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A new physical simulation system of drilling mud invasion in formation module
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 44, Issue 1, February 2017 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2017, 44(1): 127–131.
RESEARCH PAPER
A new physical simulation system of drilling mud invasion in formation module FAN Yiren1, 2, *, WU Junchen1, 2, WU Fei3, ZHOU Cancan4, LI Chaoliu4 1. College of Geosciences in China University of Petroleum, Qingdao 266580, China; 2. CNPC Key Well Logging Laboratory in China University of Petroleum, Qingdao 266580, China; 3. Suzhou Niumag Analytical Instrument Corporation, Suzhou 215163, China; 4. Department of Well Logging and Remote Sensing Technology, PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
Abstract: After several key technologies, including the saturation and pressurization of large sample, design of invasion room, sealing and pressure maintaining of the apparatus, and dynamic monitoring of the entire invasion process were solved, a multifunctional physical simulation system of drilling mud invasion in formation module has been designed and built. By using the system, the variation patterns of formation module resistivity, pressure and mud cake parameters during the invasion are summarized. The design idea, components, and functions of each component of the system are introduced in this paper; and a drilling mud invasion experiment in the sandstone formation module was done. The experiment results show that the radial resistivity of formation module decreases in turn during brine drilling mud invasion; at the beginning of invasion, the mud cake is gradually formed with the increase of its thickness and the decrease of its porosity and permeability, and the radial pressure gradient of formation module decreases rapidly; in the middle and late periods of invasion, the mud cake properties and the radial pressure gradient of formation module become stable. The designed system, with high simulation degree, high utilization ratio and good maneuverability, can be used to reveal the invasion mechanism of drilling mud in sandstone reservoirs, analyze invasion characteristics, and improve invasion correction method for logging response and other researches. Key words: drilling mud invasion; physical simulation; formation module; mud cake parameters
Introduction Domestic and foreign researchers have conducted many researches on drilling mud invasion mechanism, numerical simulation of drilling mud invasion, seepage mechanism of multiphase fluid in complex reservoir and mud invasion correction of logging response, etc[17]. According to the scale of reservoir model, the existing typical physical simulation devices of drilling mud invasion are divided into three categories: (1) simulation devices at core plug scale, for example, Jiao et al[811] designed experimental instruments of dynamic/static drilling mud filtration to study the filtration law; (2) simulation devices at sand packing model scale, for example, Chen et al[12] simulated drilling mud invasion in oil, gas, and water layers, and obtained the radial resistivity profile of high-invasion, low-invasion, low-resistivity zone, and non-step type of invasion, provided an experimental basis for improving logging interpretation model of invasion profile; (3) simulation devices at simulation well scale, for
example, Ferguson et al[1316] used full hole experimental devices to research drilling fluid invasion, some of the devices are even equipped with a bit to simulate drilling process. For the physical simulation devices of drilling mud invasion at core plug or sand packing model scale, the seepage form of drilling fluid filtrate is horizontal linear flow during experiments, but the actual seepage form downhole is plane radial flow, as the experimental simulation conditions are very different from the actual formation conditions, the experimental results can’t be directly used to explain the phenomenon of drilling mud invasion in reservoirs. For the physical simulation devices of drilling mud invasion at simulation well scale, although the hydrodynamic environment of borehole and the formation condition of mud cake are close to the actual drilling process, but the experimental systems, complex and costly, can't be used widely. Therefore, based on the physical characteristics of sandstone reservoir and the drilling environment, the authors designed and made a multifunctional physical
Received date: 15 Aug. 2016; Revised date: 23 Nov. 2016. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2011ZX05020-008); the National Natural Science Foundation of China (41174099). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
FAN Yiren et al. / Petroleum Exploration and Development, 2017, 44(1): 127–131
simulation system of drilling mud invasion at formation module scale independently to simulate the drilling mud invasion process in sandstone formation module, and analyzed the variation patterns of resistivity, pressure and mud cake parameters during the invasion.
