Water-based drilling fluids can cause hydration of the wellbore rocks, thereby leading to instability. This study aimed to synthesize a hydrophobic small-molecule polymer (HLMP) as an inhibitor to suppress mud shale hydration. An infrared spectral method and a thermogravimetric technique were used to characterize the chemical composition of the HLMP and evaluate its heat stability. Experiments were conducted to measure the linear swelling, rolling recovery rate, and bentonite inhibition rate and evaluate accordingly the inhibition performance of the HLMP. Moreover, the HLMP was characterized through measurements of the zeta potential, particle size distribution, contact angles, and interlayer space testing. As confirmed by the results, the HLMP could successfully be synthesized with a favorable heat stability. Furthermore, favorable results were found for the inhibitory processes of the HLMP on swelling and dispersed hydration during mud shale hydration. The positively charged HLMP could be electrically neutralized with clay particles, thereby inhibiting diffusion in the double electron clay layers. The hydrophobic group in the HLMP molecular structure resulted in the formation of a hydrophobic membrane on the rock surface, enhancing the hydrophobicity of the rock. In addition, the small molecules of the HLMP could plug the spaces between the layers of bentonite crystals, thereby reducing the entry of water molecules and inhibiting shale hydration.
“Drilling fluid” is a general term for the various types of circulatory fluids used in oil and gas drilling. When using water-based drilling fluids in drilling work, the liquid in the drilling fluids will create adverse conditions for wellbore rocks, especially in mud shale, which will affect the formation stability and drilling operations. This can lead to wellbore collapse, wellbore enlargement, and drill jamming, and this lack of stability is often called wellbore instability [
The use of mud shale inhibitors in drilling fluids is an important measure for suppressing dispersed hydration and swelling in mudshale or clay and stabilizing wellbores [
The inhibitors mentioned above have unsatisfactory high-temperature resistance and limited inhibition abilities, so they cannot meet the actual drilling requirements. Some previously used high-molecular-weight polymers were found to have a large impact on the performance of drilling fluid systems, while low-molecular-weight polymers had a much smaller effect on the rheology of the drilling fluid system. Low-molecular-weight polyethylene glycols were found to exhibit some inhibitory properties, and have been used as inhibitors [
In this study, acrylamide (AM), (3-acrylamide propyl) trimethylammonium chloride (ATAC), and butyl methacrylate (BMA) were combined to form a low-molecular-weight ternary copolymer. Inhibition was evaluated by testing the linear swelling, rolling recovery rate, and bentonite inhibition rate. Zeta potential, particle size, particle size distribution, contact angle, and interlayer space measurements, as well as scanning electron microscopy (SEM) observations, were obtained to study the mechanism of this hydrophobic low-molecular-weight polymer (HLMP). This study may guide the future development of hydrophobic polymer inhibitors.
Acrylamide (AM, 99.0 wt%), (3-acrylamide propyl) trimethylammonium chloride (ATAC, 75.0 wt%), butyl methacrylate (BMA, 95.0 wt%), ethanol (99.9 wt%), potassium chloride (99.5 wt%), and benzoyl peroxide (BPO, 99.0 wt%) were purchased from Aladdin Chemical Reagents (Shanghai, China). Polyetheramine D230 (PAD230) was purchased from Shanghai Juxin Chemical (China). Bentonite and rocks were provided by the Xinjiang Oilfield Company (China). The detailed mineralogical composition of the shale is presented in
Component | wt% |
---|---|
Plagioclase | 24.1 |
Potassium feldspar | 1.5 |
Calcite | 2.4 |
Siderite | 1.2 |
Quartz | 24.2 |
Chlorite | 3.8 |
Illite | 14.4 |
Kaolinite | 11.1 |
Halite | 4.5 |
Illite/smectite mixed layer | 12.8 |
First, 100 mL of ethanol was added to a 250 ml three-neck flask and then stirred after the addition of 3 g of AM. After the AM was completely dissolved, 1.8 g of ATAC and 0.2 g of BMA were added in turn and stirred well to allow for complete dissolution. Subsequently, an oil bath pot was used to heat up the reaction system to 80°C, and nitrogen was introduced into the flask for 30 min to remove oxygen. Then, 0.1 g of BPO was added to trigger the reaction. After 4 h (a full reaction), a large volume of ethanol was used to precipitate the product of interest. The product was dried at 70°C and crushed at 25 ± 2°C to produce a powdered polymer, which was the HLMP. The chemical reaction equation of HLMP synthesis is shown in
The purified HLMP was freeze-dried and then ground into a powder, and its infrared spectrum was obtained by an Antaris II Fourier Transform Near-Infrared Spectrometer (USA) using the potassium bromide tableting method. Under a nitrogen atmosphere, the thermogravimetric curves of the HLMP at 40°C–700°C were obtained using an HTG-2 Thermogravimetric Analyzer (Mettler, Switzerland), with a heating rate of 10 °C/min.
