DETERMINATION OF THE SUITABLE DRILLING OPERATING PARAMETERS IN DIFFERENT GEOLOGICAL FORMATIONS EL-SHARARA OIL FIELD ( SOUTH-WESTERN LIBYA )

Purpose. The objective of this study is to determine the best operating conditions for the most notable drilling parameters (i.e. weight on bit (WOB), rotary drilling speed (RPM), and characteristics of drilling fluid) using field data obtained from El-Sharara Oil Field. Methods. The used data has been extracted from daily drilling reports of well named (NC-186/K04h) field. Such data contains information about the geological formations, casing strings, drill-bits, fuel consumption, flow rate of drilling fluid and other drilling parameters. Findings. The results reveal that, the lower geological formations of El-Sharara Oil Field, the harder are the upper formations. Therefore, it is recommended to apply heavy loads (i.e. WOB of 45000 lb) with low drilling speed (i.e. 100 rpm) in the lower formations; and to apply small loads (i.e. WOB of 19000 lb) with high drilling speed (i.e. 160 rpm) in the upper formations. Originality. This study evaluates the performance of drilling operation based on the interaction between rock formations and machine drilling parameters. Practical implications. Understanding such interaction between rock formations and machine drilling parameters will remarkably improve the rate of penetration (ROP) in the related geological formations. Consequently, the overall drilling costs will be reduced in terms of drilling time, life of drill-bit and fuel consumption.


INTRODUCTION
Drilling operation is frequently practised in geotechnical engineering, e.g.civil, petroleum and mining.The performance efficiency of drilling process mainly depends on rock properties, machine and operating parameters, particularly rate of penetration.The latter, e.g.ROP, can be used as a tool to estimate the strength properties of rock (Ali, Abdellah, & Abd El-Aal, 2016).
Rock drillability could be predicted using different methods of analysis such as computer programming/simulator, regression analysis, specific energy (SE) index, dimensionless index of uniaxial compressive strength of rock/specific energy (UCS/SE), and information obtained from cores and/or cuttings.Simulator facilitates rapid deriving prediction equations which assist estimating the drilling costs and help selecting the type of drilling rig according to ground conditions.The starting point for numerical analysis is to obtain field data to be used as input parameters for the simulator in order to have predic-tions of the ROP as an output para-meter (Wijk, 1991;Abouzeid & Cooper, 2003;Dahl, Grøv, & Breivik, 2007).
Regression analysis can be conducted using a computing package called "Statgraphics-Statistical Graphic System" to develop a penetration rate models and to decide which variables should be included in the model (i.e.drill power and/or properties of the rock penetrated).Consequently, hundreds of models are resulted and then statistically are tested to select the best-fit model (Selim & Bruce, 1970;Kahraman, 1999).The concept of SE is firstly suggested by Teale (Teale, 1965), Mellor (Mellor, 1972), Pathinkar and Misra (Pathinkar & Misra, 1980), and Bilgin and Kahraman (Bilgin & Kahraman, 2003) as a tool to evaluate rock drillability.SE is a function of ROP, torque (T), RPM and cross-sectional area of drill-bit (A) (Abdellah, 2007).The dimensionless index (UCS/SE) can also be employed to distinctly classify rock types by plotting that index against the obtained ROP (EL-Biblawi, Sayed, Mohamed, & EL-Rawy, 2007a).