1. Multifunctional physical simulation system of drilling mud invasion 1.1.
Design idea
The basic principle of the physical simulation of drilling mud invasion is: choose a permeable medium with properties similar to the underground reservoir as the experiment object under laboratory condition, use the experimental instrument to simulate the invasion of drilling mud into the permeable medium, and acquire corresponding data during the experiments, finally, analyze the dynamic invasion data to find out the law of drilling mud invasion. The overall design ideas of the multifunctional physical simulation system of drilling mud invasion are as follows: first, under the premise of not affecting the physical simulation results and ensuring the model has enough invasion space, the thickness and radial length of formation model are optimized by numerical simulation of drilling mud invasion; then, in order to simulate drilling mud invasion under complex drilling conditions realistically and meet the needs of invasion mechanism research, the whole system takes modular design, so that each part of the system has specific functions. The functions of each part of the system are as follows: (1) Main unit of the physical simulation of drilling mud invasion. In this unit, the invasion room is connected with the simulation wellbore (Fig. 1) to form a sealed and pressure maintaining space, to ensure the drilling fluid filtrate invading into the formation module in the form of plane radial flow, to realize comparison of drilling mud invasion under various experimental conditions, acquire formation module resistance and pressure, record fluid-loss velocity in real time during the invasion process, and obtain mud cake samples at different stages of the invasion. (2) Saturation device of formation module. Saturate oven-dried formation module with simulated formation water. (3) Drilling mud preparation device. Prepare drilling mud for the experiment. (4) Dynamic invasion data acquisition device. Use data acquisition software to realize real-time monitoring of the dynamic data of drilling mud invasion, temperature and pressure environment of borehole on computer. (5) Test device of mud cake parameters. Test the thickness, porosity and permeability of mud cake samples. (6) Constant temperature room. Provide constant external temperature environment for the experimental device. 1.2.
Fig. 1. Structural diagram of the main unit of physical simulation of drilling mud invasion. 1—Drilling mud filling valve; 2— Drilling mud agitating device; 3—Drilling mud pressure pump; 4 —Jack and hydraulic pressure control device; 5—Fluid-loss velocity measurement device (video record filtrate volume and fluid-loss velocity of drilling mud); 6—Simulation wellbore (provide drilling mud circulating, temperature and pressure environment for the experiment); 7—Invasion room for resistance measurement (acquire formation module resistance during the invasion process, obtain mud cake sample); 8—Invasion room for pressure measurement (acquire formation module pressure during the invasion process, obtain mud cake sample); 9 — Conventional invasion room (only load and seal formation module, obtain mud cake sample).
state of drilling fluid filtrate in permeable medium similar to that in the actual drilling environment, sandstone in surface outcrop was chosen to make the formation modules. The formation module adopts a sector shaped block structure, its shape and size are as follows: thickness of 100.0 mm, radial length of 570.0 mm, top surface width of 51.7 mm, bottom surface width of 278.5 mm, and sector angle of 22.5°. 1.3.
Invasion room for resistance measurement
In order to obtain the dynamic resistance profile of the for-
Main unit
The actual main unit of physical simulation of drilling mud invasion is shown in Fig. 2. In order to make the seepage
Fig. 2. The factual picture of the main unit of physical simulation of drilling mud invasion.
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FAN Yiren et al. / Petroleum Exploration and Development, 2017, 44(1): 127–131
mation module during drilling mud invasion process, an invasion room with the function of resistance measurement has been designed. The invasion room is composed of stainless steel cover, invasion room cavity, electrodes, electrode leads, and aviation plugs, etc. In addition, an insulation rubber sleeve matches the size of formation module has been designed to insulate and seal the formation module from the side, top and bottom surface of the invasion room. Eight pairs of electrodes are arranged in the structure of dense inside but sparse outside (indicated by A0B0, A1B1, A2B2, A3B3, A4B4, A5B5, A6B6, and A7B7). The position of each paired electrodes on the top and bottom of the invasion room is symmetrical. The distances between the eight electrodes on the same side are 32, 40, 48, 56, 60, 80 and 100 mm. 1.4.
Invasion room for pressure measurement
In order to obtain the dynamic pressure profile of formation module during drilling mud invasion process, an invasion room with the function of pressure measurement has been designed. The invasion room is composed of stainless steel cover, invasion room cavity, threaded holes, pressure sensors, and pressure measurement holes of rubber sleeve, etc. The positions of the eight pressure sensors are the same as that of the electrodes to ensure the consistency of the pressure data and the resistance data. 1.5.
