Abstract: Waste drilling fluids impose immense negative influence on water quality and soil because of their high pollution, diversity and difficulty in disposal. Application of more rigorous environmental protection regulations has imposed new requirements on the disposal of waste muds. This paper summarizes nine different methods of waste mud disposal and their development. Solidification method, thermal desorption method, chemically enhanced solid-liquid separation method, the so-called “waste mud producing while disposing”, and the integration of these technologies are emphasized in this paper, and several popular technologies are compared. The development trend of each technology is predicted and it is concluded that the integration of these technologies is of the most potential technology. The prospects of these technologies are presented based on the current policies and regulations as well as literature research. It is found that the future research work will be focused on the recycling of waste muds at low energy consumption and high efficiency. It is important to not only control the source of pollution, the processing of pollution and the result of waste mud treatment, but also reinforce management and establishment of working system, and develop new technologies.
Abstract: Mud losses are frequently encountered in the west Shaanbei area, especially in Tiebian town of Wuqi County. Severe mud losses sometimes resulted in hole abandonment or sidetrack, hence high drilling cost in this area. By analyzing the characteristics of the cores taken from Tiebian town and performing mechanical testing on the cores, it was found that what caused the mud losses were mainly the inter-connected flow channels formed by propagation and extension of micro-fractures as well as the communication of natural fractures. Based on this understanding of the mud losses, a lost circulation control slurry was formulated with rigid particles (for bridging), flexible deformable agents (for packing) and fiber materials (for intertwined connection) through simulated experiment. In laboratory experiment the lost circulation control slurry was able to bear pressure of more than 7 MPa. Three times of trials of this slurry in field application were all succeeded in the first try, and the pressure bearing capacity (equivalent density) of the thief zones was increased by 0.22 g/cm3. The successful application of the lost circulation control slurry provided a technical support to the prevention and control of lost circulation in the area of application.
Abstract: Geological study, drilling fluid measure and optimization study and field application of the research results were conducted to resolve the problems such as wellbore instability, pipe sticking and drags encountered in drilling the fractured formations in several wells in Shunbei oilfield. It was found in the studies that the lithology of the fractured formations was non-clay minerals, and invasion of the mud filtrates into the fractured formations makes the in-situ pore- and fissure-network communicated with each other and produce induced pressure inside the pores and fissures, reducing the cementation strength between bedding plains. The formations with mud filtrate invasion are thus becoming loosened, hence resulting in large-scale caving, sloughing and even pipe sticking. To drill this non-clay fractured formations with stress concentration, effective plugging of the formations with submicron-micron (0.1-100 μm) sized particles to contain the invasion of mud and/or mud filtrates into the formations is necessary, and the drilling fluid should also have good cuttings carrying capacity. Laboratory optimization of the drilling fluid previously used to drill the fourth interval of the well gave birth to a drilling fluid suitable for drilling the fractured formations. In the whole process of drilling the fourth interval, no sloughing or caving, drag and over-pull have ever happened, and the well was successfully drilled to the designed depth of 8014 m in a record-setting short time of reservoir drilling in this area. Filed application of the optimized drilling fluid on the well Shunbei-Y showed that the borehole wall stabilizing drilling fluid had excellent properties. It can be used to stabilize the fractured formation during drilling, and is worth popularizing.
