Study of a strong inhibitory solid-free drilling fluid

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Introduction
The solid-free drilling fluid is a kind of polymer water solution mainly prepared by water soluble polymer.Compared with the solid drilling fluid system, the system has many advantages, such as lower flow resistance, lower static shear force, good rheology and easy viscosity adjustment.In the drilling process, it has good ability of carrying cuttings, protecting well wall and preventing well collapse.It can protect oil and gas reservoir very well.Solid-phase drilling fluid is highly recognized and applied in oilfield drilling industry because of its advantages of better filtration reduction performance, high lubrication coefficient, low friction between drill bit and wellbore, simple operation and low production cost.[1][2][3][4] Generally, the solid-phase drilling fluid is used as viscosity enhancer with biopolymer, cellulose derivative, synthetic polymer and weak gel, and the modified starch, polyanionic cellulose (PAC), sulfonated phenol formaldehyde resin (SMP) are used to improve its filtration loss reduction, adjust the density with weighting agent and salt, and use corrosion inhibitor, anti collapse inhibitor.The mixture of other drilling fluid treatment agents such as bactericides is made.[5][6][7][8] In recent years, people have more and more demand for oil and gas production.Many kinds of conditions encountered during drilling process also promote the development of drilling technology, and the requirements on its performance are also more stringent.Well under different conditions and various complex drilling processes require drilling fluid to have good strong inhibition performance.Whether the well wall is stable or not, whether it has high temperature resistance stability, and whether it causes pollution to the oil reservoir is closely related to the strength and strength of the drilling fluid inhibition.The research on high performance strong inhibitory solid-phase drilling fluid and matching with different treatment agents to find the suitable single agent can better maintain the shale wall fixation, prevent borehole expansion, find out the simple formula with green environment protection and low cost, adapt to the current environment, and improve the drilling efficiency of various environments, reduce drilling cost and improve enterprise income and benefit.[9][10] In this paper, the inhibitor with better inhibition was screened by linear expansion experiment, and the inhibition performance was tested by compounding with xanthan gum, HPAM, CMC Na and HV PAC.Through the high temperature evaluation experiment and salt resistance evaluation experiment, the viscosity enhancer with good anti high temperature and salt resistance is selected.The best concentration of viscosity enhancer and the screened inhibitor were used to optimize the filtrating loss agent.Finally, mud ball experiment and drilling fluid performance evaluation are carried out to evaluate the performance of drilling fluid formula.

Screening of inhibitors
Linear expansion experiments were carried out in 1831, polydimethylammonium chloride and 1227 at different concentration gradients to investigate the inhibition of inhibitors on the hydration expansion of clay, so as to select the inhibitors in the final formula.The experimental results show that the linear expansion rate of 1831 solution with concentration of 0.2% is the lowest after 120 min, indicating that the clay expansion inhibition performance of 1831 solution with concentration of 0.2% is the best.Therefore, 0.2% 1831 solution is used as the inhibitor of the formula.

Tackifier screening
The viscosity increasing properties of CMC Na, xanthan gum, HV-PAC and HPAM were evaluated through experiments, and the optimal viscosity increasing agent with good viscosity and low filtration was selected.

Drilling fluid stability
It can be seen from Table 1 that with the increasing dosage of tackifier, the plastic viscosity, dynamic shear force and apparent viscosity of other tackifiers fluctuate and are relatively unstable, except that the plastic viscosity, dynamic shear force and apparent viscosity of xanthan gum increase steadily.

Evaluation of high temperature resistance[24]
After each thickener is prepared into a solution, it is put into a frequency conversion roller heating furnace and heated for 16 h at 80 ℃, 90 ℃, 100 ℃, 110 ℃ and 120 ℃ respectively.After the solution is cooled, the performance of drilling fluid is evaluated as shown in Table 2.It can be seen from the data in Table 2 that with the increase of temperature, the plastic viscosity, dynamic shear force, apparent viscosity and dynamic plastic ratio of xanthan gum gradually decrease, but the decrease range is not obvious, while the filtration loss gradually increases, but the increase range is not obvious, indicating that the drilling fluid has good high temperature resistance.For CMC Na, with the gradual increase of temperature, the plastic viscosity of drilling fluid first increases and then decreases, the dynamic shear force first increases and then decreases, and then suddenly increases, the apparent viscosity gradually increases, the dynamic plastic ratio first increases and then decreases and then increases, and the filtration rate gradually increases.The high temperature resistance of this type of drilling fluid is general.For HV-PAC, with the increase of temperature, the plastic viscosity of the drilling fluid does not change significantly, the dynamic shear force first increases and then suddenly decreases, while the apparent viscosity decreases, the filtration rate first increases and then suddenly decreases, and the temperature resistance is general.For HPAM, the plastic viscosity and plastic viscosity of drilling fluid decrease with the increase of temperature, the dynamic shear force and dynamic plastic ratio first increase and then decrease, and the filtration rate increases with the increase of temperature.At 110 ℃, the filtration rate changes suddenly, and the filtration reduction effect fails.

