Introduction
Fractures and vugs are well developed in carbonate fractured-vuggy reservoirs with abnormally high temperature and high pressure in the Central Tarim Basin. These reservoirs are typical fractured-vuggy double-leakage formations. In the drilling process, severe leakage of drilling fluid leads to a huge increase in the drilling cycle and enormous economic loss. Therefore, efficient plugging technology must be adopted[1,2,3,4]. As the most widely used plugging technology, the bridging plugging technology mixes inert materials in different shapes and sizes into the drilling fluid system at a certain proportion and injects them into the leakage formation section for plugging[3]. At present, the conventional bridging particles mainly include rigid particles (such as walnut shells) and fiber materials (such as sawdust and bagasse). The fiber materials can form plugging layers under certain conditions, but the plugging layers they form are likely to fail under high pressure[5]. The rigid particles are characterized by high pressure-bearing capacity and high resistance to deformation, and has good application effect, but the conventional rigid materials are prone to carbonization under high temperature, and thus decreasing in strength significantly, so the plugging layer would also reduce in strength, and couldn’t effectively plug and prevent the leakage, and the reservoir could be seriously damaged. The lost circulation materials resistant to high temperature mainly include mica, vermiculite, asbestos and shells at home and abroad[6]. Zhan Junyang[7], Guo Limei[8], Hu Ziqiao[9], Zhang Wei[10], Wang Xianbing[11], Guo[12] and Karcher[13] et al researched other types of lost circulation materials resistant to high temperature, including gel plugging materials and high-temperature consolidation plugging materials, but these materials have some limitations when used to plug the reservoirs with abnormally high temperature and pressure.
In order to effectively solve the double loss of fractures and vugs during the drilling of carbonate fractured-vuggy reservoirs with abnormally high temperature and high pressure in the Central Tarim Basin, this paper introduces a rigid plugging material GZD with high acid solubility and serrated surface. According to the 1/2-2/3 bridging plugging principle[1, 3], it is compounded with the lignin fiber, elastic material SQD-98 and calcium carbonate in mixture mesh to form a new compound lost circulation material for plugging high- temperature and high-pressure formation. Through drilling fluid compatibility experiment, static state pressure-bearing plugging experiment of fissure and rock debris casting bed experiment, its performance was evaluated to provide technical support and theoretical guidance for drilling and plugging reservoirs with fracture and vug double leakage risks in the central Tarim Basin.
1. The reservoirs in central Tarim Basin
The Ordovician carbonate reservoirs in the Central Tarim Basin have abundant fractures and vugs. The density of fractures in the upper part of the Yingshan Formation is 1.54-5.41 fractures/m. The lower part of the Yingshan Formation has obvious bedding characteristic, and the density of fracture of 1.47-4.25 fractures/m. The fractures include flat ones, oblique ones and vertical ones, intersecting with each other. The occurrence of fractures is shown in Fig. 1. According to the imaging logging diagram of the central Tarim Basin (Fig. 2) and the statistical analysis of geological data, it is concluded that: (1) in this area, most fractures are small ones, large and medium fractures are few (Fig. 3a); (2) the vugs are also dominated by small ones, with few large and medium ones (Fig. 3b). The fractures and vugs provide flow paths and reservoir space for oil and gas migration and storage, and also low resistance channels for drilling fluid loss. In addition, the formation pressure and temperature of this section are both abnormal. The actual measurement results show that the lower part of the Yingshan Formation has a pressure coefficient of as high as 1.9-2.1, and a geothermal gradient of 2.08-2.50 °C/100 m, representing typical reservoir with abnormally high temperature and high pressure. In this kind of formation, the plugging material is susceptible to high temperature carbonization and high pressure breaking, so the plugging layer would be lower in strength, and severe leakages could happen repeatedly.
Fig. 1.
Fig. 1.
Photos of cores showing fracture occurrence in the Central Tarim Basin.
Fig. 2.
Fig. 2.
Imaging logging chart of two wells in the central Tarim Basin.
Fig. 3.
Fig. 3.
Size distribution of fractures and vugs.
2. Experiments
2.1. Experiment equipment and materials
The experiment equipment included a static plugging assessment instrument (Fig. 4), FA visualized medium pressure sand-bed filter instrument, ZNN-D6B six-rotation viscometer, roller heating furnace BGRL-7 and triple medium pressure filtration instrument SD3. The experimental materials were distilled water, sodium bentonite, sodium carbonate, sodium hydroxide, rigid material GZD, calcium carbonate, lignin fiber, elastic material SQD-98 and weighting agent (barite) etc.
Fig. 4.
Fig. 4.
Schematic diagram of static pressure evaluation instrument.