Abstract: Nanomaterials have been used to improve the stability of foam drilling fluids, and the effects of the wettability of the nanomaterials on the foam quality were studied using three-factor three-level response surface methodology (RSM) in accordance with the Box-Behnken Design principle. In the studies the concentrations of the foaming agent BS-12, the hydrophilic nanoSiO2 and the viscosifier XC were optimized, and the effects of their interaction on the comprehensive performance of the foam were investigated. A new foam drilling fluid was designed based on these studies. It was found in the studies that the wettability of the nanomaterials remarkably affected the foam quality of the foaming agents. Hydrophobic nanomaterials was able to enhance the stability of the foam produced with an ionic foaming agent SDS. Increased treatment of the hydrophobic nanomaterials decreased the foam quality and halflife of the foam produced with an amphoteric foaming agent BS-12. The optimized foam mud was obtained using RSM design, which is: 0.6%BS-12 + 4%Nano SiO2+0.3%XC. RSM analysis showed that order of magnitude of effect of the concentrations of the three agents on the comprehensive performance of the foam is: nanomaterial > foaming agent > XC. Laboratory evaluation showed that this foam drilling fluid had apparent viscosity of 42 mPa·s, density of 0.81 g/cm3, half life of 60 h, meaning that the foam drilling fluid is able to sustain its stability for a long time. The foam drilling fluid also showed strong shale inhibitive capacity; the linear expansion rate is 65% lower than that of clean water. The recovery rate of coal rock core gas permeability is over 90%, and settling velocity of rock and coal cuttings in the foam drilling fluid was reduced by at least 92% compared with clear water, indicating that the foam drilling fluid had both good reservoir protection performance and good cuttings carrying capacity. The results of the performance evaluation test showed that this foam drilling fluid is able to satisfy the needs of coal-bed methane drilling.
Abstract: An amidated dimer acid derivative (a flow-pattern optimizer) and organophilic montmorillonite are two core additives used in formulating the FLAT-PRO constant rheology fluid. By studying these two additives from macroscopic and microscopic perspective and rheological analyses, the constant rheology mechanisms of the FLAT-PRO fluid was extensively investigated. Rheological experiment under controlled temperatures and long-time standing test results showed that, at low temperatures, the volume of the oil phase is reduced, the dispersibility of the organophilic clay in the oil phase becomes poor, and the viscosity and gel strengths were increased greatly. With temperature increasing, the volume of the oil phase expands and the organophilic clay becomes well dispersed in the oil phase, the viscosity and gel strength of the drilling fluid were correspondingly reduced. When the flow-pattern optimizer was added into the drilling fluid, changing of the viscosity and gel strength of the drilling fluid with temperature becomes less progressive, and the drilling fluid thus showed to some extent a constant rheology characteristics. Microscope observation, XRD analysis and SEM observation showed that the molecules of the flow-pattern optimizer can intercalate in between the micro crystal layers of the organophilic montmorillonite, thereby increasing the c-space of the clay and improving the dispersion of the clay in oil. In this way a temperature-responsive tight network structure is formed in a drilling fluid by highly dispersed clay platelets, molecules of the flow-pattern optimizer and emulsified droplets. The molecular chains of the flow-pattern optimizer become stretched at elevated temperatures; compared with the viscosity and gel strength of the drilling fluid at 2 ℃, the viscosity and gel strength of the drilling fluid at 65 ℃ were increased with much higher magnitude, making the rheology of the drilling fluid change more gently between 2 ℃ and 65 ℃, showing a characteristics of constant rheology.
Abstract: The Zhaotong Demonstration Zone of shale gas drilling is an important area favorable to shale gas exploration and development. Affected by the surface and underground conditions, mud losses have generally been encountered during drilling operations. Mud losses generally occurred in the shallow section, the medium-deep (vertical)section and the horizontal section. The mud losses were difficult to predict and become more severe with mud density. PetroChina Zhejiang Oilfield Company started studies on several topics such as predict of pore pressure and stability of natural fractures, prevention of low-density mud losses in horizontal section and mud loss control with deformable gel lost circulation materials (LCM). It was found through observation of drilling practice on several drilling platforms that a 3D geo-mechanical model can be used to predict the potential mud losses in medium-deep fractured formations. Technologies for preventing low-density mud losses and the deformable LCM have been tried on the well YS112 and the well YS131. By reducing the mud density to a reasonable level and enhancing the capacity of controlling mud losses with the deformable LCM, mud losses in horizontal drilling were successfully prevented from occurring. This paper, based on the statistical analyses of the mud looses encountered in previous drilling operations, discusses the current technologies for and practice of mud losses control, taking into account the geological characteristics of the Zhaotong demonstration zone of shale gas drilling. The technologies discussed in this paper can be used as references to the prevention of mud loss control in the demonstration zone.
