Calculation of Pressure Loss of Two Drilling Muds in Noor Oil Field

*Corresponding author  Peer review under the responsibility of University of Baghdad.  https://doi.org/10.31026/j.eng.2020.02.05  2520-3339 © 2019 University of Baghdad. Production and hosting by Journal of Engineering.  ). / http://creativecommons.org/licenses/by /4.0 license  4 This is an open access article under the CC BY  Article received: 24/12/2018  Article accepted: 10/3/2019  Article published: 1/2/2020


INTRODUCTION
Noor Oilfield is located in the southeast of Iraq, about 15 km northeast of Amara city, Missan Governorate. The field is NW-SE trending anticline and is about 18.9 km long and 5.9 Km wide, (Midhat, et al., 2014). Fifteen oil wells have been drilled in Noor oilfield and this research is made on the last. Oil is produced from Mishrif formation. Many problems face drilling in Noor oil field, so it became more important to make studies that can prevent these difficulties or reduce them, especially when drilling 12 1/4" and 8 3/4" holes. A lithology and drilling parameters of these holes are shown in Fig.1 and Fig. 2. The 12 1/4" hole is characterized by abnormal high formation pressure, high temperature and contains layers of salt rock. Saltsaturated mud is used to drill this hole. One of the problems that occur is the flow of fluids into the well after the process of cementing, which leads to pollution of the new drilling fluid that used in the next hole with salt. Therefore, it is important to calculate the loss of pressure inside the well to see if there is a flow of fluid or not before replacing SSM by FCL-CL . 8 3/4" hole is the last, and FCL-CL mud is used in it. It's the production hole and drilling fluid losses may happen in addition to pipe sticking, therefore; rheological model of drilling fluid must be detected, and pressure losses must be calculated accurately.

PROPERTIES OF DRILLING MUDS
There are many types of drilling mud used in oil field. A classification of it is given by (Caenn, et al., 2011) is shown in Table 1. Drilling fluids studied in these researches are saltsaturated mud and FCL-CL mud. First one is used for drilling hole of 12 1/4". The length of hole is about 700 m, and it contains a salt rock. Salt formations are distinctive. Porosity and permeability of salt are very little. It can flow plastically through other geological rock beds under stress with "salt creep" and that leads to reducing in wellbore size and collapse in casing. Also, salt dissolves in water, therefore; the salinity of a water-based mud must be kept near or at saturation to prevent or minimize wellbore enlargement that can lead to bad cementing of the casing and incomplete zonal isolation, (Amer, et al., 2016). Boreholes in salt layers tend to be overbalanced, (Weijermars, et al., 2013). After completing the drilling of this hole, casing is placed and then the process of cementing is done. Float shoe and float collar are drilled as well excess cement by using the same drilling fluid, which specifications may be affected by these successive processes. The specifications of SSM are shown in Table 2. The second type of drilling fluid is used for drilling hole of 8 3/4". FCL-CL mud has been used because of its resistant to contamination. A ferrochrome lignosulfonate called Q-BROXIN had the unusual property of thinning gyp muds and salty muds. In 1955, Roy Dawson introduced Q-BROXIN to oil field drilling. Chrome lignite (CL) with chrome lignosulfonate afforded a simple chemical system that was widely applicable. This system supplied control on both filtration and flow properties over a wide range in pH, salinity, and solids content, (Caenn, et al., 2011). The length of hole is about 1300 m. Drilling fluid specifications are shown in Table 3. All tests of drilling muds are made according to, (API RP 13B-1, 2003). The devices used are shown in Fig.3.
There are many models that represent the relationship between shear rate and shear stress. In this research, three models:

3.1Bingham Plastic Model
Bingham model can be known through two parameters: plastic viscosity and yield point. The plastic viscosity depends on the size, concentration, shape of solids and the viscosity of the liquid phase. Te yield point is formed by the power of attraction between solid particles as a result of the existing charge on their surfaces. It can be represented mathematically as follows:

Ʈ= μp * γ + Ʈy
(3) Type of the flow of the drilling fluid is determined by Reynolds number. If flow is laminar, the pressure loss is calculated by the following equation, Rabia, 1985: Inside drill pipe, In annulus ∆ = 1500 2 + 225 (4) ∆ = 1000 ( ℎ − ) 2 + 200 ( ℎ − ) Pressure loss equation of turbulent flow is, Rabia, 1985: Inside dill pipe, In annulus 3.2 Power Law Model, (API 13D, 1980). Power-law model can be known through two variables: consistency index (k) and power-law index (n) (dimensionless). It can be represented mathematically as follows: Since drilling fluids are shear-thinning to some degree, the viscosity of the fluid changes with a change in the shear rate. In order to calculate pressure loss, the effective viscosity at a given rate of shear must be known, therefore, effective viscosity is calculated by using Eq.(9) and by using Reynolds Number Eq.(10) the flow regime is determined. Based on the flow regime; friction factor is calculated by using Eq.(11)(12)(13)(14)(15) and by Eq.(6) and (7) pressure loss is measured.

Herschel-Bulkley Model
The yield-power law (Herschel-Bulkley) fluid combines Power-law and Bingham plastic behaviours of fluids. It can be represented mathematically as follows, (Hemphill, et al., 1993): Pressure loss is estimated by using the same equation of Power-law model, but Ф600 and Ф300 is calculated from Eq.(16). Power law exponent and Consistency index in Eq.(16) are obtained from Fig.6 in the case of SSM and Fig.9 in the case of FCL-CL.

EXPERIMENTAL WORK
For two type of drilling fluid, their properties are measured and listed in Table 2 and Table 3. Rheological data are obtained from Fann V-G Meter readings. Linear regression Journal of Engineering Volume 26 February 2020 Number 2 analysis is run for the experimental data to select the model closest to the actual flow curve. This is done for Cartesian coordinates for the Bingham model and logarithmic coordinates for the Power Law and Herschel-Bulkley models. The model with squared correlation coefficient closest to unity will be chosen. R squared formula is given by, (Lenschow, 1992): The flow data, wellbore specifications and the specifications of drill string consisting from drill pipe, heavyweight drill pipe, drill collar and drilling bit are shown in Table 4 and Table 5. By using all these parameters, pressure loss inside wellbore for the three rheological models can be calculated and make a comparison between them. data specification of SSM.

RESULTS
The three models (Bingham plastic, Power law, and Herschel-Bulkley) of rheological data of SSM with its R 2 are shown in Fig.4, Fig.5 and Fig.6 respectively and in Fig.7, Fig.8 and Fig.9 for FCL-CL mud. According to R 2 results, Herschel-Bulkley model is the best one that represents the relationship between shear stress and shear rate for two types of drilling mud. Pressure loss in each part is measured according to the three models and the results are listed in Table 6 for SSM and in Table 7 for FCL-CL mud. Herschel-Bulkley model gives the minimum difference in pressure between standpipe pressure and total pressure loss. There is a difference (about 93 psi) in pressure between standpipe pressure and total pressure loss in the case of FCL-CL mud. This is because the efficiency of mud pumps is assumed to be 90% but in fact, it is constantly changing due to drilling operations and continuous change of damaged parts of mud pumps so assuming the efficiency to be 93.5%, the difference in pressure reduces to 3 psi only. In this way we can roughly measure the efficiency of mud pumps easily and quickly.