Test device of mud cake parameters
After the experiment of drilling mud invasion, the mud cake samples can be taken from the end face of formation module connected with the simulation wellbore. Mud cake parameters (including thickness, porosity, permeability) are the main indexes to study the formation law of mud cake and the filtration characteristics of drilling mud, and also important parameters for mathematical model in the numerical simulation of drilling mud invasion. The designed test device of mud cake parameters is composed of depth micrometer, support plate, pedestal, cyclic mud cake sampler, electronic balance, oven, and mud cake permeability measuring instrument, etc. The thickness, porosity and permeability of the mud cake are measured by depth micrometer, weighing method, and steady state method respectively.
(4) load and seal formation module into the invasion room; (5) add drilling mud into the simulation wellbore, stir and pressurize drilling mud (start-up experiment); (6) acquire dynamic invasion data, record fluid-loss velocity in real time; (7) isolate the contact between the formation module and drilling mud at different stages of the invasion separately, take mud cake samples after completion of experiment; (8) test mud cake parameters. In the experiment, a kind of fine sandstone with a porosity of 14.21%, and a permeability of 10.07×103 μm2 was selected to make the formation modules. The designed experimental conditions were as follows: the drilling mud type of brine drilling mud, the number of formation modules of 8, the formation water salinity of 1 g/L, drilling mud pressure difference of 1 MPa, and filtration type of dynamic filtration, mud cake samples were taken at 0.82, 1.67, 2.63, 4.68, 7.75, 12.78 and 44.00 h respectively, and the experimental temperature was 25 C. The tested parameters include the formation module resistance and pressure, and fluid-loss velocity. The formula of drilling mud used in the experiment was as follows: 2.1 kg/m3 sodium carbonate, 42.3 kg/m3 bentonite clay, 2.1 kg/m3 PAM, and 10.6 kg/m3 sodium chloride.
3. Invasion of drilling mud in sandstone formation module 3.1.
Variation of resistivity
Fig. 3 shows the relationship between the formation module resistivity at the positions of the 8 paired electrodes and invasion time. It is clearly seen that the radial resistivity of formation module decreases gradually away from the wellbore, which accords with the characteristic of decreased resistance invasion during brine drilling mud invasion. Fig. 4 shows the relationship between invasion depth, invasion depth change rate and invasion time calculated from Fig. 3. It is clearly seen that at the beginning of the invasion, the mud cake is in growth period (when the invasion time is less than 2 h), the fluid-loss velocity of drilling fluid filtrate is fast, and the invasion depth increases quickly; with the ongoing of invasion, the mud cake is gradually formed, to plug the sidewall, consequently, cause rapid decrease of fluid-loss
2. Experiment of drilling mud invasion in sandstone formation module The main purposes of the experiment are to investigate the invasion characteristics of brine drilling mud in sandstone formation module, analyze the variation patterns of formation module resistivity and pressure during the invasion, use the test results of mud cake samples at different stages of the invasion to research the variation patterns of mud cake parameters during its formation process. The experimental procedures are as follows: (1) select material and finish formation module; (2) oven dry and saturate formation module at high pressure; (3) prepare drilling mud;
Fig. 3. The relationship between formation module resistivity and invasion time.
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Fig. 4. The relationship between invasion depth, invasion depth change rate and invasion time.
velocity and reduction of increase velocity of invasion depth; after a period of time, the invasion reaches a dynamic equilibrium, the fluid-loss velocity tends to be stable, and the increase velocity of invasion depth becomes slower and slower. 3.2.
Variation of formation pressure
It can be seen from Figs. 5-7 that: (1) At the beginning of the invasion, the pressure difference on the mud cake increases gradually during the formation process of mud cake, which leads to the rapid decrease of radial pressure gradient of sandstone formation module; when the invasion reaches a dynamic equilibrium, the pressure difference on the mud cake tends to be stable, and the radial pressure gradient of the
Fig. 5. The relationship between formation module pressure and radial depth.
Fig. 6. The relationship between cumulative filtrate volume, fluid-loss velocity and invasion time.