2.2. Main plugging materials
GZD is a kind of rigid temporary plugging particle with serrated surface and wide size range. Mainly composed of the calcite, it has a density of 2.8 g/cm3 and Mohs hardness of 2.7-3.0. The particle diameter ranges from 0.2 mm to 2.0 mm, mainly in 4 grades, including A (0.9-2.0 mm), B (0.45-0.90 mm), C (0.30-0.45 mm) and D (0.2-0.3 mm), as shown in Fig. 5. It has the function of bridging and filling and can resist high temperature of over 200 °C. Experimental measurement shows that the acid solubility of GZD is more than 98%, which is beneficial to the later plugging removal with acidizing, and protection of the reservoir.
Fig. 5.
Fig. 5.
Size distribution of GZD.
The lignin fiber used in experiment was an organic fiber obtained by screening, splitting, and chemical and high temperature treatment of natural timbers. Treated at the temperature of up to 250 °C, it has good temperature resistance and is a non-toxic, harmless and environment-friendly product. Its microscopic physical structure is characterized by strip bending, uneven surface and dispersed pores, and it has good water absorption, dispersibility and flexibility. Compared with conventional fiber materials, the acid solubility of the modified lignin fiber is more than 39%, which is beneficial to the later plugging removal with acidizing and efficient development of the oil field.
As a rigid plugging material, calcium carbonate has been widely used in the oil industry. It has a density of 2.6-2.9 g/cm3, Mohs hardness of 3.0-3.5, and acid solubility of more than 99% which is conducive to reservoir protection. The mixing ratio of calcium carbonate in different meshes used herein was 0.019 mm: 0.002 1 mm (800 mesh: 1 200 mesh) = 1:1.
SQD-98 is an elastic filling particle with a particle size of 0.07-4.20 mm. It has good plugging and temperature resistance and is often used in the oil industry as a plugging material in drilling.
2.3. Experiment contents
2.3.1. Assessment of temperature resistance and high temperature abrasiveness of GZD
The commonly used plugging material walnut shell was selected to compare with GZD. The experimental bentonite base mud was prepared according to the formula of 2% bentonite + clean water + 0.2% NaOH + 0.3% Na2CO3 for the temperature resistance assessment experiment. The experimental steps are as follows: (1) 30 g of the sample after drying at (105±3) °C was weighed and coded as M1, and the appearance of sample was recorded by taking pictures; (2) the sample was added into the prepared base mud and stirred thoroughly and then put it into the aging tank. The drilling mud was heated for 16 h at 200 °C, then the sample material was separated from the drilling mud after cooling and washed clean, dried at (105±3) °C until reached constant weight and weighed the mass, which was recorded as M2; (3) The mass loss rate K after the hot rolling was calculated with $\frac{{{M}_{1}}-{{M}_{2}}}{{{M}_{1}}}\times $ $100\text{ }\!\!%\!\!\text{ }$; and (4) the temperature resistance and high temperature abrasiveness of GZD were analyzed and evaluated according to the appearance change and mass loss rate of the sample after the high temperature heat treatment.
2.3.2. Experiment to select materials
The rigid particle GZD, lignin fiber, SQD-98 and calcium carbonate of different meshes were sequentially added to the base mud to test the critical pressure value of the system with different dosages and different compounds, and the leakage under the maximum pressure and cumulative leakage to find out the optimal dosage of various plugging materials in the plugging drilling fluid. The specific experiment steps are as follows: (1) the fracture module was put into the static plugging assessment instrument, and the measuring cylinder was placed at the lower liquid outlet; (2) the base mud with the single plugging material or multiple plugging materials was poured into the static plugging assessment device slowly after stirring fully; (3) the corresponding pipelines were connected to pressurize step by step according to the specified pressure gradient and the leakage and cumulative leakage under different pressures were measured; (4) the maximum pressure and critical pressure were analyzed and recorded according to the volume and state of leakage; and (5) the optimal single agent dosage and compounding dosage were worked out according to the relevant parameters.
2.3.3. Compatibility experiment of drilling fluid
On the basis of the above experiments, the optimal dosage of rigid particle GZD, lignin fiber, elastic filling particle SQD-98 and calcium carbonate were sequentially added to the drilling fluid system currently used in the central Tarim Basin, and the ZNN-D6B rotary viscometer and triple medium- pressure filtration instrument SD3 were used to measure the rheological parameters and fluid loss parameters of the system before and after the plugging materials were added to analyze the compatibility of the plugging materials with the drilling fluid.
2.3.4. Experiment to assess crack plugging effect
The optimal dosage of lost circulation materials were sequentially added to the current drilling fluid used in central Tarim to form a new plugging drilling fluid, and plugging experiments on wedge-shaped fracture (2.0 mm × 1.5 mm) and parallel fracture (2 mm × 2 mm) were carried out respectively under static pressure at room temperature (25 °C) in a static plugging assessment instrument, and the experimental data was recorded to analyze the plugging effect of the plugging drilling fluid on the fractures. The steel fracture cores used in the experiment were columnar steel blocks with cracks in different shapes cut physically to represent wedge- shaped fractures and parallel fractures of specified sizes, and the structural schematic diagram of them are shown in Fig. 6.