Major factors influencing ROP
Resistance to penetration, shearing action of drill-bit during its rotation and the degree of abrasiveness are the major phenomena that would have an influence on the drilling efficiency.Estimating the drilling costs and identifying rock type and its properties based on prediction of ROP have become a big challenge issues in petroleum engineering.Such prediction might help drilling engineers to improve the capability of drilling operation.Well logging or wireline logging is used to measure some characteristics of geological formations such as electrical conductivity, radioactivity and porosity.Bit condition (i.e.wear degree, new and/or dull) has a great influence on the rate of drilling.Consequently, it affects the overall drilling costs.The bit wear is defined as removal of material from solid surfaces as a result of relative sliding motion at the contact surface.The life of drill-bit, replacement and/or maintenance costs are usually hard to be accurately predicted, because many factors influencing them are hard to be obtained (Sayed, 1992;Sayed, 1994;Sayed, 2009;Sayed, 2014).
In order to drill a well, three parameters have to be simultaneously combined.These are: -certain load has to be applied on the drill-bit (i.e.thrust, WOB); -the drill-bit has to be rotated (i.e.RPM); -the drilling fluid has to be circulated within the well bore (i.e.fluid/mud characteristics).
Man power and hardware systems are the two principal components required to drill borehole.The first component (i.e.man power) consists of group of drilling engineers and rig operators.Such group is involved in many drilling activities like mud design, drill-bit/rig selection, casing and well-cementation, and well control and monitoring.The second component (i.e.hardware systems) includes power generation, hoisting, mud/fluid circulation; blow out control and drilling data acquisition system (Sayed, 2001;El-Biblawi, Sayed, Mohamed, & El-Rawy, 2007;Osgouei, 2007;Sayed & Boghdady, 2010;El-Biblawi, Sayed, Boghdady, & Hamdallah, 2012;Seifabad & Ehteshami, 2013).
Therefore, it is necessary to combine these two components (i.e.man power and hardware systems) together to improve the drilling operating parameters for an efficient drilling process.Improvement of the significant drilling operating parameters are necessary to reach reasonable drilling rate related to different geological formations in oil well drilling.Such improvement is extensively influenced by several factors some of which are controllable by the operator (i.e.thrust or weight on bit, drilling speed, fluid/mud properties, and bit type/conditions) and some are beyond the control of the operator (i.e.characteristics of geological formations).Thereby, thorough understanding of the relationship between these two parameters (i.e.controllable and noncontrollable) at any given site is mandatory to achieve such improvement (Libya Oil & Gas Exploration Lows and Regulation Handbook).Therefore, this paper will use the information from El-Sharara Oil Field to estimate the suitable drilling operating conditions related to geological formations.The next section briefly presents the information about the area under the study.

STUDYING AREA EL-SHARARA OIL FIELD
The giant El-Sharara Oil Field is located in Block NC-115 of the Murzuq basin, about 730 km south of Tripoli (Biggest Producing Fields in Libya; History of the petroleum industry).The map that illustrates the location of the area under the study is shown in Figure 1.The field was discovered by Petroleum Romanian Oil and Gas Company in the 1980's and later began production in December 1996.By 2006, it was producing about 200000 barrels per day (bpd) of high quality crude oil.Since mid of 1998, the light and sweet crude which was produced at El-Sharara has been exported by pipeline through the Zawia terminal west of Tripoli (Biggest Producing Fields in Libya).

Figure 1. Location of El-Sharara Oil Field (Murzuq basin, Libya) (Biggest Producing Fields in Libya)
The field has a capacity of 400000 bpd and before the outbreak of conflict in 2011 accounted for a quarter of Libyan production.Although, other sources site the maximum capacity lower at 340000 bpd.By February 2012, the National Organization Company announced that production at the site had again reached 300000 bpd, despite delays over security concerns (History of the petroleum industry).
Akakus Oil Operations Company is one of the largest oil companies operating in Libya and has pioneered, explored and eventually developed two major blocks in the Murzuq basin in the South-Western area of the Libyan Desert adjacent to the famous Akakus Mountain strip and the old city of Ubari.The company's field of activities is limited to the Exploration and Oil Production of the National Concessions (i.e.NC-115 and NC-186) including any other unforeseen new developments for the benefit of both parties.

National Concession NC-115
The primary Concession is initially developed by Akakus (i.e.formerly is known by Repsol Oil Operations) and is comprised of the Main Gas Oil Separation Plant (GOSP) "A" and two other smaller GOSP's "B" and "H".This concession is the backbone of the field operations where all control and monitoring is done.The main Central Complex, Airport, warehouses, workshops and other supporting facilities are in this concession.