A sample of bentonite was oven-dried to a constant weight. An electronic scale was used to accurately weigh 10.0 g of bentonite, which was loaded into a core mold and pressurized to 10 MPa using a hydraulic press. The stress was relieved after 10 min, and the rock samples were obtained. The rock samples were loaded into a CPZ-II dual-channel linear swelling tester and soaked in different inhibitor solutions. The expansion of the rock samples was recorded, and the experiment was completed after 16 h.
Red mud shale selected from an outcrop in Songlinzhen, Sichuan, China, was crushed into rock chip sieves with mesh numbers of 6–10 and dried to a constant weight in a drying box. Five 50 g portions of rock chips were placed in separate aging tanks. An inhibitor solution (350 mL) with a mass friction of 1.0% was prepared and added to the aging tanks containing the rock chips and aged at 120°C for 16 h. After aging was complete, the obtained samples were cooled to 25 ± 2°C, rinsed several times with tap water, and passed through a standard 40-mesh sieve. The chips that remained on the sieve were dried at 100°C to a constant weight in the drying box, and the mass value was recorded as
Five samples of 350 mL of aqueous solutions containing 1.0 wt% HLMP were prepared using goblets, with 350 mL of water used as the blank control group. Next, 35 g of bentonite was slowly added to each HLMP sample and stirred at a speed of 5000 rpm for 20 min, sealed, and left at 25 ± 2°C for 16 h. After that, the obtained HLMP–bentonite system was stirred for 20 min at 5000 rpm, and viscosity readings were obtained at 100 rpm (Φ100) using a 6-speed rotary viscometer. After measurement, the HLMP–bentonite system was again loaded into the aging tanks and aged at different temperatures for 16 h. Following that, the system was cooled to 25 ± 2°C and stirred at 5000 rpm for 20 min. The Φ100 viscosity was measured again using the viscometer. The following equation was used to calculate the relative inhibition rate after aging:
A 4.0 wt% bentonite slurry was prepared with 16 g of bentonite dispersed in 400 mL of water, which was left to stand at 25 ± 2°C for 24 h. Different concentrations (0, 0.5, 1.0, 1.5, and 2 wt%) of the HLMP inhibitor were added to 100 mL of the prepared base slurry and stirred for 24 h to allow the HLMP to fully adsorb on the clay surface. Subsequently, a Zetasizer Nano ZS (Malvern Instruments, UK) was used to test for changes in the electrical potential of the clay.
After different concentrations of HLMP were added to the bentonite slurry prepared as described above, they were poured into an aging tank and sealed. The aging tank was placed into a roller heating furnace and aged at 150°C for 16 h. After cooling, the change in the particle size of the bentonite following the addition of HLMP was measured using a Mastersizer 3000E Particle Size Analyzer (Malvern, UK).
Cut chips were placed in different aging tanks, and then aqueous HLMP solutions with different concentrations (0, 0.5, 1.0, 1.5, and 2 wt%) were added. After aging at 150°C for 16 h, the rock chips were removed. After drying at ambient temperature, changes in the contact angle of the rock chips were tested using a contact angle measuring instrument (OCA 255, Shimadzu, Japan).
HLMP was added to 100 mL of the prepared bentonite slurry and aged at 150°C for 16 h. After aging, the slurry was centrifuged for 5 min at 8000 rpm. After centrifugation, a layer of sediment was removed, dried at 105°C, ground into a powder, and the change in the grain spacing of the bentonite was measured using X-ray diffraction (D8 DISCOVER, Bruker, Germany).
The surface morphology of the shale before and after HLMP treatment was observed by scanning electron microscopy (Nova NanoSEM 450, USA). The treatment process of the shale was the same as for contact angle measurement.
The thermal stability of the synthesized products was studied using an HTG-2 thermogravimetric synchronization analyzer, and the results are shown in
The effects of different inhibitors on the expansion of bentonite rock samples are shown in
The effects of different inhibitors on the rolling recovery rate of red mud shale rock chips are shown in
The performance results of 1.0 wt% HLMP on the hydration of the bentonite slurry after aging for 16 h at different temperatures are shown in
The influence of the HLMP concentration on the electrical potential of bentonite is shown in
The influence of HLMP concentration on the contact angle of the rock is shown in
SEM was used to analyze the surface morphology of the shale, and the results are shown in
The schematic diagram of the inhibition mechanism of HLMP is shown in
In this paper, a hydrophobic ternary copolymer with a low molecular weight, HLMP, was successfully prepared/synthesized. The experimental results indicated that the HLMP had an excellent inhibitory effect and could effectively inhibit hydration-induced swelling and dispersed hydration in shale and clay. The synthesized HLMP also had excellent application potential. In this work, we only conducted experiments to evaluate the inhibitory properties of HLMP at 120°C. The study of its heat resistance should be conducted in future work.
The authors thank the Integration and Testing of Safe and Fast Drilling and Completion Technologies for Complex Ultra-Deep Wells (2020F-46) and Major Technology Field Test of Joint-Stock Company (Drilling and Production Engineering).
The work is supported by the
The authors declare that they have no conflicts of interest to report regarding the present study.