Abstract: A high performance water base drilling fluid was required to drill the well EYIYE-1 from which a high flow rate of shale gas was obtained. This well penetrated the Shuijingtuo formation of the Cambrian system. A series of laboratory work has been conducted to formulate an appropriate drilling fluid for the designed shale gas drilling. An outcrop of the Shuijingtuo shale formation, obtained from Zigui County, Hubei Province, was analyzed with XRD and SEM methods. Mud additives, such as clay, salts, nanomaterials, filter loss reducer, MMH and surfactants were selected through experiments. A high performance water base drilling fluid, SWDF, was formulated based on electric inhibition and wettability control. The SWDF drilling fluid was evaluated for its rheology, filtration property, lubricity, high temperature stability, shale inhibitive capacity and contamination resistance. It was found that 1) the Shuijingtuo shale formation is mainly composed of quartz, feldspar, and calcite etc.; the total amount of these brittle minerals accounts for 55%-67% of the shale sample. Illite is the main component of the clay minerals, which accounts for 13%-27% of the total clay. The shale formation contains no montmorillonite, and is therefore a weak-swelling formation. 2) the shale formation is developed with pores and micro-fissures which provided channels to drilling fluid invasion. 3) the SWDF drilling fluid has appropriate viscosity and gel strength, API filter loss of only 7.2 mL and friction coefficient of 0.21. SWDF is resistant to high temperature of 120 ℃, and shows good inhibitive capacity to shale hydration and high resistance to salt, calcium and formation clay (drilled cuttings) contamination; it is therefore suitable for use in drilling the horizontal Shuijingtuo shale formation in west Hubei area.
Abstract: The lithology of the carboniferous system in the Hudson area is mainly mudstone interbedded with thin sandstones and breccia. Serious sloughing of borehole wall during drilling operation has caused overpull and pipe-sticking. Drilled cuttings sampled by mudlogger have poor representativity, reducing the quality of lithological recognition. In drilling the build-up section and horizontal section of a well, well sloughing has caused inability to exert weight on bit, making well angle buildup and drilling low efficient. Plugging of pore throats through filming of drilling fluid on the surface of borehole wall was introduced to address these problems. In laboratory experiment, a filming agent FDM-1 was evaluated for its filming efficiency using a test method in which volume of solution lost and the volume of filtrate through the filter membrane formed by the dissolved filming agent were used as the evaluation parameters. The functioning mechanisms of FDM-1 in clay slurries were studied. Laboratory evaluation and the practices of field application showed that, as a nanometer-micrometer-sized high polymer emulsion, FDM-1 was able to form a dense polymeric membrane on the surface of porous media after filtration of the liquid in the polymer solution into the media. This polymeric membrane is non-permeable and deformable, and is able to form dense plugging layers inside the formation pores, thus effectively reducing filtration rate of drilling fluid and stabilizing borehole wall. Borehole wall sloughing problem was resolved successfully using FDM-1, and by improving the quality of the mud cakes, good lubrication was achieved, which was beneficial to resolving the problem of inability to exert weight on bit.
Abstract: With increasingly rigorous requirement imposed on environment protection, use of conventional drilling fluid additives such as sulfonates and mineral oils is restricted. With wells drilled deeper and deeper, formation temperatures are also becoming higher. The highest temperature that the drilling fluid additives presently used in China can stand is 180 ℃, and the application of these additives will be restricted in drilling wells with higher bottom hole temperatures. To address these problems, two new environmentally friendly high temperature additives, BZ-ZNJ, a viscosifier, and BZ-KGJ, a filter loss reducer, have been developed. An environmentally friendly drilling fluid was then formulated with these two new additives and other environmentally friendly additives. This new drilling fluid has rheology that is controllable at 230 ℃, and its viscosity and gel strength satisfy the needs of drilling operations. The new mud can also be made saturated with salt. It can withstand the contamination of calcium to 1.5% and the contamination of clay to 10%. EC50 of the drilling fluid is greater than 30 000 mg/L, meaning that it is nontoxic. Concentration of metal ions in the drilling fluids is less than the requirements of relevant standards. This new drilling fluid should find its application in drilling operations in environment sensitive areas.