National Concession NC-186
This is the recently developed Concession and is comprised of the main gas and oil separation plant (GOSP) "A" and the minor gathering satellite stations GS "B", "D", "H", "J" and "K".All these facilities are interconnected by a network of trunk lines for facilitating oil movement and water injection mechanisms around the fields.The "I/R" development project, which is currently ongoing, is distributed among these two Concession due to the physical existence of the oil reservoir shared between the two concessions (Fello, 2001;Clark-Lowes, 2008).The drilling information that collected from a well named "NC-186/K04h" (i.e.) daily drilling reports and information about the drill-bits that are used in the drilling through a certain period is presented in the next section.

DATA COLLECTION
The daily drilling reports contain information about the geological formations, casing strings, drill-bits, fuel consumption, drilling fluid flow rate and drilling parameters.This research focuses on the operating parameter (i.e.WOB, RPM) and some related drilling parameters (i.e.ROP, fuel consumption and drilling fluid flow rate).Daily reports, from well named (NC-186/K04h), give the important drilling parameters as listed in Table 1.Similarly, other daily drilling reports for the same well (i.e.reports No. 57 to 91) are used to extract important drilling parameters and reports No. 13 to 33 are used from another well named "NC-115/M16i", as well as from well named "NC.115/WSW.37".

RESULTS AND DISCUSSIONS
The collected data indicate that the wells were drilled through different geological formations.The Murzuq basin, South-west Libya, is formed due to series of Palaeozoic intracratonic sag basins on the North African Saharan Platform.The structural fabric of the basin has been developed during the late Proterozoic Pan-African orogenic event.Thus, it is greatly influenced the depositional patterns and stratigraphy of the basin.According to the wireline log response, grain-size and the proportion of sandstone to shale, the Murzuq formations are divided into lower and upper.
The latter, upper formations, comprises mainly from sandstone with coarse to very coarse grained.Whereas, the lower formation is characterized by the presence of sandstone dominated coarsening-upward sequence accompanied by increased bioturbation towards the northeast (Fello, 2001).Therefore, in this paper the two formations are called upper and lower.The average values of drilling parameters for upper formations, in terms of ROP, flow rate of drilling fluid, fuel consumption at constant high speed and varied WOB, are given in Table 2.

Effect of WOB on ROP -upper formations
The relationship between WOB and ROP, at high RPM and WOB, is depicted in Figure 2. It can be shown that, the ROP increases with the increase of WOB.The ROP gradually increases until it reaches the maximum value of 21.4 ft/hr at WOB of 19000 lb.Afterwards, the ROP becomes constant or gradually decreases (i.e.due to wear occurrence to drill-bit).

Effect of RPM on the fuel consumption -upper formations
The fuel consumption exceeds as RPM increases as shown in Figure 3.It can be shown that, the suitable drilling speed which gives the lowest amount of fuel consumption is 55 rpm.The values of the ROP and fuel consumption at various RPMs and WOB, for the upper geological formations, is given in Table 3.

Effect of RPM on ROP and fuel consumption -upper formations
The relationship between ROP and RPM is displayed in Figure 4.It is shown that, the maximum value of ROP (i.e.55.4 ft/hr) is obtained at low RPM.However, ROP decreases if the RPM and WOB increase.The reason for that is the soft formations are more sensitive to RPM variations than WOB.On contrary, the hardest formations are likely sensitive to WOB change than RPM.In addition, the wear of the drill-bit increases as WOB and RPM increase.

Effect of WOB on ROPlower group of formations
The relationship between fuel consumption at various RPMs is shown in Figure 5.It is obvious that, the fuel consumption increases as rotary drilling speed increases.Also, the optimum RPM that gives the maximum value of ROP is 100 rpm, as shown in Figure 4 and 5.
The average values of ROP, fluid flow rate and fuel consumption at constant high speed and different weight on bit for the lower formations are listed in Table 4.
The relationship between WOB and ROP, at constant high speed, is depicted as shown in Figure 6.It can be shown that, the ROP increases as WOB increases.