Fig. 6.
Fig. 6.
Schematic diagram of wedge-shaped fracture and parallel fracture.
2.3.5. Hole plugging effect assessment experiment
(1) Rock debris sand beds with different vug diameters were made by adding 0.850-2.000 mm (10-20 mesh), 0.425- 0.850 mm (20-40 mesh), 0.250-0.425 mm (40-60 mesh), 0.180-0.250 mm (60-80 mesh) and 0.150-0.180 mm (80-100 mesh) of drill cuttings into transparent rigid plastic round tubes respectively. (2) The prepared plugging drilling fluid was added into the medium pressure sand bed filter loss instrument filled with rock debris slowly and a certain pressure was exerted to measure the filter loss of drilling fluid and invasion depth of sand bed at specific pressure (0.69 MPa) and specific moments (1.0 min, 7.5 min and 30.0 min). (3) According to the measured data, the plugging performance of the plugging drilling fluid was analyzed and evaluated.
3. The results and discussions
3.1. Results of lost circulation material selection
Fig. 7.
Fig. 7.
Schematic diagram before and after heat treatment.
Table 1 Results of temperature resistance experiments of rigid particles.
| Sample name | No | M1/g | M2/g | K/% |
|---|---|---|---|---|
| GZD | 1 | 30 | 28.65 | 4.50 |
| 2 | 30 | 28.63 | 4.57 | |
| 3 | 30 | 28.66 | 4.47 | |
| Average value | 30 | 28.64 | 4.51 | |
| Walnut shell | 1 | 30 | 20.11 | 32.97 |
| 2 | 30 | 20.42 | 31.93 | |
| 3 | 30 | 20.39 | 32.03 | |
| Average value | 30 | 20.31 | 32.30 |
It can be seen from the experiment results that the temperature resistance and high-temperature abrasiveness of the walnut shell are poorer. After high-temperature hot rolling, the walnut shell changed from the original brownish yellow to reddish brown, showing obvious high-temperature carbonization. It has an average mass loss rate of 32.30%, that is up to 1/3 amount of loss, suggesting it has serious wear at high temperature and can’t be used as lost circulation control material for high temperature and high pressure formations. Under the same conditions, the rigid particle GZD has hardly any carbonization and wear, and is always milk white before and after hot rolling. After high-temperature hot rolling, its average mass loss rate is only 4.51%, proving it has good resistance and abrasiveness at high temperature, and can meet the needs of plugging reservoirs with abnormally high temperature.
On the basis of high-temperature resistance experiment and high-temperature abrasiveness experiment, the rigid particles GZD of different sizes, i.e. A, B and X (mixing ratio C:D=1:1) were added to the base mud for static pressure-bearing plugging experiment, to test the optimal dosage and the optimal particle size ratio of GZD. The experimental results are shown in Table 2.
Table 2 Results of rigid particle GZD dosage optimization.
| Dosage/% | Maximum pressure/ MPa | Leakage/ mL | Cumulative leakage/mL | Critical pressure/ MPa |
|---|---|---|---|---|
| 1% A | 0 | / | / | 0.5 |
| 2% A | 0 | 168 | 168 | 0.5 |
| 3% A | 0.5 | 102 | 122 | 1.0 |
| 4% A | 0.5 | 87 | 199 | 1.0 |
| 4% A+1% B | 0.5 | 80 | 186 | 1.0 |
| 4% A+2% B | 1.0 | 52 | 216 | 1.5 |
| 4% A+3% B | 1.0 | 48 | 200 | 1.5 |
| 4% A+2% B+1% X | 1.0 | 50 | 194 | 1.5 |
| 4% A+2% B+2% X | 3.0 | 10 | 193 | 3.5 |
| 4% A+2% B+3% X | 3.0 | 12 | 186 | 3.5 |
Note: “/”—full loss; maximum pressure—the maximum pressure bearing value before the plugging layer completely fails or the plugging layer does not break or fail under this pressure; the leakage—the leakage under the maximum pressure; the cumulative leakage— total leakage before the plugging layer completely fails; critical pressure - the minimum pressure at which the plugging layer fails completely.
From the experimental results in Table 2, it can be seen that: (1) When the GZD particles of single size is added, the leakage and the cumulative leakage are not negatively correlated with the addition amount of the particle, and when the addition amount increases from 3% to 4%, the cumulative leakage increases significantly rather than decreases, and the maximum pressure and critical pressure are 0.5 MPa and 1.0 MPa, respectively, indicating that the increase of the particle volume of a single size not only fails to achieve efficient plugging, but also results in reduction of the compactness of the plugging layer and increase of the leakage. This has a good consistency with the bridging plugging mechanism[14,15]. (2) With the addition of GZD particles in Grade B and Grade X , the pressure bearing of the base mud system improves and the leakage drops substantially, the critical pressure increases from 0.5 MPa to 3.5 MPa, and the leakage goes down from the original full loss at 0 MPa (400 mL, single-group experimental mud amount) to 10 mL at 3 MPa, which is a reduction of 97.5%, indicating that GZD particles of different sizes are synergized excellently, and the plugging layer with strong pressure bearing capacity and small leakage loss can be formed through combination of different sizes of the particle itself.