Fig. 7. The relationship between pressure gradient, fluid-loss velocity and invasion time.
sandstone formation module changes very little. (2) The rapid decrease zone of radial pressure gradient (when the invasion time is less than 2 h) is corresponding to the growth period of mud cake, in this period, the fluid-loss velocity of drilling fluid filtrate decreases rapidly, and so does the increase rate of cumulative filtrate volume; when the invasion reaches a dynamic equilibrium, the fluid-loss velocity tends to be stable, and the cumulative filtrate volume increases linearly with time. 3.3.
Variation of mud cake parameters
It can be seen from Figs. 8-10 that: in the growth period of mud cake (when the invasion time is less than 2 h), the invasion velocity of drilling fluid filtrate is fast, the mud cake is loose, and higher in permeability, and increases in thickness because the precipitation rate of solid particles on mud cake
Fig. 8. The relationship between mud cake thickness and invasion time.
Fig. 9. The relationship between mud cake porosity and invasion time.
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the low permeable mud cake formed by dehydration of drilling mud plugs the sidewall and effectively delays the invasion of drillingmud filtrate.
References [1]
AL-MALKI N, POURAFSHARY P, AL-HADRAMI H, et al. Controlling bentonite-based drilling mud properties using sepiolite nanoparticles. Petroleum Exploration and Development, 2016, 43(4): 656–661.
Fig. 10. The relationship between mud cake permeability and invasion time.
surface is greater than the erosion rate; with the ongoing of invasion, the mud cake is gradually compacted under the drilling mud pressure difference, the mud cake decreases in thickness, porosity, permeability, the pressure difference on the mud cake increases, the invasion velocity of drilling fluid filtrate decreases, the precipitation rate of solid particles on mud cake surface gradually decreases till it is equal to the erosion rate, that is the dynamic equilibrium of the invasion, and the thickness, porosity and permeability of mud cake tend to be stable.
4.
[2]
KRUGER R. Evaluation of drilling-fluid filter-loss additives under dynamic conditions. Journal of Petroleum Technology, 1963, 15(1): 90–98.
[3]
WARREN B K, SMITH T R, RAVI K M. Static and dynamic fluid-loss characteristics of drilling fluids in a full-scale wellbore. SPE 26069, 1993.
[4]
SHAW J C, CHEE T. Laboratory evaluation of drilling mud systems for formation damage prevention in horizontal wells. SPE 37121, 1996.
[5]
BEDRIKOVETSKY P, SIQUEIRA F D, FURTADO C, et al. Quantitative theory for fines migration and formation damage. SPE 128384, 2010.
[6]
SALAZAR J M, MARTIN A J. Rock quality assessment using the effect of mud-filtrate invasion on conflicting borehole re-
Conclusions
sistivity measurements. Geophysics, 2012, 77(3): 65–78.
The multifunctional physical simulation system of drilling mud invasion with sandstone module designed in this paper has the advantages of high simulation degree, high utilization ratio and good maneuverability. The overall layout of the system and the dimension of the formation module ensure the seepage form of drilling fluid filtrate is the same as that in the actual downhole environment, the invasion room can be loaded repeatedly with formation module to realize the simulation comparison of drilling mud invasion under various experimental conditions, and real-time monitoring of the experimental data enables the experimental operators to understand the running state of the device and know the experimental process intuitively. This system has effectively overcome the shortcomings of other physical simulation devices, and broken through the traditional design ideas of experimental instrument of drilling mud invasion. The variation patterns of formation module resistivity, pressure and mud cake parameters have been found out through the experiment. During the dynamic filtration of drilling mud, in the growth period of mud cake, the mud cake increases in thickness, and decreases in porosity and permeability, the radial pressure gradient of sandstone formation module and the fluid-loss velocity of drilling fluid filtrate decrease rapidly, the filtrate invasion depth is shallow, and the increase rate of cumulative filtrate volume slows down; after a period of time, when the invasion reaches a dynamic equilibrium, the mud cake properties, radial pressure gradient of formation module and fluid-loss velocity of drilling fluid filtrate tend to be stable, the cumulative filtrate volume increases linearly with time, the 131
[7]
FAN Yiren, HU Yunyun, LI Hu, et al. Numerical simulation of mud-cake dynamic formation and reservoir mud filtrate invasion. Well Logging Technology, 2013, 37(5): 466–471.
[8]
TOVAR J G, MARTINEZ A, BOCKH A, et al. Mud cake compressibility and mobility of fluid loss evaluation. SPE 26980, 1994.
