With the rapid development of the petroleum industry, the types of drilling fluids and additives are increasing, making their composition extremely complex.
Some of these ingredients are toxic to humans and the environment.
The drilling fluid waste generated on site must be transported to land by ship for treatment. The transportation cost of a large amount of drilling fluid waste is extremely high. The reduction treatment will be the development trend of offshore drilling fluid waste treatment. It is urgent to solve the problem of seawater drilling fluid. The problem of solid-liquid separation and reuse of waste liquid.
In this paper, the solid-liquid separation of seawater-based drilling fluid waste liquid is carried out, and its solid-liquid separation mechanism is analyzed. Recover costs and meet environmental protection requirements and production operation needs.
Experimental materials and instruments
Coagulant PF-PCF, homemade, cationic gemini polyacrylamide (molecular weight 3 million, cationic degree 15%); coagulant polyaluminum ferric chloride, ferric chloride, polyaluminum chloride; partially hydrolyzed polyacrylamide , Xanthan gum, sea water, NaOH, NaOH, NaCl, KCl, barite, etc.
Centrifuge, stirrer, analytical balance, pH meter, Materials Studio2017R2 software.
Results and discussion
2.1 Preparation of seawater drilling fluid waste
The basic ratio of KCl/PHPA seawater drilling fluid system used in Bohai Oilfield is shown in Table 1.
It can be seen from Table 1 that the main treatment agents in the KCl/PHPA seawater drilling fluid system are partially hydrolyzed polyacrylamide, low-viscosity anionic cellulose, xanthan gum, starch and bentonite, and partially hydrolyzed polyacrylamide is an anionic polymer. , Xanthan gum, starch is a non-ionic polymer.
For the flocculation of the above seawater drilling fluid system, the indoor synthetic coagulant PF-PCF was selected for comparison with the other three coagulants: polyaluminum ferric chloride, ferric chloride and polyaluminum chloride.
2.2 The flocculation effect of different coagulants
Take four beakers, each take 60mL of simulated seawater drilling fluid, add the same concentration and different types of coagulants for flocculation separation, the types of coagulants are PF-PCF, polyaluminum ferric chloride, ferric chloride, polychlorinated 4 kinds of aluminum, the basic formula is:
60mL of simulated seawater drilling fluid + 4mL of coagulant solution with a concentration of 100000mg/L, after adding coagulant to the waste liquid of simulated seawater drilling fluid, stir well, the concentration of coagulant in the system is 6250mg/L. Four sets of experiments were centrifuged as shown in Figure 1.
It can be seen from Figure 1 that when the concentration of coagulants is the same, PF-PCF can achieve complete separation of solid and liquid from the waste liquid of simulated seawater drilling fluid, and the flocculation effect of the other three coagulants is not ideal at the same concentration. The supernatant obtained by centrifugation was taken out (see Figure 2), the volume and pH of the supernatant were respectively measured, and the dehydration rate was calculated. The results are shown in Table 2.
Dehydration rate=(volume of supernatant-volume of added solution)/volume of treated drilling fluid waste.
It can be seen from Table 2 that the simulated seawater drilling fluid waste liquid directly flocculated with coagulant, PF-PCF has a good flocculation separation effect at a concentration of 6250mg/L, and at the same concentration, the remaining coagulants can flocculate sedimentation drilling fluid. Part of the solid in the waste liquid, but can not completely separate the solid and liquid.
2.3 Determination of the concentration of coagulant PF-PCF
Take 4 beakers, take 60mL of simulated seawater drilling fluid waste liquid, and add PF-PCF solutions of the same volume and different concentrations respectively. The specific formula is as follows:
60mL of waste drilling fluid + 4mL of PF-PCF solution with concentrations of 60,000, 80,000, 100,000, and 120,000 mg/L, respectively, and stir well.
At this time, the concentrations of coagulant PF-PCF in systems No. 1-4 were 3750, 5000, 6250, and 7500 mg/L, respectively. Four sets of experiments were centrifuged as shown in Figure 3.
It can be seen from Figure 3 that when the concentration of PF-PCF reaches 6250 mg/L, a better flocculation effect can be achieved. As the concentration increased, the supernatant obtained by flocculation separation became clearer. The supernatant obtained by centrifugation was taken out, and various data were measured, as shown in Table 3.
It can be seen from Table 3 that when the concentration of PF-PCF in the system reaches 6250 mg/L, solid-liquid separation can be achieved, and as the concentration increases, the dehydration rate of solid-liquid separation also increases to a certain extent. The concentration of PF-PCF was selected to be 7500 mg/L, the pH of the supernatant after flocculation separation was 6.94, the dehydration rate was 55.7%, and the dehydrated water was relatively clear.
2.4 Destabilization mechanism of seawater drilling fluid waste
The simulation uses Materials Studio 2017R2 software to optimize the structure of the partially hydrolyzed polyacrylamide single-molecule model through the GeometryOptimization tool.
Molecules are brought to an energy minimization model using the SmartMinimization algorithm. The partially hydrolyzed polyacrylamide single molecule model is shown in Figure 4.
The optimized layer is calculated using the Dynamics tool in the Forcite module.
Select Ensemble as NVT (canonical ensemble), Temperature: 278K, TimeStep: 1fs, TotalSimulationTime: 500ps, NumberofSteps: 5000, perform molecular dynamics simulation under the Compass force field, and repeat the calculation for each model multiple times to make each set of data The deviation is within 5%.
Two partially hydrolyzed polyacrylamide molecules are combined with 100 water molecules, and the conformational model is shown in Figure 5.
The energies in the above molecular conformations were simulated using Materials Studio 2017R2 software, and the data are shown in Table 4.
Two partially hydrolyzed polyacrylamide molecules, a coagulant molecule and a water molecule at a ratio of 2:1:100 to construct the model are shown in Figure 6.
The energies in the above molecular conformations were simulated using Materials Studio 2017R2 software, and the data are shown in Table 5.
Comparing the energy changes in Table 4 and Table 5, before adding the coagulant, the total energy of the partially hydrolyzed polyacrylamide and water molecule system is -937.733kcal/mol. After adding the coagulant, the total energy of the mixed system is -390.518kcal/mol.
The absolute value of the energy in the system decreased by 547.215kcal/mol, and the decrease rate was 58.4%. The decrease of the system energy leads to the decrease of the repulsion energy when the two partially hydrolyzed polyacrylamide molecules are close to each other, and the system becomes unstable and flocculation occurs.
It can also be seen that when the partially hydrolyzed polyacrylamide adsorbs the coagulant, the partially hydrolyzed polyacrylamide has a negative charge, while the coagulant has a positive charge.
When the two are adsorbed, the partial positive and negative charges are electrically neutralized, which reduces the negative charge of the partially hydrolyzed polyacrylamide and reduces the zeta potential, resulting in a decrease in the repulsion between the two partially hydrolyzed polyacrylamide molecules.
Combining the changes in the energy and zeta potential between the two partially hydrolyzed polyacrylamide molecules, both show a decreasing trend. Therefore, the two partially hydrolyzed polyacrylamide molecules are close to each other and are easy to aggregate. flocculation.
(1) Through comparative experiments, for seawater drilling fluid waste, the preferred coagulant is PF-PCF, the concentration is 7500mg/L, the pH of the supernatant after flocculation separation is 6.94, and the dehydration rate is 55.7%.
(2)Materials Studio2017R2 software analyzed the absolute value of the energy of the seawater drilling fluid waste liquid system before and after adding coagulant from 937.733kcal/mol to 390.518kcal/mol. The decline rate is 58.4%, which is beneficial to the solid-liquid separation of seawater drilling fluid waste.