In the lab study, the GZD particles A and B of large sizes were used as the skeleton particles, and the particle X of smaller size was used as the filling particle to test the optimal addition amount of the lignin fiber, and the experimental results are shown in Table 3.
Table 3 Optimization results of composite dosage of GZD and lignin fiber.
| Dosage/% | Maximum pressure/ MPa | Leakage/ mL | Cumulative leakage/ mL | Critical pressure/ MPa |
|---|---|---|---|---|
| 4% A+2% B+2% X | 3 | 10.0 | 193 | 3.5 |
| 4% A+2% B+2% X+ 0.5% lignin fiber | 4 | 2.0 | 92 | 4.5 |
| 4% A+2% B+2% X+ 1.0% lignin fiber | 4 | 1.8 | 89 | 4.5 |
It can be concluded from the experimental results that after adding a small amount of lignin fiber, the leakage loss of the original system (4% A+2% B+2% X added) reduces apparently from the original 10.0 mL to 1.8 mL, which is a drop of 82%, and the pressure bearing capacity of the plugging layer increases from 3 MPa to 4 MPa. This is because lignin has good water absorption property and can absorb 6-8 times of water of its own weight. Its favorable water transfer function and water dispersing capacity can enhance the surface strength of the plugging layer and the bonding strength between other materials and its own particles, thus the fracture resistance[16,17,18,19] of the material, and the strength and compactness of the mud cake, which functions like “reinforced ribbing and fracture arresting”[16]. However, when the addition amount of lignin fiber increases by one time, the filter loss only reduces by 0.2 mL, and the maximum pressure and critical pressure remains unchanged. Therefore, the addition amount of lignocellulose shall be controlled at about 0.5%.
On the basis of the optimization experiment of addition amount of lignin fiber, the addition amount of the elastic filling material SQD-98 was further tested and the results are shown in Table 4.
Table 4 Optimization results of composite dosage of GZD, lignin fiber and SQD-98.
| Dosage/% | Maximum pressure/MPa | Leakage/ mL | Cumulative leakage/mL | Critical pressure/MPa |
|---|---|---|---|---|
| 1#+1% SQD-98 (large) | 4.0 | 12 | 134 | 4.5 |
| 1#+2% SQD-98 (large) | 4.0 | 9 | 123 | 4.5 |
| 1#+3% SQD-98 (large) | 4.5 | 7 | 125 | 5.0 |
| 2#+1% SQD-98 (middle) | 5.5 | 13 | 153 | 6.0 |
| 2#+2% SQD-98 (middle) | 6.5 | 10 | 123 | 7.0 |
| 2#+3% SQD-98 (middle) | 7.0 | 2 | 114 | 7.5 |
Note: 1#—4% A+2% B+2% X+0.5% lignin fiber; 2#—4% A+2% B+2% X+0.5% lignin fiber+3% SQD-98 (coarse).
It can be seen from Table 4 that compared with the 1# system, with the increase of amount of the elastic plugging material SQD-98, the minimum leakage of the two are similar, but the pressure bearing capacity of the layer increases by 75%, and the maximum pressure value reaches up to 7 MPa. This is because SQD-98 is an elastic deformable material that enters the gaps under pressure and bonds the entire plugging layer more firmly, so it is harder for the skeleton particles, or the skeleton particles and the filling particles to move relatively, and the shear resistance performance of the entire plugging layer is greatly improved[20,21].
Calcium carbonate, as a rigid plugging particle, is widely used in the oil industry[22,23]. Its more than 99% of ultra-high acid solubility is beneficial to the later plugging removal with acidizing and the efficient development of oil fields, which can protect the reservoir to a certain extent. Calcium carbonate was added to the 3# formula (2#+3% SQD-98 (middle)) for static pressure bearing and plugging experiment, and the experimental results are shown in Table 5.
Table 5 Optimization results of composite dosage of GZD, lignin fiber, SQD-98 and CaCO3.
| Dosage /% | Maximum pressure/MPa | Leakage/ mL | Cumulative leakage/mL | Critical pressure/MPa |
|---|---|---|---|---|
| 3#+1%CaCO3 | 7 | 10 | 98 | 7.5 |
| 3#+2%CaCO3 | 7 | 6 | 88 | 7.5 |
Note: 3#—2#+3% SQD-98 (middle)
It can be seen from Table 5 that after the calcium carbonate is added to the System 3#, the cumulative leakage of the drilling fluid reduces substantially from 114 mL to 88 mL, and the pressure bearing capacity remains unchanged. This indicates that calcium carbonate synergizes with the elastic material SQD-98, filling the tiny pores formed by the rigid particles GZD and lignin fiber, further strengthening the plugging layer compactness.