[9]
JIAO D, SHARMA M. An experimental investigation of the resistivity profile in the flushed zone. Journal of Nanchang University, 1991(1): 59–63.
[10] JIAO D, SHARMA M. Mechanism of cake buildup in crossflow filtration of colloidal suspensions. Journal of Colloid and Interface Science, 1994, 162(1): 454–462. [11] DEWAN J T, CHENEVERT M E. A model for filtration of water-based mud during drilling: Determination of mud cake parameters. Petrophysics, 2001, 42(3): 237–250. [12] CHEN Fuxuan, SUN Jiaxu. Simulation test of radial electric conductivity during the mud filtrate invading porous formation. Acta Geophycica Sinica, 1996, 39(Supp.): 371–378. [13] FERGUSON C K, KLOTZ J A, AIME M. Filtration from mud during drilling. Journal of Petroleum Technology, 1954, 6(2): 30–43. [14] PEDEN J M, ARTHUR K G, AVALOS M. Analysis of filtration under dynamic and static conditions. SPE 12503, 1984. [15] VAUSSARD A, MARTIN M, KONIRSCH O, et al. An experimental study of drilling fluids dynamic filtration. SPE 15412, 1986. [16] WARREN B K, SMITH T R, RAVI K M. Static and dynamic fluid-loss characteristics of drilling fluids in a full-scale wellbore. SPE 26069, 1993.
RESEARCH PAPER
A new physical simulation system of drilling mud invasion in formation module FAN Yiren1, 2, *, WU Junchen1, 2, WU Fei3, ZHOU Cancan4, LI Chaoliu4 1. College of Geosciences in China University of Petroleum, Qingdao 266580, China; 2. CNPC Key Well Logging Laboratory in China University of Petroleum, Qingdao 266580, China; 3. Suzhou Niumag Analytical Instrument Corporation, Suzhou 215163, China; 4. Department of Well Logging and Remote Sensing Technology, PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China
Abstract: After several key technologies, including the saturation and pressurization of large sample, design of invasion room, sealing and pressure maintaining of the apparatus, and dynamic monitoring of the entire invasion process were solved, a multifunctional physical simulation system of drilling mud invasion in formation module has been designed and built. By using the system, the variation patterns of formation module resistivity, pressure and mud cake parameters during the invasion are summarized. The design idea, components, and functions of each component of the system are introduced in this paper; and a drilling mud invasion experiment in the sandstone formation module was done. The experiment results show that the radial resistivity of formation module decreases in turn during brine drilling mud invasion; at the beginning of invasion, the mud cake is gradually formed with the increase of its thickness and the decrease of its porosity and permeability, and the radial pressure gradient of formation module decreases rapidly; in the middle and late periods of invasion, the mud cake properties and the radial pressure gradient of formation module become stable. The designed system, with high simulation degree, high utilization ratio and good maneuverability, can be used to reveal the invasion mechanism of drilling mud in sandstone reservoirs, analyze invasion characteristics, and improve invasion correction method for logging response and other researches. Key words: drilling mud invasion; physical simulation; formation module; mud cake parameters
Introduction Domestic and foreign researchers have conducted many researches on drilling mud invasion mechanism, numerical simulation of drilling mud invasion, seepage mechanism of multiphase fluid in complex reservoir and mud invasion correction of logging response, etc[17]. According to the scale of reservoir model, the existing typical physical simulation devices of drilling mud invasion are divided into three categories: (1) simulation devices at core plug scale, for example, Jiao et al[811] designed experimental instruments of dynamic/static drilling mud filtration to study the filtration law; (2) simulation devices at sand packing model scale, for example, Chen et al[12] simulated drilling mud invasion in oil, gas, and water layers, and obtained the radial resistivity profile of high-invasion, low-invasion, low-resistivity zone, and non-step type of invasion, provided an experimental basis for improving logging interpretation model of invasion profile; (3) simulation devices at simulation well scale, for
example, Ferguson et al[1316] used full hole experimental devices to research drilling fluid invasion, some of the devices are even equipped with a bit to simulate drilling process. For the physical simulation devices of drilling mud invasion at core plug or sand packing model scale, the seepage form of drilling fluid filtrate is horizontal linear flow during experiments, but the actual seepage form downhole is plane radial flow, as the experimental simulation conditions are very different from the actual formation conditions, the experimental results can’t be directly used to explain the phenomenon of drilling mud invasion in reservoirs. For the physical simulation devices of drilling mud invasion at simulation well scale, although the hydrodynamic environment of borehole and the formation condition of mud cake are close to the actual drilling process, but the experimental systems, complex and costly, can't be used widely. Therefore, based on the physical characteristics of sandstone reservoir and the drilling environment, the authors designed and made a multifunctional physical
Received date: 15 Aug. 2016; Revised date: 23 Nov. 2016. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the China National Science and Technology Major Project (2011ZX05020-008); the National Natural Science Foundation of China (41174099). Copyright © 2017, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
FAN Yiren et al. / Petroleum Exploration and Development, 2017, 44(1): 127–131
simulation system of drilling mud invasion at formation module scale independently to simulate the drilling mud invasion process in sandstone formation module, and analyzed the variation patterns of resistivity, pressure and mud cake parameters during the invasion.