Abstract: Waste drilling fluid is one of the major pollutants of the drilling operations. In the development of environmentally friendly water base drilling fluids, controlling the production of waste drilling fluid from the source is becoming an important means of achieving “green drilling and clean production”. An environmentally friendly high temperature filter loss reducer, HBFR, has been developed from modified starch to satisfy the needs of environmentally friendly water base drilling fluids in Shengli Oilfield. HBFR has good filtration control property in both fresh water and saltwater drilling fluids, and is resistant to high temperature (150 ℃). As one of the additives used for formulating environmentally friendly water base drilling fluids, HBFR has been used to formulate a high temperature environmentally friendly drilling fluid SLHB. SLHB in laboratory experiment showed good rheological properties, filtration control capacity, shale inhibitive capacity and contamination resistance, and can be used at 150 ℃. The HTHP filter loss of SLHB was 10.2 mL. SLHB has satisfied the level 2 requirement for sewage discharge because it has EC50 of greater than 1×105 mg/L and BOD5/CODCr of 16.2%. SLHB has been used on more than 10 wells in Shengli Oilfield, satisfying the needs for environment protection and providing a technical support for the “green development” in Shengli Oilfield with environmentally friendly drilling fluids.
Abstract: As well depth is becoming deeper, high temperature stability of foam mud becomes one of the restricting factors to foam drilling. Synergistic effect between different types of surfactants can be used to enhance the high temperature stability of foams. In our study six kinds of surfactants at the optimum concentration of 0.6% have been chosen to formulate a foam. Among these six surfactants, four of them have better foaming performance, and were evaluated for their ability to resist salt contamination and high temperature stability. The evaluation results showed that dodecyl dimethyl ammonium oxide (OA) had a stable foam comprehensive value at varied salt concentration, indicating that OA has good salt contamination resistance. At 100 ℃ or above, the α-sodium olefinsulfonate (AOS) had high foam comprehensive value, indicating that AOS has better high temperature stability. Study on the compounding of OA and AOS showed that the optimum ratio of the two was AOS：OA = 4：1. The optimum concentration of the compounded AOS and OA was 0.4%. Evaluation test on a BZY-1 surface tensiometer showed that the surface tension of the compounded surfactants is decreasing with temperature, indicating that when temperature is increasing, the foam fluid formulated with the compounded surfactants is becoming more and more stable. It was found in the evaluation test that the synergistic effect of the different surfactants reaches maximum at 150 ℃, and the foam comprehensive value at 100 ℃ reached 21.96×104 mL·s, indicating that the compounded surfactants have good high temperature stability.
Abstract: Oil well drilling in the subtropic rain forest climate in Bangladesh is facing high temperature environment through the whole year. To prevent the negative effects of high temperature on the performance of drilling fluid, a compound bactericide HCA101 was selected to treat the seawater drilling fluid (seawater + 6% salt clay A1/B1/E1) for directional drilling. HCA101 is compatible very well with the drilling fluid; treatment of the drilling fluid with 0.5% HCA101 does not cause the mud to foam when agitating at high speeds. Time for the HCA101 treated mud to ferment is longer than 10 d, satisfying the needs of fermentation control in field application. HCA101 was first used in March of 2019 in a single-point mooring project in Bangladesh with good operational results; the mud properties were stable after successively 20 d of operation, no fermentation at high temperatures was found. This bactericide is worth spreading in operation with seawater drilling fluids in an environment that is similar to that in Bangladesh.
