncheva Elena Evgenevna, assistant professor
Azerbaijan State University of Oil and Industry (Baku, Azerbaijan)
Offshore drilling has a high risk that complicates the design of drilling processes. The traditional method of calculating offshore drilling fluid flow does not consider the effect of the extended reach limit in offshore drilling. As a result, an extended reach well may not reach the target length of the well trajectory, creating a potential safety hazard. Therefore, there is an acceptable range of drilling fluid flow in offshore conditions.
The article describes three characteristic features for the hydraulics design of offshore horizontal wells. The design criteria that were used to optimize well cleaning and, consequently, the rate of penetration are substantiated.
Keywords: drilling hydraulics, well cleaning, penetration rate, maximum hydraulic power, maximum impact force, maximum nozzle speed.
Offshore drilling has the characteristics of a high-risk investment, which complicates the design of offshore drilling [1]. The traditional method of calculating offshore drilling fluid flow does not consider the effect of the extended reach limit in offshore drilling. As a result, an extended reach well may not reach the planned length of the well trajectory, which creates a potential safety hazard [6,8]. Therefore, there is an acceptable range of drilling fluid flow in offshore conditions.
There are three characteristic features for the hydraulics design of offshore horizontal wells [3,4,5]:
1) The length of the horizontal section of the sea is usually very large, from several hundred meters to thousands of meters. Failure to remove sludge in a timely manner will result in sludge accumulation and may pose a safety hazard.
2) The riser section must be considered when designing an offshore horizontal well casing program. The annular size of a riser section is usually larger than other sections. As a result, the minimum consumption of drilling fluid for cleaning the hole in the riser may be greater than in other sections of the well, otherwise there may be potential threats to drilling safety due to untimely removal of cuttings in the riser.
3) Mud circulation often results in large annulus pressure losses due to the large, measured depth of offshore horizontal wells, which will pose a great threat to the well stability of the formation being drilled and the bearing capacity of the mud pump. In addition, the mud flow rate is often improved to increase the efficiency of cuttings removal from the riser section, which further affects the bearing capacity of the formation being drilled and the mud pump.
Drilling hydraulics is considered the most important factor in drilling performance [2,3,7]. ROP can be significantly increased using modern hydraulic optimization technologies to minimize drilling costs. The goal of optimization is to maximize the use of pump power so that the bit can drill at maximum efficiency. This is achieved by minimizing energy losses due to friction in the circulating system and using the saved energy to improve the hydraulics of the bit.
The main task of the operator when working with drilling fluid hydraulics is to ensure adequate cleaning of the hole under the bit.
This is important for the following reasons:
– Penetration rate depends on hole clearance below the bit.
– Some bits can be damaged by overheating if cuttings accumulate underneath.
Poor cleanup below the bit prevents detection of changes in formation properties that could otherwise be determined from ROP. Achieving the proper level of well cleanout requires maximum utilization of the power of the mud pumps on the bit hydraulics. This means maximum use of pump pressure and flow. For these fixed power pumps, the available pump pressure and flow rate are determined by the piston size used. Thus, the optimal choice of pressure and flow rate is not simple. This article describes the criteria used by the drilling industry to optimize drilling fluid hydraulics in order to achieve maximum ROP. There are different theories regarding the mechanism for clearing barrels. Various design criteria have been used to optimize the hydraulic fluid for maximum hole cleanout and therefore ROP. These criteria include maximum bit hydraulic power, maximum bit hydraulic shock, and maximum bit stream velocity.
Determination of criteria. The criterion for the maximum hydraulic power of the bit can be formulated as follows: within the maximum available pressure of the pump, the flow rate of the drilling fluid and the size of the nozzle should be chosen so that the bit gains the maximum possible power to clean the bottom of the well. It is known that the efficiency of jet bits can be improved by increasing the hydraulic power of the pump. ROP will increase with hydraulic power until cuttings are removed as quickly as they form. Once this level of «perfect cleaning» is reached, there should be no further increase in ROP with hydraulic power. However, due to the loss of pressure due to friction in the drill string and the annulus, the hydraulic power developed at the bottom of the well is different from the hydraulic power developed by the pump. Therefore, the important parameter is the power of the bit, and not the power of the pump. It is clear, however, that bit power is not necessarily maximized by operating the pump at its maximum possible power.
The criterion for maximum jet impact can be formulated as follows: within the maximum available pump pressure, the drilling fluid flow rate and nozzle size should be chosen so that the bit generates the maximum possible jet impact for bottom hole cleaning.
The criterion for the maximum nozzle speed can be formulated as follows: within the maximum available pump pressure, the drilling fluid flow rate and nozzle size should be chosen so that the bit will create the maximum possible jet speed for bottomhole cleaning.
Nozzle speed can be increased by reducing the flow rate, which reduces parasitic pressure loss. In the field, the flow rate is set to the minimum flow rate, which is determined by the minimum annulus velocity required to lift cuttings.
As for the question of which criterion is best for optimizing bit hydraulics, most people use the maximum bit hydraulic power criterion or the maximum bit hydraulic force criterion at shallow to medium depths, and then switch to maximum nozzle speed at greater depths.
Between the criteria for maximum bit hydraulic power and maximum bit hydraulic shock force, none of the criteria turned out to be the best in all cases, because the difference in applying these two procedures is negligible. If the jet impact force is at its maximum, the hydraulic power will be within 90 % of the maximum, and vice versa. Another argument is that in many cases the bits provide hydraulic performance in excess of what is required, so the effect of design using different criteria is masked.