1. Multifunctional physical simulation system of drilling mud invasion 1.1.
Design idea
The basic principle of the physical simulation of drilling mud invasion is: choose a permeable medium with properties similar to the underground reservoir as the experiment object under laboratory condition, use the experimental instrument to simulate the invasion of drilling mud into the permeable medium, and acquire corresponding data during the experiments, finally, analyze the dynamic invasion data to find out the law of drilling mud invasion. The overall design ideas of the multifunctional physical simulation system of drilling mud invasion are as follows: first, under the premise of not affecting the physical simulation results and ensuring the model has enough invasion space, the thickness and radial length of formation model are optimized by numerical simulation of drilling mud invasion; then, in order to simulate drilling mud invasion under complex drilling conditions realistically and meet the needs of invasion mechanism research, the whole system takes modular design, so that each part of the system has specific functions. The functions of each part of the system are as follows: (1) Main unit of the physical simulation of drilling mud invasion. In this unit, the invasion room is connected with the simulation wellbore (Fig. 1) to form a sealed and pressure maintaining space, to ensure the drilling fluid filtrate invading into the formation module in the form of plane radial flow, to realize comparison of drilling mud invasion under various experimental conditions, acquire formation module resistance and pressure, record fluid-loss velocity in real time during the invasion process, and obtain mud cake samples at different stages of the invasion. (2) Saturation device of formation module. Saturate oven-dried formation module with simulated formation water. (3) Drilling mud preparation device. Prepare drilling mud for the experiment. (4) Dynamic invasion data acquisition device. Use data acquisition software to realize real-time monitoring of the dynamic data of drilling mud invasion, temperature and pressure environment of borehole on computer. (5) Test device of mud cake parameters. Test the thickness, porosity and permeability of mud cake samples. (6) Constant temperature room. Provide constant external temperature environment for the experimental device. 1.2.
Fig. 1. Structural diagram of the main unit of physical simulation of drilling mud invasion. 1—Drilling mud filling valve; 2— Drilling mud agitating device; 3—Drilling mud pressure pump; 4 —Jack and hydraulic pressure control device; 5—Fluid-loss velocity measurement device (video record filtrate volume and fluid-loss velocity of drilling mud); 6—Simulation wellbore (provide drilling mud circulating, temperature and pressure environment for the experiment); 7—Invasion room for resistance measurement (acquire formation module resistance during the invasion process, obtain mud cake sample); 8—Invasion room for pressure measurement (acquire formation module pressure during the invasion process, obtain mud cake sample); 9 — Conventional invasion room (only load and seal formation module, obtain mud cake sample).
state of drilling fluid filtrate in permeable medium similar to that in the actual drilling environment, sandstone in surface outcrop was chosen to make the formation modules. The formation module adopts a sector shaped block structure, its shape and size are as follows: thickness of 100.0 mm, radial length of 570.0 mm, top surface width of 51.7 mm, bottom surface width of 278.5 mm, and sector angle of 22.5°. 1.3.
Invasion room for resistance measurement
In order to obtain the dynamic resistance profile of the for-
Main unit
The actual main unit of physical simulation of drilling mud invasion is shown in Fig. 2. In order to make the seepage
Fig. 2. The factual picture of the main unit of physical simulation of drilling mud invasion.
