Drilling muds
Drilling muds are prepared in such a manner that it resembles the muds that are used according to the daily mud report of the South Oil Company, Iraq. Tables 1 and 2 show the composition of water-based ferrochrome lignosulfonate mud (FCLS) and lime mud, while Tables 3 and 4 show the physical properties of drilling muds, respectively.
Table 1 Composition of FCLS drilling mud
Table 2 Composition of lime drilling mud
Table 3 Characteristics of FCLS drilling mud
Table 4 Characteristics of lime drilling mud
Basic components of drilling muds
Several components are used to docile the rheological properties of drilling muds as follows:
Water
Water is the most important substance involved in drilling fluid technology. It is usually readily available at relatively low cost. Among the unusual properties of water in comparison with other liquids are the highest surface tension, dielectric constant, heat of fusion, heat of vaporization, and the superior ability of water to dissolve different substances.
Barite
One of the important functions of drilling mud is the control of formation fluid pressure to prevent blowouts. The density of the mud must be raised at times to stabilize fragile formations. Barite (barium sulfate, BaSO4) contains 58.8 % barium and has a specific gravity of (4.2–4.5). Commercial barite, which is usually impure, is of lower specific gravity because of the presence of other minerals such as quartz, chert, calcite, anhydrite, celestite, and various silicates. In addition, it usually contains several iron minerals, some of which may increase the average specific gravity of the product. Barite virtually is insoluble in water and does not react with other components of the mud. It has been used to raise the density to 1.44 gm/cc to control the gas inflow and stop caving and in pulling off dry pipe (Darley and Gray 1988).
Bentonite
Bentonite is the only commercial clay, which is now used in significant amounts in freshwater muds. It has been defined as consisting of fine-grained clays that contain not less than (85 %) montmorillonite (Abdou et al. 2013). Bentonite is added to freshwater muds in order to:
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1.
Reduce water filtration into permeable formations.
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2.
Increase hole-cleaning capability.
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3.
Form a low permeable filter cake.
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4.
Promote hole stability in unconsolidated formations.
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5.
Prevent or overcome loss of circulation.
Carboxymethylcellulose (CMC)
CMC is an anionic polymer and is adsorbed on clay particles. Filtration is sharply reduced by low concentrations of CMC, and thermal degradation of CMC is accelerated as the temperature approaches 300 °F. Also, it is used for raising viscosity. The effectiveness of CMC in reducing filtration and raising viscosity decreases as salt concentration increases.
Lignite
Lignite is an organic thinner that serves water muds for filtration reduction, oil emulsification, and stabilization of properties against high-temperature effects. It is not used as a thinner for salty muds. It maintains stable filtration rates in drilling hot holes. A major application for lignite is in conjunction with chrome lignosulfonates for improving the filtration property and thermal stability of mud.
Ferrochrome lignosulfonates
Ferrochrome lignosulfonates is an effective dispersing agent at a high pH for different mud systems. It is commonly used to prevent salt flocculate of bentonite and minimize the effect of high-temperature gelatin in bentonite fluid and sometimes added to get better filtration control.
Caustic soda
Caustic soda or sodium hydroxide (NaOH) is used in water-based mud to raise its pH; to solve lignite, lignosulfonate, and tannin substances; to counteract corrosion; and to neutralize hydrogen sulfide. Also, some improvement in performance can be achieved by adding caustic soda to the freshwater along with the bentonite to act as a dispersing agent.
Soda ash
The principal use of soda ash or sodium carbonate (Na2CO3) is for the removal of soluble salts from makeup of waters and muds and to enhance the yield of clay.
Lime
The function of the lime is to furnish sufficient calcium to prevent hydration and dispersion of drilled shales and clays. Lime is an inorganic compound with the chemical formula Ca(OH)2. It is a white powder and is derived from heating limestone (mainly calcium carbonate). This reaction produces calcium oxide (quick lime). On adding water this forms calcium hydroxide (slaked lime). Table 5 shows basic information about lime.
Table 5 Basic information of lime
The environmental protection agency recognizes that lime is the most available and cost of the blending material. It does an excellent job in solidifying the particles and recommends it for use in oil field and industrial applications.