Abstract: The well Shanan-3 is a key exploratory well located at the No. Shanan-3 structural high in Shanan tectonic zone of the north sag in Tarim Basin. This well penetrated the Kumugeliemu Group of the Paleogene System in which multiple salt zones are developed. Above the salt zones there are multiple sandstones with low leak pressures (coefficient of leak pressure ≤ 1.4). To guarantee the safe drilling of the salt zones, the pressure-bearing capacity of these sandstones should be greatly enhanced. To control the mud losses in this hole section, a new mud loss control slurry was formulated with two core additives, one was a particle material with excellent detainability and the other, a compound lost circulation material which swells at hydration. The mud loss control slurry was optimized to make it adaptable to loss zones with different sizes of mud loss channels. Loss zones treated with the mud loss control slurry had pressure-bearing capacity of at least 15 MPa and resistance to the flow-back of the mud loss control slurry of at least 5 MPa. In field operations, mud loss control techniques of various kinds were used, the mud density was gradually increased from 1.4 g/cm3 to 1.75 g/cm3, and the pressure-bearing capacity was increased to 19 MPa, ensuring the smooth running of the casing string.
Abstract: A low temperature low density spacer fluid LL-CSF has been developed to satisfy the needs of “zero discharge” in offshore operations and to minimize the length of free casing string in annular space. LL-CSF can be used to increase the displacement efficiency in deviated wells (especially in the extended-displacement well section and horizontal section) and improve the cementing quality between the wellbore and the cement sheath. Performance evaluation experimental results showed that: 1) LL-CSF had good compatibility with drilling fluids and cement slurries, and can satisfy the needs of well cementing operations. 2) The solidified mixture of LL-CSF, drilling fluid and cement slurry had compressive strength of greater than 1.6 MPa, and the solidified mixture LL-CSF and drilling fluid also had some compress strength. 3) At 20 ℃, LL-CSF had the ability to increase the shear strength of the interface between the borehole wall and the cement sheath; at 40 ℃, this ability was increased by two times. When the temperature was higher than 60 ℃, the ability of LL-CSF to increase the shear strength of the interface between the borehole wall and the cement sheath was lost. 4) The compressive strength of the solidified LL-CSF first increased and then decreased with temperature and time. The compressive strength of the solidified LL-CSF increased generally with its density. 5) The solidified LL-CSF had good durability and stability. The solidification mechanism of LL-CSF is believed to be the hydration products generated by the re-combination of all kinds of ions and chemical bonds produced by the decomposition of the calcium-rich phase and the silicon-rich phase in the slag. LL-CSF is cheap and easy to use in field operation, and is able to satisfy the needs of well cementing in offshore drilling operations, in which low temperatures (< 40 ℃) are always encountered, the cement slurry is unable to return to the surface and solidified spacer fluid is required to fill the annular space.
Abstract: A high temperature retarder ZRT-1 was synthesized with a nonmetallic material and an organic polymer. The development of ZRT-1 is to overcome the high temperature stability limitation of water soluble polymers used in cement slurries. The optimum synthesis process was determined using orthogonal test and single factor method; the molar ratio of the monomers was: AMPS：IA：AA = 7.0：1.5：1.5. The concentrations of the coupling agent, the inorganic material and the initiator were 0.5%, 1% and 1.5%. The mass fraction of the monomers was 30%. The pH of the reaction was 4, the reaction temperature 65 ℃, and the reaction time 8 h. Characterization of the synthetic product by FT-IR, DSC/DTG and GPC showed that ZRT-1 had the molecular structure and functional groups required. The molecular structure and functional groups render ZRT-1 good thermal stability. ZRT-1 had a weight average molecular weight (Mw) of 10091, a number average molecular weight (Mn) of 2020, and a molecular weight distribution of 4.99. Evaluation of ZRT-1 showed that the thickening time of a cement slurry linearly increased the concentration of ZRT-1, and linearly decreased with temperature. A cement slurry treated with 0.4%-1.5% ZRT-1 had a thickening time between 200 min and 400 min at 114-230 ℃/54-90 MPa, and the thickening curve changed smoothly. The cement slurry also showed short transition time and rightangled thickening curve.