Figure 6. ROP versus WOB at constant RPM of 175 rpm
The values of penetration rate increases gradually until the maximum RPM reaches 8.8 ft/hr at WOB of 20000 lb.

Effect of RPM on ROP and fuel consumption -lower formations
The results show that the harder formations (i.e.lower formations) are more sensitive to the increase in the WOB rather than increase in RPM as shown in Figure 7. High RPM leads to decrease in the values of ROP and increase the bit wear (i.e. increase friction with formations).It is found that, the suitable RPM is 160 rpm which gives the suitable ROP in hard formations.
The relationship between fuel consumption and RPM is displayed as shown in Figure 8.  and 8).The relationship between flow rate of fluid and RPM is given in Figure 9. From these three figures (Figs. 7 -9), it can be shown that the suitable value of drilling speed is 160 rpm which gives best ROP of 11.2 ft/hr, minimum fuel consumption of 30 gal/hr and minimum fluid flow rate of 400 gpm.

CONCLUSIONS
It is utmost important to optimize the drilling opera-ting parameters (i.e.WOB, RPM and fluid flow rate) for the best drilling conditions (i.e.maximum ROP and minimum fuel consumption).In this paper, drilling information from El-Sharara Oil Field in Libya is used to determine the suitable drilling conditions with respect to different geological formations.Generally, the ROP increases when WOB increases for both two formations type.In the upper formations, the maximum value of ROP of 21.40 ft/hr is obtained at low WOB of 19000 lb, and high RPM of 175 rpm.Whilst, the ROP of 55.4 ft/hr is reached at high thrust or WOB of 45000 lb, and low RPM of 100 rpm.Alternatively, ROP is directly proportional to WOB and inversely with RPM.But, ROP decreases when both WOB and RPM increase.
The most suitable value of drilling speed is found to be 100 rpm to produce best ROP (Figs. 4 and 5).Also, the results indicate that, the ROP in the upper formations is 2.5 times that in the lower formations, (i.e.ROP of 21.4 ft/hr) compared to 8.8 ft/hr, at the same drilling conditions (i.e.WOB, RPM and fluid flow rate).Thereby, lower formations are harder than upper formations.
When RPM increases the fuel consumption (i.e.high energy consumed) increases for both two different geological formations.Finally, it is recommended to apply heavy WOB with lower RPM of 100 rpm in hard formations (i.e.lower formations) and small WOB with high RPM of 160 rpm in soft formations (i.e.upper formations) to get maximum ROP, minimum fluid flow rate and minimum fuel consumption.

RECOMMENDATIONS
The following two recommendations are highly advisable: 1.It is not recommended to apply both high WOB and RPM together, it would accelerate drill-bit wear, e.g., heavy loads of weight on bit would create huge friction with rock, and increase the fuel consumption, e.g., due to high speed.
2. For oil well, e.g., NC-186/K.04H,to obtain the best drilling conditions, e.g., maximum ROP, minimum fluid flow rate and minimum fuel consumption, heavy loads on drill-bit, e.g.weight on bit of 45000 lb, should be applied at low drilling speed of 100 rpm particularly in hard formations, e.g., lower formations, and small loads of thrust of 19000 lb, are used at high drilling speed 160 rpm in the soft formations, e.g.upper formations.

Figure 2 .
Figure 2. Variation of ROP at different WOB at low WOB and high RPM

Figure 3 .
Figure 3. Fuel consumption against RPM at low WOB of 20000 lb

Figure 4 .
Figure 4. Relationship between ROP and RPM at high WOB 45000 lb

Figure 5 .
Figure 5. Relationship between fuel consumption and RPM at low WOB of 20000 lb (ROP), ft/hr Weight on bit (WOB), 1000 lb

Figure 8 .
Figure 7. ROP against the RPM at constant WOB of 20000 lb (lower formations) Figure 9. Fluid flow rate versus RPM at WOB of 20000 lbIt is shown that, the fluid flow rate increases as drilling RPM increases.