Gas oil
Gas oil is used as an oil or emulsifier to obtain a large cutting size, in addition to viscosity and fluid loss control. Table 6 shows basic information about gas oil.
Table 6 Physical properties of gas oil
Core preparation
Core plugs cutting and cleaning
In this study, the core samples were provided from seven oil wells of Zubair Formation of two oil fields in southern Iraq (Basrah region): These wells are R.45, R.99, and R.181 of Rumaila North Oil Field and Ru.64, Ru.181, Ru.182, and Ru.197 of Rumaila South Oil Field. About 100 plugs were prepared from depths of over (3000) m (pay zones intervals). The core samples were cut to about one inch diameter and 1.5 inch length using a small special bit by Rockwell cutting machine. The saltwater was used as a coolant. The plugs were then cleaned using Soxhlet extraction with a mixture of equal volumes of toluene (C7H8), methanol (CH3OH), and pure benzene (C6H6). The process was repeated until the color of the solution no longer changes and becomes clear. After this, the plugs were dried in an oven for 24 h at 200 °F. A study was carried out before and after exposing the core samples to different drilling fluids. The reservoir conditions were prepared as close as possible to mimic down hole conditions, namely, hydrostatic pressure, formation pressure, and hole temperature.
Permeability measurements
Two types of permeabilities were measured by Ruska liquid permeameter:
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1.
Absolute permeability of formation water. Thus, all the core samples were saturated with the same fluid flow.
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2.
Effective permeability for gas oil was measured after measuring absolute permeability.
These two types follow Darcy’s law:
$$K = \frac{1000Q\mu L}{{\Delta pA}}$$
(1)
where K = permeability of core sample (md); Q = flow rate (cm3/s); µ = viscosity of the liquid (cp.); L = length of the core sample (cm), A = cross-sectional area of the core sample (cm2); ∆p = differential pressure across the core sample (atm.).
The saltwater was flushed by gas oil until the liquid flow rate through the core sample becomes constant and no more water was evident in the discharge liquid. This was done in order to reach irreducible water saturation (Swi). Gas oil was used as oil rock formations instead of other types of petroleum products. It does not volatile at room temperature, easily obtainable, and requires simple pressure to test the effective oil permeability. The effective oil permeabilities ranged from 36 to 1391 mD. This range is sufficient to give results for different experiments.
Saturation measurements
The reservoir rocks normally contain oil and water; this water is called connate water. Fluid saturation can be determined by direct method from small rock samples. The retort method had been used to determine fluid saturation. The small sample is heated to (1000 °F) to evaporate all oil and water. The vapor is condensed and collected in small vessel after waiting for a period of 30 min. All saturations are calculated as a percent to pore volume, according to following equations:
$${\text{sw}} = \frac{{{\text{WV}}\left( {\text{cc}} \right)}}{{{\text{PV}}\left( {\text{cc}} \right)}} * 100$$
(2)
$${\text{So}} = \frac{{{\text{OV}}\left( {\text{cc}} \right)}}{{{\text{PV}}\left( {\text{cc}} \right)}} * 100$$
(3)
where Sw = water saturation (%); So = oil saturation (%); WV = water volume (cc); OV = oil volume (cc); PV = pore volume (cc).
Saturation method had been used to measure pore volume for core samples. Formation water was used as a liquid saturation, whose density is (1.1188 gm/cc).
X-ray diffraction
X-ray diffraction analysis is that auxiliary measurement was taken in order to obtain more information of various minerals in core samples.
The static immersion test
This test measures the swelling and spalling of core sample in shale intervals due to its exposure to the different drilling fluids. The procedure test is as follows:
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1.
Core samples were prepared and dried in the oven at 200 °F. Initial dry weights were measured.
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2.
Core samples immersed in two drilling muds. These muds were FCLS mud and lime mud.
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3.
The core samples were left static for 15 days in these muds. After period termination, the samples were collected and their final wet weight was measured.
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4.
The core samples were dried in the oven for 4 h at 200 °F. The final dry weights were measured.
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5.