Abstract: To improve the gas-channeling prevention performance of regular silica water suspension in ultra-deep well drilling, the silica suspension was treated with a white carbon black and grounded with sand mill. The particle size of the solids in the suspension was optimized and the properties of the suspension evaluated in laboratory experiment. Basic performance evaluation was performed to evaluate the compatibility of the optimized silica water suspension with cement slurries. Other tests such as compressive strength test, porosity measuring and cementing strength test were performed to evaluate the gas-channeling prevention performance of the silica water suspension. Using XRD and SEM, the gas-channeling prevention mechanisms of the silica water suspension in cement slurries at 180 ℃ were analyzed and explained. It was found in the experiment that the average particle size of the silica was 300 nm, and was compatible very well with cement slurries. Cement slurries treated with 15% silica water suspension had compressive strength that can sustain for a very long time. The porosity of the set cement was less than 0.02 mD and the cementing strength was greater than 4 MPa. The amount of calcium hydroxide produced during hydration of the cement was greatly reduced and the micro structure of the set cement turned from massy to strip-shaped and fibrous. The study showed that the anti gas channeling agent formulated with particlesize optimized silica-water suspension was suitable for use in cement slurries commonly in use and performed better than regular silica water suspension.
Abstract: To understand the factors affecting the dispersibility of the carbon nanotubes (CNTs) in cement slurries containing no dispersants, the CNTs was amide-modified on its surface and the modification was characterized by IR. The dispersibility of CNTs was evaluated through the electro-conductivity difference in different zones of set cement, and the effects of the amidated CNTs on the mechanical property and the overall performance of the set cement were also evaluated. The studies showed that dispersant for CNTs (gum Arabic) at high concentrations rendered negative effects on the retardation and settling properties of cement slurries, reducing the strength of the set cement by 51.4% and the density difference between the top and the bottom of a cement column was as high as 1.309 g/cm3. Amidated CNTs can be evenly dispersed in a cement slurry with no dispersant. CNTs after amidation had its spatial conductivity variance reduced by at least 1000 times. The amidated CNTs had stronger water wettability. A cement slurry treated with 0.06% amidated CNTs had its compressive strength, rupture strength, tensile strength and impact resistance increased by 33%, 30%, 61% and 33%, respectively. The amidated CNTs had no effect on the thickening, free water and filtration rate of cement slurries. It reduced the permeability and porosity of a set cement by 22.9% and 25.5% respectively. It was concluded that in cement slurries with no dispersants, the amidated CNTs can be evenly dispersed and can improve the mechanical performance of the cement slurries. This property renders the amidated CNTs application prospects in well cementing.
Abstract: Hydraulic fracturing experiment was performed using a large-scale real tri-axial simulation experiment system to extensively investigate the effects of perforation phasing and geo-stress on the fracture-initiation pressure and fracture propagation of thin interbeds. By scanning the fracture section, the hydraulic fracture propagation and distribution status were described, the effects of perforation phasing and geo-stress on the fracture-initiation pressure and fracture propagation as well as the basic mechanisms were analyzed. These researches can be used to provide support to fracturing design and operations. Experimental results showed that, ① rock samples in the test fractured at the end of perforation section, and the fracture diverted to propagate along the direction perpendicular to the direction of the minimum horizontal principal stress. A minimum fracture-initiation pressure existed under the same geo-stress and at 60° perforation phase, and the time spent in initial fracturing and in fracturing process was the shortest. Also under the same conditions, the fractures propagated most extensively, the number of fractures was the highest and the forms of the fractures were complex. ② when the difference between the vertical principal stress and the maximum horizontal principal stress was high, and the differential horizontal principal stresses was low, the fracture-initiation pressure was then low. The process of fracture propagation was steady, and was only weakly affected by rock breakdown. When the difference between the vertical principal stress and the maximum horizontal principal stress was low, the higher the differential horizontal principal stresses, the higher the fracture-initiation pressure, times for the fractures to propagate became less and time spent for fracturing was short. ③ When the differential horizontal principal stresses was high, the propagation of the fractures was evidently along the vertical direction. The fracturing section was flat and perpendicular to the minimum horizontal principal stress. When the differential horizontal principal stresses was low, the direction of the propagation of the fractures was difficult to control; it was easy for the fractures to deflect or to propagate along transverse direction. 4) When the hydraulic fracture met with structural plane, sub-fractures as well as bifurcation, deflection and cross-layer were generated. These were the necessary conditions for a complex fracture network to form. Formation bedding affects cross-layer of fractures, and micro-fissures and micro-pores all affects fracture-initiation pressure and fracture propagation.