Through the above-mentioned temperature resistance experiment, high temperature abrasiveness experiment and material optimization experiment, it can be concluded that the rigid lost circulation material GZD has good temperature resistance and high temperature abrasiveness, and can form compact plugging layer with lignin fiber, SQD-98 and calcium carbonate. The optimal complex ratio of them is 8% GZD (A: B: X = 2: 1:1) + 0.5% lignin fiber + 6% SQD (medium: coarse = 1: 1) +2% calcium carbonate (0.019 mm: 0.002 1 mm (800 mesh: 1 200 mesh) = 1:1).
3.2. Compatibility experiment results of drilling fluid system
According to the optimal composite amount, the lost circulation control materials were sequentially added to the drilling fluid system used in the Central Tarim Basin, and the rheological parameters and fluid loss parameters of the drilling fluid system before and after the lost circulation control materials were added were measured to analyze the compatibility of the lost circulation control materials and the on-site drilling fluid, and the experimental results are shown in Table 6.
Table 6 Experimental results of compatibility between plugging materials and drilling fluid.
| Formula | Test condition | Density/ (g•cm-3) | Apparent viscosity/(mPa·s) | Plastic viscosity/(mPa·s) | Shear stress/Pa | Initial stress (final stress)/Pa | API filter loss/mL |
|---|---|---|---|---|---|---|---|
| Mud | Room temperature | 1.80 | 65 | 57 | 8 | 1.5(2.0) | 3.8 |
| Mud & LCM | 1.86 | 75 | 64 | 11 | 2.0(2.5) | 3.6 | |
| Mud | High temperature (200 °C) | 2.00 | 88 | 76 | 12 | 2.0(2.6) | 3.6 |
| Mud & LCM | 2.05 | 97 | 80 | 16 | 2.0(3.0) | 3.2 |
It can be seen from Table 6 that the lost circulation control materials selected through experiment have good compatibility with the on-site drilling fluid system, and the lost circulation control materials have little influence on the rheological properties of the drilling fluid. Moreover, they improve the filtration reduction property of the current drilling fluid used in the Central Tarim Basin and reduce the filter loss.
3.3. Results of fracture plugging experiments
In order to further analyze and evaluate the plugging effect of the drilling fluid system after adding the lost circulation control material, a set of wedge-shaped fractures (2.0 mm×1.5 mm) and parallel fractures (2 mm×2 mm) were used for static plugging and assessment experiments. The experimental results are shown in Fig. 8.
Fig. 8.
Fig. 8.
Static pressure test results on fractures.
It can be seen from Fig. 8a that as the pressure increases, the leakage at a certain pressure first increases and then decreases, and the cumulative leakage gradually increases and finally stabilizes. When the pressure is less than 7 MPa, the leakage tends to decrease on the whole but increases transiently at 0.5 MPa. This is because an effective plugging layer has not been formed before the pressure is applied, and the plugging layer is compacted after the pressure is applied, so the leakage loss increases transiently; as the pressure acting on the plugging layer gets higher and higher, the plugging layer is further compacted, and the leakage loss is reduced to zero finally. At this point, the cumulative leakage no longer changes any more. When the pressure is greater than 7 MPa, the leakage is always zero with the increase of pressure, and the cumulative leakage remains unchanged. This is because a dense and high-strength plugging layer is formed at the mouth and middle of the fracture, even if the pressure continues to increase, mud will not lose any more. It can be seen from Fig. 8b that the turning point with zero leakage is also present at 7 MPa, that is, when the pressure is greater than 7 MPa, the fracture is completely plugged and no leakage occurs; however, unlike the curve in Fig. 8a, when the pressure is at 2-6 MPa, the leakage is in nonlinear relationship with the pressure. Due to the different shapes of the fractures, the wedge-shaped fracture is favorable for the accumulation and compaction of the lost circulation control particles, so an effective plugging layer can be formed in a short time in this kind of fracture; while it takes longer time and under higher pressure to form a plugging layer of the same effect in the parallel fracture. In summary, it can be concluded that the plugging drilling fluid system has excellent plugging effect on fractures, and regardless in wedge-shaped fracture or parallel fracture, once the plugging layer is formed, it would have high pressure bearing capacity and extremely low leakage. No damage or leakage occurs under the pressure of up to 9 MPa, and the cumulative leakage is only 13.4 mL, which can meet the plugging needs of abnormally high-pressure fractured formations in the central Tarim Basin.
3.4. Results of sand bed plugging experiments
By measuring the intrusion depth and leakage to the rock debris sand bed at different moments, the plugging effect of the plugging drilling fluid on vugs was analyzed and assessed (Fig. 9).
Fig. 9.
Fig. 9.