Evaluation of salt resistance
Salt layer and gypsum layer will inevitably be encountered in the process of drilling construction, so salt resistance is also a very important evaluation index of drilling fluid.Add KCl in different proportions such as 5%, 10%, 15%, 20%, 25% and 30% into each solution respectively, and then evaluate the performance of drilling fluid.The data are shown in Table 3.It can be seen from the data in Table 3 that for xanthan gum, with the increase of salt concentration, the plastic viscosity, dynamic shear force, apparent viscosity and dynamic plastic ratio of drilling fluid remain stable, and the filtration rate is relatively stable.After adding 30% KCl, the filtration rate is only 10ml, indicating that this type of drilling fluid has good salt resistance.For CMC Na, the plastic viscosity, dynamic shear force, apparent viscosity and dynamic plastic ratio of drilling fluid decrease with the increase of salt concentration, and the filtration loss increases gradually.The salt resistance of this type of drilling fluid is general.For HV-PAC, the plastic viscosity and apparent viscosity of the drilling fluid first increase and then decrease with the increase of salt concentration, while the dynamic shear force decreases continuously, and the filtration loss first decreases and then suddenly increases.At 15%, the filtration loss suddenly changes to 40 mL, and the filtration reduction effect is invalid.For HPAM, the plastic viscosity and apparent viscosity of drilling fluid decrease with the increase of salt concentration, the dynamic shear force and dynamic plastic ratio first increase and then decrease, and the filtration loss increases slowly with the increase of salt concentration.
In conclusion, through the above experiments, it can be found that among the four tackifiers, xanthan gum has the best comprehensive performance of high temperature resistance and salt resistance.Therefore, xanthan gum is used as the tackifier in the final solid-free drilling fluid system in this formula.

Determination of fluid loss reducer
The 1% xanthan gum solution was compounded with monoxanthate polymer, potato starch, phenolic resin and lignin respectively, and then its viscosity and filtration loss were measured respectively.The filtration reduction performance was compared, and the single agent with the best performance was selected.It can be seen from Table 4 that when the concentration of monoxanthate polymer increases gradually, AV and YP first increase and then decrease.The filtration loss is too large and the filtration reduction effect is general.With the increase of potato starch concentration, PV remains stable and basically unchanged.Compared with monoxanthic acid, the filtration loss is greater, and the filtration reduction effect is not ideal, so it is not considered.With the increase of lignin concentration, AV and YP decreased first and then increased, PV was also relatively unstable, and the filtration rate decreased compared with the previous two filtration reducing agents, but it was still not ideal.As the concentration of phenolic resin increases, the filtration loss decreases to the ideal range, and the PV increases steadily.Therefore, phenolic resin has good filtration loss reduction performance, so it is selected as the filtration loss reducer of the final formula.

Lubricant determination
Lubricant can reduce the friction between drilling tool and borehole wall, so as to reduce the rotating torque and additional tension, and avoid the occurrence of downhole complex accidents such as sticking.After compounding the optimal concentration of tackifier and fluid loss reducer with various lubricants, the viscosity coefficient and extreme pressure lubrication coefficient were measured respectively to select the best lubricant.The selected tackifier, i.e. 1% xanthan gum and filtration reducing phenolic resin, are compounded with three lubricants respectively, and their viscosity coefficient and extreme pressure lubrication coefficient are measured to determine their lubrication performance, as shown in Table 5.It can be seen from Table 5 that for vegetable oil, with the increase of dosage, the viscosity coefficient of mud cake first decreases and then increases, and the extreme pressure lubrication coefficient also decreases gradually, so the lubrication effect is not ideal.For sodium benzene sulfonate, with the increase of its dosage, the viscosity coefficient of mud cake decreases gradually, and the extreme pressure lubrication coefficient also increases gradually.Therefore, it can be seen that its lubrication performance is excellent.For KD03, with the increase of dosage, the viscosity coefficient of mud cake first decreases and then increases, the extreme pressure lubrication coefficient increases slowly, and the lubrication effect is not very ideal.To sum up, the lubricant with the best lubricating performance is sodium benzenesulfonate solution with a concentration of 1%.3.5 Formulation determination and performance characterization of solid free drilling fluid The formulation of solid-free drilling fluid was determined, and the performance of strongly inhibitory solid-free drilling fluid system was characterized by thermogravimetric analysis, mud ball experiment and scanning electron microscope.After screening, the final formula of solid-free drilling fluid is determined as: 0.2% 1831+1% xanthan gum+1% phenolic resin+1% sodium benzene sulfonate.