Abstract: The result of fracturing nonMarine shale by conventional slick-water with poor anti-swelling effect and without dissolving calcium function is unsatisfactory. Therefore, it is urgent to develop a novel multifunctional slick-water system integrating drag reduction, dissolution, fracture, anti-swelling and sand carrying. In this study, a new type of acid-resistant drag reduction agent was synthesized, which has good viscosity-increasing and stable performance in acid. Corrosion inhibitor with the dual function was optimized to greatly improve the anti-swelling and corrosion mitigation. The multifunctional acidic slick-water was formed by adding clay stabilizer and the cleanup additive. The results show that the acidic slick-water with drag reduction rate up to 69%, anti-swelling rate higher than 90%, surface tension less than 28 mN/m, and the corrosion rate less than 5 g·m-2/h meets various performance requirements. After field test, it shows good effect of reducing pressure and resistance, and plays an important role in dissolving, expanding fracture and pressure reduction in fracturing. Thus, the acidic slick-water has good promoted application prospect.
Abstract: In underbalanced well completion with gel valve, gel breaking has always been performed with drill bit or by injecting gel breaker through the drill pipe. In this gel breaking operation, a delayed gel breaker is not available right now. If gel breaking can be performed automatically after well completion, time spent in tripping the drill pipe can be saved. To realize automatic gel breaking, a delayed gel breaker has been developed through sol-gel method, with tetraethyl orthosilicate (TEOS) as the precursor, SiO2 as the shell and ammonium persulfate as the core. Study on the effects of concentrations of TEOS, water and acetone, reaction temperature and pH on the coat effect showed that delayed gel breaker that satisfies the need of delaying gel breaking for 5 days at 120 ℃ can be obtained in the following conditions: molar ratio of TEOS, water, acetone and ammonium persulfate of 3：30：10：1, temperature of 40 ℃, pH of 6 and reaction time of 12 h. Characterization of the delayed gel breaker obtained with SEM has proved the existence of the coating SiO2.
Abstract: Long time development of horizontal wells have their formation pressure and formation energy are gradually depleted because of the physical properties of the reservoirs and the difficulties setting up the injection-production displacement system. Depletion of reservoir pressure often results in severe mud losses during workover operations and reestablishment of circulation is generally quite difficult. These in turn result in long operation time and low operation efficiency. Based on the properties of foam fluid, a highly stable nanometer compound gel foam workover fluid has been developed to deal with this problem. Compared with conventional workover fluids formulated with active water, this foam workover fluid has higher stability, lower density and better capacity in preventing mud losses. The composition of the foam is as follows: 1.4% foaming agent + 0.25% nanometer SiO2 + 0.5% polymer + 0.05% crosslinking agent + 0.02% stabilizer. This workover fluid uses micro foams as dispersed phase and gel as foam stabilizer, the loss of this workover fluid can be minimized by its strong retaining capacity and the combined ability of mud loss control of foam and gel which can help temporarily bridge the channels in the formations through which the workover fluid is lost. In laboratory simulation test, the initial fluid loss of the foam workover fluid through artificial cores was almost zero and the stable rate of loss was less than 10 mL/min, showing itself good pressure-bearing capacity and excellent performance of minimizing mud loss rate. Field test of the workover fluid showed that it had stable properties and was able to temporarily plug the loss zones. This study shows that the nanometer compound gel workover fluid has excellent temporary plugging capacity and ability of minimizing mud losses, it will find wide application in low-pressure horizontal well workover operations.
Competent Authorities：China National Petroleum Corporation Ltd
Sponsored by：CNPC Bohai Drilling Engineering Co. LtdPetroChina Huabei Oilfield Company
Address： Editorial Department of Drilling Fluid and Completion Fluid, Bohai Drilling Engineering Institute, Yanshan South Road, Renqiu City, Hebei Province