Results of sand bed experiment
It can be seen from Fig. 9 that as the particle size of the rock debris decreases, (i.e. the vug radius of the sand bed decreases) the intrusion depth of the sand bed also decreases. When the particle size of the rock debris is larger, the intrusion depth of mud into the sand bed decreases more obviously with the decrease of the particle size. When the particle size is smaller, the intrusion depth of drilling mud into the sand bed decreases slowly. As time goes by, the invasion depth increases, but the overall invasion depth (less than 2.5 cm) is small. When the particle size of the rock debris is 0.850-2.000 mm (10-20 mesh), and the cumulative intrusion time increases from 1 min to 30 min, the intrusion depth into the sand bed only increases by 1 cm, which indicates that the plugging drilling fluid can form a dense plugging layer in a short time, and the damage to reservoir caused by intrusion of liquid phase can be mitigated. When the vug radius of the sand bed is the largest, the intrusion depth is only 2.5 cm in 30 min. When the vug radius of the sand bed is smaller, the intrusion depth is less than 1.0 cm in 30 min, indicating that the plugging drilling fluid system can not only plug fractures and vugs of large sizes, but also effectively prevent the filtration leakage.
4. Conclusions
The rigid lost circulation control material GZD with high temperature resistance, high pressure bearing capacity and high temperature abrasiveness is introduced, which can form a new type of composite lost circulation material with high temperature resistance and high pressure bearing capacity with lignin fiber, elastic lost circulation control material SQD-98 and calcium carbonate. It is named SXM-I, and the main components and their mixing proportion are: (8% to 10%) GZD (A: B: X = 2: 1:1) + (0.5% to 1%) lignin fiber + (6% to 8%) SQD-98 (medium: coarse = 1:1) + (1% to 2%) calcium carbonate (0.019 0 mm: 0.0021 mm (800 mesh: 1 200 mesh) = 1:1).
The lost circulation material has good compatibility with the drilling fluid system currently used in the central Tarim Basin, and the formed plugging drilling fluid can resist the high temperature of more than 180 °C and has a density of 1.80 g/cm3 or more, meeting the requirements of drilling abnormally high temperature and high pressure wells in the block. The newly prepared plugging drilling fluid system has good plugging effect on fractures and vugs. The plugging layer formed in fracture can bear the pressure of above 9 MPa, and the cumulative leakage reduces to 13.4 mL. The cumulative invasion depth of the large-diameter rock debris sand bed is only 2.5 cm/30 min, the cumulative intrusion amount of small-diameter rock debris sand bed is less than 1.0 cm/30 min. A plugging layer with high pressure bearing capacity and ultra-low-permeability can be formed in a short time. This plugging material provides technical support for the drilling of abnormally high pressure and high temperature reservoirs in the central Tarim Basin.
Reference
Mechanism of temporary sealing for fractured-vuggy carbonate reservoir
For the problem of co-existence of micron to millimeter-level loss channel in fractured-vuggy reservoirs,this paper suggested that good and fast crack plugging is the key to leakage control and reservoir protection.Drilling fluids dynamic damage evaluation experiments in zone 12 of Tahe Oilfield were conducted,the results indicated this drilling fluid could not meet the requirement of mud loss and formation damage control in fractured-vuggy reservoir.The drilling fluid was improved using temporary plugging theory.The temporary sealing rate of the modified drilling fluid reached 99.999% after 3 mins.The accumulated mud loss was less than 0.5 mL after an hour.The ability was increased by 2.3 times and the recovery rate was improved by 60%.All these indicate the temporary sealing is feasible.The temporary sealing has the characteristics of broad-range of particle size,fast bridging,compact sealing,moderate invasion,bi-directional pressure bearing and acid-removing etc.The differences between temporary sealing and temporarily shield sealing are mud loss formation,sealing form duration,press resistance,seal thickness,and filter cake removing methods.
A new plugging-while-drilling fluid of high bearing strength and acid solubility with multifaceted zigzag metal particles as a skeleton material
When a high-density and oil-based drilling fluid is used in ultradeep tight sandstone fractured gas reservoirs,the conventional lost circulation materials could not meet the requirements of oil phase dispersion,acid solubility and high pressure in the case of complicated lost circulation.In this paper,the aluminum alloy material with the properties of oil phase dispersion,high rigidity and high acid solubility was introduced to solve the problem.This material is composed of multifaceted zigzag aluminum alloy particles(GYD) and its density is 1.60 g/cm~3.The foamed aluminum alloy is ground into 3 80 meshes.Compared with the currently used marble bridge particles,GYD are dispersed completely in oil phase,its Mohs hardness is two times higher(up to 5.0 6.0) and acid solubility is over 90%.According to bridge rules of 1/2 2/3 opening degree of formation fractures,laboratory HTHP static and dynamic lost circulation simulation and evaluation were conducted on the slurry which is composed of high-density oil based drilling fluids(as the base slurry),GYD(as the skeleton particles),fiber material and variable filling particles.It is shown that the plugging pressure is over 25 MPa,acid solubility is 65%,the slurry is stable and the density difference(48 hrs.) is less than 0.03 g/cm~3 when the dosage of GYD is 8%.This material was tested on site at a certain well in the piedmont zone of the Tarim Basin.The gas reservoir in this well is 7 220 m deep.It is shown that the daily leakage was about 50 m~3 when the high-density and oil-based drilling fluid was used.When the GYD drilling fluid was adopted,however,the reservoir is drilled smoothly without any leakage.And thus,the sealing function of "high-rigidity bridge + fiber network +deformable filling" is better exerted on the formation fractures with a high flow resistance and a high loading capacity.