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FAN Yiren et al. / Petroleum Exploration and Development, 2017, 44(1): 127–131
mation module during drilling mud invasion process, an invasion room with the function of resistance measurement has been designed. The invasion room is composed of stainless steel cover, invasion room cavity, electrodes, electrode leads, and aviation plugs, etc. In addition, an insulation rubber sleeve matches the size of formation module has been designed to insulate and seal the formation module from the side, top and bottom surface of the invasion room. Eight pairs of electrodes are arranged in the structure of dense inside but sparse outside (indicated by A0B0, A1B1, A2B2, A3B3, A4B4, A5B5, A6B6, and A7B7). The position of each paired electrodes on the top and bottom of the invasion room is symmetrical. The distances between the eight electrodes on the same side are 32, 40, 48, 56, 60, 80 and 100 mm. 1.4.
Invasion room for pressure measurement
In order to obtain the dynamic pressure profile of formation module during drilling mud invasion process, an invasion room with the function of pressure measurement has been designed. The invasion room is composed of stainless steel cover, invasion room cavity, threaded holes, pressure sensors, and pressure measurement holes of rubber sleeve, etc. The positions of the eight pressure sensors are the same as that of the electrodes to ensure the consistency of the pressure data and the resistance data. 1.5.
Test device of mud cake parameters
After the experiment of drilling mud invasion, the mud cake samples can be taken from the end face of formation module connected with the simulation wellbore. Mud cake parameters (including thickness, porosity, permeability) are the main indexes to study the formation law of mud cake and the filtration characteristics of drilling mud, and also important parameters for mathematical model in the numerical simulation of drilling mud invasion. The designed test device of mud cake parameters is composed of depth micrometer, support plate, pedestal, cyclic mud cake sampler, electronic balance, oven, and mud cake permeability measuring instrument, etc. The thickness, porosity and permeability of the mud cake are measured by depth micrometer, weighing method, and steady state method respectively.
(4) load and seal formation module into the invasion room; (5) add drilling mud into the simulation wellbore, stir and pressurize drilling mud (start-up experiment); (6) acquire dynamic invasion data, record fluid-loss velocity in real time; (7) isolate the contact between the formation module and drilling mud at different stages of the invasion separately, take mud cake samples after completion of experiment; (8) test mud cake parameters. In the experiment, a kind of fine sandstone with a porosity of 14.21%, and a permeability of 10.07×103 μm2 was selected to make the formation modules. The designed experimental conditions were as follows: the drilling mud type of brine drilling mud, the number of formation modules of 8, the formation water salinity of 1 g/L, drilling mud pressure difference of 1 MPa, and filtration type of dynamic filtration, mud cake samples were taken at 0.82, 1.67, 2.63, 4.68, 7.75, 12.78 and 44.00 h respectively, and the experimental temperature was 25 C. The tested parameters include the formation module resistance and pressure, and fluid-loss velocity. The formula of drilling mud used in the experiment was as follows: 2.1 kg/m3 sodium carbonate, 42.3 kg/m3 bentonite clay, 2.1 kg/m3 PAM, and 10.6 kg/m3 sodium chloride.
3. Invasion of drilling mud in sandstone formation module 3.1.
Variation of resistivity
Fig. 3 shows the relationship between the formation module resistivity at the positions of the 8 paired electrodes and invasion time. It is clearly seen that the radial resistivity of formation module decreases gradually away from the wellbore, which accords with the characteristic of decreased resistance invasion during brine drilling mud invasion. Fig. 4 shows the relationship between invasion depth, invasion depth change rate and invasion time calculated from Fig. 3. It is clearly seen that at the beginning of the invasion, the mud cake is in growth period (when the invasion time is less than 2 h), the fluid-loss velocity of drilling fluid filtrate is fast, and the invasion depth increases quickly; with the ongoing of invasion, the mud cake is gradually formed, to plug the sidewall, consequently, cause rapid decrease of fluid-loss
2. Experiment of drilling mud invasion in sandstone formation module The main purposes of the experiment are to investigate the invasion characteristics of brine drilling mud in sandstone formation module, analyze the variation patterns of formation module resistivity and pressure during the invasion, use the test results of mud cake samples at different stages of the invasion to research the variation patterns of mud cake parameters during its formation process. The experimental procedures are as follows: (1) select material and finish formation module; (2) oven dry and saturate formation module at high pressure; (3) prepare drilling mud;
Fig. 3. The relationship between formation module resistivity and invasion time.