The concept of hydraulic bit power was introduced as a design criterion in the early 1950s. This is a measure of the work required to force drilling fluid through bit nozzles. This work is related to the removal of cuttings from under the bit. Hydraulic bit power is the most common design procedure, probably because it was the first to be used.
The concept of hydraulic shock force as a design criterion was introduced in the mid-1950s. The hydraulic hammer force is a measure of the force exerted by the fluid as it exits the nozzles of the bit. This fluid forces cleans the bottom hole through direct erosion and cross flow under the bit.
The force of the water hammer under the bit seems to be more logical than the hydraulic power of the bit when considering bottomhole cleanup design techniques. Bits with jet nozzles extended closer to the bottom of the well are widely used.
Since the extension of the nozzles does not change the hydraulic power of the bit but changes the force of the hydraulic shock on the bottom of the well, it is believed that the latter is directly related to the cleaning of the well.
The criteria for bit hydraulic power and water hammer strength are widely used in the development of drilling fluid hydraulics programs. The debate over which design criterion to use can be contentious, as any of them can be used to optimize bottom hole cleaning requirements. Drilling tests in real drilling conditions determine the optimal cleaning requirements. Thus, if bottom hole cleaning requirements are determined using bit hydraulic power, then that hydraulic power should be the basis for the calculation. The same applies to the force of a water hammer.
In seams of normal hardness, where there is no definite point of failure, the amount of bottomhole cleaning required can be determined directly in field work. It can be difficult to determine the cleanout required to achieve maximum ROP in very soft formations.
In these formations, the maximum penetration rates are achieved with the maximum hydrodynamic impact on the bottomhole. Thus, the problem is to use the maximum jet action that is economically feasible. The economic feasibility depends on the maximum ROP possible, the condition of the well and other factors such as connection time and maintenance of ancillary equipment.
With high-capacity pumps, it is possible to achieve a higher level of bit hydraulics (power, impact force, or nozzle speed) than is necessary to adequately clean the bottom hole. Using higher than necessary bit hydraulics is not only wasteful but can also be harmful.
This is because high flow rates in the system can lead to well and pipe erosion, as well as premature failure of pump parts.
It is important to know that pump maintenance costs increase as pump pressure increases. Showing a direct mathematical relationship between pump pressure and maintenance costs is difficult because many other variables also have a direct effect on pump maintenance costs. In fact, pump maintenance costs are rising much faster than increasing pump pressure. For example, the cost of maintaining a pump often more than doubles when the pump pressure increases from 17 000 to 20 000 kPa). Exact figures for specific drilling rigs or operations should be determined depending on the operating conditions in the field.
Conclusions. If the need for cleanout can be determined from ROP data obtained with similar lithology under different bit hydraulic conditions, then pump energy consumption should be reduced by reducing flow until the desired level of bit hydraulics is achieved if the pump is running. at the maximum allowable pressure. The same logic can be applied using either hydraulic power or impact force as the hydraulic parameter.
In addition, when the mud pump does not meet the drilling fluid circulation requirements, it is necessary to replace the mud pump with a pump of a higher-pressure rating or power rating.
References:
- Abahusayn, M., B. Foster, J. Brink, M. Kuck, and J. Longo. 2012. Nikaitchuq Extended-Reach Drilling: Designing for Success on the North Slope of Alaska. SPE Drilling & Completion. 27(04), p. 501–515.
- Erge, O., Ozbayoglu, E. M., Miska, S., Yu, M., Takach, N., Saasen, A., & May, R., 2015. The Effects of Drillstring-Eccentricity, -Rotation, and-Buckling Configurations on Annular Frictional Pressure Losses While Circulating Yield-Power-Law Fluids. SPE Drilling & Completion.
- Guo, B., & Liu, G. (2011). Mud Hydraulics Optimization. Applied Drilling Circulation Systems, p. 81–105.
- Kelessidis, V. C., Dalamarinis, P., & Maglione, R., 2011. Experimental study and predictions of pressure losses of fluids modeled as Herschel-Bulkley in concentric and eccentric annuli in laminar, transitional and turbulent flows. Journal of Petroleum Science and Engineering. 77(3), p. 305–312.
- Li, X., Gao, D., Tan, L., Zhang, H., Chen, X., & Zhou, Y. (2017). Study on the Drilling Fluid Flow Rate Allowable Range in Offshore Drilling Considering the Extended-Reach Limit. Abu Dhabi International Petroleum Exhibition & Conference. doi:10.2118/188435-ms
- Xin, Li, Deli, Gao, Yingcao, Zhou, et al., 2016a. A model for extended-reach limit analysis in offshore horizontal drilling based on formation fracture pressure. Journal of Petroleum Science and Engineering. 146, p. 400–408.
- Xin, Li, Deli, Gao, Yingcao, Zhou, et al., 2016b. Study on open-hole extended-reach limit model analysis for horizontal drilling in shales. Journal of Natural Gas Science and Engineering. 34, p. 520–533.
- Xin, Li, Deli, Gao, Yingcao, Zhou, et al., 2016c. General approach for the calculation and optimal control of the extended-reach limit in horizontal drilling based on the mud weight window. Journal of Natural Gas Science and Engineering. 35, p. 964–979.