Testing and application of high temperature drilling for leakage material
Development and application of chemical consolidation plugging agent HDL-1
DOI:10.3969/j.issn.10010890.2014.02.014
URL
[Cited within: 1]
In order to effectively seal fractured thief zone,and avoid poor plugging effect caused by particle size match error,chemical consolidation plugging agent HDL-1has been developed with features of high strength and high temperature resistance.In view of shortcomings of conventional consolidation plugging agents such as poor resistance to high temperature and low density adjustability,the authors selected positron bonding material,chemical coagulant,reology control material,triggering material and density modifiers after extensive laboratory tests on retention ability,consolidation time,strength and density,determined the optimal proportion of these materials,and developed the chemical consolidation plugging agent HDL-1.The plugging agent HDL-1had advantages such as adjustable thickening time(4-12h),adjustable density(1.3-1.9g/cm3),and high pressure bearing capacity(48hstrength17MPa).The chemical consolidation plugging agent has been successfully applied to improve the formation bearing capacity by 8.2MPa for Well SX2Permian fractured thief zone in Tahe Oilfield,resulting in satisfactory drilling results.The chemical consolidation plugging agent HDL-1could effectively seal fractured thief zone,which provided technical support for the sealing of Permian igneous leakage zones.
Study on grain swelling plugging agents with high temperature and salt tolerance
A new polymer gel plugging agent with high-temperature and salt resistance is successfully synthesized with polyethylene glycol( PEG-8000) as interpenetrating agent and diethylene glycol divinyl ether as crosslinking agent.The influences of monomer ratio,the monomer concentration,PEG dosage and crosslinking agent are discussed. Texture analyzer,FT-IR,SEM and TGA are applied to characterize the compressive strength and structure of the obtained plugging agent. The carrier fluid system is also optimized. The plugging performance of the gel is investigated by core permeability tester. The results show that good gel strength and water absorption properties can be achieved in the oil bath at 160 for 40 days. In the simulated underground water with 25% salt,the water absorption rate is approximately10 g / g. At the same time,the gel has good salt tolerance. In the core experiment,the plugging rate is over 95% with breakthrough pressure above 2 MPa.
Development of a novel two-step crosslinking gel with high temperature resistance for plugging lost circulation
For plugging lost circulation in high temperature formations,normal gels with lower thermal stability are unable to be applied. To solve the problem,a novel two-step crosslinking gel containing thermal stable polymer GN-1 was developed,which performed high temperature resistance. The optimized gel system is fresh water + 1. 5% GN-1 + 0. 15%ground crosslinkers + 1% downhole crosslinkers + 0. 2% antioxidant. Thermal stability of the gel was evaluated via hot rolling and high temperature rheological measurement. After hot rolling at 180℃ for 24 h,the shear viscosity of the gel at the rate of 7. 34s-1was above 10000 m Pa·s. The viscoelasticity of the high-density gel system remained good after aging for24 h under 180℃. The results showed that the gel had excellent heat-resisting property. The influence of environmental temperature and p H value to gelling performance and the compatibility with polysulfonate drilling fluid system was also discussed,which was very helpful to operation in future.
Preparation and evaluation of new borehole wall strengthening agent
The preparation and evaluation of a new lost circulation material ZTC-1
A new lost circulation additive,ZTC-1,is developed using almond shell as the main raw material through crushing,degreasing,drying,comminuting,and pelleting.Laboratory experiments show that ZTC-1 generates an effective,high strength sealing on borehole wall instantaneously with shallow particle invasion in the formation.ZTC-1 has lagged swelling,with a larger swelling volume compared with conventional LCM.ZTC-1,when mixed with conventional LCM,can be used in sealing mud losses in fracturing, cracked,porous and permeable formations,as well as in loss zones that are difficult to locate.