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Fig. 4. The relationship between invasion depth, invasion depth change rate and invasion time.
velocity and reduction of increase velocity of invasion depth; after a period of time, the invasion reaches a dynamic equilibrium, the fluid-loss velocity tends to be stable, and the increase velocity of invasion depth becomes slower and slower. 3.2.
Variation of formation pressure
It can be seen from Figs. 5-7 that: (1) At the beginning of the invasion, the pressure difference on the mud cake increases gradually during the formation process of mud cake, which leads to the rapid decrease of radial pressure gradient of sandstone formation module; when the invasion reaches a dynamic equilibrium, the pressure difference on the mud cake tends to be stable, and the radial pressure gradient of the
Fig. 5. The relationship between formation module pressure and radial depth.
Fig. 6. The relationship between cumulative filtrate volume, fluid-loss velocity and invasion time.
Fig. 7. The relationship between pressure gradient, fluid-loss velocity and invasion time.
sandstone formation module changes very little. (2) The rapid decrease zone of radial pressure gradient (when the invasion time is less than 2 h) is corresponding to the growth period of mud cake, in this period, the fluid-loss velocity of drilling fluid filtrate decreases rapidly, and so does the increase rate of cumulative filtrate volume; when the invasion reaches a dynamic equilibrium, the fluid-loss velocity tends to be stable, and the cumulative filtrate volume increases linearly with time. 3.3.
Variation of mud cake parameters
It can be seen from Figs. 8-10 that: in the growth period of mud cake (when the invasion time is less than 2 h), the invasion velocity of drilling fluid filtrate is fast, the mud cake is loose, and higher in permeability, and increases in thickness because the precipitation rate of solid particles on mud cake
Fig. 8. The relationship between mud cake thickness and invasion time.
Fig. 9. The relationship between mud cake porosity and invasion time.
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the low permeable mud cake formed by dehydration of drilling mud plugs the sidewall and effectively delays the invasion of drillingmud filtrate.
References [1]
AL-MALKI N, POURAFSHARY P, AL-HADRAMI H, et al. Controlling bentonite-based drilling mud properties using sepiolite nanoparticles. Petroleum Exploration and Development, 2016, 43(4): 656–661.
Fig. 10. The relationship between mud cake permeability and invasion time.
surface is greater than the erosion rate; with the ongoing of invasion, the mud cake is gradually compacted under the drilling mud pressure difference, the mud cake decreases in thickness, porosity, permeability, the pressure difference on the mud cake increases, the invasion velocity of drilling fluid filtrate decreases, the precipitation rate of solid particles on mud cake surface gradually decreases till it is equal to the erosion rate, that is the dynamic equilibrium of the invasion, and the thickness, porosity and permeability of mud cake tend to be stable.
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Conclusions
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The multifunctional physical simulation system of drilling mud invasion with sandstone module designed in this paper has the advantages of high simulation degree, high utilization ratio and good maneuverability. The overall layout of the system and the dimension of the formation module ensure the seepage form of drilling fluid filtrate is the same as that in the actual downhole environment, the invasion room can be loaded repeatedly with formation module to realize the simulation comparison of drilling mud invasion under various experimental conditions, and real-time monitoring of the experimental data enables the experimental operators to understand the running state of the device and know the experimental process intuitively. This system has effectively overcome the shortcomings of other physical simulation devices, and broken through the traditional design ideas of experimental instrument of drilling mud invasion. The variation patterns of formation module resistivity, pressure and mud cake parameters have been found out through the experiment. During the dynamic filtration of drilling mud, in the growth period of mud cake, the mud cake increases in thickness, and decreases in porosity and permeability, the radial pressure gradient of sandstone formation module and the fluid-loss velocity of drilling fluid filtrate decrease rapidly, the filtrate invasion depth is shallow, and the increase rate of cumulative filtrate volume slows down; after a period of time, when the invasion reaches a dynamic equilibrium, the mud cake properties, radial pressure gradient of formation module and fluid-loss velocity of drilling fluid filtrate tend to be stable, the cumulative filtrate volume increases linearly with time, the 131
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