The equations, which were used to calculate the swelling percentage and spalling percentage, are:
$${\text{Swelling}},\;{\text{wt}}\% = \frac{{W - W_{0} }}{{W_{0} }}$$
(4)
$${\text{Spalling}},\;{\text{wt}}\% = \frac{{W_{1} - W_{0} }}{{W_{0} }}$$
(5)
where W = final wet weight (gm); W
1 = initial dry weight (gm); W
0 = final dry weight (gm).
Experimental rig
A laboratory rig was prepared for evaluating the extent of damage caused by drilling fluids. It consists of a core holder similar to Hassler core holder, which can accommodate a core plug of 1 inch diameter. The core holder tolerates high pressure and temperature. The core sample was placed inside a rubber sleeve and the two sealed together by overburden pressure, which applied by a hydraulic pump. The pressure of nitrogen gas was used to push the drilling fluid through the core sample at constant pressure for 1-h testing time. This pressure was considered as the differential pressure between hydrostatic and formation pressures. Figure 1 shows a schematic diagram of the Hassler core holder. The parameters that were kept constant in this study are the differential pressure (300 psi), the formation temperature (200 °F), and the confining pressure (5000 psi).
The other components of the rig consist of the following apparatuses:
Viscometer Viscosity of gas oil and formation water was measured by using (Ubbelhode-Holde) viscometer. It has a constant of “0.00845” cp/s. Viscosity is determined as the time of flow through the apparatus from the upper limit until the marked lower limit. The following equation is used to calculate viscosity.
$$\mu = 0.00845 * t$$
(6)
where μ = dynamic viscosity (cp); t = time of fluid flow (s).
Pycnometer It was used to measure the specific gravity of gas oil and formation water.
Mud Mixer Hamilton beach mixer was used in laboratory to mix the mud materials.
Mud Balance Baroid mud balance was used to determine the densities of all drilling muds that were used. The mud balance may be calibrated with freshwater. The reading at room temperature should be 8.33 lb/gal.
Filter Pressure Test Baroid filter press was used to determine the filtration properties of drilling muds at room temperature and 100 psi pressure at static conditions. The amount of filtrate discharged in 30 min is measured.
Fann V–G meter (Model 35A) was used to measure two constant speeds, θ600 and θ300 rpm. The plastic viscosity (PV) is calculated by the following equation:
$${\text{PV}} = \theta 600 - \theta 300$$
(7)
where PV = plastic viscosity in centipoises (cp); θ600 and θ300 = dial reading at speeds of 600 and 300 rpm, respectively.
The yield point (YP) is calculated from the following equation:
$${\text{YP}} = \theta 300 - {\text{PV}}$$
(8)
where YP = yield point (lb/100 ft2); θ300 = dial reading (at a speed of 300 rpm); PV = plastic viscosity in (cp).
Soil Hydrometer A (152 H-62 ASTM Soil Hydrometer 0–60 gm/l) was used to determine particle size distribution of muds. This test was used to calculate the diameters of particles by hydrometer analysis.
Experimental procedure
All essential calibrations for pumps, pressure gauges, and other devices were carried out before starting up any experiment. The experiments are carried out as follows:
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1.
The core is evacuated of air over a period of 12 h using a vacuum pump and then slowly saturated with NaCl solution (formation water) until atmospheric pressure is attained.
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2.
The absolute permeability is measured by the Ruska liquid permeameter with formation water.
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3.
The effective permeability of the core is determined by flowing gas oil at a constant pressure. Also, initial fluid saturation is determined by the direct Retort method.
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4.
The plug is mounted in a Hassler core holder, and the pressure is raised to the confining pressure (5000 psi).
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5.
The Hassler core holder is covered with a heater jacket to raise the temperature to 200 °F (reservoir formation temperature).
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6.
The core is damaged by a mud penetrating across its face for the duration of 1 h at a constant differential pressure of 300 psi using nitrogen gas pressure.
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7.
The sample is left for 1 day to backflow with gas oil until no further permeability is obtained.
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8.
The percent of formation damage can be determined by comparison, between effective permeability and fluid saturation before and after mud exposed.