Investigation of the mitigation of lost circulation in oil-based drilling fluids by use of gilsonite
Study on drilling fluid bridge plugging technology for fractured formation
New theory and method for optimizing the particle size distribution of bridging agents in drilling fluids
The crack resistance influence mechanism of lignin fiber on warm-high modulus asphalt and its mixture
Study on pavement performance of asphalt mixture with fiber
DOI:10.1300/J091v15n02_04
URL
[Cited within: 1]
A series of test are carried out to analyze the characteristics of high temperature rutting ,low temperature crack resistance ,water stability and fatigue resistance of asphalt mixture with fiber. The results show that fiber can perfect pavement performance of asphalt mixture. So asphalt mixture with fiber has great economic benefits.
Study on properties of lignin fiber reinforced friction material
A lignin fiber reinforced friction material was prepared,and its mechanical properties and tribological properties were investigated by using the impact tester and hardness meter,and the friction and wear tester,respectively. The morphology of the cross section and the worn surface of the friction materials were analyzed by Scanning Electron Microscope( SEM) and X-ray diffractometer,and the wear mechanism of friction materials was investigated. The results show that,compared with steel fiber,the lignin fiber can improve the impact toughness and fracture toughness of friction materials,and reduce the hardness of friction materials. A layer of dense friction and transition film is formed by the lignin fiber on the surface of friction material during the friction process,so the lignin fiber reinforced friction material has stable friction coefficient,has no significant decrease of friction coefficient when the temperature is above 300 ,and its wear rate is within the provisions of national standard. The analysis results by SEM and X-ray diffractometer show that lignin fiber has high combining strength with matrix,and has a obvious enhancement effect to friction materials. The main wear mechanisms of lignin fiber reinforced friction material are abrasive wear and fatigue abrasion at high temperature.
Application of granular lignin fiber
Mud loss control under pressure in No.1 Structure of Nanpu: Study and application
DOI:10.3969/j.issn.1001-5620.2014.05.016
URL
[Cited within: 1]
Abstract The Structure No.1 of Nanpu, an artificial island, has found formation pressure depletion because of long period of production. Mud losses have been a major problem for safe and economic drilling operation. A high performance LCM slurry, which is aimed at controlling mud losses under pressure, is recently developed. This LCM slurry uses large bridging particles, flake LCM NTS-M and NTS-S and fiber LCM SQD-98 as well as sized packing LCM in a certain mass ratio, can hold pressures up to 7 MPa in laboratory test. Mud losses in well Nanpu 13-1170 was completely controlled at the first use of this LCM slurry, and subsequent jobs such as casing running, cementing were all successful.
Mechanism simulation of oil displacement by imbibition in fractured reservoirs
DOI:10.1016/S1876-3804(17)30091-5
URL
[Cited within: 1]
The mechanism model of both static and dynamic imbibition considering capillary pressure and gravity was presented based on the imbibition mechanisms and seepage theory.The validation of the model was performed using published experiment data.Then,this model was employed to study the impacts of oil viscosity,matrix permeability,core size,interface tension,and displacement rate on imbibitions.The results show that,the recovery decreases as oil viscosity increases,and the initial imbibition rate is much faster for lower viscosity oil.Imbibitions recovery is positively related to matrix permeability,the differences of oil recovery for low-permeability to tight oil reservoirs are obvious.Imbibitions effect is negatively related to core size.If the interface tension is low,imbibitions cannot occur without consideration of gravity.But it can occur even in very low interface tension scenario with consideration of gravity.On the whole,the recovery first increases and then decreases as the interface tension decreases.The gravity and capillary play different roles at different ranges of interface tension.There exists an optimal value range of displacement rate in fractured reservoir,which should be optimized with a sufficient oil production rate to achieve higher recovery.
Plugging performance of sepiolite drilling mud with calcium carbonate
DOI:10.1016/S1876-3804(17)30051-4 URL [Cited within: 1]
Application of plugging-while-drilling technology in long open-hole interval of the Well Yueman-3-3
Yueman Block is a key development target in Tarim Oilfield in the past few years. Although the open-hole second spud-in interval is up to 5 500 m, and the well penetrates multiple layers, the safe density window is only 0.01~0.02 g/cm3. During drilling operation, lost circulation and stability of sidewalls are frequent problems. Using conventional polysulfonate drilling fluid, daily consumption of the drilling fluid may up to 100 m3, which significantly limits the application of polysulfonate drilling fluid in the area. To eliminate these problems, plugging-while-drilling agents, ultra-fine calcium carbonate, nanometer plugging agents and other plugging materials with desirable properties have been used in accordance with specific formation conditions in the Yueman Block. Assessment on HTHP filtration loss, HTHP water loss through filtration, HTHP sand-bed filtration, HTHP sand-bed water loss through filtration and core displacement capacities in both directions provides competent plugging formula suitable for specific geologic conditions in the Block. These drilling fluids have been deployed in combination with existing drilling fluid system in Well Yueman-3-3. Compared with adjacent wells, this well is suitable for the drilling fluid having a density higher by 0.02~0.04 g/cm3, but with no outstanding lost circulation. The innovative technology may provide satisfactory solution to filtration loss, sidewall instability and some other technical problems in the Yueman Block.