Mud ball experiment
At room temperature, sodium bentonite was put into oven, adjusted to 105 ℃ and dried for 5 h.After mixing the sodium bentonite and tap water at a mass ratio of 2:1, the ball was weighed about 10 g.The balls were put into the formula solution and soaked in tap water for 72 h.The appearance of the mud balls was observed and photographed at 24 h intervals.Then the inhibition effect of each clay inhibitor is evaluated by the surface morphology of the expanded clay ball.The experimental results are shown in Fig. 3.It can be seen from Fig. 3

Conclusions
Through the linear expansion experiments of 1831, 1227, polydiene dimethyl ammonium chloride and polymer thickener, the results show that 1831 has the best inhibition performance.Among them, the linear expansion rate of clay measured by 1831 with concentration of 0.2% is the lowest.1831 is compounded with xanthan gum, HV-PAC, CMC Na and HPAM respectively for thermogravimetric experiment and clay ball experiment.It can be seen that 1831 has a strong effect on inhibiting the hydration expansion of clay.XG was selected as the single agent in the later compounding by the screening experiment, high temperature resistance experiment and salt resistance experiment, among which the xanthan gum with 1% concentration had the best performance.Xanthan gum was compounded with monosulfonate, potato starch, phenolic resin and lignin respectively, and phenolic resin was selected as a single agent with better filtration reduction performance.Then xanthan gum and phenolic resin were compounded with vegetable oil, KD03 and sodium benzenesulfonate respectively, and the single agent with better lubrication performance, namely sodium benzenesulfonate, was screened out.The best formula of viscosity enhancer and each treatment agent is 0.2% 1831+1% xanthan gum, 1% phenolic resin and 1% sodium benzenesulfonate.
Scanning electron microscopy (SEM) was used to observe the micro morphology of the dried soil samples prepared by adding 1831 inhibitor to the clean water, and then adding CMC Na, xanthan gum, HV-PAC, HPAM, etc. as thickeners, respectively, and the dried soil samples prepared by adding thickener and only tap water, as shown in Fig.1.It can be seen from Fig.1that through the microscopic analysis of soil samples before and after treatment with different thickeners by scanning electron microscope, 1831 inhibitor is added to the drilling fluid base slurry, and then sodium carboxymethyl cellulose, xanthan gum, HV-PAC and HPAM are added as thickeners respectively.The dried soil sample particles are significantly larger than those after hydration and drying of tap water,It can be shown that 1831 inhibitor can play a good inhibitory effect and inhibit the hydration and dispersion of clay.

Fig. 1
Fig. 1 Microstructure of clay before and after treatment with different thickeners

Fig. 2
Fig. 2 Appearance of mud ball in aqueous solution with different inhibitors (48 h) that the mud ball put into the clean water has completely collapsed, while the surface of the mud ball placed in the drilling fluid solution has no cracks and hydration expansion.It shows that adding 1831 inhibitor into drilling fluid has good inhibition on the hydration expansion of clay, which is consistent with the linear expansion rate of bentonite.(a) Clear water (b) Drilling fluid solution

Fig. 3
Fig. 3 Appearance of mud ball in clear water and drilling fluid formula (48 h)

Table . 1
Effect of different tackifier dosage on drilling fluid performance

Table . 2
Performance evaluation of drilling fluids of various systems at different temperatures

Table . 3
Performance evaluation of drilling fluids of various systems under different concentrations

Table . 4
Performance evaluation of 1% XG under different concentrations of filtrate reducer

Table 5
Performance evaluation of 1% XG and 1% phenolic resin under different lubricants